script
elementnoscript
elementtemplate
elementcanvas
elementDrawingStyle
objectsPath2D
objectscanvas
elementsScripts allow authors to add interactivity to their documents.
Authors are encouraged to use declarative alternatives to scripting where possible, as declarative mechanisms are often more maintainable, and many users disable scripting.
For example, instead of using script to show or hide a section to show more details, the
details
element could be used.
Authors are also encouraged to make their applications degrade gracefully in the absence of scripting support.
For example, if an author provides a link in a table header to dynamically resort the table, the link could also be made to function without scripts by requesting the sorted table from the server.
script
elementsrc
attribute, depends on the value of the type
attribute, but must match
script content restrictions.src
attribute, the element must be either empty or contain only
script documentation that also matches script
content restrictions.src
— Address of the resourcetype
— Type of embedded resourcecharset
— Character encoding of the external script resourceasync
— Execute script when available, without blockingdefer
— Defer script executioncrossorigin
— How the element handles crossorigin requestsinterface HTMLScriptElement : HTMLElement { attribute DOMString src; attribute DOMString type; attribute DOMString charset; attribute boolean async; attribute boolean defer; attribute DOMString? crossOrigin; attribute DOMString text; };
The script
element allows authors to include dynamic script and data blocks in
their documents. The element does not represent content for the
user.
When used to include dynamic scripts, the scripts may either be embedded inline or may be
imported from an external file using the src
attribute. If
the language is not that described by "text/javascript
", then the type
attribute must be present, as described below. Whatever
language is used, the contents of the script
element must conform with the
requirements of that language's specification.
When used to include data blocks (as opposed to scripts), the data must be embedded inline, the
format of the data must be given using the type
attribute,
the src
attribute must not be specified, and the contents of
the script
element must conform to the requirements defined for the format used.
The type
attribute gives the language of the
script or format of the data. If the attribute is present, its value must be a valid MIME
type. The charset
parameter must not be specified. The default, which
is used if the attribute is absent, is "text/javascript
".
The src
attribute, if specified, gives the
address of the external script resource to use. The value of the attribute must be a valid
non-empty URL potentially surrounded by spaces identifying a script resource of the type
given by the type
attribute, if the attribute is present, or
of the type "text/javascript
", if the attribute is absent. A resource is a
script resource of a given type if that type identifies a scripting language and the resource
conforms with the requirements of that language's specification.
The charset
attribute gives the character
encoding of the external script resource. The attribute must not be specified if the src
attribute is not present. If the attribute is set, its value
must be an ASCII case-insensitive match for one of the labels of an encoding, and must specify the same encoding as
the charset
parameter of the Content-Type
metadata of the external file, if any. [ENCODING]
The async
and defer
attributes are boolean attributes that indicate how the script should be executed. The defer
and async
attributes
must not be specified if the src
attribute is not
present.
There are three possible modes that can be selected using these attributes. If the async
attribute is present, then the script will be executed
as soon as it is available, but without blocking further parsing of the page. If the async
attribute is not present but the defer
attribute is
present, then the script is executed when the page has finished parsing. If neither attribute is
present, then the script is fetched and executed immediately, before the user agent continues
parsing the page.
The exact processing details for these attributes are, for mostly historical
reasons, somewhat non-trivial, involving a number of aspects of HTML. The implementation
requirements are therefore by necessity scattered throughout the specification. The algorithms
below (in this section) describe the core of this processing, but these algorithms reference and
are referenced by the parsing rules for script
start and end tags in HTML, in foreign content,
and in XML, the rules for the document.write()
method, the handling of scripting, etc.
The defer
attribute may be specified even if the async
attribute is specified, to cause legacy Web browsers that
only support defer
(and not async
) to fall back to the defer
behaviour instead of the blocking behaviour that
is the default.
The crossorigin
attribute is a
CORS settings attribute. It controls, for scripts that are obtained from other origins, whether error information will be exposed.
Changing the src
, type
, charset
, async
, defer
, and crossorigin
attributes dynamically has no direct effect;
these attribute are only used at specific times described below.
A script
element has several associated pieces of state.
The first is a flag indicating whether or not the script block has been "already
started". Initially, script
elements must have this flag unset (script blocks,
when created, are not "already started"). The cloning
steps for script
elements must set the "already started" flag on the copy if
it is set on the element being cloned.
The second is a flag indicating whether the element was "parser-inserted".
Initially, script
elements must have this flag unset. It is set by the HTML
parser and the XML parser on script
elements they insert and
affects the processing of those elements.
The third is a flag indicating whether the element will "non-blocking". Initially,
script
elements must have this flag set. It is unset by the HTML parser
and the XML parser on script
elements they insert. In addition, whenever
a script
element whose "non-blocking" flag is set has a async
content attribute added, the element's
"non-blocking" flag must be unset.
The fourth is a flag indicating whether or not the script block is "ready to be
parser-executed". Initially, script
elements must have this flag unset (script
blocks, when created, are not "ready to be parser-executed"). This flag is used only for elements
that are also "parser-inserted", to let the parser know when to execute the
script.
The last few pieces of state are the script block's
type, the script block's character
encoding, and the script block's
fallback character encoding. They are determined when the script is prepared, based on
the attributes on the element at that time, and the
script
element's node document.
When a script
element that is not marked as being "parser-inserted"
experiences one of the events listed in the following list, the user agent must immediately
prepare the script
element:
script
element gets inserted
into a document, at the time the node is inserted
according to the DOM, after any other script
elements inserted at the same time that
are earlier in the Document
in tree order.script
element is in a Document
and a node or
document fragment is inserted into the
script
element, after any script
elements inserted at that time.script
element is in a Document
and has a src
attribute set where previously the element had no such
attribute.To prepare a script, the user agent must act as follows:
If the script
element is marked as having "already started", then
the user agent must abort these steps at this point. The script is not executed.
If the element has its "parser-inserted" flag set, then set was-parser-inserted to true and unset the element's "parser-inserted" flag. Otherwise, set was-parser-inserted to false.
This is done so that if parser-inserted script
elements fail to run
when the parser tries to run them, e.g. because they are empty or specify an unsupported
scripting language, another script can later mutate them and cause them to run again.
If was-parser-inserted is true and the element does not have an async
attribute, then set the element's
"non-blocking" flag to true.
This is done so that if a parser-inserted script
element fails to
run when the parser tries to run it, but it is later executed after a script dynamically updates
it, it will execute in a non-blocking fashion even if the async
attribute isn't set.
If the element has no src
attribute, and its child
nodes, if any, consist only of comment nodes and empty Text
nodes, then the user
agent must abort these steps at this point. The script is not executed.
If the element is not in a Document
, then the user agent must abort
these steps at this point. The script is not executed.
If either:
script
element has a type
attribute
and its value is the empty string, orscript
element has no type
attribute
but it has a language
attribute and that
attribute's value is the empty string, orscript
element has neither a type
attribute nor a language
attribute, then...let the script block's type for this
script
element be "text/javascript
".
Otherwise, if the script
element has a type
attribute, let the
script block's type for this script
element be the value of that attribute
with any leading or trailing sequences of space characters
removed.
Otherwise, the element has a non-empty language
attribute; let the script block's type for this
script
element be the concatenation of the string "text/
"
followed by the value of the language
attribute.
The language
attribute is never
conforming, and is always ignored if there is a type
attribute present.
If the user agent does not support the scripting language given by the script block's type for this script
element,
then the user agent must abort these steps at this point. The script is not executed.
If was-parser-inserted is true, then flag the element as "parser-inserted" again, and set the element's "non-blocking" flag to false.
The user agent must set the element's "already started" flag.
The state of the element at this moment is later used to determine the script source.
If the element is flagged as "parser-inserted", but the element's
node document is not the Document
of the parser that created the element,
then abort these steps.
If scripting is disabled for the script
element, then the user agent must abort these steps at this point. The script is not
executed.
The definition of scripting is disabled
means that, amongst others, the following scripts will not execute: scripts in
XMLHttpRequest
's responseXML
documents, scripts in DOMParser
-created documents, scripts in documents created by
XSLTProcessor
's transformToDocument
feature, and scripts
that are first inserted by a script into a Document
that was created using the
createDocument()
API. [XHR]
[DOMPARSING] [DOM]
If the script
element has an event
attribute and a for
attribute, then run these substeps:
Let for be the value of the for
attribute.
Let event be the value of the event
attribute.
Strip leading and trailing whitespace from event and for.
If for is not an ASCII case-insensitive match for the
string "window
", then the user agent must abort these steps at this
point. The script is not executed.
If event is not an ASCII case-insensitive match for
either the string "onload
" or the string "onload()
", then the user agent must abort these steps at this point. The script
is not executed.
If the script
element has a charset
attribute, then let the script block's character
encoding for this script
element be the result of getting an
encoding from the value of the charset
attribute.
Otherwise, let the script block's fallback
character encoding for this script
element be the same as the encoding of the document itself.
Only one of these two pieces of state is set.
If the element has a src
content attribute, run these
substeps:
Let src be the value of the element's src
attribute.
If src is the empty string, queue a task to fire
a simple event named error
at the element, and abort
these steps.
Resolve src relative to the element.
If the previous step failed, queue a task to fire a simple
event named error
at the element, and abort these
steps.
Do a potentially CORS-enabled fetch of the resulting
absolute URL, with the mode being the current state of the element's crossorigin
content attribute, the origin being
the origin of the script
element's node document, and the
default origin behaviour set to taint.
The resource obtained in this fashion can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.
For performance reasons, user agents may start fetching the script (as defined above) as
soon as the src
attribute is set, instead, in the hope
that the element will be inserted into the document (and that the crossorigin
attribute won't change value in the
meantime). Either way, once the element is inserted into the document, the load must have started as described in this
step. If the UA performs such prefetching, but the element is never inserted in the document,
or the src
attribute is dynamically changed, or the crossorigin
attribute is dynamically changed, then the
user agent will not execute the script so obtained, and the fetching process will have been
effectively wasted.
Then, the first of the following options that describes the situation must be followed:
src
attribute, and the element has a defer
attribute, and
the element has been flagged as "parser-inserted", and the element does not have
an async
attributeThe element must be added to the end of the list of scripts that will execute when the
document has finished parsing associated with the Document
of the parser
that created the element.
The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.
src
attribute, and the element has been flagged as
"parser-inserted", and the element does not have an async
attributeThe element is the pending parsing-blocking script of the
Document
of the parser that created the element. (There can only be one such
script per Document
at a time.)
The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.
src
attribute, and the element has been flagged as
"parser-inserted", and either the parser that created the script
is
an XML parser or it's an HTML parser whose script nesting
level is not greater than one, and the Document
of the HTML
parser or XML parser that created the script
element has
a style sheet that is blocking scriptsThe element is the pending parsing-blocking script of the
Document
of the parser that created the element. (There can only be one such
script per Document
at a time.)
Set the element's "ready to be parser-executed" flag. The parser will handle executing the script.
src
attribute, does not have an async
attribute, and does not have the
"non-blocking" flag setThe element must be added to the end of the list of scripts that will execute in order
as soon as possible associated with the node document of the script
element at the time the prepare a script algorithm started.
The task that the networking task source places on the task queue once the fetching algorithm has completed must run the following steps:
If the element is not now the first element in the list of scripts that will execute in order as soon as possible to which it was added above, then mark the element as ready but abort these steps without executing the script yet.
Execution: Execute the script block corresponding to the first script element in this list of scripts that will execute in order as soon as possible.
Remove the first element from this list of scripts that will execute in order as soon as possible.
If this list of scripts that will execute in order as soon as possible is still not empty and the first entry has already been marked as ready, then jump back to the step labeled execution.
src
attributeThe element must be added to the set of scripts that will execute as soon as
possible of the node document of the script
element at the time the
prepare a script algorithm started.
The task that the networking task source places on the task queue once the fetching algorithm has completed must execute the script block and then remove the element from the set of scripts that will execute as soon as possible.
Fetching an external script must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined above) has been run.
The pending parsing-blocking script of a Document
is used by the
Document
's parser(s).
If a script
element that blocks a parser gets moved to another
Document
before it would normally have stopped blocking that parser, it nonetheless
continues blocking that parser until the condition that causes it to be blocking the parser no
longer applies (e.g. if the script is a pending parsing-blocking script because there
was a style sheet that is blocking scripts when it was parsed, but then the script is
moved to another Document
before the style sheet loads, the script still blocks the
parser until the style sheets are all loaded, at which time the script executes and the parser is
unblocked).
When the user agent is required to execute a script block, it must run the following steps:
If the element is flagged as "parser-inserted", but the element's
node document is not the Document
of the parser that created the element,
then abort these steps.
Jump to the appropriate set of steps from the list below:
Executing the script block must just consist of firing
a simple event named error
at the element.
Executing the script block must consist of running the following steps. For the purposes of
these steps, the script is considered to be from an external file if, while the
prepare a script algorithm above was running for this script, the
script
element had a src
attribute
specified.
Initialise the script block's source as follows:
The contents of that file, interpreted as a Unicode string, are the script source.
To obtain the Unicode string, the user agent run the following steps:
If the resource's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.
If the algorithm above set the script block's character encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.
Let character encoding be the script block's fallback character encoding.
If the specification for the script block's type gives specific rules for decoding files in that format to Unicode, follow them, using character encoding as the character encoding specified by higher-level protocols, if necessary.
Otherwise, decode the file to Unicode, using character encoding as the fallback encoding.
The decode algorithm overrides character encoding if the file contains a BOM.
The external file is the script source. When it is later executed, it must be interpreted in a manner consistent with the specification defining the language given by the script block's type.
The value of the text
IDL attribute at the time
the element's "already started" flag was last set is the script source.
The child nodes of the script
element at the time the element's
"already started" flag was last set are the script source.
Fire a simple event named beforescriptexecute
that bubbles and is cancelable
at the script
element.
If the event is canceled, then abort these steps.
If the script is from an external file, then increment the
ignore-destructive-writes counter of the script
element's
node document. Let neutralised doc be that
Document
.
Let old script element be the value to which the script
element's node document's currentScript
object was most recently
initialised.
Initialise the script
element's node document's currentScript
object to the script
element.
Create a script, using the script
block's source, the URL from which the script was obtained, the script block's type as the scripting language, and
the environment settings object of the script
element's
node document's Window
object.
If the script came from a resource that was fetched in the steps above, and the resource was CORS-cross-origin, then pass the muted errors flag to the create a script algorithm as well.
This is where the script is compiled and actually executed.
Initialise the script
element's node document's currentScript
object to old script
element.
Decrement the ignore-destructive-writes counter of neutralised doc, if it was incremented in the earlier step.
Fire a simple event named afterscriptexecute
that bubbles (but is not
cancelable) at the script
element.
If the script is from an external file, fire a simple event named load
at the script
element.
Otherwise, the script is internal; queue a task to fire a simple
event named load
at the script
element.
The IDL attributes src
, type
, charset
, defer
, each must reflect the respective
content attributes of the same name.
The crossOrigin
IDL attribute must
reflect the crossorigin
content attribute.
The async
IDL attribute controls whether the
element will execute asynchronously or not. If the element's "non-blocking" flag is
set, then, on getting, the async
IDL attribute must return
true, and on setting, the "non-blocking" flag must first be unset, and then the
content attribute must be removed if the IDL attribute's new value is false, and must be set to
the empty string if the IDL attribute's new value is true. If the element's
"non-blocking" flag is not set, the IDL attribute must reflect
the async
content attribute.
text
[ = value ]Returns the contents of the element, ignoring child nodes that aren't Text
nodes.
Can be set, to replace the element's children with the given value.
The IDL attribute text
must return a
concatenation of the contents of all the Text
nodes that are children of the
script
element (ignoring any other nodes such as comments or elements), in tree
order. On setting, it must act the same way as the textContent
IDL attribute.
When inserted using the document.write()
method, script
elements execute (typically blocking further script execution or HTML parsing), but when inserted using
innerHTML
and outerHTML
attributes, they do not execute at all.
In this example, two script
elements are used. One embeds an external script, and
the other includes some data.
<script src="game-engine.js"></script> <script type="text/x-game-map"> ........U.........e o............A....e .....A.....AAA....e .A..AAA...AAAAA...e </script>
The data in this case might be used by the script to generate the map of a video game. The data doesn't have to be used that way, though; maybe the map data is actually embedded in other parts of the page's markup, and the data block here is just used by the site's search engine to help users who are looking for particular features in their game maps.
The following sample shows how a script element can be used to define a function that is then
used by other parts of the document. It also shows how a script
element can be used
to invoke script while the document is being parsed, in this case to initialise the form's
output.
<script> function calculate(form) { var price = 52000; if (form.elements.brakes.checked) price += 1000; if (form.elements.radio.checked) price += 2500; if (form.elements.turbo.checked) price += 5000; if (form.elements.sticker.checked) price += 250; form.elements.result.value = price; } </script> <form name="pricecalc" onsubmit="return false" onchange="calculate(this)"> <fieldset> <legend>Work out the price of your car</legend> <p>Base cost: £52000.</p> <p>Select additional options:</p> <ul> <li><label><input type=checkbox name=brakes> Ceramic brakes (£1000)</label></li> <li><label><input type=checkbox name=radio> Satellite radio (£2500)</label></li> <li><label><input type=checkbox name=turbo> Turbo charger (£5000)</label></li> <li><label><input type=checkbox name=sticker> "XZ" sticker (£250)</label></li> </ul> <p>Total: £<output name=result></output></p> </fieldset> <script> calculate(document.forms.pricecalc); </script> </form>
A user agent is said to support the scripting language if each component of the script block's type is an ASCII case-insensitive match for the corresponding component in the MIME type string of a scripting language that the user agent implements.
The following lists the MIME type strings that user agents must recognize, and the languages to which they refer:
application/ecmascript
application/javascript
application/x-ecmascript
application/x-javascript
text/ecmascript
text/javascript
text/javascript1.0
text/javascript1.1
text/javascript1.2
text/javascript1.3
text/javascript1.4
text/javascript1.5
text/jscript
text/livescript
text/x-ecmascript
text/x-javascript
User agents may support other MIME types for other languages, but must not support other MIME types for the languages in the list above. User agents are not required to support the languages listed above.
The following MIME types (with or without parameters) must not be interpreted as scripting languages:
These types are explicitly listed here because they are poorly-defined types that are nonetheless likely to be used as formats for data blocks, and it would be problematic if they were suddenly to be interpreted as script by a user agent.
When examining types to determine if they represent supported languages, user agents must not ignore MIME parameters. Types are to be compared including all parameters.
For example, types that include the charset
parameter will
not be recognised as referencing any of the scripting languages listed above.
script
elementsThe easiest and safest way to avoid the rather strange restrictions described in
this section is to always escape "<!--
" as "<\!--
", "<script
" as "<\script
", and "</script
" as "<\/script
" when these sequences appear in literals in scripts (e.g. in
strings, regular expressions, or comments), and to avoid writing code that uses such constructs in
expressions. Doing so avoids the pitfalls that the restrictions in this section are prone to
triggering: namely, that, for historical reasons, parsing of script
blocks in HTML is
a strange and exotic practice that acts unintuitively in the face of these sequences.
The textContent
of a script
element must match the script
production in the following ABNF, the character set for which is Unicode.
[ABNF]
script = outer *( comment-open inner comment-close outer ) outer = < any string that doesn't contain a substring that matches not-in-outer > not-in-outer = comment-open inner = < any string that doesn't contain a substring that matches not-in-inner > not-in-inner = comment-close / script-open comment-open = "<!--" comment-close = "-->" script-open = "<" s c r i p t tag-end s = %x0053 ; U+0053 LATIN CAPITAL LETTER S s =/ %x0073 ; U+0073 LATIN SMALL LETTER S c = %x0043 ; U+0043 LATIN CAPITAL LETTER C c =/ %x0063 ; U+0063 LATIN SMALL LETTER C r = %x0052 ; U+0052 LATIN CAPITAL LETTER R r =/ %x0072 ; U+0072 LATIN SMALL LETTER R i = %x0049 ; U+0049 LATIN CAPITAL LETTER I i =/ %x0069 ; U+0069 LATIN SMALL LETTER I p = %x0050 ; U+0050 LATIN CAPITAL LETTER P p =/ %x0070 ; U+0070 LATIN SMALL LETTER P t = %x0054 ; U+0054 LATIN CAPITAL LETTER T t =/ %x0074 ; U+0074 LATIN SMALL LETTER T tag-end = %x0009 ; U+0009 CHARACTER TABULATION (tab) tag-end =/ %x000A ; U+000A LINE FEED (LF) tag-end =/ %x000C ; U+000C FORM FEED (FF) tag-end =/ %x0020 ; U+0020 SPACE tag-end =/ %x002F ; U+002F SOLIDUS (/) tag-end =/ %x003E ; U+003E GREATER-THAN SIGN (>)
When a script
element contains script documentation, there are
further restrictions on the contents of the element, as described in the section below.
The following script illustrates this issue. Suppose you have a script that contains a string, as in:
var example = 'Consider this string: <!-- <script>'; console.log(example);
If one were to put this string directly in a script
block, it would violate the
restrictions above:
<script> var example = 'Consider this string: <!-- <script>'; console.log(example); </script>
The bigger problem, though, and the reason why it would violate those restrictions, is that
actually the script would get parsed weirdly: the script block above is not terminated.
That is, what looks like a "</script>
" end tag in this snippet is
actually still part of the script
block. The script doesn't execute (since it's not
terminated); if it somehow were to execute, as it might if the markup looked as follows, it would
fail because the script (highlighted here) is not valid JavaScript:
<script> var example = 'Consider this string: <!-- <script>'; console.log(example); </script> <!-- despite appearances, this is actually part of the script still! --> <script> ... // this is the same script block still... </script>
What is going on here is that for legacy reasons, "<!--
" and "<script
" strings in script
elements in HTML need to be balanced
in order for the parser to consider closing the block.
By escaping the problematic strings as mentioned at the top of this section, the problem is avoided entirely:
<script> var example = 'Consider this string: <\!-- <\script>'; console.log(example); </script> <!-- this is just a comment between script blocks --> <script> ... // this is a new script block </script>
It is possible for these sequences to naturally occur in script expressions, as in the following examples:
if (x<!--y) { ... } if ( player<script ) { ... }
In such cases the characters cannot be escaped, but the expressions can be rewritten so that the sequences don't occur, as in:
if (x < !--y) { ... } if (!--y > x) { ... } if (!(--y) > x) { ... } if (player < script) { ... } if (script > player) { ... }
Doing this also avoids a different pitfall as well: for related historical reasons, the string "<!--" in JavaScript is actually treated as a line comment start, just like "//".
If a script
element's src
attribute is
specified, then the contents of the script
element, if any, must be such that the
value of the text
IDL attribute, which is derived from the
element's contents, matches the documentation
production in the following
ABNF, the character set for which is Unicode. [ABNF]
documentation = *( *( space / tab / comment ) [ line-comment ] newline ) comment = slash star *( not-star / star not-slash ) 1*star slash line-comment = slash slash *not-newline ; characters tab = %x0009 ; U+0009 CHARACTER TABULATION (tab) newline = %x000A ; U+000A LINE FEED (LF) space = %x0020 ; U+0020 SPACE star = %x002A ; U+002A ASTERISK (*) slash = %x002F ; U+002F SOLIDUS (/) not-newline = %x0000-0009 / %x000B-10FFFF ; a Unicode character other than U+000A LINE FEED (LF) not-star = %x0000-0029 / %x002B-10FFFF ; a Unicode character other than U+002A ASTERISK (*) not-slash = %x0000-002E / %x0030-10FFFF ; a Unicode character other than U+002F SOLIDUS (/)
This corresponds to putting the contents of the element in JavaScript comments.
This requirement is in addition to the earlier restrictions on the syntax of
contents of script
elements.
This allows authors to include documentation, such as license information or API information,
inside their documents while still referring to external script files. The syntax is constrained
so that authors don't accidentally include what looks like valid script while also providing a
src
attribute.
<script src="cool-effects.js"> // create new instances using: // var e = new Effect(); // start the effect using .play, stop using .stop: // e.play(); // e.stop(); </script>
script
elements and XSLTThis section is non-normative.
This specification does not define how XSLT interacts with the script
element.
However, in the absence of another specification actually defining this, here are some guidelines
for implementors, based on existing implementations:
When an XSLT transformation program is triggered by an <?xml-stylesheet?>
processing instruction and the browser implements a
direct-to-DOM transformation, script
elements created by the XSLT processor need to
be marked "parser-inserted" and run in document order (modulo scripts marked defer
or async
),
immediately, as the transformation is occurring.
The XSLTProcessor.transformToDocument()
method
adds elements to a Document
that is not in a browsing context, and,
accordingly, any script
elements they create need to have their "already
started" flag set in the prepare a script algorithm and never get executed
(scripting is disabled). Such script
elements still need to be marked "parser-inserted", though, such that their async
IDL attribute will return false in the absence of an async
content attribute.
The XSLTProcessor.transformToFragment()
method
needs to create a fragment that is equivalent to one built manually by creating the elements
using document.createElementNS()
. For instance,
it needs to create script
elements that aren't "parser-inserted" and
that don't have their "already started" flag set, so that they will execute when the
fragment is inserted into a document.
The main distinction between the first two cases and the last case is that the first two
operate on Document
s and the last operates on a fragment.
noscript
elementhead
element of an HTML document, if there are no ancestor noscript
elements.noscript
elements.head
element: in any order, zero or more link
elements, zero or more style
elements, and zero or more meta
elements.head
element: transparent, but there must be no noscript
element descendants.HTMLElement
.The noscript
element represents nothing if scripting is enabled, and represents its children if
scripting is disabled. It is used to present different
markup to user agents that support scripting and those that don't support scripting, by affecting
how the document is parsed.
When used in HTML documents, the allowed content model is as follows:
head
element, if scripting is
disabled for the noscript
elementThe noscript
element must contain only link
, style
,
and meta
elements.
head
element, if scripting is enabled
for the noscript
elementThe noscript
element must contain only text, except that invoking the
HTML fragment parsing algorithm with
the noscript
element as the context
element and the text contents as the input must result in a list of nodes
that consists only of link
, style
, and meta
elements that
would be conforming if they were children of the noscript
element, and no parse errors.
head
elements, if scripting is
disabled for the noscript
elementThe noscript
element's content model is transparent, with the
additional restriction that a noscript
element must not have a noscript
element as an ancestor (that is, noscript
can't be nested).
head
elements, if scripting is
enabled for the noscript
elementThe noscript
element must contain only text, except that the text must be such
that running the following algorithm results in a conforming document with no
noscript
elements and no script
elements, and such that no step in the
algorithm throws an exception or causes an HTML parser to flag a parse
error:
All these contortions are required because, for historical reasons, the
noscript
element is handled differently by the HTML parser based on
whether scripting was enabled or not when the parser was
invoked.
The noscript
element must not be used in XML documents.
The noscript
element is only effective in the HTML
syntax, it has no effect in the XHTML syntax. This is because the way it works
is by essentially "turning off" the parser when scripts are enabled, so that the contents of the
element are treated as pure text and not as real elements. XML does not define a mechanism by
which to do this.
The noscript
element has no other requirements. In particular, children of the
noscript
element are not exempt from form submission, scripting, and so
forth, even when scripting is enabled for the element.
In the following example, a noscript
element is
used to provide fallback for a script.
<form action="calcSquare.php"> <p> <label for=x>Number</label>: <input id="x" name="x" type="number"> </p> <script> var x = document.getElementById('x'); var output = document.createElement('p'); output.textContent = 'Type a number; it will be squared right then!'; x.form.appendChild(output); x.form.onsubmit = function () { return false; } x.oninput = function () { var v = x.valueAsNumber; output.textContent = v + ' squared is ' + v * v; }; </script> <noscript> <input type=submit value="Calculate Square"> </noscript> </form>
When script is disabled, a button appears to do the calculation on the server side. When script is enabled, the value is computed on-the-fly instead.
The noscript
element is a blunt instrument. Sometimes, scripts might be enabled,
but for some reason the page's script might fail. For this reason, it's generally better to avoid
using noscript
, and to instead design the script to change the page from being a
scriptless page to a scripted page on the fly, as in the next example:
<form action="calcSquare.php"> <p> <label for=x>Number</label>: <input id="x" name="x" type="number"> </p> <input id="submit" type=submit value="Calculate Square"> <script> var x = document.getElementById('x'); var output = document.createElement('p'); output.textContent = 'Type a number; it will be squared right then!'; x.form.appendChild(output); x.form.onsubmit = function () { return false; } x.oninput = function () { var v = x.valueAsNumber; output.textContent = v + ' squared is ' + v * v; }; var submit = document.getElementById('submit'); submit.parentNode.removeChild(submit); </script> </form>
The above technique is also useful in XHTML, since noscript
is not supported in
the XHTML syntax.
template
elementcolgroup
element that doesn't have a span
attribute.ol
and ul
elements.dl
elements.figure
elements.ruby
elements.object
elements.video
and audio
elements.table
elements.colgroup
elements.thead
, tbody
, and tfoot
elements.tr
elements.fieldset
elements.select
elements.details
elements.menu
elements whose type
attribute is in the popup menu state.interface HTMLTemplateElement : HTMLElement { readonly attribute DocumentFragment content; };
The template
element is used to declare fragments of HTML that can be cloned and
inserted in the document by script.
In a rendering, the template
element represents nothing.
content
Returns the contents of the template
, which are stored in a
DocumentFragment
associated with a different Document
so as to avoid
the template
contents interfering with the main Document
. (For
example, this avoids form controls from being submitted, scripts from executing, and so
forth.)
Each template
element has an associated DocumentFragment
object that
is its template contents. When a template
element is created, the user
agent must run the following steps to establish the template contents:
Let doc be the template
element's node document's appropriate template contents owner
document.
Create a DocumentFragment
object whose node document is doc.
Set the template
element's template contents to the newly
created DocumentFragment
object.
A Document
doc's appropriate template contents owner
document is the Document
returned by the following algorithm:
If doc is not a Document
created by this algorithm, run
these substeps:
If doc does not yet have an associated inert template document then run these substeps:
Let new doc be a new Document
(that does not have a
browsing context). This is "a Document
created by this algorithm"
for the purposes of the step above.
If doc is an HTML document, mark new doc as an HTML document also.
Let doc's associated inert template document be new doc.
Set doc to doc's associated inert template document.
Each Document
not created by this algorithm thus gets a single
Document
to act as its proxy for owning the template contents of all
its template
elements, so that they aren't in a browsing context and
thus remain inert (e.g. scripts do not run). Meanwhile, template
elements inside
Document
objects that are created by this algorithm just reuse the same
Document
owner for their contents.
Return doc.
When a template
element's node document changes, the user agent must run the following
steps:
Let doc be the template
element's new node document's appropriate template contents owner
document.
Adopt the template
element's
template contents (a DocumentFragment
object) into doc.
The content
IDL attribute must return the
template
element's template contents.
The cloning steps for a template
element node being cloned to a copy copy must run the
following steps:
If the clone children flag is not set in the calling clone algorithm, abort these steps.
Let copied contents be the result of cloning all the children of node's template contents, with document set to copy's template contents's node document, and with the clone children flag set.
Append copied contents to copy's template contents.
In this example, a script populates a table four-column with data from a data structure, using
a template
to provide the element structure instead of manually generating the
structure from markup.
<!DOCTYPE html> <title>Cat data</title> <script> // Data is hard-coded here, but could come from the server var data = [ { name: 'Pillar', color: 'Ticked Tabby', sex: 'Female (neutered)', legs: 3 }, { name: 'Hedral', color: 'Tuxedo', sex: 'Male (neutered)', legs: 4 }, ]; </script> <table> <thead> <tr> <th>Name <th>Colour <th>Sex <th>Legs <tbody> <template id="row"> <tr><td><td><td><td> </template> </table> <script> var template = document.querySelector('#row'); for (var i = 0; i < data.length; i += 1) { var cat = data[i]; var clone = template.content.cloneNode(true); var cells = clone.querySelectorAll('td'); cells[0].textContent = cat.name; cells[1].textContent = cat.color; cells[2].textContent = cat.sex; cells[3].textContent = cat.legs; template.parentNode.appendChild(clone); } </script>
This example uses cloneNode()
on the
template
's contents; it could equivalently have used document.importNode()
, which does the same thing. The
only difference between these two APIs is when the node document is updated: with
cloneNode()
it is updated when the nodes are appended
with appendChild()
, with document.importNode()
it is updated when the nodes are
cloned.
template
elements with XSLT and XPathThis section is non-normative.
This specification does not define how XSLT and XPath interact with the template
element. However, in the absence of another specification actually defining this, here are some
guidelines for implementors, which are intended to be consistent with other processing described
in this specification:
An XSLT processor based on an XML parser that acts as described
in this specification needs to act as if template
elements contain as
descendants their template contents for the purposes of the transform.
An XSLT processor that outputs a DOM needs to ensure that nodes that would go into a
template
element are instead placed into the element's template
contents.
XPath evaluation using the XPath DOM API when applied to a Document
parsed
using the HTML parser or the XML parser described in this specification
needs to ignore template contents.
canvas
elementa
elements, img
elements with usemap
attributes, button
elements, input
elements whose type
attribute are in the Checkbox or Radio Button states, input
elements that are buttons, select
elements with a multiple
attribute or a display size greater than 1, sorting interface th
elements, and elements that would not be interactive content except for having the tabindex
attribute specified.width
— Horizontal dimensionheight
— Vertical dimensiontypedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext; interface HTMLCanvasElement : HTMLElement { attribute unsigned long width; attribute unsigned long height; RenderingContext? getContext(DOMString contextId, any... arguments); boolean probablySupportsContext(DOMString contextId, any... arguments); void setContext(RenderingContext context); CanvasProxy transferControlToProxy(); DOMString toDataURL(optional DOMString type, any... arguments); void toBlob(FileCallback? _callback, optional DOMString type, any... arguments); };
The canvas
element provides scripts with a resolution-dependent bitmap canvas,
which can be used for rendering graphs, game graphics, art, or other visual images on the fly.
Authors should not use the canvas
element in a document when a more suitable
element is available. For example, it is inappropriate to use a canvas
element to
render a page heading: if the desired presentation of the heading is graphically intense, it
should be marked up using appropriate elements (typically h1
) and then styled using
CSS and supporting technologies such as Web Components.
When authors use the canvas
element, they must also provide content that, when
presented to the user, conveys essentially the same function or purpose as the
canvas
' bitmap. This content may be placed as content of the canvas
element. The contents of the canvas
element, if any, are the element's fallback
content.
In interactive visual media, if scripting is enabled for
the canvas
element, and if support for canvas
elements has been enabled,
the canvas
element represents embedded content consisting
of a dynamically created image, the element's bitmap.
In non-interactive, static, visual media, if the canvas
element has been
previously associated with a rendering context (e.g. if the page was viewed in an interactive
visual medium and is now being printed, or if some script that ran during the page layout process
painted on the element), then the canvas
element represents
embedded content with the element's current bitmap and size. Otherwise, the element
represents its fallback content instead.
In non-visual media, and in visual media if scripting is
disabled for the canvas
element or if support for canvas
elements
has been disabled, the canvas
element represents its fallback
content instead.
When a canvas
element represents embedded content, the
user can still focus descendants of the canvas
element (in the fallback
content). When an element is focused, it is the target of keyboard interaction
events (even though the element itself is not visible). This allows authors to make an interactive
canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no
effect on mouse interaction events.) [DOMEVENTS]
An element whose nearest canvas
element ancestor is being rendered
and represents embedded content is an element that is being used as
relevant canvas fallback content.
The canvas
element has two attributes to control the size of the element's bitmap:
width
and height
. These attributes, when specified, must have
values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their
numeric values. If an attribute is missing, or if parsing its value returns an error, then the
default value must be used instead. The width
attribute defaults to 300, and the height
attribute
defaults to 150.
The intrinsic dimensions of the canvas
element when it represents
embedded content are equal to the dimensions of the element's bitmap.
The user agent must use a square pixel density consisting of one pixel of image data per
coordinate space unit for the bitmaps of a canvas
and its rendering contexts.
A canvas
element can be sized arbitrarily by a style sheet, its
bitmap is then subject to the 'object-fit' CSS property. [CSSIMAGES]
The bitmaps of canvas
elements, as well as some of the bitmaps of rendering
contexts, such as those described in the section on the CanvasRenderingContext2D
object below, have an origin-clean flag, which can
be set to true or false. Initially, when the canvas
element is created, its bitmap's
origin-clean flag must be set to true.
A canvas
bitmap can also have a hit region list, as described in the
CanvasRenderingContext2D
section below.
A canvas
element can have a rendering context bound to it. Initially, it does not
have a bound rendering context. To keep track of whether it has a rendering context or not, and
what kind of rendering context it is, a canvas
also has a canvas context mode, which is initially none but can be changed to either direct-2d, direct-webgl, indirect, or proxied by algorithms defined in this specification.
When its canvas context mode is none, a canvas
element has no rendering context,
and its bitmap must be fully transparent black with an intrinsic width equal to the numeric value
of the element's width
attribute and an intrinsic height
equal to the numeric value of the element's height
attribute, those values being interpreted in CSS pixels, and being updated as the attributes are
set, changed, or removed.
When a canvas
element represents embedded content, it provides
a paint source whose width is the element's intrinsic width, whose height is the element's
intrinsic height, and whose appearance is the element's bitmap.
Whenever the width
and height
content attributes are set, removed, changed, or
redundantly set to the value they already have, if the canvas context mode is direct-2d, the user agent must set bitmap dimensions to the numeric values of
the width
and height
content attributes.
The width
and height
IDL attributes must reflect the
respective content attributes of the same name, with the same defaults.
getContext
(contextId [, ... ] )Returns an object that exposes an API for drawing on the canvas. The first argument specifies
the desired API, either "2d
" or "webgl
". Subsequent arguments are handled by that API.
This specification defines the "2d
" context below.
There is also a specification that defines a "webgl
"
context. [WEBGL]
Returns null if the given context ID is not supported, if the canvas has already been
initialised with the other context type (e.g. trying to get a "2d
" context after getting a "webgl
" context).
Throws an InvalidStateError
exception if the setContext()
or transferControlToProxy()
methods have been
used.
probablySupportsContext
(contextId [, ... ] )Returns false if calling getContext()
with the
same arguments would definitely return null, and true otherwise.
This return value is not a guarantee that getContext()
will or will not return an object, as
conditions (e.g. availability of system resources) can vary over time.
Throws an InvalidStateError
exception if the setContext()
or transferControlToProxy()
methods have been
used.
setContext
(context)Sets the canvas
' rendering context to the given object.
Throws an InvalidStateError
exception if the getContext()
or transferControlToProxy()
methods have been
used.
There are two ways for a canvas
element to acquire a rendering context: the
canvas
element can provide one via the getContext()
method, and one can be assigned to it via the
setContext()
method. In addition, the whole issue of a
rendering context can be taken out of the canvas
element's hands and passed to a
CanvasProxy
object, which itself can then be assigned a rendering context using its
setContext()
method.
These three methods are mutually exclusive; calling any of the three makes the other two start
throwing InvalidStateError
exceptions when called.
Each rendering context has a context bitmap mode, which is one of fixed, unbound, or bound. Initially, rendering contexts must be in the unbound mode.
The getContext(contextId, arguments...)
method of the canvas
element, when invoked,
must run the steps in the cell of the following table whose column header describes the
canvas
element's canvas context mode
and whose row header describes the method's first argument.
none | direct-2d | direct-webgl | indirect | proxied | |
---|---|---|---|---|---|
"2d "
|
Follow the 2D
context creation algorithm defined in the section below, passing it the
canvas element and the method's arguments..., to obtain a CanvasRenderingContext2D object;
if this does not throw an exception, then
set the canvas element's context
mode to direct-2d, set
the CanvasRenderingContext2D object's context bitmap mode to
fixed, and return the
CanvasRenderingContext2D object
| Return the same object as was return the last time the method was invoked with this same first argument. | Return null. |
Throw an InvalidStateError exception.
|
Throw an InvalidStateError exception.
|
"webgl ", if the user agent supports the WebGL feature in its current configuration
|
Follow the instructions given in the WebGL specification's Context Creation section to
obtain either a WebGLRenderingContext or null; if the returned value is null,
then return null and abort these steps, otherwise, set the canvas element's context mode to direct-webgl, set the new
WebGLRenderingContext object's context bitmap mode to fixed, and return the WebGLRenderingContext
object [WEBGL]
| Return null. | Return the same object as was return the last time the method was invoked with this same first argument. |
Throw an InvalidStateError exception.
|
Throw an InvalidStateError exception.
|
A vendor-specific extension* | Behave as defined for the extension. | Behave as defined for the extension. | Behave as defined for the extension. |
Throw an InvalidStateError exception.
|
Throw an InvalidStateError exception.
|
An unsupported value† | Return null. | Return null. | Return null. |
Throw an InvalidStateError exception.
|
Throw an InvalidStateError exception.
|
* Vendors may define experimental contexts using the syntax vendorname-context
, for example, moz-3d
.
† For example, the "webgl
" value in the case of a user agent having exhausted the
graphics hardware's abilities and having no software fallback implementation.
The probablySupportsContext(contextId,
arguments...)
method of the canvas
element, when
invoked, must return false if calling getContext()
on
the same object and with the same arguments would definitely return null at this time, and true
otherwise.
The setContext(context)
method of the canvas
element, when invoked, must
run the following steps:
If the canvas
element's canvas
context mode is neither none nor indirect, throw an InvalidStateError
exception and abort these steps.
If context's context
bitmap mode is fixed, then throw an
InvalidStateError
exception and abort these steps.
If context's context bitmap mode is bound, then run context's unbinding steps and set its context's context bitmap mode to unbound.
Run context's binding
steps to bind it to this canvas
element.
Set the canvas
element's context
mode to indirect and the context's context bitmap
mode to bound.
toDataURL
( [ type, ... ] )Returns a data:
URL for the image in the
canvas.
The first argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is image/png
; that type is also used if the given type
isn't supported. The other arguments are specific to the type, and control the way that the
image is generated, as given in the table
below.
When trying to use types other than "image/png
", authors can check if the image
was really returned in the requested format by checking to see if the returned string starts
with one of the exact strings "data:image/png,
" or "data:image/png;
". If it does, the image is PNG, and thus the requested type was
not supported. (The one exception to this is if the canvas has either no height or no width, in
which case the result might simply be "data:,
".)
toBlob
(callback [, type, ... ] )Creates a Blob
object representing a file containing the image in the canvas,
and invokes a callback with a handle to that object.
The second argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is image/png
; that type is also used if the given type
isn't supported. The other arguments are specific to the type, and control the way that the
image is generated, as given in the table
below.
The toDataURL()
method must run the
following steps:
If the canvas
element's bitmap's origin-clean flag is set to false, throw a
SecurityError
exception and abort these steps.
If the canvas
element's bitmap has no pixels (i.e. either its horizontal
dimension or its vertical dimension is zero) then return the string "data:,
" and abort these steps. (This is the shortest data:
URL; it represents the empty string in a text/plain
resource.)
Let file be a
serialisation of the canvas
element's bitmap as a file, using the method's
arguments (if any) as the arguments.
The toBlob()
method must run the following
steps:
If the canvas
element's bitmap's origin-clean flag is set to false, throw a
SecurityError
exception and abort these steps.
Let callback be the first argument.
Let arguments be the second and subsequent arguments to the method, if any.
If the canvas
element's bitmap has no pixels (i.e. either its horizontal
dimension or its vertical dimension is zero) then let result be null.
Otherwise, let result be a Blob
object representing a serialisation of the canvas
element's
bitmap as a file, using arguments. [FILEAPI]
Return, but continue running these steps in parallel.
If callback is null, abort these steps.
Queue a task to invoke the FileCallback
callback with
result as its argument. The task source for this task is the canvas
blob serialisation task source.
Since DOM nodes cannot be accessed across worker boundaries, a proxy object is needed to enable
workers to render to canvas
elements in Document
s.
[Exposed=(Window,Worker)] interface CanvasProxy { void setContext(RenderingContext context); }; // CanvasProxy implements Transferable;
transferControlToProxy
()Returns a CanvasProxy
object that can be used to transfer control for this
canvas over to another document (e.g. an iframe
from another origin)
or to a worker.
Throws an InvalidStateError
exception if the getContext()
or setContext()
methods have been used.
setContext
(context)Sets the CanvasProxy
object's canvas
element's rendering context to
the given object.
Throws an InvalidStateError
exception if the CanvasProxy
has been
transfered.
The transferControlToProxy()
method of the canvas
element, when invoked, must run the following steps:
If the canvas
element's canvas
context mode is not none, throw an
InvalidStateError
exception and abort these steps.
Set the canvas
element's context
mode to proxied.
Return a CanvasProxy
object bound to this canvas
element.
A CanvasProxy
object can be neutered (like any Transferable
object),
meaning it can no longer be transferred, and
can be disabled, meaning it can no longer be bound
to rendering contexts. When first created, a CanvasProxy
object must be neither.
A CanvasProxy
is created with a link to a canvas
element. A
CanvasProxy
object that has not been disabled must have a strong reference to its canvas
element.
The setContext(context)
method of CanvasProxy
objects, when invoked,
must run the following steps:
If the CanvasProxy
object has been disabled, throw an InvalidStateError
exception and abort these steps.
If the CanvasProxy
object has not been neutered, then neuter it.
If context's context
bitmap mode is fixed, then throw an
InvalidStateError
exception and abort these steps.
If context's context bitmap mode is bound, then run context's unbinding steps and set its context's context bitmap mode to unbound.
Run context's binding
steps to bind it to this CanvasProxy
object's canvas
element.
Set the context's context bitmap mode to bound.
To transfer a
CanvasProxy
object old to a new owner owner,
a user agent must create a new CanvasProxy
object linked to the same
canvas
element as old, thus obtaining new,
must neuter and disable the old object, and must
finally return new.
Here is a clock implemented on a worker. First, the main page:
<!DOCTYPE HTML> <title>Clock</title> <canvas></canvas> <script> var canvas = document.getElementsByTagName('canvas')[0]; var proxy = canvas.transferControlToProxy(); var worker = new Worker('clock.js'); worker.postMessage(proxy, [proxy]); </script>
Second, the worker:
onmessage = function (event) { var context = new CanvasRenderingContext2D(); event.data.setContext(context); // event.data is the CanvasProxy object setInterval(function () { context.clearRect(0, 0, context.width, context.height); context.fillText(new Date(), 0, 100); context.commit(); }, 1000); };
typedef (HTMLImageElement or HTMLVideoElement or HTMLCanvasElement or CanvasRenderingContext2D or ImageBitmap) CanvasImageSource; enum CanvasFillRule { "nonzero", "evenodd" }; dictionary CanvasRenderingContext2DSettings { boolean alpha = true; }; [Constructor(), Constructor(unsigned long width, unsigned long height), Exposed=(Window,Worker)] interface CanvasRenderingContext2D { // back-reference to the canvas readonly attribute HTMLCanvasElement canvas; // canvas dimensions attribute unsigned long width; attribute unsigned long height; // for contexts that aren't directly fixed to a specific canvas void commit(); // push the image to the output bitmap // state void save(); // push state on state stack void restore(); // pop state stack and restore state // transformations (default transform is the identity matrix) attribute SVGMatrix currentTransform; void scale(unrestricted double x, unrestricted double y); void rotate(unrestricted double angle); void translate(unrestricted double x, unrestricted double y); void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f); void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f); void resetTransform(); // compositing attribute unrestricted double globalAlpha; // (default 1.0) attribute DOMString globalCompositeOperation; // (default source-over) // image smoothing attribute boolean imageSmoothingEnabled; // (default true) // colours and styles (see also the CanvasDrawingStyles interface) attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black) attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black) CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1); CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1); CanvasPattern createPattern(CanvasImageSource image, [TreatNullAs=EmptyString] DOMString repetition); // shadows attribute unrestricted double shadowOffsetX; // (default 0) attribute unrestricted double shadowOffsetY; // (default 0) attribute unrestricted double shadowBlur; // (default 0) attribute DOMString shadowColor; // (default transparent black) // rects void clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h); void fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h); void strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h); // path API (see also CanvasPathMethods) void beginPath(); void fill(optional CanvasFillRule fillRule = "nonzero"); void fill(Path2D path, optional CanvasFillRule fillRule = "nonzero"); void stroke(); void stroke(Path2D path); void drawFocusIfNeeded(Element element); void drawFocusIfNeeded(Path2D path, Element element); void scrollPathIntoView(); void scrollPathIntoView(Path2D path); void clip(optional CanvasFillRule fillRule = "nonzero"); void clip(Path2D path, optional CanvasFillRule fillRule = "nonzero"); void resetClip(); boolean isPointInPath(unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero"); boolean isPointInPath(Path2D path, unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero"); boolean isPointInStroke(unrestricted double x, unrestricted double y); boolean isPointInStroke(Path2D path, unrestricted double x, unrestricted double y); // text (see also the CanvasDrawingStyles interface) void fillText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth); void strokeText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth); TextMetrics measureText(DOMString text); // drawing images void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy); void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh); void drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy, unrestricted double sw, unrestricted double sh, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh); // hit regions void addHitRegion(optional HitRegionOptions options); void removeHitRegion(DOMString id); void clearHitRegions(); // pixel manipulation ImageData createImageData(double sw, double sh); ImageData createImageData(ImageData imagedata); ImageData getImageData(double sx, double sy, double sw, double sh); void putImageData(ImageData imagedata, double dx, double dy); void putImageData(ImageData imagedata, double dx, double dy, double dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight); }; CanvasRenderingContext2D implements CanvasDrawingStyles; CanvasRenderingContext2D implements CanvasPathMethods; [NoInterfaceObject, Exposed=(Window,Worker)] interface CanvasDrawingStyles { // line caps/joins attribute unrestricted double lineWidth; // (default 1) attribute DOMString lineCap; // "butt", "round", "square" (default "butt") attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter") attribute unrestricted double miterLimit; // (default 10) // dashed lines void setLineDash(sequence<unrestricted double> segments); // default empty sequence<unrestricted double> getLineDash(); attribute unrestricted double lineDashOffset; // text attribute DOMString font; // (default 10px sans-serif) attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start") attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic") attribute DOMString direction; // "ltr", "rtl", "inherit" (default: "inherit") }; [NoInterfaceObject, Exposed=(Window,Worker)] interface CanvasPathMethods { // shared path API methods void closePath(); void moveTo(unrestricted double x, unrestricted double y); void lineTo(unrestricted double x, unrestricted double y); void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy, unrestricted double x, unrestricted double y); void bezierCurveTo(unrestricted double cp1x, unrestricted double cp1y, unrestricted double cp2x, unrestricted double cp2y, unrestricted double x, unrestricted double y); void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radius); void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation); void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h); void arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false); void ellipse(unrestricted double x, unrestricted double y, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false); }; [Exposed=(Window,Worker)] interface CanvasGradient { // opaque object void addColorStop(double offset, DOMString color); }; [Exposed=(Window,Worker)] interface CanvasPattern { // opaque object void setTransform(SVGMatrix transform); }; [Exposed=(Window,Worker)] interface TextMetrics { // x-direction readonly attribute double width; // advance width readonly attribute double actualBoundingBoxLeft; readonly attribute double actualBoundingBoxRight; // y-direction readonly attribute double fontBoundingBoxAscent; readonly attribute double fontBoundingBoxDescent; readonly attribute double actualBoundingBoxAscent; readonly attribute double actualBoundingBoxDescent; readonly attribute double emHeightAscent; readonly attribute double emHeightDescent; readonly attribute double hangingBaseline; readonly attribute double alphabeticBaseline; readonly attribute double ideographicBaseline; }; dictionary HitRegionOptions { Path2D? path = null; CanvasFillRule fillRule = "nonzero"; DOMString id = ""; DOMString? parentID = null; DOMString cursor = "inherit"; // for control-backed regions: Element? control = null; // for unbacked regions: DOMString? label = null; DOMString? role = null; }; [Constructor(unsigned long sw, unsigned long sh), Constructor(Uint8ClampedArray data, unsigned long sw, optional unsigned long sh), Exposed=(Window,Worker)] interface ImageData { readonly attribute unsigned long width; readonly attribute unsigned long height; readonly attribute Uint8ClampedArray data; }; [Constructor(optional Element scope), Exposed=(Window,Worker)] interface DrawingStyle { }; DrawingStyle implements CanvasDrawingStyles; [Constructor, Constructor(Path2D path), Constructor(Path2D[] paths, optional CanvasFillRule fillRule = "nonzero"), Constructor(DOMString d), Exposed=(Window,Worker)] interface Path2D { void addPath(Path2D path, optional SVGMatrix? transformation = null); void addPathByStrokingPath(Path2D path, CanvasDrawingStyles styles, optional SVGMatrix? transformation = null); void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth); void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth); void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path2D path, optional unrestricted double maxWidth); void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path2D path, optional unrestricted double maxWidth); }; Path2D implements CanvasPathMethods;
getContext
('2d' [, { [ alpha
: false ] } ] )Returns a CanvasRenderingContext2D
object that is permanently bound to a
particular canvas
element.
If the alpha
setting is
provided and set to false, then the canvas is forced to always be opaque.
CanvasRenderingContext2D
( [ width, height ] )Returns an unbound CanvasRenderingContext2D
object with an implied bitmap with
the given dimensions in CSS pixels (300x150, if the arguments are omitted).
canvas
Returns the canvas
element, if the rendering context was obtained using the
getContext()
method.
width
height
Return the dimensions of the bitmap, in CSS pixels.
Can be set, to update the bitmap's dimensions. If the rendering context is bound to a canvas, this will also update the canvas' intrinsic dimensions.
commit
()If the rendering context is bound to a canvas
, display the current frame.
A CanvasRenderingContext2D
object can be obtained in two ways: the getContext()
method on a canvas
element (which
invokes the 2D context creation algorithm), and the CanvasRenderingContext2D()
constructor.
A CanvasRenderingContext2D
object has a scratch bitmap and can be bound
to an output bitmap. These are initialised when the object is created, and can be
subsequently adjusted when the rendering context is bound or unbound. In some cases, these bitmaps are the same
underlying bitmap. In general, the scratch bitmap is what scripts interact with, and
the output bitmap is what is being displayed. These bitmaps always have the same
dimensions.
Each such bitmap has an origin-clean flag, which can be set to true or false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.
These bitmaps also have a hit region list, which is described in a later section. Initially, this list is empty.
Scratch bitmaps have a list of pending interface actions, which can contain instructions to draw the user's attention to a location on the bitmap, and instructions to scroll to a location on the bitmap. Initially, this list is also empty.
Scratch bitmaps also have an alpha flag, which can be set to true or false. Initially, when one of these bitmaps is created, its alpha flag must be set to true. When a scratch bitmap has its alpha flag set to false, then its alpha channel must be fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel must be silently ignored.
Thus, such a bitmap starts off as fully-opaque black instead of fully-tranparent
black; clearRect()
always results in fully-opaque
black pixels, every fourth byte from getImageData()
is always 255, the putImageData()
method effectively ignores every fourth
byte in its input, and so on. However, the alpha component of styles and images drawn onto the
canvas are still honoured up to the point where they would impact the scratch
bitmap's alpha channel; for instance, drawing a 50% transparent white square on a freshly
created scratch bitmap with its alpha flag
set to false will result in a fully-opaque gray square.
The CanvasRenderingContext2D
2D rendering context represents a flat linear
Cartesian surface whose origin (0,0) is at the top left corner, with the coordinate space having
x values increasing when going right, and y values
increasing when going down. The x-coordinate of the right-most edge is equal to
the width of the rendering context's scratch bitmap in CSS pixels; similarly, the
y-coordinate of the bottom-most edge is equal to the height of the rendering
context's scratch bitmap in CSS pixels.
The size of the coordinate space does not necessarily represent the size of the actual bitmaps that the user agent will use internally or during rendering. On high-definition displays, for instance, the user agent may internally use bitmaps with two device pixels per unit in the coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can similarly be implemented using over-sampling with bitmaps of a higher resolution than the final image on the display.
The 2D context creation algorithm, which is passed a target (a
canvas
element) and optionally some arguments, consists of running the following steps:
If the algorithm was passed some arguments, let arg be the first such argument. Otherwise, let arg be undefined.
Let settings be the result of coercing the arg context arguments for 2D.
Create a new CanvasRenderingContext2D
object.
Initialise its canvas
attribute to point to
target.
Let the new CanvasRenderingContext2D
object's output bitmap and
scratch bitmap both be the same bitmap as target's bitmap (so
that they are shared).
Set bitmap dimensions to the
numeric values of target's width
and
height
content attributes.
Process each of the members of settings as follows:
alpha
Return the new CanvasRenderingContext2D
object.
The CanvasRenderingContext2D()
constructor, when
invoked, must run the following steps:
Create a new CanvasRenderingContext2D
object.
Initialise its canvas
attribute to
null.
Let the new CanvasRenderingContext2D
object's scratch bitmap be
a new bitmap.
If the constructor was called with arguments, let width and height be the first and second arguments, respectively. Otherwise, let width and height be 300 and 150, respectively.
Set bitmap dimensions to width and height.
Let the new CanvasRenderingContext2D
object have no output
bitmap.
Return the new CanvasRenderingContext2D
object.
When a user agent is required to coerce context arguments for 2D, it must run the following steps:
Let input be the argument to coerce.
Let jsval be the result of converting input to an ECMAScript value. If this throws an exception, then propagate the exception and abort these steps.
Let dict be the result of converting
jsval to the dictionary type CanvasRenderingContext2DSettings
. If this
throws an exception, then propagate the exception and abort these steps.
Return dict.
When the user agent is required to commit the scratch bitmap for a rendering context, it must run the following steps:
Let bitmap copy be a copy of the rendering context's scratch bitmap.
Let origin-clean flag copy be a copy of the rendering context's scratch bitmap's origin-clean flag.
Let hit region list copy be a copy of the rendering context's scratch bitmap's hit region list.
Let list of pending interface actions copy be a copy of the rendering context's scratch bitmap's list of pending interface actions.
Empty the scratch bitmap's list of pending interface actions.
If the rendering context has no output bitmap, abort these steps.
Let output bitmap be the rendering context's output bitmap.
Let canvas be the canvas
element to which the rendering
context was most recently bound.
Queue a task associated with canvas' Document
to perform the following substeps:
Overwrite output bitmap with bitmap copy.
Overwrite output bitmap's origin-clean flag with origin-clean flag copy.
Overwrite output bitmap's hit region list with hit region list copy.
Follow the directions in the list of pending interface actions copy.
The algorithm above must use the canvas updating task source (which is just used by this algorithm).
The commit()
method must run the
following steps:
If the rendering context's context bitmap
mode is fixed, throw an
InvalidStateError
exception and abort these steps.
Commit the scratch bitmap for the rendering context.
The scratch bitmap is only committed when the commit()
method is
called. (This doesn't matter for canvas
elements in direct-2d mode, since there the scratch
bitmap is also the canvas
element's bitmap so every drawing operation is
immediately drawn.)
When the user agent is to set bitmap dimensions to width and height, it must run the following steps:
Clear the scratch bitmap's hit region list and its list of pending interface actions.
Resize the scratch bitmap to the new width and height and clear it to fully transparent black.
If the rendering context has an output bitmap, and the scratch bitmap is a different bitmap than the output bitmap, then resize the output bitmap to the new width and height and clear it to fully transparent black.
If the rendering context's context bitmap mode is fixed, then run these substeps:
Let canvas be the canvas
element to which the rendering
context's canvas
attribute was initialised.
If the rendering context's context
bitmap mode is fixed and the numeric value of
the canvas' width
content attribute
differs from width, then set canvas' width
content attribute to the shortest possible string
representing width as a valid non-negative integer.
If the rendering context's context
bitmap mode is fixed and the numeric value of
the canvas' height
content attribute
differs from height, then set canvas' height
content attribute to the shortest possible string
representing height as a valid non-negative integer.
Only one square appears to be drawn in the following example:
// canvas is a reference to a <canvas> element var context = canvas.getContext('2d'); context.fillRect(0,0,50,50); canvas.setAttribute('width', '300'); // clears the canvas context.fillRect(0,100,50,50); canvas.width = canvas.width; // clears the canvas context.fillRect(100,0,50,50); // only this square remains
When the user agent is to run the unbinding steps for a rendering context, it must run the following steps:
Clear the scratch bitmap's hit region list and its list of pending interface actions.
Clear the CanvasRenderingContext2D
object's scratch bitmap to a
transparent black.
Set the CanvasRenderingContext2D
object's scratch bitmap's origin-clean flag to true.
Let the CanvasRenderingContext2D
object have no output
bitmap.
When the user agent is to run the binding steps
to bind the rendering context to the canvas
element target, it
must run the following steps:
Clear the scratch bitmap's hit region list and its list of pending interface actions.
Resize the CanvasRenderingContext2D
object's scratch bitmap to
the dimensions of target's bitmap and clear it to fully transparent
black.
Set the CanvasRenderingContext2D
object's scratch bitmap's origin-clean flag to true.
Let the CanvasRenderingContext2D
object's output bitmap be target's bitmap.
The canvas
attribute must return the
value it was initialised to when the object was created.
The width
attribute, on getting, must
return the width of the rendering context's scratch bitmap, in CSS pixels. On
setting, it must set bitmap dimensions
to the new value and the current height of the rendering context's scratch bitmap in
CSS pixels, respectively.
The height
attribute, on getting, must
return the height of the rendering context's scratch bitmap, in CSS pixels. On
setting, it must set bitmap dimensions
to the current width of the rendering context's scratch bitmap in CSS pixels and the
new value, respectively.
Except where otherwise specified, for the 2D context interface, any method call with a numeric argument whose value is infinite or a NaN value must be ignored.
Whenever the CSS value currentColor
is used as a colour in the
CanvasRenderingContext2D
API, the "computed value of the 'color' property" for the
purposes of determining the computed value of the currentColor
keyword is
the value described by the appropriate entry in the following list:
canvas
element is being renderedThe "computed value of the 'color' property" for the purposes of determining the computed
value of the currentColor
keyword is the computed value of the 'color'
property on the canvas
element at the time that the colour is specified (e.g. when
the appropriate attribute is set, or when the method is called; not when the colour is rendered or
otherwise used). [CSSCOLOR]
The "computed value of the 'color' property" for the purposes of determining the computed
value of the currentColor
keyword is fully opaque black. [CSSCOLOR]
In the case of addColorStop()
on
CanvasGradient
, the "computed value of the 'color' property" for the purposes of
determining the computed value of the currentColor
keyword is always fully
opaque black (there is no associated element). [CSSCOLOR]
This is because CanvasGradient
objects are
canvas
-neutral — a CanvasGradient
object created by one
canvas
can be used by another, and there is therefore no way to know which is the
"element in question" at the time that the colour is specified.
Similar concerns exist with font-related properties; the rules for those are described in detail in the relevant section below.
The CanvasFillRule
enumeration is used to select the fill rule
algorithm by which to determine if a point is inside or outside a path.
The value "nonzero
" value
indicates the non-zero winding rule, wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path going in one direction is equal to the
number of times it crosses the path going in the other direction.
The "evenodd
" value indicates
the even-odd rule, wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path is even.
If a point is not outside a shape, it is inside the shape.
This section is non-normative.
Although the way the specification is written it might sound like an implementation needs to
track up to four bitmaps per canvas or rendering context — one scratch bitmap,
one output bitmap for the rendering context, one bitmap for the canvas
,
and one bitmap for the actually currently rendered image — user agents can in fact generally
optimise this to only one or two.
The scratch bitmap, when it isn't the same bitmap as the output
bitmap, is only directly observable if it is read, and therefore implementations can,
instead of updating this bitmap, merely remember the sequence of drawing operations that have been
applied to it until such time as the bitmap's actual data is needed (for example because of a call
to commit()
, drawImage()
, or the createImageBitmap()
factory method). In many cases, this will be more memory efficient.
The bitmap of a canvas
element is the one bitmap that's pretty much always going
to be needed in practice. The output bitmap of a rendering context, when it has one,
is always just an alias to a canvas
element's bitmap.
Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its intrinsic size, or to enable double buffering so that the rendering commands from the scratch bitmap can be applied without the rendering being updated midway.
Each CanvasRenderingContext2D
rendering context maintains a stack of drawing
states. Drawing states consist of:
strokeStyle
, fillStyle
, globalAlpha
, lineWidth
, lineCap
, lineJoin
, miterLimit
, lineDashOffset
, shadowOffsetX
, shadowOffsetY
, shadowBlur
, shadowColor
, globalCompositeOperation
, font
, textAlign
, textBaseline
, direction
, imageSmoothingEnabled
.The current default path and the rendering context's bitmaps are not
part of the drawing state. The current default path is persistent, and can only be
reset using the beginPath()
method. The bitmaps
depend on whether and how the rendering context is bound to a canvas
element.
save
()Pushes the current state onto the stack.
restore
()Pops the top state on the stack, restoring the context to that state.
The save()
method must push a copy of the
current drawing state onto the drawing state stack.
The restore()
method must pop the top
entry in the drawing state stack, and reset the drawing state it describes. If there is no saved
state, the method must do nothing.
When the user agent is to reset the rendering context to its default state, it must clear the drawing state stack and everything that drawing state consists of to initial values.
DrawingStyle
objectsAll the line styles (line width, caps, joins, and dash patterns) and text styles (fonts)
described in the next two sections apply to CanvasRenderingContext2D
objects and to
DrawingStyle
objects. This section defines the constructor used to obtain a
DrawingStyle
object. This object is then used by methods on Path2D
objects to control how text and paths are rasterised and stroked.
DrawingStyle
( [ element ] )Creates a new DrawingStyle
object, optionally using a specific element for
resolving relative keywords and sizes in font specifications.
Each DrawingStyle
object can have a styles scope object.
The DrawingStyle()
constructor, when invoked,
must return a newly created DrawingStyle
object. If the constructor was passed an
argument, then the DrawingStyle
object's styles scope object is that
element. Otherwise, if the JavaScript global environment is a document
environment, the object's styles scope object is the Document
object of the active document of the browsing context of the
Window
object on which the interface object of the invoked constructor is found.
Otherwise, the JavaScript global environment is a worker environment,
and the styles scope object is the worker.
lineWidth
[ = value ]lineWidth
[ = value ]Returns the current line width.
Can be set, to change the line width. Values that are not finite values greater than zero are ignored.
lineCap
[ = value ]lineCap
[ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
The possible line cap styles are "butt"
", "round"
", and "square"
". Other values are ignored.
lineJoin
[ = value ]lineJoin
[ = value ]Returns the current line join style.
Can be set, to change the line join style.
The possible line join styles are "bevel"
", "round"
", and "miter"
". Other values are ignored.
miterLimit
[ = value ]miterLimit
[ = value ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
setLineDash
(segments)setLineDash
(segments)Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.
getLineDash
()getLineDash
()Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).
lineDashOffset
lineDashOffset
Returns the phase offset (in the same units as the line dash pattern).
Can be set, to change the phase offset. Values that are not finite values are ignored.
Objects that implement the CanvasDrawingStyles
interface have attributes and
methods (defined in this section) that control how lines are treated by the object.
The lineWidth
attribute gives the
width of lines, in coordinate space units. On getting, it must return the current value. On
setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged;
other values must change the current value to the new value.
When the object implementing the CanvasDrawingStyles
interface is created, the
lineWidth
attribute must initially have the value
1.0.
The lineCap
attribute defines the type
of endings that UAs will place on the end of lines. The three valid values are "butt"
", "round"
", and "square"
".
On getting, it must return the current value. On setting, if the new value is one of the
literal strings "butt"
", "round"
", and "square"
", then the current value must be changed to the new value; other values
must ignored, leaving the value unchanged.
When the object implementing the CanvasDrawingStyles
interface is created, the
lineCap
attribute must initially have the value
"butt"
".
The lineJoin
attribute defines the type
of corners that UAs will place where two lines meet. The three valid values are
"bevel"
", "round"
", and "miter"
".
On getting, it must return the current value. On setting, if the new value is one of the
literal strings "bevel"
", "round"
", and "miter"
", then the current value must be changed to the new value; other values
must be ignored, leaving the value unchanged.
When the object implementing the CanvasDrawingStyles
interface is created, the
lineJoin
attribute must initially have the value
"miter"
".
When the lineJoin
attribute has the value "miter"
", strokes use the miter limit ratio to decide how to render joins. The
miter limit ratio can be explicitly set using the miterLimit
attribute. On getting, it must return
the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving
the value unchanged; other values must change the current value to the new value.
When the object implementing the CanvasDrawingStyles
interface is created, the
miterLimit
attribute must initially have the value
10.0.
Each CanvasDrawingStyles
object has a dash list, which is either empty
or consists of an even number of non-negative numbers. Initially, the dash list must
be empty.
When the setLineDash()
method is
invoked, it must run the following steps:
Let a be the argument.
If any value in a is not finite (e.g. an Infinity or a NaN value), or if any value is negative (less than zero), then abort these steps (without throwing an exception; user agents could show a message on a developer console, though, as that would be helpful for debugging).
If the number of elements in a is odd, then let a be the concatentation of two copies of a.
Let the object's dash list be a.
When the getLineDash()
method is
invoked, it must return a sequence whose values are the values of the object's dash
list, in the same order.
It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching
ants" effect. The phase can be set using the lineDashOffset
attribute. On getting, it must
return the current value. On setting, infinite and NaN values must be ignored, leaving the value
unchanged; other values must change the current value to the new value.
When the object implementing the CanvasDrawingStyles
interface is created, the
lineDashOffset
attribute must initially have
the value 0.0.
When a user agent is to trace a path, given an object style
that implements the CanvasDrawingStyles
interface, it must run the following
algorithm. This algorithm returns a new path.
Let path be a copy of the path being traced.
Prune all zero-length line segments from path.
Remove from path any subpaths containing no lines (i.e. subpaths with just one point).
Replace each point in each subpath of path other than the first point and the last point of each subpath by a join that joins the line leading to that point to the line leading out of that point, such that the subpaths all consist of two points (a starting point with a line leading out of it, and an ending point with a line leading into it), one or more lines (connecting the points and the joins), and zero or more joins (each connecting one line to another), connected together such that each subpath is a series of one or more lines with a join between each one and a point on each end.
Add a straight closing line to each closed subpath in path connecting the last point and the first point of that subpath; change the last point to a join (from the previously last line to the newly added closing line), and change the first point to a join (from the newly added closing line to the first line).
If the styles dash list is empty, jump to the step labeled convert.
Let pattern width be the concatenation of all the entries of the styles dash list, in coordinate space units.
For each subpath subpath in path, run the following substeps. These substeps mutate the subpaths in path in vivo.
Let subpath width be the length of all the lines of subpath, in coordinate space units.
Let offset be the value of the styles lineDashOffset
, in coordinate space
units.
While offset is greater than pattern width, decrement it by pattern width.
While offset is less than zero, increment it by pattern width.
Define L to be a linear coordinate line defined along all lines in subpath, such that the start of the first line in the subpath is defined as coordinate 0, and the end of the last line in the subpath is defined as coordinate subpath width.
Let position be zero minus offset.
Let index be 0.
Let current state be off (the other states being on and zero-on).
Dash on: Let segment length be the value of the styles dash list's indexth entry.
Increment position by segment length.
If position is greater than subpath width, then end these substeps for this subpath and start them again for the next subpath; if there are no more subpaths, then jump to the step labeled convert instead.
If segment length is non-zero, let current state be on.
Increment index by one.
Dash off: Let segment length be the value of the styles dash list's indexth entry.
Let start be the offset position on L.
Increment position by segment length.
If position is less than zero, then jump to the step labeled post-cut.
If start is less than zero, then let start be zero.
If position is greater than subpath width, then let end be the offset subpath width on L. Otherwise, let end be the offset position on L.
Jump to the first appropriate step:
Do nothing, just continue to the next step.
Cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two; remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.
The point has a directionality for the purposes of drawing line caps (see below). The directionality is the direction that the original line had at that point (i.e. when L was defined above).
Cut the line on which start finds itself into two at start and place a point there, cutting the subpath that it was in in two, and similarly cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two, and then remove all line segments, joins, points, and subpaths that are between start and end.
If start and end are the same point, then this results in just the line being cut in two and two points being inserted there, with nothing being removed, unless a join also happens to be at that point, in which case the join must be removed.
Post-cut: If position is greater than subpath width, then jump to the step labeled convert.
If segment length is greater than zero, let positioned-at-on-dash be false.
Increment index by one. If it is equal to the number of entries in the styles dash list, then let index be 0.
Return to the step labeled dash on.
Convert: This is the step that converts the path to a new path that represents its stroke.
Create a new path that describes the edge of the areas
that would be covered if a straight line of length equal to the styles
lineWidth
was swept along each subpath in path while being kept at an angle such that the line is orthogonal to the path
being swept, replacing each point with the end cap necessary to satisfy the styles lineCap
attribute as
described previously and elaborated below, and replacing each join with the join necessary to
satisfy the styles lineJoin
type, as defined below.
Caps: Each point has a flat edge perpendicular to the direction of the line
coming out of it. This is them augmented according to the value of the styles lineCap
. The "butt"
" value means that no additional line cap is added. The "round"
" value means that a semi-circle with the diameter equal to the styles lineWidth
width must
additionally be placed on to the line coming out of each point. The "square"
" value means that a rectangle with the length of the styles lineWidth
width and the
width of half the styles lineWidth
width, placed flat against the edge
perpendicular to the direction of the line coming out of the point, must be added at each
point.
Points with no lines coming out of them must have two caps placed back-to-back as if it was really two points connected to each other by an infinitesimally short straight line in the direction of the point's directionality (as defined above).
Joins: In addition to the point where a join occurs, two additional points are relevant to each join, one for each line: the two corners found half the line width away from the join point, one perpendicular to each line, each on the side furthest from the other line.
A triangle connecting these two opposite corners with a straight line, with the third point
of the triangle being the join point, must be added at all joins. The lineJoin
attribute controls whether anything else is
rendered. The three aforementioned values have the following meanings:
The "bevel"
" value means that this is all that is rendered at
joins.
The "round"
" value means that an arc connecting the two aforementioned
corners of the join, abutting (and not overlapping) the aforementioned triangle, with the
diameter equal to the line width and the origin at the point of the join, must be added at
joins.
The "miter"
" value means that a second triangle must (if it can given
the miter length) be added at the join, with one line being the line between the two
aforementioned corners, abutting the first triangle, and the other two being continuations of
the outside edges of the two joining lines, as long as required to intersect without going over
the miter length.
The miter length is the distance from the point where the join occurs to the intersection of
the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of
the miter length to half the line width. If the miter length would cause the miter limit ratio
(as set by the style miterLimit
attribute) to be exceeded, this second
triangle must not be added.
The subpaths in the newly created path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.
Return the newly created path.
font
[ = value ]font
[ = value ]Returns the current font settings.
Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.
Relative keywords and lengths are computed relative to the font of the canvas
element.
textAlign
[ = value ]textAlign
[ = value ]Returns the current text alignment settings.
Can be set, to change the alignment. The possible values are and their meanings are given
below. Other values are ignored. The default is "start
".
textBaseline
[ = value ]textBaseline
[ = value ]Returns the current baseline alignment settings.
Can be set, to change the baseline alignment. The possible values and their meanings are
given below. Other values are ignored. The default is "alphabetic
".
direction
[ = value ]direction
[ = value ]Returns the current directionality.
Can be set, to change the directionality. The possible values and their meanings are given
below. Other values are ignored. The default is "inherit
".
Objects that implement the CanvasDrawingStyles
interface have attributes (defined
in this section) that control how text is laid out (rasterised or outlined) by the object. Such
objects can also have a font style source object. For
CanvasRenderingContext2D
objects whose context bitmap mode is fixed, this is their canvas
element; for other
CanvasRenderingContext2D
objects, if the JavaScript global environment
is a document environment, the object's font style source object is the
Document
object of the active document of the browsing
context of the Window
object on which the interface object of the
CanvasRenderingContext2D
object is found; otherwise the JavaScript global
environment is a worker environment and the font style source
object is the worker. For DrawingStyle
objects, it's the styles scope
object.
The font
IDL attribute, on setting, must
be parsed as a CSS <'font'> value (but without supporting property-independent
style sheet syntax like 'inherit'), and the resulting font must be assigned to the context, with
the 'line-height' component forced to 'normal', with the 'font-size' component converted to CSS
pixels, and with system fonts being computed to explicit values. If the new value is syntactically
incorrect (including using property-independent style sheet syntax like 'inherit' or 'initial'),
then it must be ignored, without assigning a new font value. [CSS]
Font family names must be interpreted in the context of the font style source
object when the font is to be used; any fonts embedded using @font-face
or loaded using FontFace
objects that are visible to the
font style source object must therefore be available once they are loaded. (Each font style source
object has a font source, which determines what fonts are available.) If a font
is used before it is fully loaded, or if the font style source object does not have
that font in scope at the time the font is to be used, then it must be treated as if it was an
unknown font, falling back to another as described by the relevant CSS specifications. [CSSFONTS] [CSSFONTLOAD]
On getting, the font
attribute must return the serialised form of the current font of the context (with
no 'line-height' component). [CSSOM]
For example, after the following statement:
context.font = 'italic 400 12px/2 Unknown Font, sans-serif';
...the expression context.font
would evaluate to the string "italic 12px "Unknown Font", sans-serif
". The "400"
font-weight doesn't appear because that is the default value. The line-height doesn't appear
because it is forced to "normal", the default value.
When the object implementing the CanvasDrawingStyles
interface is created, the
font of the context must be set to 10px sans-serif. When the 'font-size' component is set to
lengths using percentages, 'em' or 'ex' units, or the 'larger' or 'smaller' keywords, these must
be interpreted relative to the computed value of the 'font-size' property of the font style
source object at the time that the attribute is set, if it is an element. When the
'font-weight' component is set to the relative values 'bolder' and 'lighter', these must be
interpreted relative to the computed value of the 'font-weight' property of the font style
source object at the time that the attribute is set, if it is an element. If the computed
values are undefined for a particular case (e.g. because the font style source object
is not an element or is not being rendered), then the relative keywords must be
interpreted relative to the normal-weight 10px sans-serif default.
The textAlign
IDL attribute, on
getting, must return the current value. On setting, if the value is one of start
, end
, left
, right
, or center
, then the value must be changed to the new
value. Otherwise, the new value must be ignored. When the object implementing the
CanvasDrawingStyles
interface is created, the textAlign
attribute must initially have the value start
.
The textBaseline
IDL attribute, on
getting, must return the current value. On setting, if the value is one of top
, hanging
, middle
, alphabetic
, ideographic
, or bottom
, then the value must be changed to the
new value. Otherwise, the new value must be ignored. When the object implementing the
CanvasDrawingStyles
interface is created, the textBaseline
attribute must initially have the value
alphabetic
.
The direction
IDL attribute, on
getting, must return the current value. On setting, if the value is one of "ltr
", "rtl
", or "inherit
", then the value must be changed to the
new value. Otherwise, the new value must be ignored. When the object implementing the
CanvasDrawingStyles
interface is created, the direction
attribute must initially have the value "inherit
".
The textAlign
attribute's allowed keywords are
as follows:
start
Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).
end
Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).
left
Align to the left.
right
Align to the right.
center
Align to the center.
The textBaseline
attribute's allowed keywords correspond to alignment points in the
font:
The keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
The direction
attribute's allowed keywords are
as follows:
ltr
Treat input to the text preparation algorithm as left-to-right text.
rtl
Treat input to the text preparation algorithm as right-to-left text.
inherit
Default to the directionality of the canvas
element or Document
as appropriate.
The text preparation algorithm is as follows. It takes as input a string text, a CanvasDrawingStyles
object target, and an
optional length maxWidth. It returns an array of glyph shapes, each positioned
on a common coordinate space, a physical alignment whose value is one of
left, right, and center, and an inline box. (Most callers of this
algorithm ignore the physical alignment and the inline box.)
If maxWidth was provided but is less than or equal to zero, return an empty array.
Replace all the space characters in text with U+0020 SPACE characters.
Let font be the current font of target, as given
by that object's font
attribute.
Apply the appropriate step from the following list to determine the value of direction:
direction
attribute has the value "ltr
"direction
attribute has the value "rtl
"Document
and that Document
has a root element childForm a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]
If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
The anchor point is a point on the inline box, and the physical alignment is one of the values left, right, and center. These variables are determined by the textAlign
and textBaseline
values as follows:
Horizontal position:
textAlign
is left
textAlign
is start
and direction is
'ltr'textAlign
is end
and direction is 'rtl'textAlign
is right
textAlign
is end
and direction is 'ltr'textAlign
is start
and direction is
'rtl'textAlign
is center
Vertical position:
textBaseline
is top
textBaseline
is hanging
textBaseline
is middle
textBaseline
is alphabetic
textBaseline
is ideographic
textBaseline
is bottom
Let result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin is at the anchor point.
Return result, physical alignment, and the inline box.
Each object implementing the CanvasPathMethods
interface has a path. A path has a list of zero or
more subpaths. Each subpath consists of a list of one or more points, connected by straight or
curved line segments, and a flag indicating whether the subpath is closed or not. A
closed subpath is one where the last point of the subpath is connected to the first point of the
subpath by a straight line. Subpaths with only one point are ignored when painting the path.
Paths have a need new subpath flag. When this flag is set, certain APIs create a new subpath rather than extending the previous one. When a path is created, its need new subpath flag must be set.
When an object implementing the CanvasPathMethods
interface is created, its path must be initialised to zero subpaths.
moveTo
(x, y)moveTo
(x, y)Creates a new subpath with the given point.
closePath
()closePath
()Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
lineTo
(x, y)lineTo
(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
quadraticCurveTo
(cpx, cpy, x, y)quadraticCurveTo
(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
arcTo
(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )arcTo
(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.
If two radii are provided, the first controls the width of the arc's ellipse, and the second controls the height. If only one is provided, or if they are the same, the arc is from a circle. In the case of an ellipse, the rotation argument controls the clockwise inclination of the ellipse relative to the x-axis.
Throws an IndexSizeError
exception if the given radius is negative.
arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an IndexSizeError
exception if the given radius is negative.
ellipse
(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )ellipse
(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an IndexSizeError
exception if the given radius is negative.
rect
(x, y, w, h)rect
(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
The following methods allow authors to manipulate the paths
of objects implementing the CanvasPathMethods
interface.
For CanvasRenderingContext2D
objects, the points passed to the methods, and the
resulting lines added to current default path by these methods, must be transformed
according to the current transformation matrix
before being added to the path.
The moveTo(x, y)
method must create a new subpath with the specified point as its
first (and only) point.
When the user agent is to ensure there is a subpath for a coordinate (x, y) on a path, the user
agent must check to see if the path has its need new
subpath flag set. If it does, the user agent must create a new subpath with the point (x, y) as its first (and only) point, as if the moveTo()
method had been called, and must then unset the
path's need new subpath flag.
The closePath()
method must do nothing
if the object's path has no subpaths. Otherwise, it must mark the last subpath as closed, create a
new subpath whose first point is the same as the previous subpath's first point, and finally add
this new subpath to the path.
If the last subpath had more than one point in its list of points, then this is
equivalent to adding a straight line connecting the last point back to the first point, thus
"closing" the shape, and then repeating the last (possibly implied) moveTo()
call.
New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the object's path.
The lineTo(x, y)
method must ensure there is a subpath for (x, y) if the object's path has no
subpaths. Otherwise, it must connect the last point in the subpath to the given point (x, y) using a straight line, and must then add the given point
(x, y) to the subpath.
The quadraticCurveTo(cpx, cpy, x, y)
method must ensure there is a subpath for (cpx, cpy), and then must connect the last
point in the subpath to the given point (x, y) using a
quadratic Bézier curve with control point (cpx, cpy), and must then add the given point (x, y) to the subpath. [BEZIER]
The bezierCurveTo(cp1x,
cp1y, cp2x, cp2y, x, y)
method must ensure there is a
subpath for (cp1x, cp1y), and
then must connect the last point in the subpath to the given point (x, y) using a cubic Bézier curve with control points (cp1x,
cp1y) and (cp2x, cp2y). Then, it must
add the point (x, y) to the subpath. [BEZIER]
The arcTo(x1, y1, x2, y2, radiusX,
radiusY, rotation)
method must first
ensure there is a subpath for (x1, y1). Then, the behaviour depends on the arguments and the last point in the
subpath, as described below.
Negative values for radiusX or radiusY must cause the
implementation to throw an IndexSizeError
exception. If radiusY
is omitted, user agents must act as if it had the same value as radiusX.
Let the point (x0, y0) be the last point in the subpath, transformed by the inverse of the current transformation matrix (so that it is in the same coordinate system as the points passed to the method).
If the point (x0, y0) is equal to the point (x1, y1), or if the point (x1, y1) is equal to the point (x2, y2), or if both radiusX and radiusY are zero, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, let The Arc be the shortest arc given by circumference of the ellipse that has radius radiusX on the major axis and radius radiusY on the minor axis, and whose semi-major axis is rotated rotation radians clockwise from the positive x-axis, and that has one point tangent to the half-infinite line that crosses the point (x0, y0) and ends at the point (x1, y1), and that has a different point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point (x2, y2). The points at which this ellipse touches these two lines are called the start and end tangent points respectively. The method must connect the point (x0, y0) to the start tangent point by a straight line, adding the start tangent point to the subpath, and then must connect the start tangent point to the end tangent point by The Arc, adding the end tangent point to the subpath.
The arc(x, y,
radius, startAngle, endAngle, anticlockwise)
and ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)
methods draw arcs.
The arc()
method is equivalent to the ellipse()
method in the case where the two radii are equal.
When the arc()
method is invoked, it must act as if the
ellipse()
method had been invoked with the radiusX and radiusY arguments set to the value of the radius argument, the rotation argument set to zero, and the
other arguments set to the same values as their identically named arguments on the arc()
method.
When the ellipse()
method is invoked, it must
proceed as follows. First, if the object's path has any subpaths, then the method must add a
straight line from the last point in the subpath to the start point of the arc. Then, it must add
the start and end points of the arc to the subpath, and connect them with an arc. The arc and its
start and end points are defined as follows:
Consider an ellipse that has its origin at (x, y), that has a major-axis radius radiusX and a minor-axis radius radiusY, and that is rotated about its origin such that its semi-major axis is inclined rotation radians clockwise from the x-axis.
If the anticlockwise argument is false and endAngle-startAngle is equal to or greater than 2π, or, if the anticlockwise argument is true and startAngle-endAngle is equal to or greater than 2π, then the arc is the whole circumference of this ellipse, and the point at startAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, acts as both the start point and the end point.
Otherwise, the points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start and end points respectively, and the arc is the path along the circumference of this ellipse from the start point to the end point, going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles from zero, the arc can never cover an angle greater than 2π radians.
Even if the arc covers the entire circumference of the ellipse and there are no
other points in the subpath, the path is not closed unless the closePath()
method is appropriately invoked.
Negative values for radiusX or radiusY must cause the
implementation to throw an IndexSizeError
exception.
The rect(x, y, w, h)
method must create
a new subpath containing just the four points (x, y),
(x+w, y), (x+w, y+h), (x, y+h), in that order, with those four
points connected by straight lines, and must then mark the subpath as closed. It must then create
a new subpath with the point (x, y) as the only point in
the subpath.
Path2D
objectsPath2D
objects can be used to declare paths that are then later used on
CanvasRenderingContext2D
objects. In addition to many of the APIs described in
earlier sections, Path2D
objects have methods to combine paths, and to add text to
paths.
Path2D
()Creates a new empty Path2D
object.
Path2D
(path)Creates a new Path2D
object that is a copy of the argument.
Path2D
(paths [, fillRule ] )Creates a new Path2D
object that describes a path that outlines the given paths, using the given fill rule.
Path2D
(d)Creates a new path with the path described by the argument, interpreted as SVG path data. [SVG]
addPath
(path [, transform ] )addPathByStrokingPath
(path, styles [, transform ] )Adds to the path the path given by the argument.
In the case of the stroking variants, the line styles are taken from the styles argument, which can be either a DrawingStyle
object or a
CanvasRenderingContext2D
object.
addText
(text, styles, transform, x, y [, maxWidth ] )addText
(text, styles, transform, path [, maxWidth ] )addPathByStrokingText
(text, styles, transform, x, y [, maxWidth ] )addPathByStrokingText
(text, styles, transform, path [, maxWidth ] )Adds to the path a series of subpaths corresponding to the given text. If the arguments give a coordinate, the text is drawn horizontally at the given coordinates. If the arguments give a path, the text is drawn along the path. If a maximum width is provided, the text will be scaled to fit that width if necessary.
The font, and in the case of the stroking variants, the line styles, are taken from the styles argument, which can be either a DrawingStyle
object or a
CanvasRenderingContext2D
object.
The Path2D()
constructor, when invoked, must return a
newly created Path2D
object.
The Path2D(path)
constructor, when invoked, must return a newly created Path2D
object, to which the
subpaths of the argument are added. (In other words, it returns a copy of the argument.)
The Path2D(paths, fillRule)
constructor, when invoked, must run the following
steps:
Run the appropriate step from the following list, based on the constructor's sectond argument:
Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths provided in the constructor's first argument, the number of times a half-infinite straight line drawn from that point crosses a subpath going in one direction is not equal to the number of times it crosses a subpath going in the other direction.
Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths provided in the constructor's first argument, the number of times a half-infinite straight line drawn from that point crosses that path is odd.
The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.
Add all the subpaths in merged path to the Path2D
object.
Set the Path2D
object's need new subpath flag.
The Path2D(d)
constructor must run the following steps:
Parse and interpret the d argument according to the SVG specification's rules for path data, thus obtaining an SVG path. [SVG]
The resulting path could be empty. SVG defines error handling rules for parsing and applying path data.
Let (x, y) be the last point in the SVG path.
Create a new Path2D
object and add all the subpaths in the SVG path, if any,
to that Path2D
object.
Create a new subpath in the Path2D
object with (x, y) as the only point in the subpath.
Return the Path2D
object as the constructed object.
The addPath(b, transform)
method, when invoked on a Path2D
object a, must run the following steps:
If the Path2D
object b has no subpaths, abort these
steps.
Create a copy of all the subpaths in b. Let this copy be known as c.
Transform all the coordinates and lines in c by the transform matrix transform, if it is not null.
Let (x, y) be the last point in the last subpath of c.
Add all the subpaths in c to a.
Create a new subpath in a with (x, y) as the only point in the subpath.
The addPathByStrokingPath(b, styles, transform)
method, when invoked on a Path2D
object a, must run the
following steps:
If the Path2D
object b has no subpaths, abort these
steps.
Create a copy of all the subpaths in b. Let this copy be known as c.
Transform all the coordinates and lines in c by transformation matrix transform, if it is not null.
Let a new list of subpaths d be the result of tracing c, using the styles argument for the line styles.
Let (x, y) be the last point in the last subpath of d.
Add all the subpaths in d to a.
Create a new subpath in a with (x, y) as the only point in the subpath.
The addText()
and addPathByStrokingText()
methods each
come in two variants: one rendering text at a given coordinate, and one rendering text along a
given path. In both cases, the methods take a CanvasDrawingStyles
object argument for
the text and (if appropriate) line styles to use, an SVGMatrix
object transform (which can be null), and a maximum width can optionally be provided.
When one of the addText()
and addPathByStrokingText()
variants that take as
argument an (x, y) coordinate is invoked, the method
must run the following algorithm:
Run the text preparation algorithm, passing it text, the
CanvasDrawingStyles
object argument, and, if the maxWidth
argument was provided, that argument. Let glyphs be the result.
Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.
Let glyph paths be a list of paths describing the shapes given in glyphs, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit in glyph paths. Subpaths in glyph paths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.
Transform all the coordinates and lines in glyph paths by the transformation matrix transform, if it is not null.
If the method is addPathByStrokingText()
, replace glyph paths by the result of tracing each
path in glyph paths, using the
CanvasDrawingStyles
object argument for the line styles.
Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths in glyph paths, the number of times a half-infinite straight line drawn from that point crosses that path is odd.
The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.
For example, suppose text consists of two overlapping glyphs "Q" and "p" (maybe the "Q" has a flourish that crosses into the tail of the "p"). The glyph paths therefore consist of two paths, each with two subpaths: one for the outside of the letter shape, and one for the inside of the letter shape. There are points that, according to the even-odd fill rule, are filled in both shapes simultaneously: where they overlap. As such, the subpaths from the two glyphs actually cross each other.
The resulting merged path in such a situation would have just one path for these two letters, with a total of just three subpaths (one big outer outline, one for the inside of the "Q", and one for inside of the "p"). This single path would have no subpaths that cross each other.
Add all the subpaths in merged path to the Path2D
object.
Set the Path2D
object's need new subpath flag.
When one of the addText()
and addPathByStrokingText()
variants that take as
argument a Path2D
object is invoked, the method must run the following algorithm:
Let target be the Path2D
object on which the method was
invoked.
Let path be the Path2D
object that was provided in the
method's arguments.
Run the text preparation algorithm, passing it text, the
CanvasDrawingStyles
object argument, and, if the maxWidth
argument was provided, that argument. Let glyphs be the resulting array, and
physical alignment be the resulting alignment value.
Let width be the aggregate length of all the subpaths in path, including the distances from the last point of each closed subpath to the first point of that subpath.
Define L to be a linear coordinate line for of all the subpaths in path, with additional lines drawn between the last point and the first point of each closed subpath, such that the first point of the first subpath is defined as point 0, and the last point of the last subpath, if the last subpath is not closed, or the second occurrence first point of that subpath, if it is closed, is defined as point width.
Let offset be determined according to the appropriate step below:
Move all the shapes in glyphs to the right by offset CSS pixels.
Let transformed path list be an empty list of paths.
For each glyph glyph in the glyphs array, run these substeps:
Let dx be the x-coordinate of the horizontal center of the bounding box of the shape described by glyph, in CSS pixels.
If dx is negative or greater than width, skip the remainder of these substeps for this glyph.
Recast dx to coordinate spaces units in path. (This just changes the dimensionality of dx, not its numeric value.)
Find the point p on path (or implied closing lines in path) that corresponds to the position dx on the coordinate line L.
Let θ be the clockwise angle from the positive x-axis to the side of the line that is tangential to path at the point p that is going in the same direction as the line at point p.
Rotate the shape described by glyph clockwise by θ about the point that is at the dx coordinate horizontally and the zero coordinate vertically.
Let (x, y) be the coordinate of the point p.
Move the shape described by glyph to the right by x and down by y.
Let glyph subpaths be a list of subpaths describing the shape given in glyph, with each CSS pixel in the coordinate space of glyph mapped to one coordinate space unit in glyph subpaths. Subpaths in glyph subpaths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.
Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.
If the method is addPathByStrokingText()
, replace glyph subpaths by the result of tracing glyph subpaths, using the CanvasDrawingStyles
object argument for
the line styles.
Add all the subpaths in glyph subpaths to transformed path list.
Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths in transformed path list, the number of times a half-infinite straight line drawn from that point crosses that path is odd.
The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.
See the equivalent step in the earlier algorithm for an example of this step. It's even more likely that there will be overlap with this method, since neighboring glyphs are likely to be rotated relative to each other.
Add all the subpaths in merged path to target.
Set the Path2D
object's need new subpath flag.
Each CanvasRenderingContext2D
object has a current transformation matrix,
as well as methods (described in this section) to manipulate it. When a
CanvasRenderingContext2D
object is created, its transformation matrix must be
initialised to the identity transform.
The transformation matrix is applied to coordinates when creating the current default
path, and when painting text, shapes, and Path2D
objects, on
CanvasRenderingContext2D
objects.
Most of the API uses SVGMatrix
objects rather than this API. This API
remains mostly for historical reasons.
The transformations must be performed in reverse order.
For instance, if a scale transformation that doubles the width is applied to the canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.
currentTransform
[ = value ]Returns the transformation matrix, as an SVGMatrix
object.
Can be set, to change the transformation matrix.
scale
(x, y)Changes the transformation matrix to apply a scaling transformation with the given characteristics.
rotate
(angle)Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.
translate
(x, y)Changes the transformation matrix to apply a translation transformation with the given characteristics.
transform
(a, b, c, d, e, f)Changes the transformation matrix to apply the matrix given by the arguments as described below.
setTransform
(a, b, c, d, e, f)Changes the transformation matrix to the matrix given by the arguments as described below.
resetTransform
()Changes the transformation matrix to the identity transform.
The currentTransform
, on
getting, must return the last object that it was set to. On setting, its value must be changed to
the new value, and the transformation matrix must be updated to match the matrix described by the
new value. When the CanvasRenderingContext2D
object is created, the currentTransform
attribute must be set a newly
created SVGMatrix
object. When the transformation matrix is mutated by the methods
described in this section, the last SVGMatrix
object to which the attribute has been
set must be mutated in a corresponding fashion.
The scale(x, y)
method must add the scaling transformation described by the arguments to the transformation
matrix. The x argument represents the scale factor in the horizontal direction and the
y argument represents the scale factor in the vertical direction. The factors are
multiples.
The rotate(angle)
method must
add the rotation transformation described by the argument to the transformation matrix. The
angle argument represents a clockwise rotation angle expressed in radians.
The translate(x,
y)
method must add the translation transformation described by the
arguments to the transformation matrix. The x argument represents the translation
distance in the horizontal direction and the y argument represents the translation
distance in the vertical direction. The arguments are in coordinate space units.
The transform(a, b,
c, d, e, f)
method must replace the
current transformation matrix with the result of multiplying the current transformation matrix
with the matrix described by:
a | c | e |
b | d | f |
0 | 0 | 1 |
The arguments a, b, c, d, e, and f are sometimes called m11, m12, m21, m22, dx, and dy or m11, m21, m12, m22, dx, and dy. Care should be taken in particular with the order of the second and third arguments (b and c) as their order varies from API to API and APIs sometimes use the notation m12/m21 and sometimes m21/m12 for those positions.
The setTransform(a, b,
c, d, e, f)
method must reset the current
transform to the identity matrix, and then invoke the transform(a, b, c,
d, e, f)
method with the same arguments.
The resetTransform()
method must
reset the current transform to the identity matrix.
Several methods in the CanvasRenderingContext2D
API take the union type
CanvasImageSource
as an argument.
This union type allows objects implementing any of the following interfaces to be used as image sources:
HTMLImageElement
(img
elements)HTMLVideoElement
(video
elements)HTMLCanvasElement
(canvas
elements)CanvasRenderingContext2D
ImageBitmap
The ImageBitmap
interface can be created from a number of other
image-representing types, including ImageData
.
When a user agent is required to check the usability of the image
argument, where image is a CanvasImageSource
object, the
user agent must run these steps, which return either good, bad, or
aborted:
If the image argument is an HTMLImageElement
object that
is in the broken state, then throw an
InvalidStateError
exception, return aborted, and abort these steps.
If the image argument is an HTMLImageElement
object that
is not fully decodable, or if the image
argument is an HTMLVideoElement
object whose readyState
attribute is either HAVE_NOTHING
or HAVE_METADATA
, then return bad and abort these
steps.
If the image argument is an HTMLImageElement
object with
an intrinsic width or intrinsic height (or both) equal to zero, then return bad and abort
these steps.
If the image argument is an HTMLCanvasElement
object with
either a horizontal dimension or a vertical dimension equal to zero, then return bad and
abort these steps.
Return good.
When a CanvasImageSource
object represents an HTMLImageElement
, the
element's image must be used as the source image.
Specifically, when a CanvasImageSource
object represents an animated image in an
HTMLImageElement
, the user agent must use the default image of the animation (the
one that the format defines is to be used when animation is not supported or is disabled), or, if
there is no such image, the first frame of the animation, when rendering the image for
CanvasRenderingContext2D
APIs.
When a CanvasImageSource
object represents an HTMLVideoElement
, then
the frame at the current playback position when the method with the argument is
invoked must be used as the source image when rendering the image for
CanvasRenderingContext2D
APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource
(i.e. after any aspect-ratio correction has been applied).
When a CanvasImageSource
object represents an HTMLCanvasElement
, the
element's bitmap must be used as the source image.
When a CanvasImageSource
object represents a CanvasRenderingContext2D
, the
object's scratch bitmap must be used as the source image.
When a CanvasImageSource
object represents an element that is being
rendered and that element has been resized, the original image data of the source image
must be used, not the image as it is rendered (e.g. width
and
height
attributes on the source element have no effect on how
the object is interpreted when rendering the image for CanvasRenderingContext2D
APIs).
When a CanvasImageSource
object represents an ImageBitmap
, the
object's bitmap image data must be used as the source image.
The image argument is not origin-clean if it is an
HTMLImageElement
or HTMLVideoElement
whose origin is not
the same as the origin specified by the entry
settings object, or if it is an HTMLCanvasElement
whose bitmap's origin-clean flag is false, or if it is a
CanvasRenderingContext2D
object whose scratch bitmap's origin-clean flag is false.
fillStyle
[ = value ]Returns the current style used for filling shapes.
Can be set, to change the fill style.
The style can be either a string containing a CSS colour, or a CanvasGradient
or
CanvasPattern
object. Invalid values are ignored.
strokeStyle
[ = value ]Returns the current style used for stroking shapes.
Can be set, to change the stroke style.
The style can be either a string containing a CSS colour, or a CanvasGradient
or
CanvasPattern
object. Invalid values are ignored.
The fillStyle
attribute represents the
colour or style to use inside shapes, and the strokeStyle
attribute represents the colour
or style to use for the lines around the shapes.
Both attributes can be either strings, CanvasGradient
s, or
CanvasPattern
s. On setting, strings must be parsed as CSS <color> values and the colour assigned, and
CanvasGradient
and CanvasPattern
objects must be assigned themselves. [CSSCOLOR] If the value is a string but cannot be parsed as a CSS
<color> value, then it must be ignored, and the attribute must retain its previous
value.
If the new value is a CanvasPattern
object that is marked as not origin-clean, then the scratch
bitmap's origin-clean flag must be set to
false.
When set to a CanvasPattern
or CanvasGradient
object, the assignment
is live, meaning that changes made to the object after the assignment do affect
subsequent stroking or filling of shapes.
On getting, if the value is a colour, then the serialisation of the colour must be returned. Otherwise, if it is not a colour but a
CanvasGradient
or CanvasPattern
, then the respective object must be
returned. (Such objects are opaque and therefore only useful for assigning to other attributes or
for comparison to other gradients or patterns.)
The serialisation of a colour for a colour value is a string, computed as follows: if
it has alpha equal to 1.0, then the string is a lowercase six-digit hex value, prefixed with a "#"
character (U+0023 NUMBER SIGN), with the first two digits representing the red component, the next
two digits representing the green component, and the last two digits representing the blue
component, the digits being lowercase ASCII hex digits. Otherwise, the colour value
has alpha less than 1.0, and the string is the colour value in the CSS rgba()
functional-notation format: the literal string "rgba
" (U+0072 U+0067 U+0062
U+0061) followed by a U+0028 LEFT PARENTHESIS, a base-ten integer in the range 0-255 representing
the red component (using ASCII digits in the shortest form possible), a literal
U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer
for the blue component, another comma and space, a U+0030 DIGIT ZERO, if the alpha value is
greater than zero then a U+002E FULL STOP (representing the decimal point), if the alpha value is
greater than zero then one or more ASCII digits representing the fractional part of
the alpha, and finally a U+0029
RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with
the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same
alpha value.
When the context is created, the fillStyle
and strokeStyle
attributes
must initially have the string value #000000
.
When the value is a colour, it must not be affected by the transformation matrix when used to draw on bitmaps.
There are two types of gradients, linear gradients and radial gradients, both represented by
objects implementing the opaque CanvasGradient
interface.
Once a gradient has been created (see below), stops are placed along it to define how the colours are distributed along the gradient. The colour of the gradient at each stop is the colour specified for that stop. Between each such stop, the colours and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the colour to use at that offset. Before the first stop, the colour must be the colour of the first stop. After the last stop, the colour must be the colour of the last stop. When there are no stops, the gradient is transparent black.
addColorStop
(offset, color)Adds a colour stop with the given colour to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.
Throws an IndexSizeError
exception if the offset is out of range. Throws a
SyntaxError
exception if the colour cannot be parsed.
createLinearGradient
(x0, y0, x1, y1)Returns a CanvasGradient
object that represents a
linear gradient that paints along the line given by the
coordinates represented by the arguments.
createRadialGradient
(x0, y0, r0, x1, y1, r1)Returns a CanvasGradient
object that represents a
radial gradient that paints along the cone given by the circles
represented by the arguments.
If either of the radii are negative, throws an
IndexSizeError
exception.
The addColorStop(offset,
color)
method on the CanvasGradient
interface adds a
new stop to a gradient. If the offset is less than 0 or greater than 1 then an
IndexSizeError
exception must be thrown. If the color cannot be
parsed as a CSS <color> value, then a SyntaxError
exception must
be thrown. Otherwise, the gradient must have a new stop placed, at offset offset relative to the whole gradient, and with the colour obtained by parsing color as a CSS <color> value. If multiple stops are added at the same offset
on a gradient, they must be placed in the order added, with the first one closest to the start of
the gradient, and each subsequent one infinitesimally further along towards the end point (in
effect causing all but the first and last stop added at each point to be ignored).
The createLinearGradient(x0, y0, x1, y1)
method takes four arguments that represent the start point (x0, y0) and end point (x1, y1) of the gradient. The method must return a linear CanvasGradient
initialised with the specified line.
Linear gradients must be rendered such that all points on a line perpendicular to the line that crosses the start and end points have the colour at the point where those two lines cross (with the colours coming from the interpolation and extrapolation described above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.
If x0 = x1 and y0 = y1, then the linear gradient must paint nothing.
The createRadialGradient(x0, y0, r0, x1, y1, r1)
method takes six arguments, the first
three representing the start circle with origin (x0, y0)
and radius r0, and the last three representing the end circle with origin
(x1, y1) and radius r1. The values are
in coordinate space units. If either of r0 or r1 are
negative, an IndexSizeError
exception must be thrown. Otherwise, the method must
return a radial CanvasGradient
initialised with the two specified circles.
Radial gradients must be rendered by following these steps:
If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing. Abort these steps.
Let x(ω) = (x1-x0)ω + x0
Let y(ω) = (y1-y0)ω + y0
Let r(ω) = (r1-r0)ω + r0
Let the colour at ω be the colour at that position on the gradient (with the colours coming from the interpolation and extrapolation described above).
For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the colour at ω, but only painting on the parts of the bitmap that have not yet been painted on by earlier circles in this step for this rendering of the gradient.
This effectively creates a cone, touched by the two circles defined in the creation of the gradient, with the part of the cone before the start circle (0.0) using the colour of the first offset, the part of the cone after the end circle (1.0) using the colour of the last offset, and areas outside the cone untouched by the gradient (transparent black).
The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.
Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.
Patterns are represented by objects implementing the opaque CanvasPattern
interface.
createPattern
(image, repetition)Returns a CanvasPattern
object that uses the given image and repeats in the
direction(s) given by the repetition argument.
The allowed values for repetition are repeat
(both
directions), repeat-x
(horizontal only), repeat-y
(vertical only), and no-repeat
(neither). If the repetition argument is empty, the value repeat
is used.
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an InvalidStateError
exception.
setTransform
(transform)Sets the transformation matrix that will be used when rendering the pattern during a fill or stroke painting operation.
To create objects of this type, the createPattern(image, repetition)
method is used. When the method is invoked, the user agent
must run the following steps:
Let image be the first argument and repetition be the second argument.
Check the usability of the image argument. If this returns aborted, then an exception has been thrown and the method doesn't return anything; abort these steps. If it returns bad, then return null and abort these steps. Otherwise it returns good; continue with these steps.
If repetition is the empty string, let it be "repeat
".
If repetition is not a case-sensitive match for one of
"repeat
", "repeat-x
", "repeat-y
", or "no-repeat
", throw a SyntaxError
exception and abort these steps.
Create a new CanvasPattern
object with the image image
and the repetition behaviour given by repetition.
If the image argument is not origin-clean, then mark the
CanvasPattern
object as not
origin-clean.
Return the CanvasPattern
object.
Modifying the image used when creating a CanvasPattern
object
after calling the createPattern()
method must
not affect the pattern(s) rendered by the CanvasPattern
object.
Patterns have a transformation matrix, which controls how the pattern is used when it is painted. Initially, a pattern's transformation matrix must be the identity transform.
When the setTransform()
method
is invoked on the pattern, the user agent must replace the pattern's transformation matrix with
the one described by the SVGMatrix
object provided as an argument to the method.
When a pattern is to be rendered within an area, the user agent must run the following steps to determine what is rendered:
Create an infinite transparent black bitmap.
Place a copy of the image on the bitmap, anchored such that its top left corner is at the
origin of the coordinate space, with one coordinate space unit per CSS pixel of the image, then
place repeated copies of this image horizontally to the left and right, if the repetition
behaviour is "repeat-x
", or vertically up and down, if the repetition
behaviour is "repeat-y
", or in all four directions all over the bitmap, if
the repetition behaviour is "repeat
".
If the original image data is a bitmap image, the value painted at a point in the area of the
repetitions is computed by filtering the original image data. When scaling up, if the imageSmoothingEnabled
attribute is set to
false, the image must be rendered using nearest-neighbor interpolation. Otherwise, the user agent
may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When
such a filtering algorithm requires a pixel value from outside the original image data, it must
instead use the value from wrapping the pixel's coordinates to the original image's dimensions.
(That is, the filter uses 'repeat' behaviour, regardless of the value of the pattern's repetition
behaviour.)
Transform the resulting bitmap according to the pattern's transformation matrix.
Transform the resulting bitmap again, this time according to the current transformation matrix.
Replace any part of the image outside the area in which the pattern is to be rendered with transparent black.
The resulting bitmap is what is to be rendered, with the same origin and same scale.
If a radial gradient or repeated pattern is used when the transformation matrix is singular, the resulting style must be transparent black (otherwise the gradient or pattern would be collapsed to a point or line, leaving the other pixels undefined). Linear gradients and solid colours always define all points even with singular tranformation matrices.
There are three methods that immediately draw rectangles to the bitmap. They each take four arguments; the first two give the x and y coordinates of the top left of the rectangle, and the second two give the width w and height h of the rectangle, respectively.
The current transformation matrix must be applied to the following four coordinates, which form the path that must then be closed to get the specified rectangle: (x, y), (x+w, y), (x+w, y+h), (x, y+h).
Shapes are painted without affecting the current default path, and are subject to
the clipping region, and, with the exception of clearRect()
, also shadow
effects, global alpha, and global composition operators.
clearRect
(x, y, w, h)Clears all pixels on the bitmap in the given rectangle to transparent black.
fillRect
(x, y, w, h)Paints the given rectangle onto the bitmap, using the current fill style.
strokeRect
(x, y, w, h)Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.
The clearRect(x, y, w, h)
method must run the
following steps:
Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.
Clear the pixels in pixels to a fully transparent black, erasing any previous image.
Clear regions that cover the pixels in pixels on the scratch bitmap.
If either height or width are zero, this method has no effect, since the set of pixels would be empty.
The fillRect(x, y, w, h)
method must paint the
specified rectangular area using the fillStyle
. If
either height or width are zero, this method has no effect.
The strokeRect(x, y, w, h)
method must take the
result of tracing the path described below, using the
CanvasRenderingContext2D
object's line styles, and fill it with the strokeStyle
.
If both w and h are zero, the path has a single subpath with just one point (x, y), and no lines, and this method thus has no effect (the trace a path algorithm returns an empty path in that case).
If just one of either w or h is zero, then the path has a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.
Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w, y+h), and (x, y+h), connected to each other in that order by straight lines.
fillText
(text, x, y [, maxWidth ] )strokeText
(text, x, y [, maxWidth ] )Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.
measureText
(text)Returns a TextMetrics
object with the metrics of the given text in the current
font.
width
actualBoundingBoxLeft
actualBoundingBoxRight
fontBoundingBoxAscent
fontBoundingBoxDescent
actualBoundingBoxAscent
actualBoundingBoxDescent
emHeightAscent
emHeightDescent
hangingBaseline
alphabeticBaseline
ideographicBaseline
Returns the measurement described below.
The CanvasRenderingContext2D
interface provides the following methods for
rendering text.
The fillText()
and strokeText()
methods take three or four
arguments, text, x, y, and optionally
maxWidth, and render the given text at the given (x, y) coordinates ensuring that the text isn't wider than maxWidth if specified, using the current font
, textAlign
,
and textBaseline
values. Specifically, when the
methods are called, the user agent must run the following steps:
Run the text preparation algorithm, passing it text, the
CanvasRenderingContext2D
object, and, if the maxWidth argument
was provided, that argument. Let glyphs be the result.
Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.
Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.
For fillText()
, fillStyle
must be applied to the shapes and strokeStyle
must be ignored. For strokeText()
, the reverse holds: strokeStyle
must be applied to the result of tracing the shapes using the CanvasRenderingContext2D
object for the line styles, and fillStyle
must be
ignored.
These shapes are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.
If the text preparation algorithm used a font that has an origin that is not the same as the origin specified by the entry settings object (even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font), then set the scratch bitmap's origin-clean flag to false.
The measureText()
method takes one
argument, text. When the method is invoked, the user agent must run the
text preparation algorithm, passing it text and the
CanvasRenderingContext2D
object, and then using the returned inline box must create a
new TextMetrics
object with its attributes set as described in the following list.
If doing these measurements requires using a font that has an origin that is not the
same as that of the Document
object that owns the
canvas
element (even if "using a font" means just checking if that font has a
particular glyph in it before falling back to another font), then the method must throw a
SecurityError
exception.
Otherwise, it must return the new TextMetrics
object.
[CSS]
width
attributeThe width of that inline box, in CSS pixels. (The text's advance width.)
actualBoundingBoxLeft
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the left side of the bounding
rectangle of the given text, in CSS pixels; positive numbers indicating a distance going left
from the given alignment point.
The sum of this value and the next (actualBoundingBoxRight
) can be wider than
the width of the inline box (width
), in particular
with slanted fonts where characters overhang their advance width.
actualBoundingBoxRight
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the right side of the bounding
rectangle of the given text, in CSS pixels; positive numbers indicating a distance going right
from the given alignment point.
fontBoundingBoxAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the top of the highest
bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers
indicating a distance going up from the given baseline.
This value and the next are useful when rendering a background that must have a
consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent
attribute (and
its corresponding attribute for the descent) are useful when drawing a bounding box around
specific text.
fontBoundingBoxDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the bottom of the lowest
bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers
indicating a distance going down from the given baseline.
actualBoundingBoxAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the top of the bounding
rectangle of the given text, in CSS pixels; positive numbers indicating a distance going up from
the given baseline.
This number can vary greatly based on the input text, even if the first font
specified covers all the characters in the input. For example, the actualBoundingBoxAscent
of a lowercase
"o" from an alphabetic baseline would be less than that of an uppercase "F". The value can
easily be negative; for example, the distance from the top of the em box (textBaseline
value "top
") to the top of the bounding rectangle when
the given text is just a single comma ",
" would likely (unless the font is
quite unusual) be negative.
actualBoundingBoxDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the bottom of the bounding
rectangle of the given text, in CSS pixels; positive numbers indicating a distance going down
from the given baseline.
emHeightAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the highest top of the em
squares in the line box, in CSS pixels; positive numbers indicating that the given baseline is
below the top of that em square (so this value will usually be positive). Zero if the given
baseline is the top of that em square; half the font size if the given baseline is the middle of
that em square.
emHeightDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the lowest bottom of the em
squares in the line box, in CSS pixels; positive numbers indicating that the given baseline is
below the bottom of that em square (so this value will usually be negative). (Zero if the given
baseline is the top of that em square.)
hangingBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the hanging baseline of the
line box, in CSS pixels; positive numbers indicating that the given baseline is below the hanging
baseline. (Zero if the given baseline is the hanging baseline.)
alphabeticBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the alphabetic baseline of
the line box, in CSS pixels; positive numbers indicating that the given baseline is below the
alphabetic baseline. (Zero if the given baseline is the alphabetic baseline.)
ideographicBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the ideographic baseline of
the line box, in CSS pixels; positive numbers indicating that the given baseline is below the
ideographic baseline. (Zero if the given baseline is the ideographic baseline.)
Glyphs rendered using fillText()
and
strokeText()
can spill out of the box given by the
font size (the em square size) and the width returned by measureText()
(the text width). Authors are encouraged
to use the bounding box values described above if this is an issue.
A future version of the 2D context API may provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.
The context always has a current default path. There is only one current default path, it is not part of the drawing state. The current default path is a path, as described above.
beginPath
()Resets the current default path.
fill
( [ fillRule ] )fill
(path [, fillRule ] )Fills the subpaths of the current default path or the given path with the current fill style, obeying the given fill rule.
stroke
()stroke
(path)Strokes the subpaths of the current default path or the given path with the current stroke style.
drawFocusIfNeeded
(element)drawFocusIfNeeded
(path, element)If the given element is focused, draws a focus ring around the current default path or the given path, following the platform conventions for focus rings.
scrollPathIntoView
()scrollPathIntoView
(path)Scrolls the current default path or the given path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.
clip
( [ fillRule ] )clip
(path [, fillRule ] )Further constrains the clipping region to the current default path or the given path, using the given fill rule to determine what points are in the path.
resetClip
()Unconstrains the clipping region.
isPointInPath
(x, y [, fillRule ] )isPointInPath
(path, x, y [, fillRule ] )Returns true if the given point is in the current default path or the given path, using the given fill rule to determine what points are in the path.
isPointInStroke
(x, y)isPointInStroke
(path, x, y)Returns true if the given point would be in the region covered by the stroke of the current default path or the given path, given the current stroke style.
The beginPath()
method must empty the
list of subpaths in the context's current default path so that the it once again has
zero subpaths.
Where the following method definitions use the term intended path, it means the
Path2D
argument, if one was provided, or the current default path
otherwise.
When the intended path is a Path2D
object, the coordinates and lines of its
subpaths must be transformed according to the CanvasRenderingContext2D
object's current transformation matrix when used by these
methods (without affecting the Path2D
object itself). When the intended path is the
current default path, it is not affected by the transform. (This is because
transformations already affect the current default path when it is constructed, so
applying it when it is painted as well would result in a double transformation.)
The fill()
method must fill all the
subpaths of the intended path, using fillStyle
, and
using the fill rule indicated by the fillRule argument. Open
subpaths must be implicitly closed when being filled (without affecting the actual subpaths).
The stroke()
method must trace the intended path, using the
CanvasRenderingContext2D
object for the line styles, and then fill the resulting path
using the strokeStyle
attribute, using the non-zero winding rule.
As a result of how the algorithm to trace a path is defined, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.
The stroke style is affected by the transformation during painting, even if the intended path is the current default path.
Paths, when filled or stroked, must be painted without affecting the current default
path or any Path2D
objects, and must be subject to shadow effects, global
alpha, the clipping region, and global composition operators. (The effect
of transformations is described above and varies based on which path is being used.)
The drawFocusIfNeeded(element)
method, when invoked, must run the following steps:
If element is not focused or is not a descendant of the element with whose context the method is associated, then abort these steps.
Draw a focus ring of the appropriate style along the intended path, following platform conventions.
Some platforms only draw focus rings around elements that have been focused from
the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus
rings around some elements at all unless relevant accessibility features are enabled. This API
is intended to follow these conventions. User agents that implement distinctions based on the
manner in which the element was focused are encouraged to classify focus driven by the focus()
method based on the kind of user interaction event from which
the call was triggered (if any).
The focus ring should not be subject to the shadow effects, the
global alpha, the global composition operators, the fillStyle
attribute, the strokeStyle
attribute, or any of the
CanvasDrawingStyles
members, but should be subject to the clipping
region. (The effect of transformations is described above and varies based on which path
is being used.)
Optionally, run the appropriate step from the following list:
CanvasRenderingContext2D
object's context bitmap mode is fixedInform the user that the focus is at the location given by the intended path. User agents may wait until the next time the event loop reaches its update the rendering step to optionally inform the user.
Add instructions to the scratch bitmap's list of pending interface actions that inform the user that the focus is at the location of the bitmap given by the intended path.
User agents should not implicitly close open subpaths in the intended path when drawing the focus ring.
This might be a moot point, however. For example, if the focus ring is drawn as an axis-aligned bounding rectangle around the points in the intended path, then whether the subpaths are closed or not has no effect. This specification intentionally does not specify precisely how focus rings are to be drawn: user agents are expected to honor their platform's native conventions.
The scrollPathIntoView()
method, when invoked, if the CanvasRenderingContext2D
object's context bitmap mode is fixed, must run the following steps; and otherwise, must add
instructions to the scratch bitmap's list of pending interface actions
that run the following steps:
Let the specified rectangle be the rectangle of the bounding box of the intended path.
Let notional child be a hypothetical element that is a rendered child
of the canvas
element whose dimensions are those of the specified
rectangle.
Scroll notional child into view with the align to top flag set.
Optionally, inform the user that the caret or selection (or both)
cover the specified rectangle of the canvas. If the
CanvasRenderingContext2D
object's context bitmap mode was fixed when the method was invoked, the user agent may wait
until the next time the event loop reaches its update the rendering step to
optionally inform the user.
"Inform the user", as used in this section, does not imply any persistent state
change. It could mean, for instance, calling a system accessibility API to notify assistive
technologies such as magnification tools so that the user's magnifier moves to the given area of
the canvas. However, it does not associate the path with the element, or provide a region for
tactile feedback, etc. To persistently associate a region with information provided to
accessibility tools, use the addHitRegion()
API.
The clip()
method must create a new
clipping region by calculating the intersection of the current clipping region and the
area described by the intended path, using the fill rule indicated by the fillRule argument. Open subpaths must be implicitly closed when computing the
clipping region, without affecting the actual subpaths. The new clipping region replaces the
current clipping region.
When the context is initialised, the clipping region must be set to the largest infinite surface (i.e. by default, no clipping occurs).
The resetClip()
method must create a
new clipping region that is the largest infinite surface. The new clipping region
replaces the current clipping region.
The isPointInPath()
method must
return true if the point given by the x and y coordinates
passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the
current transformation, is inside the intended path as determined by the fill rule
indicated by the fillRule argument; and must return false otherwise. Open
subpaths must be implicitly closed when computing the area inside the path, without affecting the
actual subpaths. Points on the path itself must be considered to be inside the path. If either of
the arguments is infinite or NaN, then the method must return false.
The isPointInStroke()
method
must return true if the point given by the x and y
coordinates passed to the method, when treated as coordinates in the canvas coordinate space
unaffected by the current transformation, is inside the path that results from tracing the intended path, using the non-zero winding rule, and using the
CanvasRenderingContext2D
object for the line styles; and must return false otherwise.
Points on the resulting path must be considered to be inside the path. If either of the arguments
is infinite or NaN, then the method must return false.
This canvas
element has a couple of checkboxes. The path-related commands are
highlighted:
<canvas height=400 width=750> <label><input type=checkbox id=showA> Show As</label> <label><input type=checkbox id=showB> Show Bs</label> <!-- ... --> </canvas> <script> function drawCheckbox(context, element, x, y, paint) { context.save(); context.font = '10px sans-serif'; context.textAlign = 'left'; context.textBaseline = 'middle'; var metrics = context.measureText(element.labels[0].textContent); if (paint) { context.beginPath(); context.strokeStyle = 'black'; context.rect(x-5, y-5, 10, 10); context.stroke(); context.addHitRegion({ control: element }); if (element.checked) { context.fillStyle = 'black'; context.fill(); } context.fillText(element.labels[0].textContent, x+5, y); } context.beginPath(); context.rect(x-7, y-7, 12 + metrics.width+2, 14); context.drawFocusIfNeeded(element)) { context.restore(); } function drawBase() { /* ... */ } function drawAs() { /* ... */ } function drawBs() { /* ... */ } function redraw() { var canvas = document.getElementsByTagName('canvas')[0]; var context = canvas.getContext('2d'); context.clearRect(0, 0, canvas.width, canvas.height); drawCheckbox(context, document.getElementById('showA'), 20, 40, true); drawCheckbox(context, document.getElementById('showB'), 20, 60, true); drawBase(); if (document.getElementById('showA').checked) drawAs(); if (document.getElementById('showB').checked) drawBs(); } function processClick(event) { var canvas = document.getElementsByTagName('canvas')[0]; var context = canvas.getContext('2d'); var x = event.clientX; var y = event.clientY; var node = event.target; while (node) { x -= node.offsetLeft - node.scrollLeft; y -= node.offsetTop - node.scrollTop; node = node.offsetParent; } drawCheckbox(context, document.getElementById('showA'), 20, 40, false); if (context.isPointInPath(x, y)) document.getElementById('showA').checked = !(document.getElementById('showA').checked); drawCheckbox(context, document.getElementById('showB'), 20, 60, false); if (context.isPointInPath(x, y)) document.getElementById('showB').checked = !(document.getElementById('showB').checked); redraw(); } document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true); document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true); document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true); document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false); redraw(); </script>
To draw images, the drawImage
method
can be used.
This method can be invoked with three different sets of arguments:
drawImage(image, dx, dy)
drawImage(image, dx, dy, dw, dh)
drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
drawImage
(image, dx, dy)drawImage
(image, dx, dy, dw, dh)drawImage
(image, sx, sy, sw, sh, dx, dy, dw, dh)Draws the given image onto the canvas. The arguments are interpreted as follows:
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an InvalidStateError
exception.
When the drawImage()
method is invoked, the user
agent must run the following steps:
Check the usability of the image argument. If this returns aborted, then an exception has been thrown and the method doesn't return anything; abort these steps. If it returns bad, then abort these steps without drawing anything. Otherwise it returns good; continue with these steps.
Establish the source and destination rectangles as follows:
If not specified, the dw and dh arguments must default to the values of sw and sh, interpreted such that one CSS pixel in the image is treated as one unit in the scratch bitmap's coordinate space. If the sx, sy, sw, and sh arguments are omitted, they must default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively. If the image has no intrinsic dimensions, the concrete object size must be used instead, as determined using the CSS "Concrete Object Size Resolution" algorithm, with the specified size having neither a definite width nor height, nor any additional contraints, the object's intrinsic properties being those of the image argument, and the default object size being the size of the scratch bitmap. [CSSIMAGES]
The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).
The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).
When the source rectangle is outside the source image, the source rectangle must be clipped to the source image and the destination rectangle must be clipped in the same proportion.
When the destination rectangle is outside the destination image (the scratch bitmap), the pixels that land outside the scratch bitmap are discarded, as if the destination was an infinite canvas whose rendering was clipped to the dimensions of the scratch bitmap.
If one of the sw or sh arguments is zero, abort these steps. Nothing is painted.
Paint the region of the image argument specified by the source rectangle on the region of the rendering context's scratch bitmap specified by the destination rectangle, after applying the current transformation matrix to the destination rectangle.
The image data must be processed in the original direction, even if the dimensions given are negative.
When scaling up, if the imageSmoothingEnabled
attribute is set to
true, the user agent should attempt to apply a smoothing algorithm to the image data when it is
scaled. Otherwise, the image must be rendered using nearest-neighbor interpolation.
This specification does not define the precise algorithm to use when scaling an
image down, or when scaling an image up when the imageSmoothingEnabled
attribute is set to
true.
When a canvas
or CanvasRenderingContext2D
object is
drawn onto itself, the drawing model requires the source to be copied before the
image is drawn, so it is possible to copy parts of a canvas
or scratch
bitmap onto overlapping parts of itself.
If the original image data is a bitmap image, the value painted at a point in the destination rectangle is computed by filtering the original image data. The user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behaviour.) When the filtering algorithm requires a pixel value from outside the source rectangle but inside the original image data, then the value from the original image data must be used.
Thus, scaling an image in parts or in whole will have the same effect. This does
mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in
the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is
surrounded by a border of transparent black, or by copying sprites to be scaled into temporary
canvas
elements and drawing the scaled sprites from there.
Images are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.
If the image argument is not origin-clean, set the scratch bitmap's origin-clean flag to false.
A hit region list is a list of hit regions for a bitmap.
Each hit region consists of the following information:
A set of pixels on the bitmap for which this region is responsible.
A bounding circumference on the bitmap that surrounds the hit region's set of pixels as they stood when it was created.
Optionally, a non-empty string representing an ID for distinguishing the region from others.
Optionally, a reference to another region that acts as the parent for this one.
A count of regions that have this one as their parent, known as the hit region's child count.
A cursor specification, in the form
of either a CSS cursor value, or the string "inherit
" meaning that the
cursor of the hit region's parent, if any, or of the canvas
element, if
not, is to be used instead.
Optionally, either a control, or an unbacked region description.
A control is a reference to an Element
node, to which, in certain conditions, the user agent will route events, and from which the user
agent will determine the state of the hit region for the purposes of accessibility tools. (The
control is ignored when it is not a descendant of the canvas
element.)
An unbacked region description consists of the following:
Optionally, a label.
An ARIA role, which, if the unbacked region description also has a label, could be the empty string.
addHitRegion
(options)Adds a hit region to the bitmap. The argument is an object with the following members:
path
(default null)Path2D
object that describes the pixels that form part of the region. If
this member is not provided or is set to null, the current default path is used
instead.fillRule
(default "nonzero
")id
(default empty string) MouseEvent
events on the
canvas
(event.region
) and as a way to
reference this region in later calls to addHitRegion()
.parentID
(default null)cursor
(default "inherit
")inherit
" means to use the cursor for the parent region (as specified by the
parentID
member), if any, or to use the
canvas
element's cursor if the region has no parent.control
(default null)canvas
) to which events are to be
routed, and which accessibility tools are to use as a surrogate for describing and interacting
with this region.label
(default null)role
(default null)Hit regions can be used for a variety of purposes:
canvas
to automatically submit a form via a button
element.canvas
without
seeing it, e.g. by touch on a mobile device.canvas
to
have different cursors, with the user agent automatically switching between them.removeHitRegion
(id)Removes a hit region (and all its descendants) from the canvas bitmap. The argument is the ID
of a region added using addHitRegion()
.
The pixels that were covered by this region and its descendants are effectively cleared by this operation, leaving the regions non-interactive. In particular, regions that occupied the same pixels before the removed regions were added, overlapping them, do not resume their previous role.
clearHitRegions
()Removes all hit regions from the canvas bitmap.
A hit region A is an ancestor region of a hit region B if B has a parent and its parent is either A or another hit region for which A is an ancestor region.
The region identified by the ID ID in a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):
If ID is null, return nothing and abort these steps.
Let list be the hit region list associated with bitmap.
If there is a hit region in list whose ID is a case-sensitive match for ID, then return that hit region and abort these steps.
Otherwise, return nothing.
The region representing the control control for a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):
Let list be the hit region list associated with bitmap.
If there is a hit region in list whose control is control, then return that hit region and abort these steps.
Otherwise, return nothing.
The control represented by a region region for a
canvas
element ancestor is the value returned by the following
algorithm (which can return an element or nothing):
If region has no control, return nothing and abort these steps.
Let control be region's control.
If control is not a descendant of ancestor, then return nothing and abort these steps.
If control is no longer a supported interactive canvas fallback element, then return nothing and abort these steps.
Otherwise, return control.
The cursor for a hit region region of a canvas
element ancestor is the value returned by the following algorithm:
Loop: If region has a cursor specification other than "inherit
", then
return that hit region's cursor specification and abort these steps.
If region has a parent, then let region be that hit region's parent, and return to the step labeled loop.
Otherwise, return the used value of the 'cursor' property for the canvas
element, if any; if there isn't one, return 'auto'. [CSSUI]
The region for a pixel pixel on a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):
Let list be the hit region list associated with bitmap.
If there is a hit region in list whose set of pixels contains pixel, then return that hit region and abort these steps.
Otherwise, return nothing.
To clear regions that cover the pixels pixels on a bitmap bitmap, the user agent must run the following steps:
Let list be the hit region list associated with bitmap.
Remove all pixels in pixels from the set of pixels of each hit region in list.
Garbage-collect the regions of bitmap.
To garbage-collect the regions of a bitmap bitmap, the user agent must run the following steps:
Let list be the hit region list associated with bitmap.
Loop: Let victim be the first hit region in list to have an empty set of pixels and a zero child count, if any. If there is no such hit region, abort these steps.
If victim has a parent, then decrement that hit region's child count by one.
Remove victim from list.
Jump back to the step labeled loop.
Adding a new region and calling clearRect()
are the two ways this clearing algorithm can
be invoked. The hit region list itself is also reset when the rendering context is
reset, e.g. when a CanvasRenderingContext2D
object is bound to or unbound from a
canvas
, or when the dimensions of the bitmap are changed.
An element is a supported interactive canvas fallback element if it is one of the following:
an a
element that represents a
hyperlink and that does not have any img
descendants
a button
element
an input
element whose type
attribute is in one of the Checkbox or Radio Button states
an input
element that is a button but
its type
attribute is not in the Image Button state
a select
element with a multiple
attribute or a display size greater than 1
an option
element that is in a list of options of a select
element
with a multiple
attribute or a display size greater than 1
an element that would not be interactive content except for having the
tabindex
attribute specified
a non-interactive table
,
caption
, thead
, tbody
, tfoot
,
tr
, td
, or th
element
When the addHitRegion()
method is
invoked, the user agent must run the following steps:
Let arguments be the dictionary object provided as the method's argument.
If the arguments object's path
member is not null, let source
path be the path
member's value. Otherwise,
let it be the CanvasRenderingContext2D
object's current default
path.
Transform all the coordinates and lines in source path by the current
transform matrix, if the arguments object's path
member is not null.
Let specified pixels be the pixels contained in source
path, using the fill rule indicated by the fillRule
member.
Remove from specified pixels any pixels not contained within the clipping region.
If the arguments object's id
member is the empty string, let it be null
instead.
If the arguments object's id
member is not null, then let previous
region for this ID be the region identified by the ID given by the id
member's value in this scratch bitmap, if
any. If the id
member is null or no such region
currently exists, let previous region for this ID be null.
If the arguments object's parentID
member is the empty string, let it be null
instead.
If the arguments object's parentID
member is not null, then let parent region be the region identified by the ID given by the parentID
member's value in the scratch
bitmap, if any. If the parentID
member
is null or no such region currently exists, let parent region be
null.
If the arguments object's label
member is the empty string, let it be null
instead.
If any of the following conditions are met, throw a NotSupportedError
exception
and abort these steps.
The arguments object's control
and label
members are both non-null.
The arguments object's control
and role
members are both non-null.
The arguments object's role
member's value is the empty string, and the
label
member's value is either null or the
empty string.
The specified pixels has no pixels.
The arguments object's control
member is neither null nor a
supported interactive canvas fallback element.
The parent region is not null but has a control.
The previous region for this ID is the same hit region as the parent region.
The previous region for this ID is an ancestor region of the parent region.
If the parentID
member is not null but
parent region is null, then throw a NotFoundError
exception and
abort these steps.
If any of the following conditions are met, throw a SyntaxError
exception and
abort these steps.
cursor
member is not null but is neither an
ASCII case-insensitive match for the string "inherit
", nor a
valid CSS 'cursor' property value. [CSSUI]role
member is not null but its value is not an
ordered set of unique space-separated tokens whose tokens are all
case-sensitive matches for names of non-abstract WAI-ARIA roles. [ARIA]Let region be a newly created hit region, with its information configured as follows:
The specified pixels
A user-agent-defined shape that wraps the pixels contained in source path. (In the simplest case, this can just be the bounding rectangle; this specification allows it to be any shape in order to allow other interfaces.)
If the arguments object's id
member is not null: the value of the id
member. Otherwise, region has no
id.
If parent region is not null: parent region. Otherwise, region has no parent.
Initially zero.
If the arguments object's cursor
member is not null: the value of the cursor
member. Otherwise, the string "inherit
".
If the arguments object's control
member is not null: the value of the control
member. Otherwise, region has no control.
If the arguments object's label
member is not null: the value of the label
member. Otherwise, region
has no label.
If the arguments object's role
member is not null: the value of the role
member (which might be the empty string).
Otherwise, if the arguments object's label
member is not null: the empty string.
Otherwise, region has no ARIA
role.
If the arguments object's cursor
member is not null, then act as if a CSS rule
for the canvas
element setting its 'cursor' property had been seen, whose value was
the hit region's cursor specification.
For example, if the user agent prefetches cursor values, this would cause that
to happen in response to an appropriately-formed addHitRegion()
call.
If there is a previous region with this ID, remove it, and all hit regions for which it is an ancestor region, from the scratch bitmap's hit region list; then, if it had a parent region, decrement that hit region's child count by one.
If there is a parent region, increment its hit region's child count by one.
Clear regions that cover the pixels in region's set of pixels on this scratch bitmap.
Add region to the scratch bitmap's element's hit region list.
When the removeHitRegion()
method is invoked, the user agent must run the following steps:
Let region be the region identified by the ID given by the method's argument in the rendering context's scratch bitmap. If no such region currently exists, abort these steps.
If the method's argument is the empty string, then no region will match.
Remove region, and all hit regions for which it is an ancestor region, from the rendering context's scratch bitmap's hit region list; then, if it had a parent region, decrement that hit region's child count by one.
Garbage-collect the regions of the rendering context's scratch bitmap.
When the clearHitRegions()
method is invoked, the user agent must remove all the hit regions
from the rendering context's scratch bitmap's hit region list.
The MouseEvent
interface is extended to support hit regions:
partial interface MouseEvent { readonly attribute DOMString? region; }; partial dictionary MouseEventInit { DOMString? region; };
region
If the mouse was over a hit region, then this returns the hit region's ID, if it has one.
Otherwise, returns null.
The region
attribute on
MouseEvent
objects must return the value it was initialised to. When the object is
created, this attribute must be initialised to null. It represents the hit region's
ID if the mouse was over a hit region when the event was fired.
When a MouseEvent
is to be fired at a canvas
element by the user
agent in response to a pointing device action other than a click (e.g. a mousedown
event or a mousemove
event), the user agent must run the canvas
MouseEvent
rerouting steps immediately prior to dispatching the event. This does not affect default actions (so for instance, if the
event gets rerouted to an element that has a default action for mousemove
events, this default action doesn't trigger).
Actual clicks are handled by the run authentic click activation steps, which also invoke these steps.
The canvas MouseEvent
rerouting steps are as follows. If these steps
say to act as normal, that means that the event must be fired as it would have had these
requirements not been applied.
If the pointing device is not indicating a pixel on the canvas
, then act as
normal and abort these steps.
If the canvas
element has no hit region list, then act as normal
and abort these steps.
Let pixel be the pixel indicated by the pointing device.
Let region be the hit region that is the region for the pixel pixel on this
canvas
element's bitmap, if any.
If there is no region, then act as normal and abort these steps.
Let id be the region's ID, if any.
If there is an id, then initialise the event object's region
attribute to id.
Let control be the control represented by region for this canvas
element, if any.
If there is a control, then target the event object at control instead of the canvas
element.
Continue dispatching the event, but with the updated event object and target as given in the above steps.
The Touch
interface is extended to support hit regions also: [TOUCH]
partial interface Touch { readonly attribute DOMString? region; };
region
If the touch point was over a hit region when it was first placed on the surface, then this returns the hit region's ID, if it has one.
Otherwise, returns null.
The region
attribute on a Touch
object representing a touch point T must return the value
obtained by running the following algorithm when T was first placed on the
surface: [TOUCH]
If the touch point is not on a pixel on the canvas
, then return
null and abort these steps.
If the canvas
element has no hit region list, then return null
and abort these steps.
Let pixel be the pixel that the touch point is on.
Let region be the hit region that is the region for the pixel pixel on this
canvas
element's bitmap, if any.
If there is no region, then return null and abort these steps.
Let id be the region's ID, if any, or else null.
Return id.
When a user's pointing device cursor is positioned over a canvas
element, user
agents should render the pointing device cursor according to the cursor specification described by
the cursor for the hit region that is the region for the pixel that the pointing device designates
on the canvas
element's bitmap.
User agents are encouraged to make use of the information present in a canvas
element's hit region list to improve the accessibility of canvas
elements.
Each hit region should be handled in a fashion equivalent to a node in a virtual
DOM tree rooted at the canvas
element. The hierarchy of this virtual DOM tree must
match the hierarchy of the hit regions, as described by the parent of each region. Regions without a parent must be treated as children of the canvas
element for
the purpose of this virtual DOM tree. For each node in such a DOM tree, the hit region's
bounding circumference gives the region of the screen to use when representing the node (if
appropriate).
The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.
For the purposes of accessibility tools, when an element C is a descendant
of a canvas
element and there is a
region representing the control C for that canvas
element's bitmap, then the element's position relative to the document should be presented as if
it was that region in the canvas
element's virtual DOM tree.
The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.
Thus, for instance, a user agent on a touch-screen device could provide haptic
feedback when the user croses over a hit region's bounding circumference, and then
read the hit region's label to the user. Similarly, a desktop user agent with a
virtual accessibility focus separate from the keyboard input focus could allow the user to
navigate through the hit regions, using the virtual DOM tree described above to enable
hierarchical navigation. When an interactive control inside the canvas
element
gains focus, if the control has a corresponding region, then that hit region's
bounding circumference could be used to determine what area of the display to magnify.
ImageData
(sw, sh)createImageData
(sw, sh)Returns an ImageData
object with the given dimensions. All the pixels in the
returned object are transparent black.
Throws an IndexSizeError
exception if either of the width or height
arguments are zero.
createImageData
(imagedata)Returns an ImageData
object with the same dimensions as the argument. All the
pixels in the returned object are transparent black.
ImageData
(data, sw [, sh ] )Returns an ImageData
object using the data provided in the Uint8ClampedArray
argument, interpreted using the given dimensions.
As each pixel in the data is represented by four numbers, the length of the data needs to be a multiple of four times the given width. If the height is provided as well, then the length needs to be exactly the width times the height times 4.
Throws an IndexSizeError
exception if the given data and dimensions can't be
interpreted consistently, or if either dimension is zero.
getImageData
(sx, sy, sw, sh)Returns an ImageData
object containing the image data for the given rectangle of
the bitmap.
Throws an IndexSizeError
exception if the either of the width or height
arguments are zero.
width
height
Returns the actual dimensions of the data in the ImageData
object, in
pixels.
data
Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.
putImageData
(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ] )Paints the data from the given ImageData
object onto the bitmap. If a dirty
rectangle is provided, only the pixels from that rectangle are painted.
The globalAlpha
and globalCompositeOperation
attributes, as
well as the shadow attributes, are ignored for the purposes of this method call; pixels in the
canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.
Throws a NotSupportedError
exception if any of the arguments are not finite.
Throws an InvalidStateError
exception if the imagedata
object's data has been neutered.
The ImageData()
constructors and the createImageData()
methods are used to
instantiate new ImageData
objects.
When the ImageData()
constructor is invoked with two
numeric arguments sw and sh, it must return a new
ImageData
object representing a transparent black rectangle with a width equal to
sw and a height equal to sh, if both sw and sh are non-zero. If one or both of sw and sh are zero, then the constructor must throw an
IndexSizeError
exception instead.
When the ImageData()
constructor is invoked with its first
argument being an Uint8ClampedArray
source and its second and
(optionally) third argument(s) being numeric arguments sw and sh, it must run the following steps:
Let length be the number of bytes in source.
If length is not a non-zero integral multiple of four, throw an
InvalidStateError
exception and abort these steps.
Let length be length divided by four.
If length is not an integral multiple of sw,
throw an IndexSizeError
exception and abort these steps.
At this step, the length is guaranteed to be greater than zero (otherwise the second step above would have aborted the steps), so if sw is zero, this step will throw the exception and abort these steps.
Let height be length divided by sw.
If the sh argument was not omitted, and its value is not equal to
height, then throw an IndexSizeError
exception and abort these
steps.
Return a new ImageData
object whose width is sw, whose
height is height, and whose data is source.
The resulting object's data is not a copy of source, it's the actual Uint8ClampedArray
object passed as the
first argument to the constructor.
When the createImageData()
method is
invoked with two numeric arguments sw and sh, it must
return a new ImageData
object representing a transparent black rectangle with a width
equal to the absolute magnitude of sw and a height equal to the absolute
magnitude of sh, if both sw and sh
are non-zero. If one or both of sw and sh are zero, then
the constructor must throw an IndexSizeError
exception instead.
When the createImageData()
method is
invoked with a single imagedata argument, it must return a new
ImageData
object representing a transparent black rectangle with the same dimensions
as the ImageData
object passed as the argument.
The getImageData(sx,
sy, sw, sh)
method must,
if either the sw or sh arguments are zero, throw an
IndexSizeError
exception; otherwise,
if the scratch bitmap's origin-clean
flag is set to false, it must throw a SecurityError
exception;
otherwise, it must return an ImageData
object with width sw and
height sh representing the scratch bitmap for the area of that
bitmap denoted by the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw,
sy+sh), (sx,
sy+sh), in the bitmap's
coordinate space units. Pixels outside the scratch bitmap must be returned as
transparent black. Pixels must be returned as non-premultiplied alpha values.
New ImageData
objects must be initialised so that their width
attribute is set to the number of pixels per
row in the image data, their height
attribute is set to the number of rows in the image data, and their data
attribute, except where an existing array is
provided, is initialised to a new Uint8ClampedArray
object. The
Uint8ClampedArray
object must use a new Canvas Pixel
ArrayBuffer
for its storage, and must have a zero start offset and a length
equal to the length of its storage, in bytes. The Canvas Pixel
ArrayBuffer
must contain the image data. At least one pixel's worth of image
data must be returned. [ECMA262]
A Canvas Pixel ArrayBuffer
is an ArrayBuffer
whose data is
represented in left-to-right order, row by row top to bottom, starting with the top left, with
each pixel's red, green, blue, and alpha components being given in that order for each pixel. Each
component of each pixel represented in this array must be in the range 0..255, representing the 8
bit value for that component. The components must be assigned consecutive indices starting with 0
for the top left pixel's red component. [ECMA262]
The putImageData()
method writes
data from ImageData
structures back to the rendering context's scratch
bitmap. Its arguments are: imagedata, dx, dy, dirtyX, dirtyY, dirtyWidth, and dirtyHeight.
When the last four arguments to this method are omitted, they must be assumed to have the
values 0, 0, the width
member of the imagedata structure, and the height
member of the imagedata structure, respectively.
When invoked, the method must act as follows:
If the imagedata argument's data
attribute has been neutered, throw an InvalidStateError
exception and abort these steps.
If dirtyWidth is negative, let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal to the absolute magnitude of dirtyWidth.
If dirtyHeight is negative, let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be equal to the absolute magnitude of dirtyHeight.
If dirtyX is negative, let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.
If dirtyY is negative, let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.
If dirtyX+dirtyWidth is greater
than the width
attribute of the imagedata argument, let dirtyWidth be the value of that width
attribute, minus the value of dirtyX.
If dirtyY+dirtyHeight is
greater than the height
attribute of the imagedata argument, let dirtyHeight be the value of that
height
attribute, minus the value of dirtyY.
If, after those changes, either dirtyWidth or dirtyHeight is negative or zero, stop these steps without affecting any bitmaps.
For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the
four channels of the pixel with coordinate (x, y) in
the imagedata data structure's Canvas Pixel
ArrayBuffer
to the pixel with coordinate (dx+x, dy+y) in the rendering context's scratch bitmap.
The handling of pixel rounding when the specified coordinates are not integers is not defined by this specification, except that the following must result in no visible changes to the rendering:
context.putImageData(context.getImageData(x, y, w, h), p, q);
...for any value of x, y, w, and h and where p is the smaller of x and the sum of x and w, and q is the smaller of y and the sum of y and h; and except that the following two calls:
context.createImageData(w, h); context.getImageData(0, 0, w, h);
...must return ImageData
objects with the same dimensions as each other, for any
value of w and h. In other words, while user agents may
round the arguments of these methods, any rounding performed must be performed consistently for
all of the methods described in this section. (The constructors only work with integer
values.)
Due to the lossy nature of converting to and from premultiplied alpha colour
values, pixels that have just been set using putImageData()
might be returned to an equivalent
getImageData()
as different values.
The current path, transformation matrix, shadow attributes, global alpha, the clipping region, and global composition operator must not affect the methods described in this section.
In the following example, the script generates an ImageData
object so that it can
draw onto it.
// canvas is a reference to a <canvas> element var context = canvas.getContext('2d'); // create a blank slate var data = context.createImageData(canvas.width, canvas.height); // create some plasma FillPlasma(data, 'green'); // green plasma // add a cloud to the plasma AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle // paint the plasma+cloud on the canvas context.putImageData(data, 0, 0); // support methods function FillPlasma(data, color) { ... } function AddCloud(data, x, y) { ... }
Here is an example of using getImageData()
and putImageData()
to implement an edge detection
filter.
<!DOCTYPE HTML> <html> <head> <title>Edge detection demo</title> <script> var image = new Image(); function init() { image.onload = demo; image.src = "image.jpeg"; } function demo() { var canvas = document.getElementsByTagName('canvas')[0]; var context = canvas.getContext('2d'); // draw the image onto the canvas context.drawImage(image, 0, 0); // get the image data to manipulate var input = context.getImageData(0, 0, canvas.width, canvas.height); // get an empty slate to put the data into var output = context.createImageData(canvas.width, canvas.height); // alias some variables for convenience // notice that we are using input.width and input.height here // as they might not be the same as canvas.width and canvas.height // (in particular, they might be different on high-res displays) var w = input.width, h = input.height; var inputData = input.data; var outputData = output.data; // edge detection for (var y = 1; y < h-1; y += 1) { for (var x = 1; x < w-1; x += 1) { for (var c = 0; c < 3; c += 1) { var i = (y*w + x)*4 + c; outputData[i] = 127 + -inputData[i - w*4 - 4] - inputData[i - w*4] - inputData[i - w*4 + 4] + -inputData[i - 4] + 8*inputData[i] - inputData[i + 4] + -inputData[i + w*4 - 4] - inputData[i + w*4] - inputData[i + w*4 + 4]; } outputData[(y*w + x)*4 + 3] = 255; // alpha } } // put the image data back after manipulation context.putImageData(output, 0, 0); } </script> </head> <body onload="init()"> <canvas></canvas> </body> </html>
globalAlpha
[ = value ]Returns the current alpha value applied to rendering operations.
Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are ignored.
globalCompositeOperation
[ = value ]Returns the current composition operation, from the values defined in the Compositing and Blending specification. [COMPOSITE].
Can be set, to change the composition operation. Unknown values are ignored.
All drawing operations are affected by the global compositing attributes, globalAlpha
and globalCompositeOperation
.
The globalAlpha
attribute gives an
alpha value that is applied to shapes and images before they are composited onto the scratch
bitmap. The value must be in the range from 0.0 (fully transparent) to 1.0 (no additional
transparency). If an attempt is made to set the attribute to a value outside this range, including
Infinity and Not-a-Number (NaN) values, the attribute must retain its previous value. When the
context is created, the globalAlpha
attribute must
initially have the value 1.0.
The globalCompositeOperation
attribute
sets the current composition operator, which controls how shapes and images are drawn onto the
scratch bitmap, once they have had globalAlpha
and the current transformation matrix
applied. The possible values are those defined in the Compositing and Blending specification, and
include the values source-over
and copy
.
[COMPOSITE]
These values are all case-sensitive — they must be used exactly as defined. User agents must not recognise values that are not a case-sensitive match for one of the values given in the Compositing and Blending specification. [COMPOSITE]
On setting, if the user agent does not recognise the specified value, it must be ignored,
leaving the value of globalCompositeOperation
unaffected.
Otherwise, the attribute must be set to the given new value.
When the context is created, the globalCompositeOperation
attribute must
initially have the value source-over
.
imageSmoothingEnabled
[ = value ]Returns whether pattern fills and the drawImage()
method will attempt to smooth images if
their pixels don't line up exactly with the display, when scaling images up.
Can be set, to change whether images are smoothed (true) or not (false).
The imageSmoothingEnabled
attribute, on getting, must return the last value it was set to. On setting, it must be set to the
new value. When the CanvasRenderingContext2D
object is created, the attribute must be
set to true.
All drawing operations are affected by the four global shadow attributes.
shadowColor
[ = value ]Returns the current shadow colour.
Can be set, to change the shadow colour. Values that cannot be parsed as CSS colours are ignored.
shadowOffsetX
[ = value ]shadowOffsetY
[ = value ]Returns the current shadow offset.
Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
shadowBlur
[ = value ]Returns the current level of blur applied to shadows.
Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.
The shadowColor
attribute sets the
colour of the shadow.
When the context is created, the shadowColor
attribute initially must be fully-transparent black.
On getting, the serialisation of the colour must be returned.
On setting, the new value must be parsed as a CSS <color> value and the colour assigned. If the value cannot be parsed as a CSS <color> value then it must be ignored, and the attribute must retain its previous value. [CSSCOLOR]
The shadowOffsetX
and shadowOffsetY
attributes specify the distance
that the shadow will be offset in the positive horizontal and positive vertical distance
respectively. Their values are in coordinate space units. They are not affected by the current
transformation matrix.
When the context is created, the shadow offset attributes must initially have the value 0.
On getting, they must return their current value. On setting, the attribute being set must be set to the new value, except if the value is infinite or NaN, in which case the new value must be ignored.
The shadowBlur
attribute specifies
the level of the blurring effect. (The units do not map to coordinate space units, and are not
affected by the current transformation matrix.)
When the context is created, the shadowBlur
attribute must initially have the value 0.
On getting, the attribute must return its current value. On setting the attribute must be set to the new value, except if the value is negative, infinite or NaN, in which case the new value must be ignored.
Shadows are only drawn if the opacity component of
the alpha component of the colour of shadowColor
is
non-zero and either the shadowBlur
is non-zero, or
the shadowOffsetX
is non-zero, or the shadowOffsetY
is non-zero.
When shadows are drawn, they must be rendered as follows:
Let A be an infinite transparent black bitmap on which the source image for which a shadow is being created has been rendered.
Let B be an infinite transparent black bitmap, with a coordinate space and an origin identical to A.
Copy the alpha channel of A to B, offset by shadowOffsetX
in the positive x
direction, and shadowOffsetY
in the positive
y direction.
If shadowBlur
is greater than 0:
Let σ be half the value of shadowBlur
.
Perform a 2D Gaussian Blur on B, using σ as the standard deviation.
User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.
Set the red, green, and blue components of every pixel in B to the
red, green, and blue components (respectively) of the colour of shadowColor
.
Multiply the alpha component of every pixel in B by the alpha
component of the colour of shadowColor
.
The shadow is in the bitmap B, and is rendered as part of the drawing model described below.
If the current composition operation is copy
, shadows
effectively won't render (since the shape will overwrite the shadow).
When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):
Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honored, and the stroke must itself also be subjected to the current transformation matrix.
When shadows are drawn, render the shadow from image A, using the current shadow styles, creating image B.
When shadows are drawn, multiply the alpha component of every pixel in B by globalAlpha
.
When shadows are drawn, composite B within the clipping region over the current scratch bitmap using the current composition operator.
Multiply the alpha component of every pixel in A by globalAlpha
.
Composite A within the clipping region over the current scratch bitmap using the current composition operator.
When compositing onto the scratch bitmap, pixels that would fall outside of the scratch bitmap must be discarded.
When a canvas is interactive, authors should include focusable elements in the element's fallback content corresponding to each focusable part of the canvas, as in the example above.
To expose text and interactive content on a canvas
to users of accessibility
tools, authors should use the addHitRegion()
API. When rendering focus rings, to ensure that focus rings have the appearance of native focus
rings, authors should use the drawFocusIfNeeded()
method, passing it the
element for which a ring is being drawn. This method only draws the focus ring if the element is
focused, so that it can simply be called whenever drawing the element, without
checking whether the element is focused or not first.
In addition to drawing focus rings, authors should use the scrollPathIntoView()
method when an element in
the canvas is focused, to make sure it is visible on the screen (if applicable).
Authors should avoid implementing text editing controls
using the canvas
element. Doing so has a large number of disadvantages:
This is a huge amount of work, and authors are most strongly encouraged to avoid doing any of
it by instead using the input
element, the textarea
element, or the
contenteditable
attribute.
This section is non-normative.
Here is an example of a script that uses canvas to draw pretty glowing lines.
<canvas width="800" height="450"></canvas> <script> var context = document.getElementsByTagName('canvas')[0].getContext('2d'); var lastX = context.canvas.width * Math.random(); var lastY = context.canvas.height * Math.random(); var hue = 0; function line() { context.save(); context.translate(context.canvas.width/2, context.canvas.height/2); context.scale(0.9, 0.9); context.translate(-context.canvas.width/2, -context.canvas.height/2); context.beginPath(); context.lineWidth = 5 + Math.random() * 10; context.moveTo(lastX, lastY); lastX = context.canvas.width * Math.random(); lastY = context.canvas.height * Math.random(); context.bezierCurveTo(context.canvas.width * Math.random(), context.canvas.height * Math.random(), context.canvas.width * Math.random(), context.canvas.height * Math.random(), lastX, lastY); hue = hue + 10 * Math.random(); context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)'; context.shadowColor = 'white'; context.shadowBlur = 10; context.stroke(); context.restore(); } setInterval(line, 50); function blank() { context.fillStyle = 'rgba(0,0,0,0.1)'; context.fillRect(0, 0, context.canvas.width, context.canvas.height); } setInterval(blank, 40); </script>
The 2D rendering context for canvas
is often used for sprite-based games. The
following example demonstrates this:
Here is the source for this example:
<!DOCTYPE HTML> <title>Blue Robot Demo</title> <base href="https://www.whatwg.org/demos/canvas/blue-robot/"> <style> html { overflow: hidden; min-height: 200px; min-width: 380px; } body { height: 200px; position: relative; margin: 8px; } .buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; } </style> <canvas width="380" height="200"></canvas> <script> var Landscape = function (context, width, height) { this.offset = 0; this.width = width; this.advance = function (dx) { this.offset += dx; }; this.horizon = height * 0.7; // This creates the sky gradient (from a darker blue to white at the bottom) this.sky = context.createLinearGradient(0, 0, 0, this.horizon); this.sky.addColorStop(0.0, 'rgb(55,121,179)'); this.sky.addColorStop(0.7, 'rgb(121,194,245)'); this.sky.addColorStop(1.0, 'rgb(164,200,214)'); // this creates the grass gradient (from a darker green to a lighter green) this.earth = context.createLinearGradient(0, this.horizon, 0, height); this.earth.addColorStop(0.0, 'rgb(81,140,20)'); this.earth.addColorStop(1.0, 'rgb(123,177,57)'); this.paintBackground = function (context, width, height) { // first, paint the sky and grass rectangles context.fillStyle = this.sky; context.fillRect(0, 0, width, this.horizon); context.fillStyle = this.earth; context.fillRect(0, this.horizon, width, height-this.horizon); // then, draw the cloudy banner // we make it cloudy by having the draw text off the top of the // canvas, and just having the blurred shadow shown on the canvas context.save(); context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0); context.shadowColor = 'white'; context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas context.shadowBlur = '5'; context.fillStyle = 'white'; context.textAlign = 'left'; context.textBaseline = 'top'; context.font = '20px sans-serif'; context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas context.restore(); // then, draw the background tree context.save(); context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0); context.beginPath(); context.fillStyle = 'rgb(143,89,2)'; context.lineStyle = 'rgb(10,10,10)'; context.lineWidth = 2; context.rect(0, this.horizon+5, 10, -50); // trunk context.fill(); context.stroke(); context.beginPath(); context.fillStyle = 'rgb(78,154,6)'; context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves context.fill(); context.stroke(); context.restore(); }; this.paintForeground = function (context, width, height) { // draw the box that goes in front context.save(); context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0); context.beginPath(); context.rect(0, this.horizon - 5, 25, 25); context.fillStyle = 'rgb(220,154,94)'; context.lineStyle = 'rgb(10,10,10)'; context.lineWidth = 2; context.fill(); context.stroke(); context.restore(); }; }; </script> <script> var BlueRobot = function () { this.sprites = new Image(); this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells this.targetMode = 'idle'; this.walk = function () { this.targetMode = 'walk'; }; this.stop = function () { this.targetMode = 'idle'; }; this.frameIndex = { 'idle': [0], // first cell is the idle frame 'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6 'stop': [7], // last cell is the stopping animation }; this.mode = 'idle'; this.frame = 0; // index into frameIndex this.tick = function () { // this advances the frame and the robot // the return value is how many pixels the robot has moved this.frame += 1; if (this.frame >= this.frameIndex[this.mode].length) { // we've reached the end of this animation cycle this.frame = 0; if (this.mode != this.targetMode) { // switch to next cycle if (this.mode == 'walk') { // we need to stop walking before we decide what to do next this.mode = 'stop'; } else if (this.mode == 'stop') { if (this.targetMode == 'walk') this.mode = 'walk'; else this.mode = 'idle'; } else if (this.mode == 'idle') { if (this.targetMode == 'walk') this.mode = 'walk'; } } } if (this.mode == 'walk') return 8; return 0; }, this.paint = function (context, x, y) { if (!this.sprites.complete) return; // draw the right frame out of the sprite sheet onto the canvas // we assume each frame is as high as the sprite sheet // the x,y coordinates give the position of the bottom center of the sprite context.drawImage(this.sprites, this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height, x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height); }; }; </script> <script> var canvas = document.getElementsByTagName('canvas')[0]; var context = canvas.getContext('2d'); var landscape = new Landscape(context, canvas.width, canvas.height); var blueRobot = new BlueRobot(); // paint when the browser wants us to, using requestAnimationFrame() function paint() { context.clearRect(0, 0, canvas.width, canvas.height); landscape.paintBackground(context, canvas.width, canvas.height); blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1); landscape.paintForeground(context, canvas.width, canvas.height); requestAnimationFrame(paint); } paint(); // but tick every 150ms, so that we don't slow down when we don't paint setInterval(function () { var dx = blueRobot.tick(); landscape.advance(dx); }, 100); </script> <p class="buttons"> <input type=button value="Walk" onclick="blueRobot.walk()"> <input type=button value="Stop" onclick="blueRobot.stop()"> <footer> <small> Blue Robot Player Sprite by <a href="http://johncolburn.deviantart.com/">JohnColburn</a>. Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small> <small> This work is itself licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-ShareAlike 3.0 Unported License</a>.</small> </footer>
The canvas
APIs must perform colour correction at only two points: when rendering
images with their own gamma correction and colour space information onto a bitmap, to convert the
image to the colour space used by the bitmaps (e.g. using the 2D Context's drawImage()
method with an HTMLImageElement
object), and when rendering the actual canvas bitmap to the output device.
Thus, in the 2D context, colours used to draw shapes onto the canvas will exactly
match colours obtained through the getImageData()
method.
The toDataURL()
method must not include colour space
information in the resources they return. Where the output format allows it, the colour of pixels
in resources created by toDataURL()
must match those
returned by the getImageData()
method.
In user agents that support CSS, the colour space used by a canvas
element must
match the colour space used for processing any colours for that element in CSS.
The gamma correction and colour space information of images must be handled in such a way that
an image rendered directly using an img
element would use the same colours as one
painted on a canvas
element that is then itself rendered. Furthermore, the rendering
of images that have no colour correction information (such as those returned by the toDataURL()
method) must be rendered with no colour
correction.
Thus, in the 2D context, calling the drawImage()
method to render the output of the toDataURL()
method to the canvas, given the appropriate
dimensions, has no visible effect.
When a user agent is to create a serialisation of the bitmap as a file, optionally with some given arguments, and optionally with a native flag set, it must create an image file in the format given by the first value of arguments, or, if there are no arguments, in the PNG format. [PNG]
If the native flag is set, or if the bitmap has one pixel per coordinate space unit, then the image file must have the same pixel data (before compression, if applicable) as the bitmap, and if the file format used supports encoding resolution metadata, the resolution of that bitmap (device pixels per coordinate space units being interpreted as image pixels per CSS pixel) must be given as well.
Otherwise, the image file's pixel data must be the bitmap's pixel data scaled to one image pixel per coordinate space unit, and if the file format used supports encoding resolution metadata, the resolution must be given as 96dpi (one image pixel per CSS pixel).
If arguments is not empty, the first value must be interpreted as a MIME type giving the format to use. If the type has any parameters, it must be treated as not supported.
For example, the value "image/png
" would mean to generate a PNG
image, the value "image/jpeg
" would mean to generate a JPEG image, and the value
"image/svg+xml
" would mean to generate an SVG image (which would require that the
user agent track how the bitmap was generated, an unlikely, though potentially awesome,
feature).
User agents must support PNG ("image/png
"). User agents may support other types.
If the user agent does not support the requested type, it must create the file using the PNG
format. [PNG]
User agents must convert the provided type to ASCII lowercase before establishing if they support that type.
For image types that do not support an alpha channel, the serialised image must be the bitmap image composited onto a solid black background using the source-over operator.
If the first argument in arguments gives a type corresponding to one of the types given in the first column of the following table, and the user agent supports that type, then the subsequent arguments, if any, must be treated as described in the second cell of that row.
Type | Other arguments | Reference |
---|---|---|
image/jpeg
| The second argument, if it is a number in the range 0.0 to 1.0 inclusive, must be treated as the desired quality level. If it is not a number or is outside that range, the user agent must use its default value, as if the argument had been omitted. | [JPEG] |
For the purposes of these rules, an argument is considered to be a number if it is converted to
an IDL double value by the rules for handling arguments of type any
in the
Web IDL specification. [WEBIDL]
Other arguments must be ignored and must not cause the user agent to throw an exception. A future version of this specification will probably define other parameters to be passed to these methods to allow authors to more carefully control compression settings, image metadata, etc.
canvas
elementsThis section is non-normative.
Information leakage can occur if scripts from one origin can access information (e.g. read pixels) from images from another origin (one that isn't the same).
To mitigate this, bitmaps used with canvas
elements are defined to have a flag
indicating whether they are origin-clean. All
bitmaps start with their origin-clean set to
true. The flag is set to false when cross-origin images or fonts are used.
The toDataURL()
, toBlob()
, and getImageData()
methods check the flag and will
throw a SecurityError
exception rather than leak cross-origin data.
The flag can be reset in certain situations; for example, when a
CanvasRenderingContext2D
is bound to a new canvas
, the bitmap is cleared
and its flag reset.
The main content of a page — not including headers and footers, navigation links, sidebars, advertisements, and so forth — can be marked up in a variety of ways, depending on the needs of the author.
The simplest solution is to not mark up the main content at all, and just leave it as implicit.
Another way to think of this is that the body
elements marks up the main content of
the page, and the bits that aren't main content are excluded through the use of more appropriate
elements like aside
and nav
.
Here is a short Web page marked up along this minimalistic school of thought. The main content
is highlighted. Notice how all the other content in the body
is marked up
with elements to indicate that it's not part of the main content, in this case
header
, nav
, and footer
.
<!DOCTYPE HTML> <html> <head> <title> My Toys </title> </head> <body> <header> <h1>My toys</h1> </header> <nav> <p><a href="/">Home</a></p> <p><a href="/contact">Contact</a></p> </nav> <p>I really like my chained book and my telephone. I'm not such a fan of my big ball.</p> <p>Another toy I like is my mirror.</p> <footer> <p>© copyright 2010 by the boy</p> </footer> </body> </html>
If the main content is an independent unit of content that one could imagine syndicating
independently, then the article
element would be appropriate to mark up the main
content of the document.
The document in the previous example is here recast as a blog post:
<!DOCTYPE HTML> <html> <head> <title> The Boy Blog: My Toys </title> </head> <body> <header> <h1>The Boy Blog</h1> </header> <nav> <p><a href="/">Home</a></p> <p><a href="/contact">Contact</a></p> </nav> <article> <header> <h1>My toys</h1> <p>Published August 4th</p> </header> <p>I really like my chained book and my telephone. I'm not such a fan of my big ball.</p> <p>Another toy I like is my mirror.</p> </article> <footer> <p>© copyright 2010 by the boy</p> </footer> </body> </html>
If the main content is not an independent unit of content so much as a section of a larger
work, for instance a chapter, then the section
element would be appropriate to mark
up the main content of the document.
Here is the same document, but as a chapter in an online book:
<!DOCTYPE HTML> <html> <head> <title> Chapter 2: My Toys — The Book of the Boy </title> </head> <body> <header> <hgroup> <h1>The Book of the Boy</h1> <h2>A book about boy stuff</h2> </hgroup> </header> <nav> <p><a href="/">Front Page</a></p> <p><a href="/toc">Table of Contents</a></p> <p><a href="/c1">Chapter 1</a> — <a href="/c3">Chapter 3</a></p> </nav> <section> <h1>Chapter 2: My Toys</h1> <p>I really like my chained book and my telephone. I'm not such a fan of my big ball.</p> <p>Another toy I like is my mirror.</p> </section> </body> </html>
If neither article
nor section
would be appropriate, but the main
content still needs an explicit element, for example for styling purposes, then the
main
element can be used.
This is the same as the original example, but using main
for the main content
instead of leaving it implied:
<!DOCTYPE HTML> <html> <head> <title> My Toys </title> <style> body > main { background: navy; color: yellow; } </style> </head> <body> <header> <h1>My toys</h1> </header> <nav> <p><a href="/">Home</a></p> <p><a href="/contact">Contact</a></p> </nav> <main> <p>I really like my chained book and my telephone. I'm not such a fan of my big ball.</p> <p>Another toy I like is my mirror.</p> </main> <footer> <p>© copyright 2010 by the boy</p> </footer> </body> </html>
This specification does not provide a machine-readable way of describing bread-crumb navigation
menus. Authors are encouraged to just use a series of links in a paragraph. The nav
element can be used to mark the section containing these paragraphs as being navigation
blocks.
In the following example, the current page can be reached via two paths.
<nav> <p> <a href="/">Main</a> ▸ <a href="/products/">Products</a> ▸ <a href="/products/dishwashers/">Dishwashers</a> ▸ <a>Second hand</a> </p> <p> <a href="/">Main</a> ▸ <a href="/second-hand/">Second hand</a> ▸ <a>Dishwashers</a> </p> </nav>
This specification does not define any markup specifically for marking up lists
of keywords that apply to a group of pages (also known as tag clouds). In general, authors
are encouraged to either mark up such lists using ul
elements with explicit inline
counts that are then hidden and turned into a presentational effect using a style sheet, or to use
SVG.
Here, three tags are included in a short tag cloud:
<style> @media screen, print, handheld, tv { /* should be ignored by non-visual browsers */ .tag-cloud > li > span { display: none; } .tag-cloud > li { display: inline; } .tag-cloud-1 { font-size: 0.7em; } .tag-cloud-2 { font-size: 0.9em; } .tag-cloud-3 { font-size: 1.1em; } .tag-cloud-4 { font-size: 1.3em; } .tag-cloud-5 { font-size: 1.5em; } } </style> ... <ul class="tag-cloud"> <li class="tag-cloud-4"><a title="28 instances" href="/t/apple">apple</a> <span>(popular)</span> <li class="tag-cloud-2"><a title="6 instances" href="/t/kiwi">kiwi</a> <span>(rare)</span> <li class="tag-cloud-5"><a title="41 instances" href="/t/pear">pear</a> <span>(very popular)</span> </ul>
The actual frequency of each tag is given using the title
attribute. A CSS style sheet is provided to convert the markup into a cloud of differently-sized
words, but for user agents that do not support CSS or are not visual, the markup contains
annotations like "(popular)" or "(rare)" to categorise the various tags by frequency, thus
enabling all users to benefit from the information.
The ul
element is used (rather than ol
) because the order is not
particularly important: while the list is in fact ordered alphabetically, it would convey the
same information if ordered by, say, the length of the tag.
The tag
rel
-keyword is
not used on these a
elements because they do not represent tags that apply
to the page itself; they are just part of an index listing the tags themselves.
This specification does not define a specific element for marking up conversations, meeting minutes, chat transcripts, dialogues in screenplays, instant message logs, and other situations where different players take turns in discourse.
Instead, authors are encouraged to mark up conversations using p
elements and
punctuation. Authors who need to mark the speaker for styling purposes are encouraged to use
span
or b
. Paragraphs with their text wrapped in the i
element can be used for marking up stage directions.
This example demonstrates this using an extract from Abbot and Costello's famous sketch, Who's on first:
<p> Costello: Look, you gotta first baseman? <p> Abbott: Certainly. <p> Costello: Who's playing first? <p> Abbott: That's right. <p> Costello becomes exasperated. <p> Costello: When you pay off the first baseman every month, who gets the money? <p> Abbott: Every dollar of it.
The following extract shows how an IM conversation log could be marked up, using the
data
element to provide Unix timestamps for each line. Note that the timestamps are
provided in a format that the time
element does not support, so the
data
element is used instead (namely, Unix time_t
timestamps).
Had the author wished to mark up the data using one of the date and time formats supported by the
time
element, that element could have been used instead of data
. This
could be advantageous as it would allow data analysis tools to detect the timestamps
unambiguously, without coordination with the page author.
<p> <data value="1319898155">14:22</data> <b>egof</b> I'm not that nerdy, I've only seen 30% of the star trek episodes <p> <data value="1319898192">14:23</data> <b>kaj</b> if you know what percentage of the star trek episodes you have seen, you are inarguably nerdy <p> <data value="1319898200">14:23</data> <b>egof</b> it's unarguably <p> <data value="1319898228">14:23</data> <i>* kaj blinks</i> <p> <data value="1319898260">14:24</data> <b>kaj</b> you are not helping your case
HTML does not have a good way to mark up graphs, so descriptions of interactive conversations
from games are more difficult to mark up. This example shows one possible convention using
dl
elements to list the possible responses at each point in the conversation.
Another option to consider is describing the conversation in the form of a DOT file, and
outputting the result as an SVG image to place in the document. [DOT]
<p> Next, you meet a fisherman. You can say one of several greetings: <dl> <dt> "Hello there!" <dd> <p> He responds with "Hello, how may I help you?"; you can respond with: <dl> <dt> "I would like to buy a fish." <dd> <p> He sells you a fish and the conversation finishes. <dt> "Can I borrow your boat?" <dd> <p> He is surprised and asks "What are you offering in return?". <dl> <dt> "Five gold." (if you have enough) <dt> "Ten gold." (if you have enough) <dt> "Fifteen gold." (if you have enough) <dd> <p> He lends you his boat. The conversation ends. <dt> "A fish." (if you have one) <dt> "A newspaper." (if you have one) <dt> "A pebble." (if you have one) <dd> <p> "No thanks", he replies. Your conversation options at this point are the same as they were after asking to borrow his boat, minus any options you've suggested before. </dl> </dd> </dl> </dd> <dt> "Vote for me in the next election!" <dd> <p> He turns away. The conversation finishes. <dt> "Sir, are you aware that your fish are running away?" <dd> <p> He looks at you skeptically and says "Fish cannot run, sir". <dl> <dt> "You got me!" <dd> <p> The fisherman sighs and the conversation ends. <dt> "Only kidding." <dd> <p> "Good one!" he retorts. Your conversation options at this point are the same as those following "Hello there!" above. <dt> "Oh, then what are they doing?" <dd> <p> He looks at his fish, giving you an opportunity to steal his boat, which you do. The conversation ends. </dl> </dd> </dl>
In some games, conversations are simpler: each character merely has a fixed set of lines that they say. In this example, a game FAQ/walkthrough lists some of the known possible responses for each character:
<section> <h1>Dialogue</h1> <p><small>Some characters repeat their lines in order each time you interact with them, others randomly pick from amongst their lines. Those who respond in order have numbered entries in the lists below.</small> <h2>The Shopkeeper</h2> <ul> <li>How may I help you? <li>Fresh apples! <li>A loaf of bread for madam? </ul> <h2>The pilot</h2> <p>Before the accident: <ul> </li>I'm about to fly out, sorry! </li>Sorry, I'm just waiting for flight clearance and then I'll be off! </ul> <p>After the accident: <ol> <li>I'm about to fly out, sorry! <li>Ok, I'm not leaving right now, my plane is being cleaned. <li>Ok, it's not being cleaned, it needs a minor repair first. <li>Ok, ok, stop bothering me! Truth is, I had a crash. </ol> <h2>Clan Leader</h2> <p>During the first clan meeting: <ul> <li>Hey, have you seen my daughter? I bet she's up to something nefarious again... <li>Nice weather we're having today, eh? <li>The name is Bailey, Jeff Bailey. How can I help you today? <li>A glass of water? Fresh from the well! </ul> <p>After the earthquake: <ol> <li>Everyone is safe in the shelter, we just have to put out the fire! <li>I'll go and tell the fire brigade, you keep hosing it down! </ol> </section>
HTML does not have a dedicated mechanism for marking up footnotes. Here are the suggested alternatives.
For short inline annotations, the title
attribute could be used.
In this example, two parts of a dialogue are annotated with footnote-like content using the
title
attribute.
<p> <b>Customer</b>: Hello! I wish to register a complaint. Hello. Miss? <p> <b>Shopkeeper</b>: <span title="Colloquial pronunciation of 'What do you'" >Watcha</span> mean, miss? <p> <b>Customer</b>: Uh, I'm sorry, I have a cold. I wish to make a complaint. <p> <b>Shopkeeper</b>: Sorry, <span title="This is, of course, a lie.">we're closing for lunch</span>.
Unfortunately, relying on the title
attribute is
currently discouraged as many user agents do not expose the attribute in an accessible manner as
required by this specification (e.g. requiring a pointing device such as a mouse to cause a
tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a
modern phone or tablet).
If the title
attribute is used, CSS can used to
draw the reader's attention to the elements with the attribute.
For example, the following CSS places a dashed line below elements that have a title
attribute.
[title] { border-bottom: thin dashed; }
For longer annotations, the a
element should be used, pointing to an element later
in the document. The convention is that the contents of the link be a number in square
brackets.
In this example, a footnote in the dialogue links to a paragraph below the dialogue. The paragraph then reciprocally links back to the dialogue, allowing the user to return to the location of the footnote.
<p> Announcer: Number 16: The <i>hand</i>. <p> Interviewer: Good evening. I have with me in the studio tonight Mr Norman St John Polevaulter, who for the past few years has been contradicting people. Mr Polevaulter, why <em>do</em> you contradict people? <p> Norman: I don't. <sup><a href="#fn1" id="r1">[1]</a></sup> <p> Interviewer: You told me you did! ... <section> <p id="fn1"><a href="#r1">[1]</a> This is, naturally, a lie, but paradoxically if it were true he could not say so without contradicting the interviewer and thus making it false.</p> </section>
For side notes, longer annotations that apply to entire sections of the text rather than just
specific words or sentences, the aside
element should be used.
In this example, a sidebar is given after a dialogue, giving it some context.
<p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched. <p> <span class="speaker">Shopkeeper</span>: I'm sorry? <p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched. <p> <span class="speaker">Shopkeeper</span>: No no no, this's'a tobacconist's. <aside> <p>In 1970, the British Empire lay in ruins, and foreign nationalists frequented the streets — many of them Hungarians (not the streets — the foreign nationals). Sadly, Alexander Yalt has been publishing incompetently-written phrase books. </aside>
For figures or tables, footnotes can be included in the relevant figcaption
or
caption
element, or in surrounding prose.
In this example, a table has cells with footnotes that are given in prose. A
figure
element is used to give a single legend to the combination of the table and
its footnotes.
<figure> <figcaption>Table 1. Alternative activities for knights.</figcaption> <table> <tr> <th> Activity <th> Location <th> Cost <tr> <td> Dance <td> Wherever possible <td> £0<sup><a href="#fn1">1</a></sup> <tr> <td> Routines, chorus scenes<sup><a href="#fn2">2</a></sup> <td> Undisclosed <td> Undisclosed <tr> <td> Dining<sup><a href="#fn3">3</a></sup> <td> Camelot <td> Cost of ham, jam, and spam<sup><a href="#fn4">4</a></sup> </table> <p id="fn1">1. Assumed.</p> <p id="fn2">2. Footwork impeccable.</p> <p id="fn3">3. Quality described as "well".</p> <p id="fn4">4. A lot.</p> </figure>
An element is said to be actually disabled if it one of the following:
button
element that is disabledinput
element that is disabledselect
element that is disabledtextarea
element that is disabledoptgroup
element that has a disabled
attributeoption
element that is disabledmenuitem
element that has a disabled
attributefieldset
element that is a disabled fieldsetThis definition is used to determine what elements can be focused and which elements match the :disabled
pseudo-class.
The Selectors specification leaves the case-sensitivity of element names, attribute names, and attribute values to be defined by the host language. [SELECTORS]
The Selectors specification defines that ID and class selectors, when matched against elements in documents that are in quirks mode, will be work in an ASCII case-insensitive.
When comparing a CSS element type selector to the names of HTML elements in HTML documents, the CSS element type selector must first be converted to ASCII lowercase. The same selector when compared to other elements must be compared according to its original case. In both cases, the comparison is case-sensitive.
When comparing the name part of a CSS attribute selector to the names of namespace-less attributes on HTML elements in HTML documents, the name part of the CSS attribute selector must first be converted to ASCII lowercase. The same selector when compared to other attributes must be compared according to its original case. In both cases, the comparison is case-sensitive.
Everything else must be treated as entirely case-sensitive for the purposes of selector matching. This includes:
There are a number of dynamic selectors that can be used with HTML. This section defines when these selectors match HTML elements. [SELECTORS] [CSSUI]
:link
:visited
All a
elements that have an href
attribute, all area
elements that have an href
attribute, and all link
elements that have
an href
attribute, must match one of :link
and :visited
.
Other specifications might apply more specific rules regarding how these elements are to match these pseudo-classes, to mitigate some privacy concerns that apply with straightforward implementations of this requirement.
:active
The :active
pseudo-class is defined to match an element
while an
element is being activated by the user
.
To determine whether a particular element is being activated for the purposes of
defining the :active
pseudo-class only, an HTML user agent
must use the first relevant entry in the following list.
:active
pseudo-classThe element is being activated.
label
element that is
currently matching :activeThe element is being activated.
button
elementinput
element whose type
attribute is in the Submit Button, Image Button, Reset
Button, or Button stateThe element is being activated if it is in a formal activation state and it is not disabled.
For example, if the user is using a keyboard to push a button
element by pressing the space bar, the element would match this pseudo-class in between the
time that the element received the keydown
event and the
time the element received the keyup
event.
menuitem
elementThe element is being activated if it is in a formal activation state
and it does not have a disabled
attribute.
a
element that has an href
attributearea
element that has an href
attributelink
element that has an href
attributeThe element is being activated if it is in a formal activation state.
The element is being activated.
An element is said to be in a formal activation state between the time the user begins to indicate an intent to trigger the element's activation behaviour and either the time the user stops indicating an intent to trigger the element's activation behaviour, or the time the element's activation behaviour has finished running, which ever comes first.
An element is said to be being actively pointed at while the user indicates the element using a pointing device while that pointing device is in the "down" state (e.g. for a mouse, between the time the mouse button is pressed and the time it is depressed; for a finger in a multitouch environment, while the finger is touching the display surface).
:hover
The :hover
pseudo-class is defined to match an element while the
user designates an element with a pointing device
. For the purposes of defining the
:hover
pseudo-class only, an HTML user agent must consider
an element as being one that the user designates if it is:
An element that the user indicates using a pointing device.
An element that has a descendant that the user indicates using a pointing device.
An element that is the labeled control of a label
element that is
currently matching :hover.
Consider in particular a fragment such as:
<p> <label for=c> <input id=a> </label> <span id=b> <input id=c> </span> </p>
If the user designates the element with ID "a
" with their pointing
device, then the p
element (and all its ancestors not shown in the snippet above),
the label
element, the element with ID "a
", and the element
with ID "c
" will match the :hover
pseudo-class. The element with ID "a
" matches it from condition 1, the
label
and p
elements match it because of condition 2 (one of their
descendants is designated), and the element with ID "c
" matches it
through condition 3 (its label
element matches :hover). However, the element with ID "b
"
does not match :hover: its descendant is not
designated, even though it matches :hover.
:focus
For the purposes of the CSS ':focus' pseudo-class, an element has the focus when its top-level browsing context has the system focus, it is not itself a browsing context container, and it is one of the elements listed in the focus chain of the currently focused area of the top-level browsing context.
:enabled
The :enabled
pseudo-class must match any element
that is one of the following:
button
element that is not disabledinput
element that is not disabledselect
element that is not disabledtextarea
element that is not disabledoptgroup
element that does not have a disabled
attributeoption
element that is not disabledmenuitem
element that does not have a disabled
attributefieldset
element that is not a disabled fieldset:disabled
The :disabled
pseudo-class must match any element that
is actually disabled.
:checked
The :checked
pseudo-class must match any element
falling into one of the following categories:
input
elements whose type
attribute is in
the Checkbox state and whose checkedness state is trueinput
elements whose type
attribute is in
the Radio Button state and whose checkedness state is trueoption
elements whose selectedness is truemenuitem
elements whose type
attribute
is in the Checkbox state and that have a
checked
attributemenuitem
elements whose type
attribute
is in the Radio state and that have a checked
attribute:indeterminate
The :indeterminate
pseudo-class must match any
element falling into one of the following categories:
input
elements whose type
attribute is in
the Checkbox state and whose indeterminate
IDL attribute is set to trueinput
elements whose type
attribute is in
the Radio Button state and whose radio button
group contains no input
elements whose checkedness state is true.progress
elements with no value
content attribute:default
The :default
pseudo-class must match any element
falling into one of the following categories:
button
elements that are their form's default buttoninput
elements whose type
attribute is in
the Submit Button or Image Button state, and that are their form's
default buttoninput
elements to which the checked
attribute applies and that have a checked
attributeoption
elements that have a selected
attribute:valid
The :valid
pseudo-class must match any element falling
into one of the following categories:
form
elements that are not the form owner of any elements that
themselves are candidates for constraint
validation but do not satisfy their
constraintsfieldset
elements that have no descendant elements that themselves are candidates for constraint validation but do
not satisfy their constraints:invalid
The :invalid
pseudo-class must match any element
falling into one of the following categories:
form
elements that are the form owner of one or more elements
that themselves are candidates for constraint
validation but do not satisfy their
constraintsfieldset
elements that have of one or more descendant elements that themselves
are candidates for constraint
validation but do not satisfy their
constraints:in-range
The :in-range
pseudo-class must match all elements
that are candidates for constraint
validation, have range limitations, and that are neither suffering
from an underflow nor suffering from an overflow.
:out-of-range
The :out-of-range
pseudo-class must match all
elements that are candidates for constraint
validation, have range limitations, and that are either suffering from
an underflow or suffering from an overflow.
:required
The :required
pseudo-class must match any element
falling into one of the following categories:
:optional
The :optional
pseudo-class must match any element
falling into one of the following categories:
:read-only
:read-write
The :read-write
pseudo-class must match any element
falling into one of the following categories, which for the purposes of Selectors are thus
considered user-alterable: [SELECTORS]
input
elements to which the readonly
attribute applies, and that are mutable (i.e. that do not
have the readonly
attribute specified and that are not
disabled)textarea
elements that do not have a readonly
attribute, and that are not disabledinput
elements nor textarea
elementsThe :read-only
pseudo-class must match all other
HTML elements.
:dir(ltr)
The :dir(ltr)
pseudo-class must match all elements whose
directionality is 'ltr'.
:dir(rtl)
The :dir(rtl)
pseudo-class must match all elements whose
directionality is 'rtl'.
Another section of this specification defines the target element used with the :target
pseudo-class.
This specification does not define when an element matches the :lang()
dynamic pseudo-class, as it is defined in sufficient
detail in a language-agnostic fashion in the Selectors specification. [SELECTORS]