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|
<?xml version="1.0" encoding="UTF-8"?><html xml:lang="en-US-x-hixie" xmlns="http://www.w3.org/1999/xhtml"><head><title>HTML Standard</title><link href="http://www.whatwg.org/style/specification" rel="stylesheet"/><link href="http://www.whatwg.org/images/icon" rel="icon"/><style>
.proposal { border: blue solid; padding: 1em; }
.bad, .bad *:not(.XXX) { color: gray; border-color: gray; background: transparent; }
#updatesStatus { display: none; }
#updatesStatus.relevant { display: block; position: fixed; right: 1em; top: 1em; padding: 0.5em; font: bold small sans-serif; min-width: 25em; width: 30%; max-width: 40em; height: auto; border: ridge 4px gray; background: #EEEEEE; color: black; }
div.head .logo { width: 11em; margin-bottom: 20em; }
#configUI { position: absolute; z-index: 20; top: 10em; right: 0; width: 11em; padding: 0 0.5em 0 0.5em; font-size: small; background: gray; background: rgba(32,32,32,0.9); color: white; border-radius: 1em 0 0 1em; -moz-border-radius: 1em 0 0 1em; }
#configUI p { margin: 0.75em 0; padding: 0.3em; }
#configUI p label { display: block; }
#configUI #updateUI, #configUI .loginUI { text-align: center; }
#configUI input[type=button] { display: block; margin: auto; }
#configUI :link, #configUI :visited { color: white; }
#configUI :link:hover, #configUI :visited:hover { background: transparent; }
#reviewer { position: fixed; bottom: 0; right: 0; padding: 0.15em 0.25em 0em 0.5em; white-space: nowrap; overflow: hidden; z-index: 30; background: gray; background: rgba(32,32,32,0.9); color: white; border-radius: 1em 0 0 0; -moz-border-radius: 1em 0 0 0; max-width: 90%; }
#reviewer input { max-width: 50%; }
#reviewer * { font-size: small; }
#reviewer.off > :not(:first-child) { display: none; }
#alert { position: fixed; top: 20%; left: 20%; right: 20%; font-size: 2em; padding: 0.5em; z-index: 40; background: gray; background: rgba(32,32,32,0.9); color: white; border-radius: 1em; -moz-border-radius: 1em; -webkit-transition: opacity 1s linear; }
#alert.closed { opacity: 0; }
#alert button { position: absolute; top: -1em; right: 2em; border-radius: 1em 1em 0 0; border: none; line-height: 0.9; color: white; background: rgb(64,64,64); font-size: 0.6em; font-weight: 900; cursor: pointer; }
#alert :link, #alert :visited { color: white; }
#alert :link:hover, #alert :visited:hover { background: transparent; }
@media print { #configUI { display: none; } }
.rfc2119 { font-variant: small-caps; text-shadow: 0 0 0.5em yellow; position: static; }
.rfc2119::after { position: absolute; left: 0; width: 25px; text-align: center; color: yellow; text-shadow: 0.075em 0.075em 0.2em black; }
.rfc2119.m\ust::after { content: '\2605'; }
.rfc2119.s\hould::after { content: '\2606'; }
[hidden] { display: none; }
</style><style type="text/css">
.applies thead th > * { display: block; }
.applies thead code { display: block; }
.applies tbody th { whitespace: nowrap; }
.applies td { text-align: center; }
.applies .yes { background: yellow; }
.matrix, .matrix td { border: hidden; text-align: right; }
.matrix { margin-left: 2em; }
.dice-example { border-collapse: collapse; border-style: hidden solid solid hidden; border-width: thin; margin-left: 3em; }
.dice-example caption { width: 30em; font-size: smaller; font-style: italic; padding: 0.75em 0; text-align: left; }
.dice-example td, .dice-example th { border: solid thin; width: 1.35em; height: 1.05em; text-align: center; padding: 0; }
td.eg { border-width: thin; text-align: center; }
#table-example-1 { border: solid thin; border-collapse: collapse; margin-left: 3em; }
#table-example-1 * { font-family: "Essays1743", serif; line-height: 1.01em; }
#table-example-1 caption { padding-bottom: 0.5em; }
#table-example-1 thead, #table-example-1 tbody { border: none; }
#table-example-1 th, #table-example-1 td { border: solid thin; }
#table-example-1 th { font-weight: normal; }
#table-example-1 td { border-style: none solid; vertical-align: top; }
#table-example-1 th { padding: 0.5em; vertical-align: middle; text-align: center; }
#table-example-1 tbody tr:first-child td { padding-top: 0.5em; }
#table-example-1 tbody tr:last-child td { padding-bottom: 1.5em; }
#table-example-1 tbody td:first-child { padding-left: 2.5em; padding-right: 0; width: 9em; }
#table-example-1 tbody td:first-child::after { content: leader(". "); }
#table-example-1 tbody td { padding-left: 2em; padding-right: 2em; }
#table-example-1 tbody td:first-child + td { width: 10em; }
#table-example-1 tbody td:first-child + td ~ td { width: 2.5em; }
#table-example-1 tbody td:first-child + td + td + td ~ td { width: 1.25em; }
.apple-table-examples { border: none; border-collapse: separate; border-spacing: 1.5em 0em; width: 40em; margin-left: 3em; }
.apple-table-examples * { font-family: "Times", serif; }
.apple-table-examples td, .apple-table-examples th { border: none; white-space: nowrap; padding-top: 0; padding-bottom: 0; }
.apple-table-examples tbody th:first-child { border-left: none; width: 100%; }
.apple-table-examples thead th:first-child ~ th { font-size: smaller; font-weight: bolder; border-bottom: solid 2px; text-align: center; }
.apple-table-examples tbody th::after, .apple-table-examples tfoot th::after { content: leader(". ") }
.apple-table-examples tbody th, .apple-table-examples tfoot th { font: inherit; text-align: left; }
.apple-table-examples td { text-align: right; vertical-align: top; }
.apple-table-examples.e1 tbody tr:last-child td { border-bottom: solid 1px; }
.apple-table-examples.e1 tbody + tbody tr:last-child td { border-bottom: double 3px; }
.apple-table-examples.e2 th[scope=row] { padding-left: 1em; }
.apple-table-examples sup { line-height: 0; }
.details-example img { vertical-align: top; }
#base64-table {
white-space: nowrap;
font-size: 0.6em;
column-width: 6em;
column-count: 5;
column-gap: 1em;
-moz-column-width: 6em;
-moz-column-count: 5;
-moz-column-gap: 1em;
-webkit-column-width: 6em;
-webkit-column-count: 5;
-webkit-column-gap: 1em;
}
#base64-table thead { display: none; }
#base64-table * { border: none; }
#base64-table tbody td:first-child:after { content: ':'; }
#base64-table tbody td:last-child { text-align: right; }
#named-character-references-table {
white-space: nowrap;
font-size: 0.6em;
column-width: 30em;
column-gap: 1em;
-moz-column-width: 30em;
-moz-column-gap: 1em;
-webkit-column-width: 30em;
-webkit-column-gap: 1em;
}
#named-character-references-table > table > tbody > tr > td:first-child + td,
#named-character-references-table > table > tbody > tr > td:last-child { text-align: center; }
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</style><link href="data:text/css," id="complete" rel="stylesheet" title="Complete specification"/><link href="data:text/css,.impl%20{%20display:%20none;%20}%0Ahtml%20{%20border:%20solid%20yellow;%20}%20.domintro:before%20{%20display:%20none;%20}" id="author" rel="alternate stylesheet" title="Author documentation only"/><link href="data:text/css,.impl%20{%20background:%20%23FFEEEE;%20}%20.domintro:before%20{%20background:%20%23FFEEEE;%20}" id="highlight" rel="alternate stylesheet" title="Highlight implementation requirements"/><link href="http://www.whatwg.org/specs/web-apps/current-work/status.css" rel="stylesheet"/></head><body class="draft" onload="init()"><header class="head" id="head"><p><a class="logo" href="http://www.whatwg.org/"><img alt="WHATWG" height="101" src="http://www.whatwg.org/images/logo" width="101"/></a></p>
<hgroup><h1 class="allcaps">HTML</h1>
<h2 class="no-num no-toc">Living Standard — Last Updated 8 August 2011</h2>
</hgroup><p class="copyright">© Copyright 2004-2011 Apple Computer, Inc.,
Mozilla Foundation, and Opera Software ASA.</p>
<p class="copyright">You are granted a license to use, reproduce
and create derivative works of this document.</p>
</header><h4 id="the-canvas-element"><span class="secno">4.8.11 </span>The <dfn id="canvas"><code>canvas</code></dfn> element</h4>
<dl class="element"><dt>Categories</dt>
<dd><a href="#flow-content">Flow content</a>.</dd>
<dd><a href="#phrasing-content">Phrasing content</a>.</dd>
<dd><a href="#embedded-content">Embedded content</a>.</dd>
<dt>Contexts in which this element can be used:</dt>
<dd>Where <a href="#embedded-content">embedded content</a> is expected.</dd>
<dt>Content model:</dt>
<dd><a href="#transparent">Transparent</a>.</dd>
<dd><a href="#transparent">Transparent</a><!--
--><!--FORK--><!--
-->, but with no <a href="#interactive-content">interactive content</a> descendants except for <code><a href="#the-a-element">a</a></code> elements, <code><a href="#the-button-element">button</a></code> elements, <code><a href="#the-input-element">input</a></code> elements whose <code title="attr-input-type"><a href="#attr-input-type">type</a></code> attribute are in the <a href="#checkbox-state" title="attr-input-type-checkbox">Checkbox</a> or <a href="#radio-button-state" title="attr-input-type-radio">Radio Button</a> states, and <code><a href="#the-input-element">input</a></code> elements that are <a href="#concept-button" title="concept-button">buttons</a><!--
--><!--FORK--><!--
-->.</dd> <!-- and maybe <select>, I guess? I've left it in for now, since I guess authors might make accessible selects if they're simple enough, and it's not obvious how to distinguish the simple ones from the complex ones... -->
<dt>Content attributes:</dt>
<dd><a href="#global-attributes">Global attributes</a></dd>
<dd><code title="attr-canvas-width"><a href="#attr-canvas-width">width</a></code></dd>
<dd><code title="attr-canvas-height"><a href="#attr-canvas-height">height</a></code></dd>
<dt>DOM interface:</dt>
<dd>
<pre class="idl">interface <dfn id="htmlcanvaselement">HTMLCanvasElement</dfn> : <a href="#htmlelement">HTMLElement</a> {
attribute unsigned long <a href="#dom-canvas-width" title="dom-canvas-width">width</a>;
attribute unsigned long <a href="#dom-canvas-height" title="dom-canvas-height">height</a>;
DOMString <a href="#dom-canvas-todataurl" title="dom-canvas-toDataURL">toDataURL</a>(in optional DOMString type, in any... args);
void <a href="#dom-canvas-toblob" title="dom-canvas-toBlob">toBlob</a>(in <span>FileCallback</span>? callback, in optional DOMString type, in any... args);
object? <a href="#dom-canvas-getcontext" title="dom-canvas-getContext">getContext</a>(in DOMString contextId, in any... args);
};</pre>
</dd>
</dl><p>The <code><a href="#the-canvas-element">canvas</a></code> element provides scripts with a
resolution-dependent bitmap canvas, which can be used for rendering
graphs, game graphics, or other visual images on the fly.</p>
<p>Authors should not use the <code><a href="#the-canvas-element">canvas</a></code> element in a
document when a more suitable element is available. For example, it
is inappropriate to use a <code><a href="#the-canvas-element">canvas</a></code> 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 <code><a href="#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements">h1</a></code>) and then styled using CSS and
supporting technologies such as XBL.</p>
<p>When authors use the <code><a href="#the-canvas-element">canvas</a></code> element, they must also
provide content that, when presented to the user, conveys
essentially the same function or purpose as the bitmap canvas. This
content may be placed as content of the <code><a href="#the-canvas-element">canvas</a></code>
element. The contents of the <code><a href="#the-canvas-element">canvas</a></code> element, if any,
are the element's <a href="#fallback-content">fallback content</a>.</p>
<p>In interactive visual media, if <a href="#concept-n-script" title="concept-n-script">scripting is enabled</a> for the
<code><a href="#the-canvas-element">canvas</a></code> element, and if support for <code><a href="#the-canvas-element">canvas</a></code>
elements has been enabled, the <code><a href="#the-canvas-element">canvas</a></code> element
<a href="#represents">represents</a> <a href="#embedded-content">embedded content</a> consisting of
a dynamically created image.</p>
<p>In non-interactive, static, visual media, if the
<code><a href="#the-canvas-element">canvas</a></code> element has been previously painted on (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 <code><a href="#the-canvas-element">canvas</a></code> element
<a href="#represents">represents</a> <a href="#embedded-content">embedded content</a> with the
current image and size. Otherwise, the element represents its
<a href="#fallback-content">fallback content</a> instead.</p>
<p>In non-visual media, and in visual media if <a href="#concept-n-noscript" title="concept-n-noscript">scripting is disabled</a> for the
<code><a href="#the-canvas-element">canvas</a></code> element or if support for <code><a href="#the-canvas-element">canvas</a></code>
elements has been disabled, the <code><a href="#the-canvas-element">canvas</a></code> element
<a href="#represents">represents</a> its <a href="#fallback-content">fallback content</a>
instead.</p>
<!-- CANVAS-FOCUS-FALLBACK -->
<p>When a <code><a href="#the-canvas-element">canvas</a></code> element <a href="#represents">represents</a>
<a href="#embedded-content">embedded content</a>, the user can still focus descendants
of the <code><a href="#the-canvas-element">canvas</a></code> element (in the <a href="#fallback-content">fallback
content</a>). 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 elements in the <a href="#fallback-content">fallback
content</a>. (Focus has no effect on mouse interaction
events.) <a href="#DOMEVENTS">[DOMEVENTS]</a></p>
<p>The <code><a href="#the-canvas-element">canvas</a></code> element has two attributes to control the
size of the coordinate space: <dfn id="attr-canvas-width" title="attr-canvas-width"><code>width</code></dfn> and <dfn id="attr-canvas-height" title="attr-canvas-height"><code>height</code></dfn>. These
attributes, when specified, must have values that are <a href="#valid-non-negative-integer" title="valid non-negative integer">valid non-negative
integers</a>. <span class="impl">The <a href="#rules-for-parsing-non-negative-integers">rules for parsing
non-negative integers</a> 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.</span> The
<code title="attr-canvas-width"><a href="#attr-canvas-width">width</a></code> attribute defaults to
300, and the <code title="attr-canvas-height"><a href="#attr-canvas-height">height</a></code>
attribute defaults to 150.</p>
<p>The intrinsic dimensions of the <code><a href="#the-canvas-element">canvas</a></code> element equal
the size of the coordinate space, with the numbers interpreted in
CSS pixels. However, the element can be sized arbitrarily by a
style sheet. During rendering, the image is scaled to fit this layout
size.</p>
<div class="impl">
<p>The size of the coordinate space does not necessarily represent
the size of the actual bitmap that the user agent will use
internally or during rendering. On high-definition displays, for
instance, the user agent may internally use a bitmap with two device
pixels per unit in the coordinate space, so that the rendering
remains at high quality throughout.</p>
<p>When the <code><a href="#the-canvas-element">canvas</a></code> element is created, and subsequently
whenever the <code title="attr-canvas-width"><a href="#attr-canvas-width">width</a></code> and <code title="attr-canvas-height"><a href="#attr-canvas-height">height</a></code> attributes are set (whether
to a new value or to the previous value), the bitmap and any
associated contexts must be cleared back to their initial state and
reinitialized with the newly specified coordinate space
dimensions.</p>
<p>When the canvas is initialized, its bitmap must be cleared to
transparent black.</p>
<p>The <dfn id="dom-canvas-width" title="dom-canvas-width"><code>width</code></dfn> and
<dfn id="dom-canvas-height" title="dom-canvas-height"><code>height</code></dfn> IDL
attributes must <a href="#reflect">reflect</a> the respective content
attributes of the same name, with the same defaults.</p>
</div>
<div class="example">
<p>Only one square appears to be drawn in the following example:</p>
<pre> // 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</pre>
</div>
<hr/><dl class="domintro"><dt><var title="">context</var> = <var title="">canvas</var> . <code title="dom-canvas-getContext"><a href="#dom-canvas-getcontext">getContext</a></code>(<var title="">contextId</var> [, ... ])</dt>
<dd>
<p>Returns an object that exposes an API for drawing on the
canvas. The first argument specifies the desired API. Subsequent
arguments are handled by that API.</p>
<!--2DCONTEXT-->
<p>This specification defines the "<code title="canvas-context-2d"><a href="#canvas-context-2d">2d</a></code>" context below. There is also
a specification that defines a "<code title="canvas-context-webgl">webgl</code>" context. <a href="#refsWEBGL">[WEBGL]</a></p>
<!--2DCONTEXT-->
<p>The list of defined contexts is given on the <a href="http://wiki.whatwg.org/wiki/CanvasContexts">WHATWG Wiki
CanvasContexts page</a>. <a href="#refsWHATWGWIKI">[WHATWGWIKI]</a>
</p><p>Returns null if the given context ID is not supported or if the
canvas has already been initialized with some other (incompatible)
context type (e.g. trying to get a "<code title="canvas-context-2d"><a href="#canvas-context-2d">2d</a></code>" context after getting a
"<code title="canvas-context-webgl">webgl</code>" context).</p>
</dd>
</dl><div class="impl">
<p>A <code><a href="#the-canvas-element">canvas</a></code> element can have a <dfn id="primary-context">primary
context</dfn>, which is the first context to have been obtained for
that element. When created, a <code><a href="#the-canvas-element">canvas</a></code> element must not
have a <a href="#primary-context">primary context</a>.</p>
<p>The <dfn id="dom-canvas-getcontext" title="dom-canvas-getContext"><code>getContext(<var title="">contextId</var>, <var title="">args...</var>)</code></dfn>
method of the <code><a href="#the-canvas-element">canvas</a></code> element, when invoked, must run
the following steps:</p>
<ol><li><p>Let <var title="">contextId</var> be the first argument to
the method.</p></li>
<li>
<p>If <var title="">contextId</var> is not the name of a context
supported by the user agent, return null and abort these
steps.</p>
<p class="note">An example of this would be a user agent that
theoretically supports the "<code title="canvas-context-webgl">webgl</code>" 3D context, in the case
where the platform does not have hardware support for OpenGL and
the user agent does not have a software OpenGL implementation.
Despite the user agent recognising the "<code title="canvas-context-webgl">webgl</code>" name, it would return
null at this step because that context is not, in practice,
supported at the time of the call.</p>
</li>
<li><p>If the element has a <a href="#primary-context">primary context</a> and that
context's entry in the <a href="http://wiki.whatwg.org/wiki/CanvasContexts">WHATWG Wiki
CanvasContexts page</a> does not list <var title="">contextId</var>
as a context with which it is compatible, return null and abort
these steps. <a href="#refsWHATWGWIKI">[WHATWGWIKI]</a></p></li>
<li><p>If the element does not have a <a href="#primary-context">primary context</a>,
let the element's <a href="#primary-context">primary context</a> be <var title="">contextId</var>.</p></li>
<li><p>If the <code title="dom-canvas-getContext"><a href="#dom-canvas-getcontext">getContext()</a></code> method has
already been invoked on this element for the same <var title="">contextId</var>, return the same object as was returned
that time, and abort these steps. The additional arguments are
ignored.</p></li>
<li><p><dfn id="getcontext-return" title="getContext-return">Return a new object for <var title="">contextId</var></dfn>, as defined by the specification
given for <var title="">contextId</var>'s entry in the <a href="http://wiki.whatwg.org/wiki/CanvasContexts">WHATWG Wiki
CanvasContexts page</a>. <a href="#refsWHATWGWIKI">[WHATWGWIKI]</a></p></li>
</ol><p>New context types may be registered in the <a href="http://wiki.whatwg.org/wiki/CanvasContexts">WHATWG Wiki
CanvasContexts page</a>. <a href="#refsWHATWGWIKI">[WHATWGWIKI]</a></p>
<p>Anyone is free to edit the WHATWG Wiki CanvasContexts page at any
time to add a new context type. These new context types must be
specified with the following information:</p>
<dl><dt>Keyword</dt>
<dd><p>The value of <var title="">contextID</var> that will return
the object for the new API.</p></dd>
<dt>Specification</dt>
<dd><p>A link to a formal specification of the context type's
API. It could be another page on the Wiki, or a link to an external
page. If the type does not have a formal specification, an informal
description can be substituted until such time as a formal
specification is available.</p></dd>
<dt>Compatible with</dt>
<dd><p>The list of context types that are compatible with this one
(i.e. that operate on the same underlying bitmap). This list must
be transitive and symmetric; if one context type is defined as
compatible with another, then all types it is compatible with must
be compatible with all types the other is compatible with.</p></dd>
</dl><p>Vendors may also define experimental contexts using the syntax
<code><var title="">vendorname</var>-<var title="">context</var></code>, for example,
<code>moz-3d</code>. Such contexts should be registered in the
WHATWG Wiki CanvasContexts page.</p>
</div>
<hr/><dl class="domintro"><dt><var title="">url</var> = <var title="">canvas</var> . <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL</a></code>( [ <var title="">type</var>, ... ])</dt>
<dd>
<p>Returns a <a href="#data-protocol" title="data protocol"><code title="">data:</code> URL</a> for the image in the canvas.</p>
<p>The first argument, if provided, controls the type of the image
to be returned (e.g. PNG or JPEG). The default is <code title="">image/png</code>; 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.</p>
<p>When trying to use types other than "<code>image/png</code>",
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 "<code title="">data:image/png,</code>" or "<code title="">data:image/png;</code>". 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 "<code title="">data:,</code>".)</p>
</dd>
<dt><var title="">canvas</var> . <code title="dom-canvas-toBlob"><a href="#dom-canvas-toblob">toBlob</a></code>(<var title="">callback</var> [, <var title="">type</var>, ... ])</dt>
<dd>
<p>Creates a <code><a href="#blob">Blob</a></code> object representing a file
containing the image in the canvas, and invokes a callback with a
handle to that object.</p>
<p>The second argument, if provided, controls the type of the
image to be returned (e.g. PNG or JPEG). The default is <code title="">image/png</code>; 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.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-canvas-todataurl" title="dom-canvas-toDataURL"><code>toDataURL()</code></dfn> method
must run the following steps:</p>
<ol><li><p>If the canvas has no pixels (i.e. either its horizontal
dimension or its vertical dimension is zero) then return the string
"<code title="">data:,</code>" and abort these steps. (This is the
shortest <a href="#data-protocol" title="data protocol"><code title="">data:</code>
URL</a>; it represents the empty string in a <code title="">text/plain</code> resource.)</p></li>
<li><p>Let <var title="">file</var> be <a href="#a-serialization-of-the-image-as-a-file">a serialization of the
image as a file</a>, using the method's arguments (if any) as
the <var title="">arguments</var>.</p></li>
<li><p>Return a <a href="#data-protocol" title="data protocol"><code title="">data:</code> URL</a> representing <var title="">file</var>. <a href="#refsRFC2397">[RFC2397]</a></p>
<!-- should we explicitly require the URL to be base64-encoded and
not have any parameters, to ensure the same exact URL is generated
in each browser? -->
</li></ol><p>The <dfn id="dom-canvas-toblob" title="dom-canvas-toBlob"><code>toBlob()</code></dfn> method
must run the following steps:</p>
<ol><li><p>Let <var title="">callback</var> be the first
argument.</p></li>
<li><p>Let <var title="">arguments</var> be the second and
subsequent arguments to the method, if any.</p></li>
<li><p>Let <var title="">file</var> be <a href="#a-serialization-of-the-image-as-a-file">a serialization of the
image as a file</a>, using <var title="">arguments</var>.</p></li>
<li><p>Return, but continue running these steps
asynchronously.</p></li>
<li><p>If <var title="">callback</var> is null, abort these
steps.</p></li>
<li><p><a href="#queue-a-task">Queue a task</a> to invoke the
<code>FileCallback</code> <var title="">callback</var> with a
<code><a href="#blob">Blob</a></code> object representing <var title="">file</var> as
its argument. The <a href="#task-source">task source</a> for this task is the
<dfn id="canvas-blob-serialization-task-source">canvas blob serialization task source</dfn>. <a href="#refsFILESYSTEMAPI">[FILESYSTEMAPI]</a> <a href="#refsFILEAPI">[FILEAPI]</a> </p></li>
</ol><p>When a user agent is to create <dfn id="a-serialization-of-the-image-as-a-file">a serialization of the image
as a file</dfn>, optionally with some given <var title="">arguments</var>, it must create an image file in the format
given by the first value of <var title="">arguments</var>, or, if
there are no <var title="">arguments</var>, in the PNG format. <a href="#refsPNG">[PNG]</a></p>
<p>If <var title="">arguments</var> is not empty, the first value
must be interpreted as a <a href="#mime-type" title="MIME type">MIME type</a>
giving the format to use. If the type has any parameters, it must be
treated as not supported.</p>
<p class="example">For example, the value "<code>image/png</code>" would
mean to generate a PNG image, the value "<code>image/jpeg</code>"
would mean to generate a JPEG image, and the value
"<code>image/svg+xml</code>" would mean to generate an SVG image
(which would probably require that the implementation actually keep
enough information to reliably render an SVG image from the canvas).</p>
<p>User agents must support PNG ("<code>image/png</code>"). 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. <a href="#refsPNG">[PNG]</a></p>
<p>User agents must <a href="#converted-to-ascii-lowercase" title="converted to ASCII
lowercase">convert the provided type to ASCII lowercase</a>
before establishing if they support that type.</p>
<p>For image types that do not support an alpha channel, the
serialized image must be the canvas image composited onto a solid
black background using the source-over operator.</p>
<p>If the first argument in <var title="">arguments</var> 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.</p>
</div>
<table><thead><tr><th> Type </th><th> Other arguments </th><th> Reference
</th></tr></thead><tbody><tr><td> <code>image/jpeg</code>
</td><td> The second argument<span class="impl">, if it</span> is a
number in the range 0.0 to 1.0 inclusive<span class="impl">, must
be</span> treated as the desired quality level. <span class="impl">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.</span>
</td><td> <a href="#refsJPEG">[JPEG]</a>
</td></tr></tbody></table><div class="impl">
<p>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 <code title="">any</code> in the Web IDL
specification. <a href="#refsWEBIDL">[WEBIDL]</a></p>
<p>Other arguments must be ignored and must not cause the user agent
to raise 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.</p>
</div>
<!--2DCONTEXT-->
<div data-component="HTML Canvas 2D Context (editor: Ian Hickson)">
<h5 id="2dcontext"><span class="secno">4.8.11.1 </span>The 2D context</h5>
<!-- v2: we're on v4.1. suggestions for next version are marked v5, v6. -->
<p>This specification defines the <dfn id="canvas-context-2d" title="canvas-context-2d"><code>2d</code></dfn> context type, whose
API is implemented using the <code><a href="#canvasrenderingcontext2d">CanvasRenderingContext2D</a></code>
interface.</p>
<div class="impl">
<p>When the <code title="dom-canvas-getContext"><a href="#dom-canvas-getcontext">getContext()</a></code>
method of a <code><a href="#the-canvas-element">canvas</a></code> element is to <a href="#getcontext-return" title="getContext-return">return a new object for the <var title="">contextId</var></a> <code title="canvas-context-2d"><a href="#canvas-context-2d">2d</a></code>, the user agent must return a
new <code><a href="#canvasrenderingcontext2d">CanvasRenderingContext2D</a></code> object. Any additional
arguments are ignored.</p>
</div>
<p>The 2D context represents a flat Cartesian surface whose origin
(0,0) is at the top left corner, with the coordinate space having
<var title="">x</var> values increasing when going right, and <var title="">y</var> values increasing when going down.</p>
<pre class="idl">interface <dfn id="canvasrenderingcontext2d">CanvasRenderingContext2D</dfn> {
// back-reference to the canvas
readonly attribute <a href="#htmlcanvaselement">HTMLCanvasElement</a> <a href="#dom-context-2d-canvas" title="dom-context-2d-canvas">canvas</a>;
// state
void <a href="#dom-context-2d-save" title="dom-context-2d-save">save</a>(); // push state on state stack
void <a href="#dom-context-2d-restore" title="dom-context-2d-restore">restore</a>(); // pop state stack and restore state
<!--
// v6 we've also received requests for:
attribute boolean <span title="dom-context-2d-forceHighQuality">forceHighQuality</span> // (default false)
// when enabled, it would prevent the UA from falling back on lower-quality but faster rendering routines
// useful e.g. for when an image manipulation app uses <canvas> both for UI previews and the actual work
-->
// transformations (default transform is the identity matrix)
void <a href="#dom-context-2d-scale" title="dom-context-2d-scale">scale</a>(in double x, in double y);
void <a href="#dom-context-2d-rotate" title="dom-context-2d-rotate">rotate</a>(in double angle);
void <a href="#dom-context-2d-translate" title="dom-context-2d-translate">translate</a>(in double x, in double y);
void <a href="#dom-context-2d-transform" title="dom-context-2d-transform">transform</a>(in double a, in double b, in double c, in double d, in double e, in double f);
void <a href="#dom-context-2d-settransform" title="dom-context-2d-setTransform">setTransform</a>(in double a, in double b, in double c, in double d, in double e, in double f);
<!--
// v6 we've also received requests for:
void skew(...);
void reflect(...); // or mirror(...)
-->
// compositing
attribute double <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">globalAlpha</a>; // (default 1.0)
attribute DOMString <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">globalCompositeOperation</a>; // (default source-over)
<!--
// v6 we've also received requests for:
- turning off antialiasing to avoid seams when patterns are painted next to each other
- might be better to overdraw?
- might be better to just draw at a higher res then downsample, like for 3d?
- nested layers
- the ability to composite an entire set of drawing operations with one shadow all at once
http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2008-August/015567.html
-->
// colors and styles
attribute any <a href="#dom-context-2d-strokestyle" title="dom-context-2d-strokeStyle">strokeStyle</a>; // (default black)
attribute any <a href="#dom-context-2d-fillstyle" title="dom-context-2d-fillStyle">fillStyle</a>; // (default black)
<a href="#canvasgradient">CanvasGradient</a> <a href="#dom-context-2d-createlineargradient" title="dom-context-2d-createLinearGradient">createLinearGradient</a>(in double x0, in double y0, in double x1, in double y1);
<a href="#canvasgradient">CanvasGradient</a> <a href="#dom-context-2d-createradialgradient" title="dom-context-2d-createRadialGradient">createRadialGradient</a>(in double x0, in double y0, in double r0, in double x1, in double y1, in double r1);
<a href="#canvaspattern">CanvasPattern</a> <a href="#dom-context-2d-createpattern" title="dom-context-2d-createPattern">createPattern</a>(in <a href="#htmlimageelement">HTMLImageElement</a> image, in DOMString repetition);
<a href="#canvaspattern">CanvasPattern</a> <a href="#dom-context-2d-createpattern" title="dom-context-2d-createPattern">createPattern</a>(in <a href="#htmlcanvaselement">HTMLCanvasElement</a> image, in DOMString repetition);
<a href="#canvaspattern">CanvasPattern</a> <a href="#dom-context-2d-createpattern" title="dom-context-2d-createPattern">createPattern</a>(in <a href="#htmlvideoelement">HTMLVideoElement</a> image, in DOMString repetition);
// line caps/joins
attribute double <a href="#dom-context-2d-linewidth" title="dom-context-2d-lineWidth">lineWidth</a>; // (default 1)
attribute DOMString <a href="#dom-context-2d-linecap" title="dom-context-2d-lineCap">lineCap</a>; // "butt", "round", "square" (default "butt")
attribute DOMString <a href="#dom-context-2d-linejoin" title="dom-context-2d-lineJoin">lineJoin</a>; // "round", "bevel", "miter" (default "miter")
attribute double <a href="#dom-context-2d-miterlimit" title="dom-context-2d-miterLimit">miterLimit</a>; // (default 10)
// shadows
attribute double <a href="#dom-context-2d-shadowoffsetx" title="dom-context-2d-shadowOffsetX">shadowOffsetX</a>; // (default 0)
attribute double <a href="#dom-context-2d-shadowoffsety" title="dom-context-2d-shadowOffsetY">shadowOffsetY</a>; // (default 0)
attribute double <a href="#dom-context-2d-shadowblur" title="dom-context-2d-shadowBlur">shadowBlur</a>; // (default 0)
attribute DOMString <a href="#dom-context-2d-shadowcolor" title="dom-context-2d-shadowColor">shadowColor</a>; // (default transparent black)
// rects
void <a href="#dom-context-2d-clearrect" title="dom-context-2d-clearRect">clearRect</a>(in double x, in double y, in double w, in double h);
void <a href="#dom-context-2d-fillrect" title="dom-context-2d-fillRect">fillRect</a>(in double x, in double y, in double w, in double h);
void <a href="#dom-context-2d-strokerect" title="dom-context-2d-strokeRect">strokeRect</a>(in double x, in double y, in double w, in double h);
// path API
void <a href="#dom-context-2d-beginpath" title="dom-context-2d-beginPath">beginPath</a>();
void <a href="#dom-context-2d-closepath" title="dom-context-2d-closePath">closePath</a>();
void <a href="#dom-context-2d-moveto" title="dom-context-2d-moveTo">moveTo</a>(in double x, in double y);
void <a href="#dom-context-2d-lineto" title="dom-context-2d-lineTo">lineTo</a>(in double x, in double y);
void <a href="#dom-context-2d-quadraticcurveto" title="dom-context-2d-quadraticCurveTo">quadraticCurveTo</a>(in double cpx, in double cpy, in double x, in double y);
void <a href="#dom-context-2d-beziercurveto" title="dom-context-2d-bezierCurveTo">bezierCurveTo</a>(in double cp1x, in double cp1y, in double cp2x, in double cp2y, in double x, in double y);
void <a href="#dom-context-2d-arcto" title="dom-context-2d-arcTo">arcTo</a>(in double x1, in double y1, in double x2, in double y2, in double radius);
void <a href="#dom-context-2d-rect" title="dom-context-2d-rect">rect</a>(in double x, in double y, in double w, in double h);
void <a href="#dom-context-2d-arc" title="dom-context-2d-arc">arc</a>(in double x, in double y, in double radius, in double startAngle, in double endAngle, in optional boolean anticlockwise);
void <a href="#dom-context-2d-fill" title="dom-context-2d-fill">fill</a>();
void <a href="#dom-context-2d-stroke" title="dom-context-2d-stroke">stroke</a>();
void <a href="#dom-context-2d-drawsystemfocusring" title="dom-context-2d-drawSystemFocusRing">drawSystemFocusRing</a>(in <a href="#element">Element</a> element);
boolean <a href="#dom-context-2d-drawcustomfocusring" title="dom-context-2d-drawCustomFocusRing">drawCustomFocusRing</a>(in <a href="#element">Element</a> element);
void <a href="#dom-context-2d-scrollpathintoview" title="dom-context-2d-scrollPathIntoView">scrollPathIntoView</a>();
void <a href="#dom-context-2d-clip" title="dom-context-2d-clip">clip</a>();
boolean <a href="#dom-context-2d-ispointinpath" title="dom-context-2d-isPointInPath">isPointInPath</a>(in double x, in double y);
// text
attribute DOMString <a href="#dom-context-2d-font" title="dom-context-2d-font">font</a>; // (default 10px sans-serif)
attribute DOMString <a href="#dom-context-2d-textalign" title="dom-context-2d-textAlign">textAlign</a>; // "start", "end", "left", "right", "center" (default: "start")
attribute DOMString <a href="#dom-context-2d-textbaseline" title="dom-context-2d-textBaseline">textBaseline</a>; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
void <a href="#dom-context-2d-filltext" title="dom-context-2d-fillText">fillText</a>(in DOMString text, in double x, in double y, in optional double maxWidth);
void <a href="#dom-context-2d-stroketext" title="dom-context-2d-strokeText">strokeText</a>(in DOMString text, in double x, in double y, in optional double maxWidth);<!-- v6DVT
void <span title="dom-context-2d-fillVerticalText">fillVerticalText</span>(in DOMString text, in double x, in double y, in optional double maxHeight);
void <span title="dom-context-2d-strokeVerticalText">strokeVerticalText</span>(in DOMString text, in double x, in double y, in optional double maxHeight); -->
<a href="#textmetrics">TextMetrics</a> <a href="#dom-context-2d-measuretext" title="dom-context-2d-measureText">measureText</a>(in DOMString text);
// drawing images
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlimageelement">HTMLImageElement</a> image, in double dx, in double dy);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlimageelement">HTMLImageElement</a> image, in double dx, in double dy, in double dw, in double dh);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlimageelement">HTMLImageElement</a> image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlcanvaselement">HTMLCanvasElement</a> image, in double dx, in double dy);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlcanvaselement">HTMLCanvasElement</a> image, in double dx, in double dy, in double dw, in double dh);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlcanvaselement">HTMLCanvasElement</a> image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlvideoelement">HTMLVideoElement</a> image, in double dx, in double dy);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlvideoelement">HTMLVideoElement</a> image, in double dx, in double dy, in double dw, in double dh);
void <a href="#dom-context-2d-drawimage" title="dom-context-2d-drawImage">drawImage</a>(in <a href="#htmlvideoelement">HTMLVideoElement</a> image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh);
// pixel manipulation
<a href="#imagedata">ImageData</a> <a href="#dom-context-2d-createimagedata" title="dom-context-2d-createImageData">createImageData</a>(in double sw, in double sh);
<a href="#imagedata">ImageData</a> <a href="#dom-context-2d-createimagedata" title="dom-context-2d-createImageData">createImageData</a>(in <a href="#imagedata">ImageData</a> imagedata);
<a href="#imagedata">ImageData</a> <a href="#dom-context-2d-getimagedata" title="dom-context-2d-getImageData">getImageData</a>(in double sx, in double sy, in double sw, in double sh);
void <a href="#dom-context-2d-putimagedata" title="dom-context-2d-putImageData">putImageData</a>(in <a href="#imagedata">ImageData</a> imagedata, in double dx, in double dy);
void <a href="#dom-context-2d-putimagedata" title="dom-context-2d-putImageData">putImageData</a>(in <a href="#imagedata">ImageData</a> imagedata, in double dx, in double dy, in double dirtyX, in double dirtyY, in double dirtyWidth, in double dirtyHeight);
};
interface <dfn id="canvasgradient">CanvasGradient</dfn> {
// opaque object
void <a href="#dom-canvasgradient-addcolorstop" title="dom-canvasgradient-addColorStop">addColorStop</a>(in double offset, in DOMString color);
};
interface <dfn id="canvaspattern">CanvasPattern</dfn> {
// opaque object
};
interface <dfn id="textmetrics">TextMetrics</dfn> {
readonly attribute double <a href="#dom-textmetrics-width" title="dom-textmetrics-width">width</a>;
};
interface <dfn id="imagedata">ImageData</dfn> {
readonly attribute unsigned long <a href="#dom-imagedata-width" title="dom-imagedata-width">width</a>;
readonly attribute unsigned long <a href="#dom-imagedata-height" title="dom-imagedata-height">height</a>;
readonly attribute <a href="#canvaspixelarray">CanvasPixelArray</a> <a href="#dom-imagedata-data" title="dom-imagedata-data">data</a>;
};
interface <dfn id="canvaspixelarray">CanvasPixelArray</dfn> {
readonly attribute unsigned long <a href="#dom-canvaspixelarray-length" title="dom-canvaspixelarray-length">length</a>;
<a href="#dom-canvaspixelarray-get" title="dom-CanvasPixelArray-get">getter</a> octet (in unsigned long index);
<a href="#dom-canvaspixelarray-set" title="dom-CanvasPixelArray-set">setter</a> void (in unsigned long index, [Clamp] in octet value);
};</pre>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-canvas"><a href="#dom-context-2d-canvas">canvas</a></code></dt>
<dd>
<p>Returns the <code><a href="#the-canvas-element">canvas</a></code> element.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-canvas" title="dom-context-2d-canvas"><code>canvas</code></dfn>
attribute must return the <code><a href="#the-canvas-element">canvas</a></code> element that the
context paints on.</p>
<p>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.</p>
<!--
Philip Taylor wrote:
> My experience with some 3d canvas code is that infinities come up in
> naturally harmless places, e.g. having a function that scales by x then
> translates by 1/x and wanting it to work when x=0 (which ought to draw
> nothing, since anything it draws is zero pixels wide), and it's a bit
> annoying to track down and fix those issues, so I'd probably like it if
> they were harmless in canvas methods. Opera appears to silently not draw
> anything if the transformation matrix is not finite, but Firefox throws
> exceptions when passing in non-finite arguments.
-->
<p>Whenever the CSS value <code title="">currentColor</code> is used
as a color in this API, the "computed value of the 'color' property"
for the purposes of determining the computed value of the <code title="">currentColor</code> keyword is the computed value of the
'color' property on the element in question at the time that the
color is specified (e.g. when the appropriate attribute is set, or
when the method is called; not when the color is rendered or
otherwise used). If the computed value of the 'color' property is
undefined for a particular case (e.g. because the element is not
<a href="#in-a-document">in a <code>Document</code></a>), then the "computed value
of the 'color' property" for the purposes of determining the
computed value of the <code title="">currentColor</code> keyword is
fully opaque black. <a href="#refsCSSCOLOR">[CSSCOLOR]</a></p>
<p>In the case of <code title="dom-canvasgradient-addColorStop"><a href="#dom-canvasgradient-addcolorstop">addColorStop()</a></code> on
<code><a href="#canvasgradient">CanvasGradient</a></code>, the "computed value of the 'color'
property" for the purposes of determining the computed value of the
<code title="">currentColor</code> keyword is always fully opaque
black (there is no associated element). <a href="#refsCSSCOLOR">[CSSCOLOR]</a></p>
<p class="note">This is because <code><a href="#canvasgradient">CanvasGradient</a></code> objects
are <code><a href="#the-canvas-element">canvas</a></code>-neutral — a
<code><a href="#canvasgradient">CanvasGradient</a></code> object created by one
<code><a href="#the-canvas-element">canvas</a></code> can be used by another, and there is therefore
no way to know which is the "element in question" at the time that
the color is specified.</p>
</div>
<h6 id="the-canvas-state"><span class="secno">4.8.11.1.1 </span>The canvas state</h6>
<p>Each context maintains a stack of drawing states. <dfn id="drawing-state" title="drawing state">Drawing states</dfn> consist of:</p>
<ul class="brief"><li>The current <a href="#transformations" title="dom-context-2d-transformation">transformation matrix</a>.</li>
<li>The current <a href="#clipping-region">clipping region</a>.</li>
<li>The current values of the following attributes: <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code>, <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code>, <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code>, <code title="dom-context-2d-lineWidth"><a href="#dom-context-2d-linewidth">lineWidth</a></code>, <code title="dom-context-2d-lineCap"><a href="#dom-context-2d-linecap">lineCap</a></code>, <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code>, <code title="dom-context-2d-miterLimit"><a href="#dom-context-2d-miterlimit">miterLimit</a></code>, <code title="dom-context-2d-shadowOffsetX"><a href="#dom-context-2d-shadowoffsetx">shadowOffsetX</a></code>, <code title="dom-context-2d-shadowOffsetY"><a href="#dom-context-2d-shadowoffsety">shadowOffsetY</a></code>, <code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code>, <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code>, <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code>, <code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code>, <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code>, <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code>.</li>
</ul><p class="note">The current path and the current bitmap are not part
of the drawing state. The current path is persistent, and can only
be reset using the <code title="dom-context-2d-beginPath"><a href="#dom-context-2d-beginpath">beginPath()</a></code> method. The
current bitmap is a property of the canvas, not the context.</p>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-save"><a href="#dom-context-2d-save">save</a></code>()</dt>
<dd>
<p>Pushes the current state onto the stack.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-restore"><a href="#dom-context-2d-restore">restore</a></code>()</dt>
<dd>
<p>Pops the top state on the stack, restoring the context to that state.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-save" title="dom-context-2d-save"><code>save()</code></dfn>
method must push a copy of the current drawing state onto the
drawing state stack.</p>
<p>The <dfn id="dom-context-2d-restore" title="dom-context-2d-restore"><code>restore()</code></dfn> 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.</p>
<!-- v6
idea from Mihai:
> 5. Drawing states should be saveable with IDs, and for easier restoring.
>
> save(id)
> restore(id)
>
> If id is not provided, then save() works as defined now. The same for
> restore().
>
> Currently, it's not trivial to save and restore a specific state.
...and from Philip:
> I think a more convenient syntax would be:
> var state = ctx.save();
> ctx.restore(state);
> But how would it interact with normal calls to ctx.restore()?
-->
</div>
<h6 id="transformations"><span class="secno">4.8.11.1.2 </span><dfn title="dom-context-2d-transformation">Transformations</dfn></h6>
<p>The transformation matrix is applied to coordinates when creating
shapes and paths.</p> <!-- conformance criteria for actual drawing
are described in the various sections below -->
<div class="impl">
<p>When the context is created, the transformation matrix must
initially be the identity transform. It may then be adjusted using
the transformation methods.</p>
<p>The transformations must be performed in reverse order. For
instance, if a scale transformation that doubles the width is
applied, 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.</p>
</div>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-scale"><a href="#dom-context-2d-scale">scale</a></code>(<var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Changes the transformation matrix to apply a scaling transformation with the given characteristics.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-rotate"><a href="#dom-context-2d-rotate">rotate</a></code>(<var title="">angle</var>)</dt>
<dd>
<p>Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-translate"><a href="#dom-context-2d-translate">translate</a></code>(<var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Changes the transformation matrix to apply a translation transformation with the given characteristics.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-transform"><a href="#dom-context-2d-transform">transform</a></code>(<var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>, <var title="">f</var>)</dt>
<dd>
<p>Changes the transformation matrix to apply the matrix given by the arguments as described below.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-setTransform"><a href="#dom-context-2d-settransform">setTransform</a></code>(<var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>, <var title="">f</var>)</dt>
<dd>
<p>Changes the transformation matrix <em>to</em> the matrix given by the arguments as described below.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-scale" title="dom-context-2d-scale"><code>scale(<var title="">x</var>, <var title="">y</var>)</code></dfn> method must
add the scaling transformation described by the arguments to the
transformation matrix. The <var title="">x</var> argument represents
the scale factor in the horizontal direction and the <var title="">y</var> argument represents the scale factor in the
vertical direction. The factors are multiples.</p>
<p>The <dfn id="dom-context-2d-rotate" title="dom-context-2d-rotate"><code>rotate(<var title="">angle</var>)</code></dfn> method must add the rotation
transformation described by the argument to the transformation
matrix. The <var title="">angle</var> argument represents a
clockwise rotation angle expressed in radians.</p>
<p>The <dfn id="dom-context-2d-translate" title="dom-context-2d-translate"><code>translate(<var title="">x</var>, <var title="">y</var>)</code></dfn> method must
add the translation transformation described by the arguments to the
transformation matrix. The <var title="">x</var> argument represents
the translation distance in the horizontal direction and the <var title="">y</var> argument represents the translation distance in the
vertical direction. The arguments are in coordinate space units.</p>
<p>The <dfn id="dom-context-2d-transform" title="dom-context-2d-transform"><code>transform(<var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>, <var title="">f</var>)</code></dfn> method must replace the current
transformation matrix with the result of multiplying the current
transformation matrix with the matrix described by:</p>
</div>
<table class="matrix"><tbody><tr><td><var title="">a</var></td>
<td><var title="">c</var></td>
<td><var title="">e</var></td>
</tr><tr><td><var title="">b</var></td>
<td><var title="">d</var></td>
<td><var title="">f</var></td>
</tr><tr><td>0</td>
<td>0</td>
<td>1</td>
</tr></tbody></table><p class="note">The arguments <var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>, and <var title="">f</var> are sometimes called
<var title="">m11</var>, <var title="">m12</var>, <var title="">m21</var>, <var title="">m22</var>, <var title="">dx</var>,
and <var title="">dy</var> or <var title="">m11</var>, <var title="">m21</var>, <var title="">m12</var>, <var title="">m22</var>, <var title="">dx</var>, and <var title="">dy</var>. Care should be taken in particular with the order
of the second and third arguments (<var title="">b</var> and <var title="">c</var>) as their order varies from API to API and APIs
sometimes use the notation <var title="">m12</var>/<var title="">m21</var> and sometimes <var title="">m21</var>/<var title="">m12</var> for those positions.</p>
<div class="impl">
<p>The <dfn id="dom-context-2d-settransform" title="dom-context-2d-setTransform"><code>setTransform(<var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>,
<var title="">f</var>)</code></dfn> method must reset the current
transform to the identity matrix, and then invoke the <code><a href="#dom-context-2d-transform" title="dom-context-2d-transform">transform</a>(<var title="">a</var>, <var title="">b</var>, <var title="">c</var>, <var title="">d</var>, <var title="">e</var>,
<var title="">f</var>)</code> method with the same arguments.</p>
</div>
<h6 id="compositing"><span class="secno">4.8.11.1.3 </span>Compositing</h6>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current alpha value applied to rendering operations.</p>
<p>Can be set, to change the alpha value. Values outside of the
range 0.0 .. 1.0 are ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current composition operation, from the list below.</p>
<p>Can be set, to change the composition operation. Unknown values
are ignored.</p>
</dd>
</dl><div class="impl">
<p>All drawing operations are affected by the global compositing
attributes, <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code> and <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code>.</p>
<!-- conformance criteria for painting are described in the "drawing
model" section below -->
<p>The <dfn id="dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha"><code>globalAlpha</code></dfn>
attribute gives an alpha value that is applied to shapes and images
before they are composited onto the canvas. 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 <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code> attribute must
initially have the value 1.0.</p>
<p>The <dfn id="dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation"><code>globalCompositeOperation</code></dfn>
attribute sets how shapes and images are drawn onto the existing
bitmap, once they have had <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code> and the
current transformation matrix applied. It must be set to a value
from the following list. In the descriptions below, the source
image, <var title="">A</var>, is the shape or image being rendered,
and the destination image, <var title="">B</var>, is the current
state of the bitmap.</p>
</div>
<dl><dt><dfn id="gcop-source-atop" title="gcop-source-atop"><code>source-atop</code></dfn></dt>
<dd><var title="">A</var> atop <var title="">B</var>. <span class="note">Display the
source image wherever both images are opaque. Display the
destination image wherever the destination image is opaque but the
source image is transparent. Display transparency elsewhere.</span></dd>
<dt><dfn id="gcop-source-in" title="gcop-source-in"><code>source-in</code></dfn></dt>
<dd><var title="">A</var> in <var title="">B</var>. <span class="note">Display the
source image wherever both the source image and destination image
are opaque. Display transparency elsewhere.</span></dd>
<dt><dfn id="gcop-source-out" title="gcop-source-out"><code>source-out</code></dfn></dt>
<dd><var title="">A</var> out <var title="">B</var>. <span class="note">Display the
source image wherever the source image is opaque and the
destination image is transparent. Display transparency
elsewhere.</span></dd>
<dt><dfn id="gcop-source-over" title="gcop-source-over"><code>source-over</code></dfn> (default)</dt>
<dd><var title="">A</var> over <var title="">B</var>. <span class="note">Display the
source image wherever the source image is opaque. Display the
destination image elsewhere.</span></dd>
<dt><dfn id="gcop-destination-atop" title="gcop-destination-atop"><code>destination-atop</code></dfn></dt>
<dd><var title="">B</var> atop <var title="">A</var>. <span class="note">Same as <code title="gcop-source-atop"><a href="#gcop-source-atop">source-atop</a></code> but using the
destination image instead of the source image and vice versa.</span></dd>
<dt><dfn id="gcop-destination-in" title="gcop-destination-in"><code>destination-in</code></dfn></dt>
<dd><var title="">B</var> in <var title="">A</var>. <span class="note">Same as <code title="gcop-source-in"><a href="#gcop-source-in">source-in</a></code> but using the destination
image instead of the source image and vice versa.</span></dd>
<dt><dfn id="gcop-destination-out" title="gcop-destination-out"><code>destination-out</code></dfn></dt>
<dd><var title="">B</var> out <var title="">A</var>. <span class="note">Same as <code title="gcop-source-out"><a href="#gcop-source-out">source-out</a></code> but using the destination
image instead of the source image and vice versa.</span></dd>
<dt><dfn id="gcop-destination-over" title="gcop-destination-over"><code>destination-over</code></dfn></dt>
<dd><var title="">B</var> over <var title="">A</var>. <span class="note">Same as <code title="gcop-source-over"><a href="#gcop-source-over">source-over</a></code> but using the
destination image instead of the source image and vice versa.</span></dd>
<!-- no clear definition of this operator (doesn't correspond to a PorterDuff operator)
<dt><dfn title="gcop-darker"><code>darker</code></dfn></dt>
<dd><span class="note">Display the sum of the source image and destination image,
with color values approaching 0 as a limit.</span></dd>
-->
<dt><dfn id="gcop-lighter" title="gcop-lighter"><code>lighter</code></dfn></dt>
<dd><var title="">A</var> plus <var title="">B</var>. <span class="note">Display the
sum of the source image and destination image, with color values
approaching 255 (100%) as a limit.</span></dd>
<dt><dfn id="gcop-copy" title="gcop-copy"><code>copy</code></dfn></dt>
<dd><var title="">A</var> (<var title="">B</var> is
ignored). <span class="note">Display the source image instead of the destination
image.</span></dd>
<dt><dfn id="gcop-xor" title="gcop-xor"><code>xor</code></dfn></dt>
<dd><var title="">A</var> xor <var title="">B</var>. <span class="note">Exclusive OR
of the source image and destination image.</span></dd>
<dt class="impl"><code><var title="">vendorName</var>-<var title="">operationName</var></code></dt>
<dd class="impl">Vendor-specific extensions to the list of
composition operators should use this syntax.</dd>
</dl><div class="impl">
<p>The operators in the above list must be treated as described by
the Porter-Duff operator given at the start of their description
(e.g. <var title="">A</var> over <var title="">B</var>). They are to
be applied as part of the <a href="#drawing-model">drawing model</a>, at which point the
<a href="#clipping-region">clipping region</a> is also applied. (Without a clipping
region, these operators act on the whole bitmap with every
operation.) <a href="#refsPORTERDUFF">[PORTERDUFF]</a></p>
<p>These values are all case-sensitive — they must be used
exactly as shown. User agents must not recognize values that are not
a <a href="#case-sensitive">case-sensitive</a> match for one of the values given
above.</p>
<p>On setting, if the user agent does not recognize the specified
value, it must be ignored, leaving the value of <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code>
unaffected.</p>
<p>When the context is created, the <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code>
attribute must initially have the value
<code>source-over</code>.</p>
</div>
<h6 id="colors-and-styles"><span class="secno">4.8.11.1.4 </span>Colors and styles</h6>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current style used for stroking shapes.</p>
<p>Can be set, to change the stroke style.</p>
<p>The style can be either a string containing a CSS color, or a
<code><a href="#canvasgradient">CanvasGradient</a></code> or <code><a href="#canvaspattern">CanvasPattern</a></code>
object. Invalid values are ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current style used for filling shapes.</p>
<p>Can be set, to change the fill style.</p>
<p>The style can be either a string containing a CSS color, or a
<code><a href="#canvasgradient">CanvasGradient</a></code> or <code><a href="#canvaspattern">CanvasPattern</a></code>
object. Invalid values are ignored.</p>
</dd>
</dl><div class="impl">
<!-- v6 feature requests:
* Getting and setting colours by component to bypass the CSS value parsing.
Either:
context.fillStyle.red += 1;
Or:
var array = context.fillStyle;
array[1] += 1;
context.fillStyle = array;
* A more performant way of setting colours in general, e.g.:
context.setFillColor(r,g,b,a) // already supported by webkit
Or:
context.fillStyle = 0xRRGGBBAA; // set a 32bit int directly
* fill rule for deciding between winding and even-odd algorithms.
SVG has fill-rule: nonzero | evenodd
http://www.w3.org/TR/SVG/painting.html#FillProperties
-->
<p>The <dfn id="dom-context-2d-strokestyle" title="dom-context-2d-strokeStyle"><code>strokeStyle</code></dfn>
attribute represents the color or style to use for the lines around
shapes, and the <dfn id="dom-context-2d-fillstyle" title="dom-context-2d-fillStyle"><code>fillStyle</code></dfn>
attribute represents the color or style to use inside the
shapes.</p>
<p>Both attributes can be either strings,
<code><a href="#canvasgradient">CanvasGradient</a></code>s, or <code><a href="#canvaspattern">CanvasPattern</a></code>s. On
setting, strings must be <a href="#parsed-as-a-css-color-value" title="parsed as a CSS <color>
value">parsed as CSS <color> values</a> and the color
assigned, and <code><a href="#canvasgradient">CanvasGradient</a></code> and
<code><a href="#canvaspattern">CanvasPattern</a></code> objects must be assigned themselves. <a href="#refsCSSCOLOR">[CSSCOLOR]</a> If the value is a string but
cannot be <a href="#parsed-as-a-css-color-value">parsed as a CSS <color> value</a>, or is
neither a string, a <code><a href="#canvasgradient">CanvasGradient</a></code>, nor a
<code><a href="#canvaspattern">CanvasPattern</a></code>, then it must be ignored, and the
attribute must retain its previous value.</p>
<p>When set to a <code><a href="#canvaspattern">CanvasPattern</a></code> or
<code><a href="#canvasgradient">CanvasGradient</a></code> object, the assignment is
<a href="#live">live</a>, meaning that changes made to the object after the
assignment do affect subsequent stroking or filling of shapes.</p>
<p>On getting, if the value is a color, then the <a href="#serialization-of-a-color" title="serialization of a color">serialization of the color</a>
must be returned. Otherwise, if it is not a color but a
<code><a href="#canvasgradient">CanvasGradient</a></code> or <code><a href="#canvaspattern">CanvasPattern</a></code>, 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.)</p>
<p>The <dfn id="serialization-of-a-color">serialization of a color</dfn> for a color 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 in the range 0-9 a-f (U+0030 to U+0039
and U+0061 to U+0066). Otherwise, the color value has alpha less
than 1.0, and the string is the color value in the CSS <code title="">rgba()</code> functional-notation format: the literal
string <code title="">rgba</code> (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 digits 0-9, U+0030
to U+0039, 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 digits in the range 0-9
(U+0030 to U+0039) representing the fractional part of the alpha
value, and finally a U+0029 RIGHT PARENTHESIS.</p> <!-- if people
complain this is unreadable, expand it into a <dl> with two nested
<ol>s -->
<p>When the context is created, the <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> and <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> attributes must
initially have the string value <code title="">#000000</code>.</p>
</div>
<hr/><p>There are two types of gradients, linear gradients and radial
gradients, both represented by objects implementing the opaque
<code><a href="#canvasgradient">CanvasGradient</a></code> interface.</p>
<p id="interpolation">Once a gradient has been created (see below),
stops are placed along it to define how the colors are distributed
along the gradient. <span class="impl">The color of the gradient at
each stop is the color specified for that stop. Between each such
stop, the colors and the alpha component must be linearly
interpolated over the RGBA space without premultiplying the alpha
value to find the color to use at that offset. Before the first
stop, the color must be the color of the first stop. After the last
stop, the color must be the color of the last stop. When there are
no stops, the gradient is transparent black.</span></p>
<dl class="domintro"><dt><var title="">gradient</var> . <code title="dom-canvasgradient-addColorStop"><a href="#dom-canvasgradient-addcolorstop">addColorStop</a></code>(<var title="">offset</var>, <var title="">color</var>)</dt>
<dd>
<p>Adds a color stop with the given color 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.</p>
<p>Throws an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception if the offset
is out of range. Throws a <code><a href="#syntax_err">SYNTAX_ERR</a></code> exception if the
color cannot be parsed.</p>
</dd>
<dt><var title="">gradient</var> = <var title="">context</var> . <code title="dom-context-2d-createLinearGradient"><a href="#dom-context-2d-createlineargradient">createLinearGradient</a></code>(<var title="">x0</var>, <var title="">y0</var>, <var title="">x1</var>, <var title="">y1</var>)</dt>
<dd>
<p>Returns a <code><a href="#canvasgradient">CanvasGradient</a></code> object that represents a
linear gradient that paints along the line given by the
coordinates represented by the arguments.</p>
<p>If any of the arguments are not finite numbers, throws a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception.</p>
</dd>
<dt><var title="">gradient</var> = <var title="">context</var> . <code title="dom-context-2d-createRadialGradient"><a href="#dom-context-2d-createradialgradient">createRadialGradient</a></code>(<var title="">x0</var>, <var title="">y0</var>, <var title="">r0</var>, <var title="">x1</var>, <var title="">y1</var>, <var title="">r1</var>)</dt>
<dd>
<p>Returns a <code><a href="#canvasgradient">CanvasGradient</a></code> object that represents a
radial gradient that paints along the cone given by the circles
represented by the arguments.</p>
<p>If any of the arguments are not finite numbers, throws a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception. If either of the radii
are negative, throws an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-canvasgradient-addcolorstop" title="dom-canvasgradient-addColorStop"><code>addColorStop(<var title="">offset</var>, <var title="">color</var>)</code></dfn>
method on the <code><a href="#canvasgradient">CanvasGradient</a></code> interface adds a new stop
to a gradient. If the <var title="">offset</var> is less than 0,
greater than 1, infinite, or NaN, then an
<code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception must be raised. If the <var title="">color</var> cannot be <a href="#parsed-as-a-css-color-value">parsed as a CSS <color>
value</a>, then a <code><a href="#syntax_err">SYNTAX_ERR</a></code> exception must be
raised. Otherwise, the gradient must have a new stop placed, at
offset <var title="">offset</var> relative to the whole gradient,
and with the color obtained by parsing <var title="">color</var> 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).</p>
<p>The <dfn id="dom-context-2d-createlineargradient" title="dom-context-2d-createLinearGradient"><code>createLinearGradient(<var title="">x0</var>, <var title="">y0</var>, <var title="">x1</var>,
<var title="">y1</var>)</code></dfn> method takes four arguments
that represent the start point (<var title="">x0</var>, <var title="">y0</var>) and end point (<var title="">x1</var>, <var title="">y1</var>) of the gradient. If any of the arguments to <code title="dom-context-2d-createLinearGradient"><a href="#dom-context-2d-createlineargradient">createLinearGradient()</a></code>
are infinite or NaN, the method must raise a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception. Otherwise, the method must
return a linear <code><a href="#canvasgradient">CanvasGradient</a></code> initialized with the
specified line.</p>
<p>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 color at the point where those two lines cross (with the colors
coming from the <a href="#interpolation">interpolation and
extrapolation</a> described above). The points in the linear
gradient must be transformed as described by the <a href="#transformations" title="dom-context-2d-transformation">current transformation
matrix</a> when rendering.</p>
<p>If <span title=""><var title="">x0</var> = <var title="">x1</var></span> and <span title=""><var title="">y0</var> = <var title="">y1</var></span>, then
the linear gradient must paint nothing.</p>
<p>The <dfn id="dom-context-2d-createradialgradient" title="dom-context-2d-createRadialGradient"><code>createRadialGradient(<var title="">x0</var>, <var title="">y0</var>, <var title="">r0</var>,
<var title="">x1</var>, <var title="">y1</var>, <var title="">r1</var>)</code></dfn> method takes six arguments, the
first three representing the start circle with origin (<var title="">x0</var>, <var title="">y0</var>) and radius <var title="">r0</var>, and the last three representing the end circle
with origin (<var title="">x1</var>, <var title="">y1</var>) and
radius <var title="">r1</var>. The values are in coordinate space
units. If any of the arguments are infinite or NaN, a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception must be raised. If either
of <var title="">r0</var> or <var title="">r1</var> are negative, an
<code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception must be raised. Otherwise,
the method must return a radial <code><a href="#canvasgradient">CanvasGradient</a></code>
initialized with the two specified circles.</p>
<p>Radial gradients must be rendered by following these steps:</p>
<ol><li><p>If <span title=""><var title="">x<sub>0</sub></var> = <var title="">x<sub>1</sub></var></span> and <span title=""><var title="">y<sub>0</sub></var> = <var title="">y<sub>1</sub></var></span> and <span title=""><var title="">r<sub>0</sub></var> = <var title="">r<sub>1</sub></var></span>, then the radial gradient must
paint nothing. Abort these steps.</p></li>
<li>
<p>Let <span title="">x(<var title="">ω</var>) = (<var title="">x<sub>1</sub></var>-<var title="">x<sub>0</sub></var>)<var title="">ω</var> + <var title="">x<sub>0</sub></var></span></p>
<p>Let <span title="">y(<var title="">ω</var>) = (<var title="">y<sub>1</sub></var>-<var title="">y<sub>0</sub></var>)<var title="">ω</var> + <var title="">y<sub>0</sub></var></span></p>
<p>Let <span title="">r(<var title="">ω</var>) = (<var title="">r<sub>1</sub></var>-<var title="">r<sub>0</sub></var>)<var title="">ω</var> + <var title="">r<sub>0</sub></var></span></p>
<p>Let the color at <var title="">ω</var> be the color at
that position on the gradient (with the colors coming from the <a href="#interpolation">interpolation and extrapolation</a>
described above).</p>
</li>
<li><p>For all values of <var title="">ω</var> where <span title="">r(<var title="">ω</var>) > 0</span>,
starting with the value of <var title="">ω</var> nearest to
positive infinity and ending with the value of <var title="">ω</var> nearest to negative infinity, draw the
circumference of the circle with radius <span title="">r(<var title="">ω</var>)</span> at position (<span title="">x(<var title="">ω</var>)</span>, <span title="">y(<var title="">ω</var>)</span>), with the color at <var title="">ω</var>, but only painting on the parts of the
canvas that have not yet been painted on by earlier circles in this
step for this rendering of the gradient.</p></li>
</ol><p class="note">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 color of the first
offset, the part of the cone after the end circle (1.0) using the
color of the last offset, and areas outside the cone untouched by
the gradient (transparent black).</p>
<p>The points in the radial gradient must be transformed as
described by the <a href="#transformations" title="dom-context-2d-transformation">current
transformation matrix</a> when rendering.</p>
<p>Gradients must be painted only where the relevant stroking or
filling effects requires that they be drawn.</p>
<!--
<p>Support for actually painting gradients is optional. Instead of
painting the gradients, user agents may instead just paint the first
stop's color. However, <code
title="dom-context-2d-createLinearGradient">createLinearGradient()</code>
and <code
title="dom-context-2d-createRadialGradient">createRadialGradient()</code>
must always return objects when passed valid arguments.</p>
-->
</div>
<hr/><p>Patterns are represented by objects implementing the opaque
<code><a href="#canvaspattern">CanvasPattern</a></code> interface.</p>
<dl class="domintro"><dt><var title="">pattern</var> = <var title="">context</var> . <code title="dom-context-2d-createPattern"><a href="#dom-context-2d-createpattern">createPattern</a></code>(<var title="">image</var>, <var title="">repetition</var>)</dt>
<dd>
<p>Returns a <code><a href="#canvaspattern">CanvasPattern</a></code> object that uses the given image
and repeats in the direction(s) given by the <var title="">repetition</var> argument.</p>
<p>The allowed values for <var title="">repetition</var> are <code title="">repeat</code> (both directions), <code title="">repeat-x</code> (horizontal only), <code title="">repeat-y</code> (vertical only), and <code title="">no-repeat</code> (neither). If the <var title="">repetition</var> argument is empty, the value <code title="">repeat</code> is used.</p>
<p>If the image has no image data, throws an
<code><a href="#invalid_state_err">INVALID_STATE_ERR</a></code> exception. If the second argument
isn't one of the allowed values, throws a <code><a href="#syntax_err">SYNTAX_ERR</a></code>
exception. If the image isn't yet fully decoded, then the method
returns null.</p>
</dd>
</dl><div class="impl">
<p>To create objects of this type, the <dfn id="dom-context-2d-createpattern" title="dom-context-2d-createPattern"><code>createPattern(<var title="">image</var>, <var title="">repetition</var>)</code></dfn>
method is used. The first argument gives the image to use as the
pattern (either an <code><a href="#htmlimageelement">HTMLImageElement</a></code>,
<code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code>, or <code><a href="#htmlvideoelement">HTMLVideoElement</a></code>
object). Modifying this image after calling the <code title="dom-context-2d-createPattern"><a href="#dom-context-2d-createpattern">createPattern()</a></code> method
must not affect the pattern. The second argument must be a string
with one of the following values: <code title="">repeat</code>,
<code title="">repeat-x</code>, <code title="">repeat-y</code>,
<code title="">no-repeat</code>. If the empty string is specified,
<code title="">repeat</code> must be assumed. If an unrecognized value
is given, then the user agent must raise a <code><a href="#syntax_err">SYNTAX_ERR</a></code>
exception. User agents must recognize the four values described above
exactly (e.g. they must not do case folding). Except as specified
below, the method must return a <code><a href="#canvaspattern">CanvasPattern</a></code> object
suitably initialized.</p>
<p>The <var title="">image</var> argument is an instance of either
<code><a href="#htmlimageelement">HTMLImageElement</a></code>, <code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code>, or
<code><a href="#htmlvideoelement">HTMLVideoElement</a></code>.</p> <!-- drawImage() has an equivalent
paragraph -->
<p>If the <var title="">image</var> argument is an
<code><a href="#htmlimageelement">HTMLImageElement</a></code> object that is not <a href="#img-good" title="img-good">fully decodable</a>, or if the <var title="">image</var> argument is an <code><a href="#htmlvideoelement">HTMLVideoElement</a></code>
object whose <code title="dom-media-readyState"><a href="#dom-media-readystate">readyState</a></code>
attribute is either <code title="dom-media-HAVE_NOTHING"><a href="#dom-media-have_nothing">HAVE_NOTHING</a></code> or <code title="dom-media-HAVE_METADATA"><a href="#dom-media-have_metadata">HAVE_METADATA</a></code>, then the
implementation must return null.</p> <!-- drawImage() has an
equivalent paragraph -->
<p>If the <var title="">image</var> argument is an
<code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code> object with either a horizontal
dimension or a vertical dimension equal to zero, then the
implementation must raise an <code><a href="#invalid_state_err">INVALID_STATE_ERR</a></code>
exception.</p>
<!-- drawImage() has an equivalent paragraph -->
<p>Patterns must be painted so that the top left of the first image
is anchored at the origin of the coordinate space, and images are
then repeated horizontally to the left and right (if the
<code>repeat-x</code> string was specified) or vertically up and
down (if the <code>repeat-y</code> string was specified) or in all
four directions all over the canvas (if the <code>repeat</code>
string was specified). The images are not scaled by this process;
one CSS pixel of the image must be painted on one coordinate space
unit. Of course, patterns must actually be painted only where the
stroking or filling effect requires that they be drawn, and are
affected by the current transformation matrix.</p>
<p>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. 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 wrapping the
pixel's coordinates to the original image's dimensions. (That is,
the filter uses 'repeat' behavior, regardless of the value of
<var title="">repetition</var>.)
<!-- drawImage() has a similar paragraph with different rules -->
</p><p>When the <code title="dom-context-2d-createPattern"><a href="#dom-context-2d-createpattern">createPattern()</a></code> method
is passed an animated image as its <var title="">image</var>
argument, the user agent must use the poster frame of the animation,
or, if there is no poster frame, the first frame of the
animation.</p>
<!-- drawImage() has an equivalent paragraph -->
<p>When the <var title="">image</var> argument is an
<code><a href="#htmlvideoelement">HTMLVideoElement</a></code>, then the frame at the <a href="#current-playback-position">current
playback position</a> must be used as the source image, and the
source image's dimensions must be the <a href="#concept-video-intrinsic-width" title="concept-video-intrinsic-width">intrinsic width</a> and
<a href="#concept-video-intrinsic-height" title="concept-video-intrinsic-height">intrinsic height</a>
of the <a href="#media-resource">media resource</a> (i.e. after any aspect-ratio
correction has been applied).</p>
<!-- drawImage() has an equivalent paragraph -->
<!--
Requests for v6 features:
* apply transforms to patterns, so you don't have to create
transformed patterns manually by rendering them to an off-screen
canvas then using that canvas as the pattern.
-->
</div>
<h6 id="line-styles"><span class="secno">4.8.11.1.5 </span>Line styles</h6>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-lineWidth"><a href="#dom-context-2d-linewidth">lineWidth</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current line width.</p>
<p>Can be set, to change the line width. Values that are not
finite values greater than zero are ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-lineCap"><a href="#dom-context-2d-linecap">lineCap</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current line cap style.</p>
<p>Can be set, to change the line cap style.</p>
<p>The possible line cap styles are <code>butt</code>,
<code>round</code>, and <code>square</code>. Other values are
ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current line join style.</p>
<p>Can be set, to change the line join style.</p>
<p>The possible line join styles are <code>bevel</code>,
<code>round</code>, and <code>miter</code>. Other values are
ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-miterLimit"><a href="#dom-context-2d-miterlimit">miterLimit</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current miter limit ratio.</p>
<p>Can be set, to change the miter limit ratio. Values that are
not finite values greater than zero are ignored.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-linewidth" title="dom-context-2d-lineWidth"><code>lineWidth</code></dfn>
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.</p>
<p>When the context is created, the <code title="dom-context-2d-lineWidth"><a href="#dom-context-2d-linewidth">lineWidth</a></code> attribute must
initially have the value <code>1.0</code>.</p>
<hr/><p>The <dfn id="dom-context-2d-linecap" title="dom-context-2d-lineCap"><code>lineCap</code></dfn> attribute
defines the type of endings that UAs will place on the end of
lines. The three valid values are <code>butt</code>,
<code>round</code>, and <code>square</code>. The <code>butt</code>
value means that the end of each line has a flat edge perpendicular
to the direction of the line (and that no additional line cap is
added). The <code>round</code> value means that a semi-circle with
the diameter equal to the width of the line must then be added on to
the end of the line. The <code>square</code> value means that a
rectangle with the length of the line width and the width of half
the line width, placed flat against the edge perpendicular to the
direction of the line, must be added at the end of each line.</p>
<p>On getting, it must return the current value. On setting, if the
new value is one of the literal strings <code>butt</code>,
<code>round</code>, and <code>square</code>, then the current value
must be changed to the new value; other values must ignored, leaving
the value unchanged.</p>
<p>When the context is created, the <code title="dom-context-2d-lineCap"><a href="#dom-context-2d-linecap">lineCap</a></code> attribute must
initially have the value <code>butt</code>.</p>
<hr/><p>The <dfn id="dom-context-2d-linejoin" title="dom-context-2d-lineJoin"><code>lineJoin</code></dfn>
attribute defines the type of corners that UAs will place where two
lines meet. The three valid values are <code>bevel</code>,
<code>round</code>, and <code>miter</code>.</p>
<p>On getting, it must return the current value. On setting, if the
new value is one of the literal strings <code>bevel</code>,
<code>round</code>, and <code>miter</code>, then the current value
must be changed to the new value; other values must be ignored,
leaving the value unchanged.</p>
<p>When the context is created, the <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code> attribute must
initially have the value <code>miter</code>.</p>
<hr/><p>A join exists at any point in a subpath shared by two consecutive
lines. When a subpath is closed, then a join also exists at its
first point (equivalent to its last point) connecting the first and
last lines in the subpath.</p>
<p>In addition to the point where the 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.</p>
<p>A filled triangle connecting these two opposite corners with a
straight line, with the third point of the triangle being the join
point, must be rendered at all joins. The <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code> attribute controls
whether anything else is rendered. The three aforementioned values
have the following meanings:</p>
<p>The <code>bevel</code> value means that this is all that is
rendered at joins.</p>
<p>The <code>round</code> value means that a filled 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
rendered at joins.</p>
<p>The <code>miter</code> value means that a second filled triangle
must (if it can given the miter length) be rendered 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.</p>
<p>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 to be exceeded, this second triangle must not
be rendered.</p>
<p>The miter limit ratio can be explicitly set using the <dfn id="dom-context-2d-miterlimit" title="dom-context-2d-miterLimit"><code>miterLimit</code></dfn>
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.</p>
<p>When the context is created, the <code title="dom-context-2d-miterLimit"><a href="#dom-context-2d-miterlimit">miterLimit</a></code> attribute must
initially have the value <code>10.0</code>.</p>
<!--
v6: dashed lines have been requested. Philip Taylor provides these
notes on what would need to be defined for dashed lines:
> I don't think it's entirely trivial to add, to the detail that's
> necessary in a specification. The common graphics APIs (at least
> Cairo, Quartz and java.awt.Graphics, and any SVG implementation) all
> have dashes specified by passing an array of dash lengths (alternating
> on/off), so that should be alright as long as you define what units
> it's measured in and what happens when you specify an odd number of
> values and how errors are handled and what happens if you update the
> array later. But after that, what does it do when stroking multiple
> subpaths, in terms of offsetting the dashes? When you use strokeRect,
> where is offset 0? Does moveTo reset the offset? How does it interact
> with lineCap/lineJoin? All the potential issues need test cases too,
> and the implementations need to make sure they handle any edge cases
> that the underlying graphics library does differently. (SVG Tiny 1.2
> appears to skip some of the problems by leaving things undefined and
> allowing whatever behavior the graphics library has.)
Another request has been for hairline width lines, that remain
hairline width with transform. ack Shaun Morris.
-->
</div>
<h6 id="shadows"><span class="secno">4.8.11.1.6 </span><dfn>Shadows</dfn></h6>
<p>All drawing operations are affected by the four global shadow
attributes.</p>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current shadow color.</p>
<p>Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-shadowOffsetX"><a href="#dom-context-2d-shadowoffsetx">shadowOffsetX</a></code> [ = <var title="">value</var> ]</dt>
<dt><var title="">context</var> . <code title="dom-context-2d-shadowOffsetY"><a href="#dom-context-2d-shadowoffsety">shadowOffsetY</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current shadow offset.</p>
<p>Can be set, to change the shadow offset. Values that are not finite numbers are ignored.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current level of blur applied to shadows.</p>
<p>Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-shadowcolor" title="dom-context-2d-shadowColor"><code>shadowColor</code></dfn>
attribute sets the color of the shadow.</p>
<p>When the context is created, the <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code> attribute
initially must be fully-transparent black.</p>
<p>On getting, the <a href="#serialization-of-a-color" title="serialization of a
color">serialization of the color</a> must be returned.</p>
<p>On setting, the new value must be <a href="#parsed-as-a-css-color-value">parsed as a CSS
<color> value</a> and the color 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. <a href="#refsCSSCOLOR">[CSSCOLOR]</a></p>
<p>The <dfn id="dom-context-2d-shadowoffsetx" title="dom-context-2d-shadowOffsetX"><code>shadowOffsetX</code></dfn>
and <dfn id="dom-context-2d-shadowoffsety" title="dom-context-2d-shadowOffsetY"><code>shadowOffsetY</code></dfn>
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.</p>
<p>When the context is created, the shadow offset attributes must
initially have the value <code>0</code>.</p>
<p>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.</p>
<p>The <dfn id="dom-context-2d-shadowblur" title="dom-context-2d-shadowBlur"><code>shadowBlur</code></dfn>
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.)</p>
<p>When the context is created, the <code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code> attribute must
initially have the value <code>0</code>.</p>
<p>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.</p>
<p><dfn id="when-shadows-are-drawn" title="when shadows are drawn">Shadows are only drawn
if</dfn> the opacity component of the alpha component of the color
of <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code> is
non-zero and either the <code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code> is non-zero, or
the <code title="dom-context-2d-shadowOffsetX"><a href="#dom-context-2d-shadowoffsetx">shadowOffsetX</a></code>
is non-zero, or the <code title="dom-context-2d-shadowOffsetY"><a href="#dom-context-2d-shadowoffsety">shadowOffsetY</a></code> is
non-zero.</p>
<p class="critical">It is likely that this will change: browser
vendors have indicated an interest in changing the processing model
for shadows such that they only draw when the composition operator
is "source-over" (the default). <a href="http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2011-May/thread.html#31457">Read
more...</a></p>
<p><a href="#when-shadows-are-drawn">When shadows are drawn</a>, they must be rendered as follows:</p>
<ol><li> <p>Let <var title="">A</var> be an infinite transparent black
bitmap on which the source image for which a shadow is being
created has been rendered.</p> </li>
<li> <p>Let <var title="">B</var> be an infinite transparent black
bitmap, with a coordinate space and an origin identical to <var title="">A</var>.</p> </li>
<li> <p>Copy the alpha channel of <var title="">A</var> to <var title="">B</var>, offset by <code title="dom-context-2d-shadowOffsetX"><a href="#dom-context-2d-shadowoffsetx">shadowOffsetX</a></code> in the
positive <var title="">x</var> direction, and <code title="dom-context-2d-shadowOffsetY"><a href="#dom-context-2d-shadowoffsety">shadowOffsetY</a></code> in the
positive <var title="">y</var> direction.</p> </li>
<li> <p>If <code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code> is greater than
0:</p>
<ol><li> <p>Let <var title="">σ</var> be half the value of
<code title="dom-context-2d-shadowBlur"><a href="#dom-context-2d-shadowblur">shadowBlur</a></code>.</p></li>
<li> <p>Perform a 2D Gaussian Blur on <var title="">B</var>,
using <var title="">σ</var> as the standard deviation.</p>
<!-- wish i could find a reference for this --> </li>
</ol><p>User agents may limit values of <var title="">σ</var> to
an implementation-specific maximum value to avoid exceeding
hardware limitations during the Gaussian blur operation.</p>
</li>
<li> <p>Set the red, green, and blue components of every pixel in
<var title="">B</var> to the red, green, and blue components
(respectively) of the color of <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code>.</p> </li>
<li> <p>Multiply the alpha component of every pixel in <var title="">B</var> by the alpha component of the color of <code title="dom-context-2d-shadowColor"><a href="#dom-context-2d-shadowcolor">shadowColor</a></code>.</p> </li>
<li> <p>The shadow is in the bitmap <var title="">B</var>, and is
rendered as part of the <a href="#drawing-model">drawing model</a> described below.</p> </li>
</ol></div>
<p>If the current composition operation is <code title="gcop-copy"><a href="#gcop-copy">copy</a></code>, shadows effectively won't render
(since the shape will overwrite the shadow).</p>
<h6 id="simple-shapes-(rectangles)"><span class="secno">4.8.11.1.7 </span>Simple shapes (rectangles)</h6>
<p>There are three methods that immediately draw rectangles to the
bitmap. They each take four arguments; the first two give the <var title="">x</var> and <var title="">y</var> coordinates of the top
left of the rectangle, and the second two give the width <var title="">w</var> and height <var title="">h</var> of the rectangle,
respectively.</p>
<div class="impl">
<p>The <a href="#transformations" title="dom-context-2d-transformation">current
transformation matrix</a> must be applied to the following four
coordinates, which form the path that must then be closed to get the
specified rectangle: <span title="">(<var title="">x</var>, <var title="">y</var>)</span>, <span title="">(<span title=""><var title="">x</var>+<var title="">w</var></span>, <var title="">y</var>)</span>,
<span title="">(<span title=""><var title="">x</var>+<var title="">w</var></span>,
<span title=""><var title="">y</var>+<var title="">h</var></span>)</span>,
<span title="">(<var title="">x</var>, <span title=""><var title="">y</var>+<var title="">h</var></span>)</span>.</p>
<p>Shapes are painted without affecting the current path, and are
subject to the <a href="#clipping-region" title="clipping region">clipping region</a>,
and, with the exception of <code title="dom-context-2d-clearRect"><a href="#dom-context-2d-clearrect">clearRect()</a></code>, also <a href="#shadows" title="shadows">shadow effects</a>, <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, and <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>.</p>
</div>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-clearRect"><a href="#dom-context-2d-clearrect">clearRect</a></code>(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</dt>
<dd>
<p>Clears all pixels on the canvas in the given rectangle to transparent black.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-fillRect"><a href="#dom-context-2d-fillrect">fillRect</a></code>(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</dt>
<dd>
<p>Paints the given rectangle onto the canvas, using the current fill style.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-strokeRect"><a href="#dom-context-2d-strokerect">strokeRect</a></code>(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</dt>
<dd>
<p>Paints the box that outlines the given rectangle onto the canvas, using the current stroke style.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-clearrect" title="dom-context-2d-clearRect"><code>clearRect(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</code></dfn> method must clear the pixels in the
specified rectangle that also intersect the current clipping region
to a fully transparent black, erasing any previous image. If either
height or width are zero, this method has no effect.</p>
<p>The <dfn id="dom-context-2d-fillrect" title="dom-context-2d-fillRect"><code>fillRect(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</code></dfn> method must paint the specified
rectangular area using the <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code>. If either height
or width are zero, this method has no effect.</p>
<p>The <dfn id="dom-context-2d-strokerect" title="dom-context-2d-strokeRect"><code>strokeRect(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</code></dfn> method must stroke the specified
rectangle's path using the <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code>, <code title="dom-context-2d-lineWidth"><a href="#dom-context-2d-linewidth">lineWidth</a></code>, <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code>, and (if
appropriate) <code title="dom-context-2d-miterLimit"><a href="#dom-context-2d-miterlimit">miterLimit</a></code> attributes. If
both height and width are zero, this method has no effect, since
there is no path to stroke (it's a point). If only one of the two is
zero, then the method will draw a line instead (the path for the
outline is just a straight line along the non-zero dimension).</p>
</div>
<h6 id="complex-shapes-(paths)"><span class="secno">4.8.11.1.8 </span>Complex shapes (paths)</h6>
<p>The context always has a current path. There is only one current
path, it is not part of the <a href="#drawing-state">drawing state</a>.</p>
<p>A <dfn id="path">path</dfn> has a list of zero or more subpaths. Each
subpath consists of a list of one or more points, connected by
straight or curved lines, 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 fewer than two points are ignored when
painting the path.</p>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-beginPath"><a href="#dom-context-2d-beginpath">beginPath</a></code>()</dt>
<dd>
<p>Resets the current path.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-moveTo"><a href="#dom-context-2d-moveto">moveTo</a></code>(<var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Creates a new subpath with the given point.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-closePath"><a href="#dom-context-2d-closepath">closePath</a></code>()</dt>
<dd>
<p>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.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-lineTo"><a href="#dom-context-2d-lineto">lineTo</a></code>(<var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Adds the given point to the current subpath, connected to the previous one by a straight line.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-quadraticCurveTo"><a href="#dom-context-2d-quadraticcurveto">quadraticCurveTo</a></code>(<var title="">cpx</var>, <var title="">cpy</var>, <var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-bezierCurveTo"><a href="#dom-context-2d-beziercurveto">bezierCurveTo</a></code>(<var title="">cp1x</var>, <var title="">cp1y</var>, <var title="">cp2x</var>, <var title="">cp2y</var>, <var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-arcTo"><a href="#dom-context-2d-arcto">arcTo</a></code>(<var title="">x1</var>, <var title="">y1</var>, <var title="">x2</var>, <var title="">y2</var>, <var title="">radius</var>)</dt>
<dd>
<p>Adds an arc with the given control points and radius to the
current subpath, connected to the previous point by a straight
line.</p>
<p>Throws an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception if the given
radius is negative.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-arc"><a href="#dom-context-2d-arc">arc</a></code>(<var title="">x</var>, <var title="">y</var>, <var title="">radius</var>, <var title="">startAngle</var>, <var title="">endAngle</var> [, <var title="">anticlockwise</var> ] )</dt>
<dd>
<p>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.</p>
<p>Throws an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception if the given
radius is negative.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-rect"><a href="#dom-context-2d-rect">rect</a></code>(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</dt>
<dd>
<p>Adds a new closed subpath to the path, representing the given rectangle.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-fill"><a href="#dom-context-2d-fill">fill</a></code>()</dt>
<dd>
<p>Fills the subpaths with the current fill style.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-stroke"><a href="#dom-context-2d-stroke">stroke</a></code>()</dt>
<dd>
<p>Strokes the subpaths with the current stroke style.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-drawSystemFocusRing"><a href="#dom-context-2d-drawsystemfocusring">drawSystemFocusRing</a></code>(<var title="">element</var>)</dt>
<dd>
<p>If the given element is focused, draws a focus ring around the
current path, following the platform conventions for focus
rings.</p>
</dd>
<dt><var title="">shouldDraw</var> = <var title="">context</var> . <code title="dom-context-2d-drawCustomFocusRing"><a href="#dom-context-2d-drawcustomfocusring">drawCustomFocusRing</a></code>(<var title="">element</var>)</dt>
<dd>
<p>If the given element is focused, and the user has configured
his system to draw focus rings in a particular manner (for
example, high contrast focus rings), draws a focus ring around the
current path and returns false.</p>
<p>Otherwise, returns true if the given element is focused, and
false otherwise. This can thus be used to determine when to draw a
focus ring (see <a href="#drawCustomFocusRingExample">the
example</a> below).</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-scrollPathIntoView"><a href="#dom-context-2d-scrollpathintoview">scrollPathIntoView</a></code>()</dt>
<dd>
<p>Scrolls the current path into view. This is especially useful
on devices with small screens, where the whole canvas might not be
visible at once.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-clip"><a href="#dom-context-2d-clip">clip</a></code>()</dt>
<dd>
<p>Further constrains the clipping region to the given path.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-isPointInPath"><a href="#dom-context-2d-ispointinpath">isPointInPath</a></code>(<var title="">x</var>, <var title="">y</var>)</dt>
<dd>
<p>Returns true if the given point is in the current path.</p>
</dd>
</dl><div class="impl">
<p>Initially, the context's path must have zero subpaths.</p>
<p>The points and lines added to the path by these methods must be
transformed according to the <a href="#transformations" title="dom-context-2d-transformation">current transformation
matrix</a> as they are added.</p>
<p>The <dfn id="dom-context-2d-beginpath" title="dom-context-2d-beginPath"><code>beginPath()</code></dfn>
method must empty the list of subpaths so that the context once
again has zero subpaths.</p>
<p>The <dfn id="dom-context-2d-moveto" title="dom-context-2d-moveTo"><code>moveTo(<var title="">x</var>, <var title="">y</var>)</code></dfn> method must
create a new subpath with the specified point as its first (and
only) point.</p>
<p>When the user agent is to <dfn id="ensure-there-is-a-subpath">ensure there is a subpath</dfn>
for a coordinate (<var title="">x</var>, <var title="">y</var>), the
user agent must check to see if the context has any subpaths, and if
it does not, then the user agent must create a new subpath with the
point (<var title="">x</var>, <var title="">y</var>) as its first
(and only) point, as if the <code title="dom-context-2d-moveTo"><a href="#dom-context-2d-moveto">moveTo()</a></code> method had been
called.</p>
<p>The <dfn id="dom-context-2d-closepath" title="dom-context-2d-closePath"><code>closePath()</code></dfn>
method must do nothing if the context 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.</p>
<p class="note">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) <code title="dom-context-2d-moveTo"><a href="#dom-context-2d-moveto">moveTo()</a></code> call.</p>
<p>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 context's paths.</p>
<p>The <dfn id="dom-context-2d-lineto" title="dom-context-2d-lineTo"><code>lineTo(<var title="">x</var>, <var title="">y</var>)</code></dfn> method must
<a href="#ensure-there-is-a-subpath">ensure there is a subpath</a> for <span title="">(<var title="">x</var>, <var title="">y</var>)</span> if the context has
no subpaths. Otherwise, it must connect the last point in the
subpath to the given point (<var title="">x</var>, <var title="">y</var>) using a straight line, and must then add the given
point (<var title="">x</var>, <var title="">y</var>) to the
subpath.</p>
<p>The <dfn id="dom-context-2d-quadraticcurveto" title="dom-context-2d-quadraticCurveTo"><code>quadraticCurveTo(<var title="">cpx</var>, <var title="">cpy</var>, <var title="">x</var>,
<var title="">y</var>)</code></dfn> method must <a href="#ensure-there-is-a-subpath">ensure there
is a subpath</a> for <span title="">(<var title="">cpx</var>,
<var title="">cpy</var>)</span>, and then must connect the last
point in the subpath to the given point (<var title="">x</var>, <var title="">y</var>) using a quadratic Bézier curve with control
point (<var title="">cpx</var>, <var title="">cpy</var>), and must
then add the given point (<var title="">x</var>, <var title="">y</var>) to the subpath. <a href="#refsBEZIER">[BEZIER]</a></p>
<p>The <dfn id="dom-context-2d-beziercurveto" title="dom-context-2d-bezierCurveTo"><code>bezierCurveTo(<var title="">cp1x</var>, <var title="">cp1y</var>, <var title="">cp2x</var>, <var title="">cp2y</var>, <var title="">x</var>, <var title="">y</var>)</code></dfn> method must
<a href="#ensure-there-is-a-subpath">ensure there is a subpath</a> for <span title="">(<var title="">cp1x</var>, <var title="">cp1y</var>)</span>, and then must
connect the last point in the subpath to the given point (<var title="">x</var>, <var title="">y</var>) using a cubic Bézier
curve with control points (<var title="">cp1x</var>, <var title="">cp1y</var>) and (<var title="">cp2x</var>, <var title="">cp2y</var>). Then, it must add the point (<var title="">x</var>, <var title="">y</var>) to the subpath. <a href="#refsBEZIER">[BEZIER]</a></p>
<hr/><p>The <dfn id="dom-context-2d-arcto" title="dom-context-2d-arcTo"><code>arcTo(<var title="">x1</var>, <var title="">y1</var>, <var title="">x2</var>,
<var title="">y2</var>, <var title="">radius</var>)</code></dfn>
method must first <a href="#ensure-there-is-a-subpath">ensure there is a subpath</a> for <span title="">(<var title="">x1</var>, <var title="">y1</var>)</span>. Then, the behavior depends on the
arguments and the last point in the subpath, as described below.</p>
<p>Negative values for <var title="">radius</var> must cause the
implementation to raise an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code>
exception.</p>
<p>Let the point (<var title="">x0</var>, <var title="">y0</var>) be
the last point in the subpath.</p>
<p>If the point (<var title="">x0</var>, <var title="">y0</var>) is
equal to the point (<var title="">x1</var>, <var title="">y1</var>),
or if the point (<var title="">x1</var>, <var title="">y1</var>) is
equal to the point (<var title="">x2</var>, <var title="">y2</var>),
or if the radius <var title="">radius</var> is zero, then the method
must add the point (<var title="">x1</var>, <var title="">y1</var>)
to the subpath, and connect that point to the previous point (<var title="">x0</var>, <var title="">y0</var>) by a straight line.</p>
<p>Otherwise, if the points (<var title="">x0</var>, <var title="">y0</var>), (<var title="">x1</var>, <var title="">y1</var>), and (<var title="">x2</var>, <var title="">y2</var>) all lie on a single straight line, then the
method must add the point (<var title="">x1</var>, <var title="">y1</var>) to the subpath, and connect that point to the
previous point (<var title="">x0</var>, <var title="">y0</var>) by a
straight line.</p>
<p>Otherwise, let <var title="">The Arc</var> be the shortest arc
given by circumference of the circle that has radius <var title="">radius</var>, and that has one point tangent to the
half-infinite line that crosses the point (<var title="">x0</var>,
<var title="">y0</var>) and ends at the point (<var title="">x1</var>, <var title="">y1</var>), and that has a different
point tangent to the half-infinite line that ends at the point (<var title="">x1</var>, <var title="">y1</var>) and crosses the point
(<var title="">x2</var>, <var title="">y2</var>). The points at
which this circle touches these two lines are called the start and
end tangent points respectively. The method must connect the point
(<var title="">x0</var>, <var title="">y0</var>) 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 <var title="">The Arc</var>, adding the end
tangent point to the subpath.</p>
<hr/><p>The <dfn id="dom-context-2d-arc" title="dom-context-2d-arc"><code>arc(<var title="">x</var>, <var title="">y</var>, <var title="">radius</var>,
<var title="">startAngle</var>, <var title="">endAngle</var>, <var title="">anticlockwise</var>)</code></dfn> method draws an arc. If
the context 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. In any case, it must draw the arc between the start point of
the arc and the end point of the arc, and add the start and end
points of the arc to the subpath. The arc and its start and end
points are defined as follows:</p>
<p>Consider a circle that has its origin at (<var title="">x</var>,
<var title="">y</var>) and that has radius <var title="">radius</var>. The points at <var title="">startAngle</var>
and <var title="">endAngle</var> along this circle's circumference,
measured in radians clockwise from the positive x-axis, are the
start and end points respectively.</p>
<p>If the <var title="">anticlockwise</var> argument is omitted or
false and <span title=""><var title="">endAngle</var>-<var title="">startAngle</var></span> is equal to or greater than <span title="">2π</span>, or, if the <var title="">anticlockwise</var>
argument is <em>true</em> and <span title=""><var title="">startAngle</var>-<var title="">endAngle</var></span> is
equal to or greater than <span title="">2π</span>, then the arc
is the whole circumference of this circle.</p>
<p>Otherwise, the arc is the path along the circumference of this
circle from the start point to the end point, going anti-clockwise
if the <var title="">anticlockwise</var> argument is true, and
clockwise otherwise. Since the points are on the circle, as opposed
to being simply angles from zero, the arc can never cover an angle
greater than <span title="">2π</span> radians. If the two points are the
same, or if the radius is zero, then the arc is defined as being of
zero length in both directions.</p>
<p>Negative values for <var title="">radius</var> must cause the
implementation to raise an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code>
exception.</p>
<hr/><p>The <dfn id="dom-context-2d-rect" title="dom-context-2d-rect"><code>rect(<var title="">x</var>, <var title="">y</var>, <var title="">w</var>, <var title="">h</var>)</code></dfn> method must create a new subpath
containing just the four points (<var title="">x</var>, <var title="">y</var>), (<var title="">x</var>+<var title="">w</var>,
<var title="">y</var>), (<var title="">x</var>+<var title="">w</var>, <var title="">y</var>+<var title="">h</var>),
(<var title="">x</var>, <var title="">y</var>+<var title="">h</var>), 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 (<var title="">x</var>, <var title="">y</var>) as the only point in the subpath.</p>
<!-- v6 feature request:
* points as a primitive shape
http://home.comcast.net/~urbanjost/canvas/vogle4.html
-->
<p>The <dfn id="dom-context-2d-fill" title="dom-context-2d-fill"><code>fill()</code></dfn>
method must fill all the subpaths of the current path, using
<code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code>, and using
the non-zero winding number rule. Open subpaths must be implicitly
closed when being filled (without affecting the actual
subpaths).</p>
<p class="note">Thus, if two overlapping but otherwise independent
subpaths have opposite windings, they cancel out and result in no
fill. If they have the same winding, that area just gets painted
once.</p>
<p>The <dfn id="dom-context-2d-stroke" title="dom-context-2d-stroke"><code>stroke()</code></dfn> method
must calculate the strokes of all the subpaths of the current path,
using the <code title="dom-context-2d-lineWidth"><a href="#dom-context-2d-linewidth">lineWidth</a></code>,
<code title="dom-context-2d-lineCap"><a href="#dom-context-2d-linecap">lineCap</a></code>, <code title="dom-context-2d-lineJoin"><a href="#dom-context-2d-linejoin">lineJoin</a></code>, and (if
appropriate) <code title="dom-context-2d-miterLimit"><a href="#dom-context-2d-miterlimit">miterLimit</a></code> attributes, and
then fill the combined stroke area using the <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code>
attribute.</p>
<p class="note">Since the subpaths are all stroked as one,
overlapping parts of the paths in one stroke operation are treated
as if their union was what was painted.</p>
<p>Paths, when filled or stroked, must be painted without affecting
the current path, and must be subject to <a href="#shadows" title="shadows">shadow effects</a>, <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, the <a href="#clipping-region" title="clipping region">clipping region</a>, and <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>. (Transformations affect the path when the path is
created, not when it is painted, though the stroke <em>style</em> is
still affected by the transformation during painting.)</p>
<p>Zero-length line segments must be pruned before stroking a
path. Empty subpaths must be ignored.</p>
<hr/><p id="dom-context-2d-drawosfocusring">The <dfn id="dom-context-2d-drawsystemfocusring" title="dom-context-2d-drawSystemFocusRing"><code>drawSystemFocusRing(<var title="">element</var>)</code></dfn> method, when invoked, must run
the following steps:</p>
<ol><li><p>If <var title="">element</var> is not focused or is not a
descendant of the element with whose context the method is
associated, then abort these steps.</p></li>
<li>
<p>If the user has requested the use of particular focus rings
(e.g. high-contrast focus rings), or if the <var title="">element</var> would have a focus ring drawn around it,
then draw a focus ring of the appropriate style along the path,
following platform conventions, and abort these steps.</p>
<p class="note">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 <code title="dom-focus"><a href="#dom-focus">focus()</a></code> method
based on the kind of user interaction event from which the call
was triggered (if any).</p>
<p>The focus ring should not be subject to the <a href="#shadows" title="shadows">shadow effects</a>, the <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, or the <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>, but <em>should</em> be subject to the <a href="#clipping-region" title="clipping region">clipping region</a>.</p>
</li>
<li>
<p>Optionally, <a href="#inform">inform the user</a> that the
focus is at the location given by the path. User agents may wait
until the next time the <a href="#event-loop">event loop</a> reaches its
"update the rendering" step to optionally inform the user.</p>
</li>
</ol><p>The <dfn id="dom-context-2d-drawcustomfocusring" title="dom-context-2d-drawCustomFocusRing"><code>drawCustomFocusRing(<var title="">element</var>)</code></dfn> method, when invoked, must run
the following steps:</p>
<ol><li><p>If <var title="">element</var> is not focused or is not a
descendant of the element with whose context the method is
associated, then return false and abort these steps.</p></li>
<li>
<p>If the user has requested the use of particular focus rings
(e.g. high-contrast focus rings), then draw a focus ring of the
appropriate style along the path, return false, and abort these
steps.</p>
<p>The focus ring should not be subject to the <a href="#shadows" title="shadows">shadow effects</a>, the <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, or the <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>, but <em>should</em> be subject to the <a href="#clipping-region" title="clipping region">clipping region</a>.</p>
</li>
<li>
<p>Optionally, <a href="#inform">inform the user</a> that the
focus is at the location given by the path. User agents may wait
until the next time the <a href="#event-loop">event loop</a> reaches its
"update the rendering" step to optionally inform the user.</p>
</li>
<li><p>Return true.</p></li>
</ol><p>The <dfn id="dom-context-2d-scrollpathintoview" title="dom-context-2d-scrollPathIntoView"><code>scrollPathIntoView()</code></dfn>
method, when invoked, must run the following steps:</p>
<ol><li><p>Let <var title="">notional child</var> be a hypothetical
element that is a rendered child of the <code><a href="#the-canvas-element">canvas</a></code> element
whose dimensions are exactly the rectangle of the bounding box of
the current path.</p></li>
<li><p><span title="scroll an element into view">Scroll <var title="">notional child</var> into view</span> with the <var title="">align to top flag</var> set.</p>
</li><li><p>Optionally, <a href="#inform">inform the user</a> that the
caret and/or selection cover <var title="">the specified
rectangle</var> of the canvas. User agents may wait until the next
time the <a href="#event-loop">event loop</a> reaches its "update the rendering"
step to optionally inform the user.</p></li>
</ol><p class="note" id="inform">"Inform the user", as used in this
section, could mean calling a system accessibility API, which would
notify assistive technologies such as magnification tools. To
properly drive magnification based on a focus change, a system
accessibility API driving a screen magnifier needs the bounds for
the newly focused object. The methods above are intended to enable
this by allowing the user agent to report the bounding box of the
path used to render the focus ring as the bounds of the <var title="">element</var> element passed as an argument, if that
element is focused, and the bounding box of the area to which the
user agent is scrolling as the bounding box of the current
selection.</p>
<hr/><p>The <dfn id="dom-context-2d-clip" title="dom-context-2d-clip"><code>clip()</code></dfn>
method must create a new <dfn id="clipping-region">clipping region</dfn> by calculating
the intersection of the current clipping region and the area
described by the current path, using the non-zero winding number
rule. 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.</p>
<p>When the context is initialized, the clipping region must be set
to the rectangle with the top left corner at (0,0) and the width and
height of the coordinate space.</p>
<!-- v6
Jordan OSETE suggests:
* support ways of extending the clipping region (union instead of intersection)
- also "add", "subtract", "replace", "intersect" and "xor"
* support ways of resetting the clipping region without save/restore
-->
<hr/><p>The <dfn id="dom-context-2d-ispointinpath" title="dom-context-2d-isPointInPath"><code>isPointInPath(<var title="">x</var>, <var title="">y</var>)</code></dfn> method must
return true if the point given by the <var title="">x</var> and <var title="">y</var> coordinates passed to the method, when treated as
coordinates in the canvas coordinate space unaffected by the current
transformation, is inside the current path as determined by the
non-zero winding number rule; and must return false
otherwise. Points on the path itself are considered to be inside the
path. If either of the arguments is infinite or NaN, then the method
must return false.</p>
</div>
<div class="example" id="drawCustomFocusRingExample">
<p>This <code><a href="#the-canvas-element">canvas</a></code> element has a couple of checkboxes. The
path-related commands are highlighted:</p>
<pre><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) {
<strong> context.beginPath();
context.strokeStyle = 'black';
context.rect(x-5, y-5, 10, 10);
context.stroke();
</strong> if (element.checked) {
<strong> context.fillStyle = 'black';
context.fill();
</strong> }
context.fillText(element.labels[0].textContent, x+5, y);
}
<strong> context.beginPath();
context.rect(x-7, y-7, 12 + metrics.width+2, 14);
if (paint && context.drawCustomFocusRing(element)) {
context.strokeStyle = 'silver';
context.stroke();
}
</strong> 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;
while (node) {
x -= node.offsetLeft - node.scrollLeft;
y -= node.offsetTop - node.scrollTop;
node = node.offsetParent;
}
drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
if (<strong>context.isPointInPath(x, y)</strong>)
document.getElementById('showA').checked = !(document.getElementById('showA').checked);
drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
if (<strong>context.isPointInPath(x, y)</strong>)
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></pre>
<!-- http://software.hixie.ch/utilities/js/live-dom-viewer/saved/340 -->
</div>
<h6 id="text-0"><span class="secno">4.8.11.1.9 </span>Text</h6> <!-- a v3 feature -->
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current font settings.</p>
<p>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.</p>
<p>Relative keywords and lengths are computed relative to the font
of the <code><a href="#the-canvas-element">canvas</a></code> element.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current text alignment settings.</p>
<p>Can be set, to change the alignment. The possible values are
<code title="">start</code>, <code title="">end</code>, <code title="">left</code>, <code title="">right</code>, and <code title="">center</code>. Other values are ignored. The default is
<code title="">start</code>.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> [ = <var title="">value</var> ]</dt>
<dd>
<p>Returns the current baseline alignment settings.</p>
<p>Can be set, to change the baseline alignment. The possible
values and their meanings are given below. Other values are
ignored. The default is <code title="">alphabetic</code>.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-fillText"><a href="#dom-context-2d-filltext">fillText</a></code>(<var title="">text</var>, <var title="">x</var>, <var title="">y</var> [, <var title="">maxWidth</var> ] )</dt>
<dt><var title="">context</var> . <code title="dom-context-2d-strokeText"><a href="#dom-context-2d-stroketext">strokeText</a></code>(<var title="">text</var>, <var title="">x</var>, <var title="">y</var> [, <var title="">maxWidth</var> ] )</dt>
<dd>
<p>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.</p>
</dd>
<dt><var title="">metrics</var> = <var title="">context</var> . <code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText</a></code>(<var title="">text</var>)</dt>
<dd>
<p>Returns a <code><a href="#textmetrics">TextMetrics</a></code> object with the metrics of the given text in the current font.</p>
</dd>
<dt><var title="">metrics</var> . <code title="dom-textmetrics-width"><a href="#dom-textmetrics-width">width</a></code></dt>
<dd>
<p>Returns the advance width of the text that was passed to the
<code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText()</a></code>
method.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-font" title="dom-context-2d-font"><code>font</code></dfn> IDL
attribute, on setting, must be parsed the same way as the 'font'
property of CSS (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. <a href="#refsCSS">[CSS]</a></p>
<p>Font names must be interpreted in the context of the
<code><a href="#the-canvas-element">canvas</a></code> element's stylesheets; any fonts embedded using
<code title="">@font-face</code> must therefore be available once
they are loaded. (If a font is referenced before it is fully loaded,
then it must be treated as if it was an unknown font, falling back
to another as described by the relevant CSS specifications.) <a href="#refsCSSFONTS">[CSSFONTS]</a></p>
<p>Only vector fonts should be used by the user agent; if a user
agent were to use bitmap fonts then transformations would likely
make the font look very ugly.</p>
<p>On getting, the <code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code>
attribute must return the <span title="serializing a CSS
value">serialized form</span> of the current font of the context
(with no 'line-height' component). <a href="#refsCSSOM">[CSSOM]</a></p>
<div class="example">
<p>For example, after the following statement:</p>
<pre>context.font = 'italic 400 12px/2 Unknown Font, sans-serif';</pre>
<p>...the expression <code title="">context.font</code> would
evaluate to the string "<code title="">italic 12px "Unknown Font", sans-serif</code>". 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.</p>
</div>
<p>When the context 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 corresponding <code><a href="#the-canvas-element">canvas</a></code>
element at the time that the attribute is set. 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 corresponding
<code><a href="#the-canvas-element">canvas</a></code> element at the time that the attribute is
set. If the computed values are undefined for a particular case
(e.g. because the <code><a href="#the-canvas-element">canvas</a></code> element is not <a href="#in-a-document">in a
<code>Document</code></a>), then the relative keywords must be
interpreted relative to the normal-weight 10px sans-serif
default.</p>
<p>The <dfn id="dom-context-2d-textalign" title="dom-context-2d-textAlign"><code>textAlign</code></dfn> IDL
attribute, on getting, must return the current value. On setting, if
the value is one of <code title="">start</code>, <code title="">end</code>, <code title="">left</code>, <code title="">right</code>, or <code title="">center</code>, then the
value must be changed to the new value. Otherwise, the new value
must be ignored. When the context is created, the <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> attribute must
initially have the value <code title="">start</code>.</p>
<p>The <dfn id="dom-context-2d-textbaseline" title="dom-context-2d-textBaseline"><code>textBaseline</code></dfn>
IDL attribute, on getting, must return the current value. On
setting, if the value is one of <code title="dom-context-2d-textBaseline-top"><a href="#dom-context-2d-textbaseline-top">top</a></code>, <code title="dom-context-2d-textBaseline-hanging"><a href="#dom-context-2d-textbaseline-hanging">hanging</a></code>, <code title="dom-context-2d-textBaseline-middle"><a href="#dom-context-2d-textbaseline-middle">middle</a></code>, <code title="dom-context-2d-textBaseline-alphabetic"><a href="#dom-context-2d-textbaseline-alphabetic">alphabetic</a></code>,
<code title="dom-context-2d-textBaseline-ideographic"><a href="#dom-context-2d-textbaseline-ideographic">ideographic</a></code>,
or <code title="dom-context-2d-textBaseline-bottom"><a href="#dom-context-2d-textbaseline-bottom">bottom</a></code>,
then the value must be changed to the new value. Otherwise, the new
value must be ignored. When the context is created, the <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> attribute
must initially have the value <code title="">alphabetic</code>.</p>
</div>
<p>The <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code>
attribute's allowed keywords correspond to alignment points in the
font:</p>
<p><img alt="The top of the em square is roughly at the top of the glyphs in a font, the hanging baseline is where some glyphs like आ are anchored, the middle is half-way between the top of the em square and the bottom of the em square, the alphabetic baseline is where characters like Á, ÿ, f, and Ω are anchored, the ideographic baseline is where glyphs like 私 and 達 are anchored, and the bottom of the em square is roughly at the bottom of the glyphs in a font. The top and bottom of the bounding box can be far from these baselines, due to glyphs extending far outside the em square." height="300" src="http://www.whatwg.org/specs/web-apps/current-work/http://images.whatwg.org/baselines.png" width="738"/></p>
<p>The keywords map to these alignment points as follows:</p>
<dl><dt><dfn id="dom-context-2d-textbaseline-top" title="dom-context-2d-textBaseline-top"><code>top</code></dfn>
</dt><dd>The top of the em square</dd>
<dt><dfn id="dom-context-2d-textbaseline-hanging" title="dom-context-2d-textBaseline-hanging"><code>hanging</code></dfn>
</dt><dd>The hanging baseline</dd>
<dt><dfn id="dom-context-2d-textbaseline-middle" title="dom-context-2d-textBaseline-middle"><code>middle</code></dfn>
</dt><dd>The middle of the em square</dd>
<dt><dfn id="dom-context-2d-textbaseline-alphabetic" title="dom-context-2d-textBaseline-alphabetic"><code>alphabetic</code></dfn>
</dt><dd>The alphabetic baseline</dd>
<dt><dfn id="dom-context-2d-textbaseline-ideographic" title="dom-context-2d-textBaseline-ideographic"><code>ideographic</code></dfn>
</dt><dd>The ideographic baseline</dd>
<dt><dfn id="dom-context-2d-textbaseline-bottom" title="dom-context-2d-textBaseline-bottom"><code>bottom</code></dfn>
</dt><dd>The bottom of the em square</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-filltext" title="dom-context-2d-fillText"><code>fillText()</code></dfn> and
<dfn id="dom-context-2d-stroketext" title="dom-context-2d-strokeText"><code>strokeText()</code></dfn>
methods take three or four arguments, <var title="">text</var>, <var title="">x</var>, <var title="">y</var>, and optionally <var title="">maxWidth</var>, and render the given <var title="">text</var> at the given (<var title="">x</var>, <var title="">y</var>) coordinates ensuring that the text isn't wider
than <var title="">maxWidth</var> if specified, using the current
<code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code>, <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code>, and <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code>
values. Specifically, when the methods are called, the user agent
must run the following steps:</p>
<ol><li><p>If <var title="">maxWidth</var> is present but less than or
equal to zero, return without doing anything; abort these
steps.</p></li>
<li><p>Let <var title="">font</var> be the current font of the
context, as given by the <code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code> attribute.</p></li>
<li><p>Replace all the <a href="#space-character" title="space character">space
characters</a> in <var title="">text</var> with U+0020 SPACE
characters.</p></li>
<li><p>Form a hypothetical infinitely wide CSS line box containing
a single inline box containing the text <var title="">text</var>,
with all the properties at their initial values except the 'font'
property of the inline box set to <var title="">font</var>, the
'direction' property of the inline box set to <a href="#the-directionality">the
directionality</a> of the <code><a href="#the-canvas-element">canvas</a></code> element, and the
'white-space' property set to 'pre'. <a href="#refsCSS">[CSS]</a></p></li>
<!-- if you insert a step here, make sure to adjust the next step's
final words -->
<li><p>If the <var title="">maxWidth</var> argument was specified
and the hypothetical width of the inline box in the hypothetical
line box is greater than <var title="">maxWidth</var> CSS pixels,
then change <var title="">font</var> 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.</p></li>
<li>
<p>Let the <var title="">anchor point</var> be a point on the
inline box, determined by the <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> and <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> values, as
follows:</p>
<p>Horizontal position:</p>
<dl><dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">left</code></dt>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">start</code> and <a href="#the-directionality">the directionality</a> of the
<code><a href="#the-canvas-element">canvas</a></code> element is 'ltr'</dt>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">end</code> and <a href="#the-directionality">the directionality</a> of the
<code><a href="#the-canvas-element">canvas</a></code> element is 'rtl'</dt>
<dd>Let the <var title="">anchor point</var>'s horizontal
position be the left edge of the inline box.</dd>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">right</code></dt>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">end</code> and <a href="#the-directionality">the directionality</a> of the
<code><a href="#the-canvas-element">canvas</a></code> element is 'ltr'</dt>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">start</code> and <a href="#the-directionality">the directionality</a> of the
<code><a href="#the-canvas-element">canvas</a></code> element is 'rtl'</dt>
<dd>Let the <var title="">anchor point</var>'s horizontal
position be the right edge of the inline box.</dd>
<dt> If <code title="dom-context-2d-textAlign"><a href="#dom-context-2d-textalign">textAlign</a></code> is <code title="">center</code></dt>
<dd>Let the <var title="">anchor point</var>'s horizontal
position be half way between the left and right edges of the
inline box.</dd>
</dl><p>Vertical position:</p>
<dl><dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-top"><a href="#dom-context-2d-textbaseline-top">top</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be the top of the em box of the first available font of the
inline box.</dd>
<dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-hanging"><a href="#dom-context-2d-textbaseline-hanging">hanging</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be the hanging baseline of the first available font of the inline
box.</dd>
<dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-middle"><a href="#dom-context-2d-textbaseline-middle">middle</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be half way between the bottom and the top of the em box of the
first available font of the inline box.</dd>
<dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-alphabetic"><a href="#dom-context-2d-textbaseline-alphabetic">alphabetic</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be the alphabetic baseline of the first available font of the inline
box.</dd>
<dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-ideographic"><a href="#dom-context-2d-textbaseline-ideographic">ideographic</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be the ideographic baseline of the first available font of the inline
box.</dd>
<dt> If <code title="dom-context-2d-textBaseline"><a href="#dom-context-2d-textbaseline">textBaseline</a></code> is <code title="dom-context-2d-textBaseline-bottom"><a href="#dom-context-2d-textbaseline-bottom">bottom</a></code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be the bottom of the em box of the first available font of the
inline box.</dd>
</dl></li>
<li>
<p>Paint the hypothetical inline box as the shape given by the
text's glyphs, as transformed by the <a href="#transformations" title="dom-context-2d-transformation">current transformation
matrix</a>, and anchored and sized so that before applying the
<a href="#transformations" title="dom-context-2d-transformation">current transformation
matrix</a>, the <var title="">anchor point</var> is at (<var title="">x</var>, <var title="">y</var>) and each CSS pixel is
mapped to one coordinate space unit.</p>
<p>For <code title="dom-context-2d-fillText"><a href="#dom-context-2d-filltext">fillText()</a></code>
<code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> must be
applied to the glyphs and <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> must be
ignored. For <code title="dom-context-2d-strokeText"><a href="#dom-context-2d-stroketext">strokeText()</a></code> the reverse
holds and <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> must be
applied to the glyph outlines and <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> must be
ignored.</p>
<p>Text is painted without affecting the current path, and is
subject to <a href="#shadows" title="shadows">shadow effects</a>, <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, the <a href="#clipping-region" title="clipping region">clipping region</a>, and <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>.</p>
</li>
</ol><!--v6DVT - this is commented out until CSS can get its act together
enough to actual specify vertical text rendering (how long have we
been waiting now?)
WHEN EDITING THIS, FIX THE PARTS MARKED "XXX" BELOW
<p>The <dfn
title="dom-context-2d-fillVerticalText"><code>fillVerticalText()</code></dfn>
and <dfn
title="dom-context-2d-strokeVerticalText"><code>strokeVerticalText()</code></dfn>
methods take three or four arguments, <var title="">text</var>, <var
title="">x</var>, <var title="">y</var>, and optionally <var
title="">maxHeight</var>, and render the given <var
title="">text</var> as vertical text at the given (<var
title="">x</var>, <var title="">y</var>) coordinates ensuring that
the text isn't taller than <var title="">maxHeight</var> if
specified, using the current <code
title="dom-context-2d-font">font</code> and <code
title="dom-context-2d-textAlign">textAlign</code>
values. Specifically, when the methods are called, the user agent
must run the following steps:</p>
<ol>
<li><p>If <var title="">maxHeight</var> is present but less than or
equal to zero, return without doing anything; abort these
steps.</p></li>
<li><p>Let <var title="">font</var> be the current font of the
context, as given by the <code
title="dom-context-2d-font">font</code> attribute.</p></li>
<li><p>Replace all the <span title="space character">space
characters</span> in <var title="">text</var> with U+0020 SPACE
characters.</p></li>
<li><p>Form a <em class="XXX">whatever CSS ends up calling
vertical line boxes and inline boxes</em> containing the text <var
title="">text</var>, with all the properties at their initial
values except the 'font' property of the inline box set to <var
title="">font</var> and the 'direction' property of the inline
box set to <span>the directionality</span> of the <code>canvas</code>
element.</p></li>
<!- - if you insert a step here, make sure to adjust the next step's
final words - ->
<li><p>If the <var title="">maxHeight</var> argument was specified
and the hypothetical height of the <em class="XXX">box</em>
in the hypothetical line box is greater than <var
title="">maxHeight</var> CSS pixels, then change <var
title="">font</var> to have a more condensed font (if one is
available or if a reasonably readable one can be synthesized by
applying an appropriate scale factor to the font) or a smaller
font, and return to the previous step.</p></li>
<li>
<p>Let the <var title="">anchor point</var> be a point on the <em
class="XXX">inline box</var>, determined by the <code
title="dom-context-2d-textAlign">textAlign</code>, as follows:</p>
<p>Vertical position:</p>
<dl>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">start</code></dt>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">left</code> and <span>the directionality</span> of the
<code>canvas</code> element is 'ltr'</dt>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">right</code> and <span>the directionality</span> of the
<code>canvas</code> element is 'rtl'</dt>
<dd>Let the <var title="">anchor point</var>'s vertical
position be the top edge of the <em class="XXX">inline
box</em>.</dd>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">end</code></dt>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">right</code> and <span>the directionality</span> of the
<code>canvas</code> element is 'ltr'</dt>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">left</code> and <span>the directionality</span> of the
<code>canvas</code> element is 'rtl'</dt>
<dd>Let the <var title="">anchor point</var>'s vertical
position be the bottom edge of the <em class="XXX">inline
box</em>.</dd>
<dt> If <code
title="dom-context-2d-textAlign">textAlign</code> is <code
title="">center</code></dt>
<dd>Let the <var title="">anchor point</var>'s vertical position
be half way between the top and bottom edges of the <em
class="XXX">inline box</em>.</dd>
</dl>
<p>Let the horizontal position be half way between the left and
right edges of the em box of the first available font of the <em
class="XXX">inline box</em>.</p>
</li>
<li>
<p>Paint the hypothetical inline box as the shape given by the
text's glyphs, as transformed by the <span
title="dom-context-2d-transformation">current transformation
matrix</span>, and anchored and sized so that before applying the
<span title="dom-context-2d-transformation">current transformation
matrix</span>, the <var title="">anchor point</var> is at (<var
title="">x</var>, <var title="">y</var>) and each CSS pixel is
mapped to one coordinate space unit.</p>
<p>For <code
title="dom-context-2d-fillVerticalText">fillVerticalText()</code>
<code title="dom-context-2d-fillStyle">fillStyle</code> must be
applied and <code
title="dom-context-2d-strokeStyle">strokeStyle</code> must be
ignored. For <code
title="dom-context-2d-strokeVerticalText">strokeVerticalText()</code>
the reverse holds and <code
title="dom-context-2d-strokeStyle">strokeStyle</code> must be
applied and <code
title="dom-context-2d-fillStyle">fillStyle</code> must be
ignored.</p>
<p>Text is painted without affecting the current path, and is
subject to <span title="shadows">shadow effects</span>, <span
title="dom-context-2d-globalAlpha">global alpha</span>, the <span
title="clipping region">clipping region</span>, and <span
title="dom-context-2d-globalCompositeOperation">global composition
operators</span>.</p>
</li>
</ol>
v6DVT (also check for '- -' bits in the part above) --><p>The <dfn id="dom-context-2d-measuretext" title="dom-context-2d-measureText"><code>measureText()</code></dfn>
method takes one argument, <var title="">text</var>. When the method
is invoked, the user agent must replace all the <a href="#space-character" title="space
character">space characters</a> in <var title="">text</var> with
U+0020 SPACE characters, and then must form a hypothetical
infinitely wide CSS line box containing a single inline box
containing the text <var title="">text</var>, with all the
properties at their initial values except the 'white-space' property
of the inline element set to 'pre' and the 'font' property of the
inline element set to the current font of the context as given by
the <code title="dom-context-2d-font"><a href="#dom-context-2d-font">font</a></code> attribute, and
must then return a new <code><a href="#textmetrics">TextMetrics</a></code> object with its
<code title="dom-textmetrics-width"><a href="#dom-textmetrics-width">width</a></code> attribute set to
the width of that inline box, in CSS pixels. <a href="#refsCSS">[CSS]</a></p>
<p>The <code><a href="#textmetrics">TextMetrics</a></code> interface is used for the objects
returned from <code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText()</a></code>. It has one
attribute, <dfn id="dom-textmetrics-width" title="dom-textmetrics-width"><code>width</code></dfn>, which is set
by the <code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText()</a></code>
method.</p>
<p class="note">Glyphs rendered using <code title="dom-context-2d-fillText"><a href="#dom-context-2d-filltext">fillText()</a></code> and <code title="dom-context-2d-strokeText"><a href="#dom-context-2d-stroketext">strokeText()</a></code> can spill out
of the box given by the font size (the em square size) and the width
returned by <code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText()</a></code> (the text
width). This version of the specification does not provide a way to
obtain the bounding box dimensions of the text. If the text is to be
rendered and removed, care needs to be taken to replace the entire
area of the canvas that the clipping region covers, not just the box
given by the em square height and measured text width.</p>
<!-- v6: Drawing text along a given path -->
<!-- v6: Adding text to a path -->
<!-- see also: http://www.w3.org/TR/SVG11/text.html#TextpathLayoutRules -->
<!-- see also: http://developer.mozilla.org/en/docs/Drawing_text_using_a_canvas -->
</div>
<p class="note">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.</p>
<h6 id="images"><span class="secno">4.8.11.1.10 </span>Images</h6>
<p>To draw images onto the canvas, the <dfn id="dom-context-2d-drawimage" title="dom-context-2d-drawImage"><code>drawImage</code></dfn> method
can be used.</p>
<p>This method can be invoked with three different sets of arguments:</p>
<ul class="brief"><li><code title="">drawImage(<var title="">image</var>, <var title="">dx</var>, <var title="">dy</var>)</code>
</li><li><code title="">drawImage(<var title="">image</var>, <var title="">dx</var>, <var title="">dy</var>, <var title="">dw</var>, <var title="">dh</var>)</code>
</li><li><code title="">drawImage(<var title="">image</var>, <var title="">sx</var>, <var title="">sy</var>, <var title="">sw</var>, <var title="">sh</var>, <var title="">dx</var>, <var title="">dy</var>, <var title="">dw</var>, <var title="">dh</var>)</code>
</li></ul><!-- v3: drawImage() of an ImageData object might make sense (when resizing as well as filtering) - ack Charles Pritchard --><p>Each of those three can take either an
<code><a href="#htmlimageelement">HTMLImageElement</a></code>, an <code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code>, or
an <code><a href="#htmlvideoelement">HTMLVideoElement</a></code> for the <var title="">image</var>
argument.</p>
<dl class="domintro"><dt><var title="">context</var> . <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage</a></code>(<var title="">image</var>, <var title="">dx</var>, <var title="">dy</var>)</dt>
<dt><var title="">context</var> . <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage</a></code>(<var title="">image</var>, <var title="">dx</var>, <var title="">dy</var>, <var title="">dw</var>, <var title="">dh</var>)</dt>
<dt><var title="">context</var> . <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage</a></code>(<var title="">image</var>, <var title="">sx</var>, <var title="">sy</var>, <var title="">sw</var>, <var title="">sh</var>, <var title="">dx</var>, <var title="">dy</var>, <var title="">dw</var>, <var title="">dh</var>)</dt>
<dd>
<p>Draws the given image onto the canvas. The arguments are
interpreted as follows:</p>
<p><img alt="The sx and sy parameters give the x and y coordinates of the source rectangle; the sw and sh arguments give the width and height of the source rectangle; the dx and dy give the x and y coordinates of the destination rectangle; and the dw and dh arguments give the width and height of the destination rectangle." height="356" src="http://www.whatwg.org/specs/web-apps/current-work/http://images.whatwg.org/drawImage.png" width="356"/></p>
<p>If the first argument isn't an <code><a href="#the-img-element">img</a></code>,
<code><a href="#the-canvas-element">canvas</a></code>, or <code><a href="#the-video-element">video</a></code> element, throws a
<code><a href="#type_mismatch_err">TYPE_MISMATCH_ERR</a></code> exception. If the image has no
image data, throws an <code><a href="#invalid_state_err">INVALID_STATE_ERR</a></code> exception. If
the one of the source rectangle dimensions is zero, throws an
<code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception. If the image isn't yet
fully decoded, then nothing is drawn.</p>
</dd>
</dl><div class="impl">
<p>If not specified, the <var title="">dw</var> and <var title="">dh</var> arguments must default to the values of <var title="">sw</var> and <var title="">sh</var>, interpreted such that
one CSS pixel in the image is treated as one unit in the canvas
coordinate space. If the <var title="">sx</var>, <var title="">sy</var>, <var title="">sw</var>, and <var title="">sh</var> 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 <i>concrete object size</i> must be used
instead, as determined using the CSS "<a href="http://dev.w3.org/csswg/css3-images/#default-sizing">Concrete
Object Size Resolution</a>" algorithm, with the <i>specified
size</i> having neither a definite width nor height, nor any
additional contraints, the object's intrinsic properties being those
of the <var title="">image</var> argument, and the <i>default object
size</i> being the size of the <code><a href="#the-canvas-element">canvas</a></code> element. <a href="#refsCSSIMAGES">[CSSIMAGES]</a></p>
<p>The <var title="">image</var> argument is an instance of either
<code><a href="#htmlimageelement">HTMLImageElement</a></code>, <code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code>, or
<code><a href="#htmlvideoelement">HTMLVideoElement</a></code>.</p> <!-- createPattern() has an
equivalent paragraph -->
<p>If the <var title="">image</var> argument is an
<code><a href="#htmlimageelement">HTMLImageElement</a></code> object that is not <a href="#img-good" title="img-good">fully decodable</a>, or if the <var title="">image</var> argument is an <code><a href="#htmlvideoelement">HTMLVideoElement</a></code>
object whose <code title="dom-media-readyState"><a href="#dom-media-readystate">readyState</a></code>
attribute is either <code title="dom-media-HAVE_NOTHING"><a href="#dom-media-have_nothing">HAVE_NOTHING</a></code> or <code title="dom-media-HAVE_METADATA"><a href="#dom-media-have_metadata">HAVE_METADATA</a></code>, then the
implementation must return without drawing anything.</p> <!--
createPattern() has an equivalent paragraph -->
<p>If the <var title="">image</var> argument is an
<code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code> object with either a horizontal
dimension or a vertical dimension equal to zero, then the
implementation must raise an <code><a href="#invalid_state_err">INVALID_STATE_ERR</a></code>
exception.</p>
<!-- createPattern() has an equivalent paragraph -->
<p>The source rectangle is the rectangle whose corners are the four
points (<var title="">sx</var>, <var title="">sy</var>), (<span title=""><var title="">sx</var>+<var title="">sw</var></span>, <var title="">sy</var>), (<span title=""><var title="">sx</var>+<var title="">sw</var></span>, <span title=""><var title="">sy</var>+<var title="">sh</var></span>), (<var title="">sx</var>, <span title=""><var title="">sy</var>+<var title="">sh</var></span>).</p>
<p>If one of the <var title="">sw</var> or <var title="">sh</var>
arguments is zero, the implementation must raise an
<code><a href="#index_size_err">INDEX_SIZE_ERR</a></code> exception.</p>
<p>The destination rectangle is the rectangle whose corners are the
four points (<var title="">dx</var>, <var title="">dy</var>),
(<span title=""><var title="">dx</var>+<var title="">dw</var></span>, <var title="">dy</var>), (<span title=""><var title="">dx</var>+<var title="">dw</var></span>, <span title=""><var title="">dy</var>+<var title="">dh</var></span>), (<var title="">dx</var>, <span title=""><var title="">dy</var>+<var title="">dh</var></span>).</p>
<p>When <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code> is
invoked, the region of the image specified by the source rectangle
must be painted on the region of the canvas specified by the
destination rectangle, after applying the <a href="#transformations" title="dom-context-2d-transformation">current transformation
matrix</a> to the points of the destination rectangle.</p>
<p>The original image data of the source image must be used, not the
image as it is rendered (e.g. <code title="attr-dim-width"><a href="#attr-dim-width">width</a></code> and <code title="attr-dim-height"><a href="#attr-dim-height">height</a></code> attributes on the source
element have no effect). The image data must be processed in the
original direction, even if the dimensions given are negative. <!--
remove that last sentence if it causes confusion. Someone once
suggested that 5,5,-2,-2 was different than 3,3,2,2; this is trying
to clarify that this is no the case. --></p>
<p class="note">This specification does not define the algorithm to
use when scaling the image, if necessary.</p>
<p class="note">When a canvas is drawn onto itself, the <a href="#drawing-model">drawing
model</a> requires the source to be copied before the image is drawn
back onto the canvas, so it is possible to copy parts of a canvas
onto overlapping parts of itself.</p>
<p>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' behavior.)</p>
<!-- see CORE-32111 and:
http://krijnhoetmer.nl/irc-logs/whatwg/20100818#l-737
http://www.w3.org/Bugs/Public/show_bug.cgi?id=10799#c11
-->
<!-- createPattern() has a similar paragraph with different rules -->
<p>When the <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code> method
is passed an animated image as its <var title="">image</var>
argument, the user agent must use the poster frame of the animation,
or, if there is no poster frame, the first frame of the
animation.</p>
<!-- createPattern() has an equivalent paragraph -->
<p>When the <var title="">image</var> argument is an
<code><a href="#htmlvideoelement">HTMLVideoElement</a></code>, then the frame at the <a href="#current-playback-position">current
playback position</a> must be used as the source image, and the
source image's dimensions must be the <a href="#concept-video-intrinsic-width" title="concept-video-intrinsic-width">intrinsic width</a> and
<a href="#concept-video-intrinsic-height" title="concept-video-intrinsic-height">intrinsic height</a>
of the <a href="#media-resource">media resource</a> (i.e. after any aspect-ratio
correction has been applied).</p>
<!-- createPattern() has an equivalent paragraph -->
<p>Images are painted without affecting the current path, and are
subject to <a href="#shadows" title="shadows">shadow effects</a>, <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, the <a href="#clipping-region" title="clipping region">clipping region</a>, and <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operators</a>.</p>
</div>
<h6 id="pixel-manipulation"><span class="secno">4.8.11.1.11 </span><dfn>Pixel manipulation</dfn></h6>
<dl class="domintro"><dt><var title="">imagedata</var> = <var title="">context</var> . <code title="dom-context-2d-createImageData"><a href="#dom-context-2d-createimagedata">createImageData</a></code>(<var title="">sw</var>, <var title="">sh</var>)</dt>
<dd>
<p>Returns an <code><a href="#imagedata">ImageData</a></code> object with the given
dimensions in CSS pixels (which might map to a different number of
actual device pixels exposed by the object itself). All the pixels
in the returned object are transparent black.</p>
</dd>
<dt><var title="">imagedata</var> = <var title="">context</var> . <code title="dom-context-2d-createImageData"><a href="#dom-context-2d-createimagedata">createImageData</a></code>(<var title="">imagedata</var>)</dt>
<dd>
<p>Returns an <code><a href="#imagedata">ImageData</a></code> object with the same
dimensions as the argument. All the pixels in the returned object
are transparent black.</p>
</dd>
<dt><var title="">imagedata</var> = <var title="">context</var> . <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData</a></code>(<var title="">sx</var>, <var title="">sy</var>, <var title="">sw</var>, <var title="">sh</var>)</dt>
<dd>
<p>Returns an <code><a href="#imagedata">ImageData</a></code> object containing the image
data for the given rectangle of the canvas.</p>
<p>Throws a <code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception if any of the
arguments are not finite. Throws an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code>
exception if the either of the width or height arguments are
zero.</p>
</dd>
<dt><var title="">imagedata</var> . <code title="dom-imagedata-width"><a href="#dom-imagedata-width">width</a></code></dt>
<dt><var title="">imagedata</var> . <code title="dom-imagedata-height"><a href="#dom-imagedata-height">height</a></code></dt>
<dd>
<p>Returns the actual dimensions of the data in the <code><a href="#imagedata">ImageData</a></code> object, in device pixels.</p>
</dd>
<dt><var title="">imagedata</var> . <code title="dom-imagedata-data"><a href="#dom-imagedata-data">data</a></code></dt>
<dd>
<p>Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.</p>
</dd>
<dt><var title="">context</var> . <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData</a></code>(<var title="">imagedata</var>, <var title="">dx</var>, <var title="">dy</var> [, <var title="">dirtyX</var>, <var title="">dirtyY</var>, <var title="">dirtyWidth</var>, <var title="">dirtyHeight</var> ])</dt>
<dd>
<p>Paints the data from the given <code><a href="#imagedata">ImageData</a></code> object
onto the canvas. If a dirty rectangle is provided, only the pixels
from that rectangle are painted.</p>
<p>The <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code>
and <code title="dom-context-2d-globalCompositeOperation"><a href="#dom-context-2d-globalcompositeoperation">globalCompositeOperation</a></code>
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.</p>
<p>If the first argument is null, throws a
<code><a href="#type_mismatch_err">TYPE_MISMATCH_ERR</a></code> exception. Throws a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception if any of the other
arguments are not finite.</p>
</dd>
</dl><div class="impl">
<p>The <dfn id="dom-context-2d-createimagedata" title="dom-context-2d-createImageData"><code>createImageData()</code></dfn>
method is used to instantiate new blank <code><a href="#imagedata">ImageData</a></code>
objects. When the method is invoked with two arguments <var title="">sw</var> and <var title="">sh</var>, it must return an
<code><a href="#imagedata">ImageData</a></code> object representing a rectangle with a width
in CSS pixels equal to the absolute magnitude of <var title="">sw</var> and a height in CSS pixels equal to the absolute
magnitude of <var title="">sh</var>. When invoked with a single <var title="">imagedata</var> argument, it must return an
<code><a href="#imagedata">ImageData</a></code> object representing a rectangle with the same
dimensions as the <code><a href="#imagedata">ImageData</a></code> object passed as the
argument. The <code><a href="#imagedata">ImageData</a></code> object returned must be filled
with transparent black.</p>
<p>The <dfn id="dom-context-2d-getimagedata" title="dom-context-2d-getImageData"><code>getImageData(<var title="">sx</var>, <var title="">sy</var>, <var title="">sw</var>,
<var title="">sh</var>)</code></dfn> method must return an
<code><a href="#imagedata">ImageData</a></code> object representing the underlying pixel data
for the area of the canvas denoted by the rectangle whose corners are
the four points (<var title="">sx</var>, <var title="">sy</var>),
(<span title=""><var title="">sx</var>+<var title="">sw</var></span>, <var title="">sy</var>), (<span title=""><var title="">sx</var>+<var title="">sw</var></span>, <span title=""><var title="">sy</var>+<var title="">sh</var></span>), (<var title="">sx</var>, <span title=""><var title="">sy</var>+<var title="">sh</var></span>), in canvas
coordinate space units. Pixels outside the canvas must be returned
as transparent black. Pixels must be returned as non-premultiplied
alpha values.</p>
<p>If any of the arguments to <code title="dom-context-2d-createImageData"><a href="#dom-context-2d-createimagedata">createImageData()</a></code> or
<code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code>
are infinite or NaN, the method must instead raise a
<code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception. If either the <var title="">sw</var> or <var title="">sh</var> arguments are zero,
the method must instead raise an <code><a href="#index_size_err">INDEX_SIZE_ERR</a></code>
exception.</p>
<p><code><a href="#imagedata">ImageData</a></code> objects must be initialized so that their
<dfn id="dom-imagedata-width" title="dom-imagedata-width"><code>width</code></dfn> attribute
is set to <var title="">w</var>, the number of physical device
pixels per row in the image data, their <dfn id="dom-imagedata-height" title="dom-imagedata-height"><code>height</code></dfn> attribute is
set to <var title="">h</var>, the number of rows in the image data,
and their <dfn id="dom-imagedata-data" title="dom-imagedata-data"><code>data</code></dfn>
attribute is initialized to a <code><a href="#canvaspixelarray">CanvasPixelArray</a></code> object
holding the image data. At least one pixel's worth of image data
must be returned.</p>
<p>The <code><a href="#canvaspixelarray">CanvasPixelArray</a></code> object provides ordered,
indexed access to the color components of each pixel of the image
data. The data must be 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 device 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.</p>
<p>The <code><a href="#canvaspixelarray">CanvasPixelArray</a></code> object thus represents <var title="">h</var>×<var title="">w</var>×4 integers. The
<dfn id="dom-canvaspixelarray-length" title="dom-canvaspixelarray-length"><code>length</code></dfn>
attribute of a <code><a href="#canvaspixelarray">CanvasPixelArray</a></code> object must return this
number.</p>
<p>The object's <a href="#supported-property-indices">supported property indices</a> are the
numbers in the range 0 .. <span title=""><var title="">h</var>×<var title="">w</var>×4-1</span>.</p>
<p>To <dfn id="dom-canvaspixelarray-get" title="dom-CanvasPixelArray-get">determine the value of
an indexed property</dfn> <var title="">index</var>, the user agent
must return the value of the <var title="">index</var>th component
in the array.</p>
<p>To <dfn id="dom-canvaspixelarray-set" title="dom-CanvasPixelArray-set">set the value of an
existing indexed property</dfn> <var title="">index</var> to value
<var title="">value</var>, the value of the <var title="">index</var>th component in the array must be set to <var title="">value</var>.</p>
<p class="note">The width and height (<var title="">w</var> and <var title="">h</var>) might be different from the <var title="">sw</var>
and <var title="">sh</var> arguments to the above methods, e.g. if
the canvas is backed by a high-resolution bitmap, or if the <var title="">sw</var> and <var title="">sh</var> arguments are
negative.</p>
<p>The <dfn id="dom-context-2d-putimagedata" title="dom-context-2d-putImageData"><code>putImageData(<var title="">imagedata</var>, <var title="">dx</var>, <var title="">dy</var>, <var title="">dirtyX</var>, <var title="">dirtyY</var>, <var title="">dirtyWidth</var>, <var title="">dirtyHeight</var>)</code></dfn> method writes data from
<code><a href="#imagedata">ImageData</a></code> structures back to the canvas.</p>
<p>If any of the arguments to the method are infinite or NaN, the
method must raise a <code><a href="#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception.</p>
<p>When the last four arguments are omitted, they must be assumed to
have the values 0, 0, the <code title="dom-imagedata-width"><a href="#dom-imagedata-width">width</a></code> member of the <var title="">imagedata</var> structure, and the <code title="dom-imagedata-height"><a href="#dom-imagedata-height">height</a></code> member of the <var title="">imagedata</var> structure, respectively.</p>
<p>When invoked with arguments that do not, per the last few
paragraphs, cause an exception to be raised, the <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData()</a></code> method
must act as follows:</p>
<ol><li>
<p>Let <var title="">dx<sub>device</sub></var> be the x-coordinate
of the device pixel in the underlying pixel data of the canvas
corresponding to the <var title="">dx</var> coordinate in the
canvas coordinate space.</p>
<p>Let <var title="">dy<sub>device</sub></var> be the y-coordinate
of the device pixel in the underlying pixel data of the canvas
corresponding to the <var title="">dy</var> coordinate in the
canvas coordinate space.</p>
</li>
<li>
<p>If <var title="">dirtyWidth</var> is negative, let <var title="">dirtyX</var> be <span title=""><var title="">dirtyX</var>+<var title="">dirtyWidth</var></span>, and let <var title="">dirtyWidth</var> be equal to the absolute magnitude of
<var title="">dirtyWidth</var>.</p>
<p>If <var title="">dirtyHeight</var> is negative, let <var title="">dirtyY</var> be <span title=""><var title="">dirtyY</var>+<var title="">dirtyHeight</var></span>, and let <var title="">dirtyHeight</var> be equal to the absolute magnitude of
<var title="">dirtyHeight</var>.</p>
</li>
<li>
<p>If <var title="">dirtyX</var> is negative, let <var title="">dirtyWidth</var> be <span title=""><var title="">dirtyWidth</var>+<var title="">dirtyX</var></span>, and
let <var title="">dirtyX</var> be zero.</p>
<p>If <var title="">dirtyY</var> is negative, let <var title="">dirtyHeight</var> be <span title=""><var title="">dirtyHeight</var>+<var title="">dirtyY</var></span>, and
let <var title="">dirtyY</var> be zero.</p>
</li>
<li>
<p>If <span title=""><var title="">dirtyX</var>+<var title="">dirtyWidth</var></span> is greater than the <code title="dom-imagedata-width"><a href="#dom-imagedata-width">width</a></code> attribute of the <var title="">imagedata</var> argument, let <var title="">dirtyWidth</var> be the value of that <code title="dom-imagedata-width"><a href="#dom-imagedata-width">width</a></code> attribute, minus the
value of <var title="">dirtyX</var>.</p>
<p>If <span title=""><var title="">dirtyY</var>+<var title="">dirtyHeight</var></span> is greater than the <code title="dom-imagedata-height"><a href="#dom-imagedata-height">height</a></code> attribute of the <var title="">imagedata</var> argument, let <var title="">dirtyHeight</var> be the value of that <code title="dom-imagedata-height"><a href="#dom-imagedata-height">height</a></code> attribute, minus the
value of <var title="">dirtyY</var>.</p>
</li>
<li>
<p>If, after those changes, either <var title="">dirtyWidth</var>
or <var title="">dirtyHeight</var> is negative or zero, stop these
steps without affecting the canvas.</p>
</li>
<li><p>Otherwise, for all integer values of <var title="">x</var>
and <var title="">y</var> where <span title=""><var title="">dirtyX</var> ≤ <var title="">x</var> < <span title=""><var title="">dirtyX</var>+<var title="">dirtyWidth</var></span></span>
and <span title=""><var title="">dirtyY</var> ≤ <var title="">y</var> < <span title=""><var title="">dirtyY</var>+<var title="">dirtyHeight</var></span></span>, copy the four channels of
the pixel with coordinate (<var title="">x</var>, <var title="">y</var>) in the <var title="">imagedata</var> data
structure to the pixel with coordinate (<span title=""><var title="">dx<sub>device</sub></var>+<var title="">x</var></span>,
<span title=""><var title="">dy<sub>device</sub></var>+<var title="">y</var></span>) in the underlying pixel data of the
canvas.</p></li>
</ol><p>The handling of pixel rounding when the specified coordinates do
not exactly map to the device coordinate space is not defined by
this specification, except that the following must result in no
visible changes to the rendering:</p>
<pre>context.putImageData(context.getImageData(x, y, w, h), p, q);</pre>
<p>...for any value of <var title="">x</var>, <var title="">y</var>,
<var title="">w</var>, and <var title="">h</var> and where <var title="">p</var> is the smaller of <var title="">x</var> and the sum
of <var title="">x</var> and <var title="">w</var>, and <var title="">q</var> is the smaller of <var title="">y</var> and the sum
of <var title="">y</var> and <var title="">h</var>; and except that
the following two calls:</p>
<pre>context.createImageData(w, h);
context.getImageData(0, 0, w, h);</pre>
<p>...must return <code><a href="#imagedata">ImageData</a></code> objects with the same
dimensions, for any value of <var title="">w</var> and <var title="">h</var>. In other words, while user agents may round the
arguments of these methods so that they map to device pixel
boundaries, any rounding performed must be performed consistently
for all of the <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">createImageData()</a></code>, <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> and <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData()</a></code>
operations.</p>
<p class="note">Due to the lossy nature of converting to and from
premultiplied alpha color values, pixels that have just been set
using <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData()</a></code> might be
returned to an equivalent <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> as
different values.</p>
<p>The current path, <a href="#transformations" title="dom-context-2d-transformation">transformation matrix</a>,
<a href="#shadows" title="shadows">shadow attributes</a>, <a href="#dom-context-2d-globalalpha" title="dom-context-2d-globalAlpha">global alpha</a>, the <a href="#clipping-region" title="clipping region">clipping region</a>, and <a href="#dom-context-2d-globalcompositeoperation" title="dom-context-2d-globalCompositeOperation">global composition
operator</a> must not affect the <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> and <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData()</a></code>
methods.</p>
</div>
<div class="example">
<p>The data returned by <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> is at the
resolution of the canvas backing store, which is likely to not be
one device pixel to each CSS pixel if the display used is a high
resolution display.</p>
<p>In the following example, the script generates an
<code><a href="#imagedata">ImageData</a></code> object so that it can draw onto it.</p>
<pre>// 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) { ... }</pre>
</div>
<div class="example">
<p>Here is an example of using <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> and <code title="dom-context-2d-putImageData"><a href="#dom-context-2d-putimagedata">putImageData()</a></code> to
implement an edge detection filter.</p>
<pre><!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></pre>
</div>
<div class="impl">
<h6 id="drawing-model"><span class="secno">4.8.11.1.12 </span><dfn>Drawing model</dfn></h6>
<p>When a shape or image is painted, user agents must follow these
steps, in the order given (or act as if they do):</p>
<ol><li><p>Render the shape or image onto an infinite transparent black
bitmap, creating image <var title="">A</var>, 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.</p></li>
<li><p><a href="#when-shadows-are-drawn">When shadows are drawn</a>, render the shadow from
image <var title="">A</var>, using the current shadow styles,
creating image <var title="">B</var>.</p></li>
<li><p><a href="#when-shadows-are-drawn">When shadows are drawn</a>, multiply the alpha
component of every pixel in <var title="">B</var> by <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code>.</p></li>
<li><p><a href="#when-shadows-are-drawn">When shadows are drawn</a>, composite <var title="">B</var> within the <a href="#clipping-region">clipping region</a> over the
current canvas bitmap using the current composition
operator.</p></li>
<li><p>Multiply the alpha component of every pixel in <var title="">A</var> by <code title="dom-context-2d-globalAlpha"><a href="#dom-context-2d-globalalpha">globalAlpha</a></code>.</p></li>
<li><p>Composite <var title="">A</var> within the <a href="#clipping-region">clipping
region</a> over the current canvas bitmap using the current
composition operator.</p></li>
</ol></div>
<h6 id="best-practices"><span class="secno">4.8.11.1.13 </span>Best practices</h6>
<p><i>This section is non-normative.</i></p>
<p>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 <a href="#drawCustomFocusRingExample">example above</a>.</p>
<p>To indicate which focusable part of the canvas is currently
focused, authors should use the <code title="dom-context-2d-drawSystemFocusRing"><a href="#dom-context-2d-drawsystemfocusring">drawSystemFocusRing()</a></code>
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.</p>
<p>Authors should avoid implementing text editing controls using the
<code><a href="#the-canvas-element">canvas</a></code> element. Doing so has a large number of
disadvantages:</p>
<ul><li>Mouse placement of the caret has to be reimplemented.</li>
<li>Keyboard movement of the caret has to be reimplemented (possibly across lines, for multiline text input).</li>
<li>Scrolling of the text field has to be implemented (horizontally for long lines, vertically for multiline input).</li>
<li>Native features such as copy-and-paste have to be reimplemented.</li>
<li>Native features such as spell-checking have to be reimplemented.</li>
<li>Native features such as drag-and-drop have to be reimplemented.</li>
<li>Native features such as page-wide text search have to be reimplemented.</li>
<li>Native features specific to the user, for example custom text
services, have to be reimplemented. This is close to impossible
since each user might have different services installed, and there
is an unbounded set of possible such services.</li>
<li>Bidirectional text editing has to be reimplemented.</li>
<li>For multiline text editing, line wrapping has to be implemented for all relevant languages.</li>
<li>Text selection has to be reimplemented.</li>
<li>Dragging of bidirectional text selections has to be reimplemented.</li>
<li>Platform-native keyboard shortcuts have to be reimplemented.</li>
<li>Platform-native input method editors (IMEs) have to be reimplemented.</li>
<li>Undo and redo functionality has to be reimplemented.</li>
<li>Accessibility features such as magnification following the
caret or selection have to be reimplemented.</li>
</ul><p>This is a huge amount of work, and authors are most strongly
encouraged to avoid doing any of it by instead using the
<code><a href="#the-input-element">input</a></code> element, the <code><a href="#the-textarea-element">textarea</a></code> element, or
the <code title="attr-contenteditable"><a href="#attr-contenteditable">contenteditable</a></code>
attribute.</p>
<h6 id="examples"><span class="secno">4.8.11.1.14 </span>Examples</h6>
<p><i>This section is non-normative.</i></p>
<div class="example">
<p>Here is an example of a script that uses canvas to draw <a href="data:text/html;charset=utf-8;base64,PCFET0NUWVBFIEhUTUw%2BDQo8aHRtbCBsYW5nPSJlbiI%2BDQogPGhlYWQ%2BDQogIDx0aXRsZT5QcmV0dHkgR2xvd2luZyBMaW5lczwvdGl0bGU%2BDQogPC9oZWFkPg0KIDxib2R5Pg0KPGNhbnZhcyB3aWR0aD0iODAwIiBoZWlnaHQ9IjQ1MCI%2BPC9jYW52YXM%2BDQo8c2NyaXB0Pg0KDQogdmFyIGNvbnRleHQgPSBkb2N1bWVudC5nZXRFbGVtZW50c0J5VGFnTmFtZSgnY2FudmFzJylbMF0uZ2V0Q29udGV4dCgnMmQnKTsNCg0KIHZhciBsYXN0WCA9IGNvbnRleHQuY2FudmFzLndpZHRoICogTWF0aC5yYW5kb20oKTsNCiB2YXIgbGFzdFkgPSBjb250ZXh0LmNhbnZhcy5oZWlnaHQgKiBNYXRoLnJhbmRvbSgpOw0KIHZhciBodWUgPSAwOw0KIGZ1bmN0aW9uIGxpbmUoKSB7DQogICBjb250ZXh0LnNhdmUoKTsNCiAgIGNvbnRleHQudHJhbnNsYXRlKGNvbnRleHQuY2FudmFzLndpZHRoLzIsIGNvbnRleHQuY2FudmFzLmhlaWdodC8yKTsNCiAgIGNvbnRleHQuc2NhbGUoMC45LCAwLjkpOw0KICAgY29udGV4dC50cmFuc2xhdGUoLWNvbnRleHQuY2FudmFzLndpZHRoLzIsIC1jb250ZXh0LmNhbnZhcy5oZWlnaHQvMik7DQogICBjb250ZXh0LmJlZ2luUGF0aCgpOw0KICAgY29udGV4dC5saW5lV2lkdGggPSA1ICsgTWF0aC5yYW5kb20oKSAqIDEwOw0KICAgY29udGV4dC5tb3ZlVG8obGFzdFgsIGxhc3RZKTsNCiAgIGxhc3RYID0gY29udGV4dC5jYW52YXMud2lkdGggKiBNYXRoLnJhbmRvbSgpOw0KICAgbGFzdFkgPSBjb250ZXh0LmNhbnZhcy5oZWlnaHQgKiBNYXRoLnJhbmRvbSgpOw0KICAgY29udGV4dC5iZXppZXJDdXJ2ZVRvKGNvbnRleHQuY2FudmFzLndpZHRoICogTWF0aC5yYW5kb20oKSwNCiAgICAgICAgICAgICAgICAgICAgICAgICBjb250ZXh0LmNhbnZhcy5oZWlnaHQgKiBNYXRoLnJhbmRvbSgpLA0KICAgICAgICAgICAgICAgICAgICAgICAgIGNvbnRleHQuY2FudmFzLndpZHRoICogTWF0aC5yYW5kb20oKSwNCiAgICAgICAgICAgICAgICAgICAgICAgICBjb250ZXh0LmNhbnZhcy5oZWlnaHQgKiBNYXRoLnJhbmRvbSgpLA0KICAgICAgICAgICAgICAgICAgICAgICAgIGxhc3RYLCBsYXN0WSk7DQoNCiAgIGh1ZSA9IGh1ZSArIDEwICogTWF0aC5yYW5kb20oKTsNCiAgIGNvbnRleHQuc3Ryb2tlU3R5bGUgPSAnaHNsKCcgKyBodWUgKyAnLCA1MCUsIDUwJSknOw0KICAgY29udGV4dC5zaGFkb3dDb2xvciA9ICd3aGl0ZSc7DQogICBjb250ZXh0LnNoYWRvd0JsdXIgPSAxMDsNCiAgIGNvbnRleHQuc3Ryb2tlKCk7DQogICBjb250ZXh0LnJlc3RvcmUoKTsNCiB9DQogc2V0SW50ZXJ2YWwobGluZSwgNTApOw0KDQogZnVuY3Rpb24gYmxhbmsoKSB7DQogICBjb250ZXh0LmZpbGxTdHlsZSA9ICdyZ2JhKDAsMCwwLDAuMSknOw0KICAgY29udGV4dC5maWxsUmVjdCgwLCAwLCBjb250ZXh0LmNhbnZhcy53aWR0aCwgY29udGV4dC5jYW52YXMuaGVpZ2h0KTsNCiB9DQogc2V0SW50ZXJ2YWwoYmxhbmssIDQwKTsNCg0KPC9zY3JpcHQ%2BDQogPC9ib2R5Pg0KPC9odG1sPg0K">pretty glowing lines</a>.</p>
<pre><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></pre>
</div>
</div><!--data-component-->
<!--2DCONTEXT-->
<div class="impl">
<h5 id="color-spaces-and-color-correction"><span class="secno">4.8.11.2 </span>Color spaces and color correction</h5>
<p>The <code><a href="#the-canvas-element">canvas</a></code> APIs must perform color correction at
only two points: when rendering images with their own gamma
correction and color space information onto the canvas, to convert
the image to the color space used by the canvas (e.g. using the 2D
Context's <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code>
method with an <code><a href="#htmlimageelement">HTMLImageElement</a></code> object), and when
rendering the actual canvas bitmap to the output device.</p>
<p class="note">Thus, in the 2D context, colors used to draw shapes
onto the canvas will exactly match colors obtained through the <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code>
method.</p>
<p>The <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL()</a></code> method
must not include color space information in the resource
returned. Where the output format allows it, the color of pixels in
resources created by <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL()</a></code> must match those
returned by the <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code>
method.</p>
<p>In user agents that support CSS, the color space used by a
<code><a href="#the-canvas-element">canvas</a></code> element must match the color space used for
processing any colors for that element in CSS.</p>
<p>The gamma correction and color space information of images must
be handled in such a way that an image rendered directly using an
<code><a href="#the-img-element">img</a></code> element would use the same colors as one painted on
a <code><a href="#the-canvas-element">canvas</a></code> element that is then itself
rendered. Furthermore, the rendering of images that have no color
correction information (such as those returned by the <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL()</a></code> method) must be
rendered with no color correction.</p>
<p class="note">Thus, in the 2D context, calling the <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code> method to render
the output of the <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL()</a></code> method to the
canvas, given the appropriate dimensions, has no visible effect.</p>
</div>
<div class="impl">
<h5 id="security-with-canvas-elements"><span class="secno">4.8.11.3 </span>Security with <code><a href="#the-canvas-element">canvas</a></code> elements</h5>
<p><strong>Information leakage</strong> can occur if scripts from
one <a href="#origin">origin</a> can access information (e.g. read pixels)
from images from another origin (one that isn't the <a href="#same-origin" title="same origin">same</a>).</p>
<p>To mitigate this, <code><a href="#the-canvas-element">canvas</a></code> elements are defined to
have a flag indicating whether they are <i>origin-clean</i>. All
<code><a href="#the-canvas-element">canvas</a></code> elements must start with their
<i>origin-clean</i> set to true. The flag must be set to false if
any of the following actions occur:</p>
<ul><li><p>The element's 2D context's <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code> method is
called with an <code><a href="#htmlimageelement">HTMLImageElement</a></code> or an
<code><a href="#htmlvideoelement">HTMLVideoElement</a></code> whose <a href="#origin">origin</a> is not the
<a href="#same-origin" title="same origin">same</a> as that of the
<code><a href="#document">Document</a></code> object that owns the <code><a href="#the-canvas-element">canvas</a></code>
element.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-drawImage"><a href="#dom-context-2d-drawimage">drawImage()</a></code> method is
called with an <code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code> whose
<i>origin-clean</i> flag is false.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> attribute is set
to a <code><a href="#canvaspattern">CanvasPattern</a></code> object that was created from an
<code><a href="#htmlimageelement">HTMLImageElement</a></code> or an <code><a href="#htmlvideoelement">HTMLVideoElement</a></code>
whose <a href="#origin">origin</a> was not the <a href="#same-origin" title="same
origin">same</a> as that of the <code><a href="#document">Document</a></code> object
that owns the <code><a href="#the-canvas-element">canvas</a></code> element when the pattern was
created.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-fillStyle"><a href="#dom-context-2d-fillstyle">fillStyle</a></code> attribute is set
to a <code><a href="#canvaspattern">CanvasPattern</a></code> object that was created from an
<code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code> whose <i>origin-clean</i> flag was
false when the pattern was created.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> attribute is
set to a <code><a href="#canvaspattern">CanvasPattern</a></code> object that was created from an
<code><a href="#htmlimageelement">HTMLImageElement</a></code> or an <code><a href="#htmlvideoelement">HTMLVideoElement</a></code>
whose <a href="#origin">origin</a> was not the <a href="#same-origin" title="same
origin">same</a> as that of the <code><a href="#document">Document</a></code> object
that owns the <code><a href="#the-canvas-element">canvas</a></code> element when the pattern was
created.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-strokeStyle"><a href="#dom-context-2d-strokestyle">strokeStyle</a></code> attribute is
set to a <code><a href="#canvaspattern">CanvasPattern</a></code> object that was created from an
<code><a href="#htmlcanvaselement">HTMLCanvasElement</a></code> whose <i>origin-clean</i> flag was
false when the pattern was created.</p></li>
<li><p>The element's 2D context's <code title="dom-context-2d-fillText"><a href="#dom-context-2d-filltext">fillText()</a></code> or <code title="dom-context-2d-fillText"><a href="#dom-context-2d-filltext">strokeText()</a></code> methods are
invoked and consider using a font that has an <a href="#origin">origin</a>
that is not the <a href="#same-origin" title="same origin">same</a> as that of
the <code><a href="#document">Document</a></code> object that owns the <code><a href="#the-canvas-element">canvas</a></code>
element. (The font doesn't even have to be used; all that matters
is whether the font was considered for any of the glyphs
drawn.)</p></li> <!-- because fonts could consider sensitive
material, I guess; and because that sensitivity could extend to
whether or not a particular glyph is in the font in the first
place. -->
</ul><p>Whenever the <code title="dom-canvas-toDataURL"><a href="#dom-canvas-todataurl">toDataURL()</a></code> method of a
<code><a href="#the-canvas-element">canvas</a></code> element whose <i>origin-clean</i> flag is set to
false is called, the method must raise a <code><a href="#security_err">SECURITY_ERR</a></code>
exception.</p>
<p>Whenever the <code title="dom-context-2d-getImageData"><a href="#dom-context-2d-getimagedata">getImageData()</a></code> method of
the 2D context of a <code><a href="#the-canvas-element">canvas</a></code> element whose
<i>origin-clean</i> flag is set to false is called with otherwise
correct arguments, the method must raise a <code><a href="#security_err">SECURITY_ERR</a></code>
exception.</p>
<p>Whenever the <code title="dom-context-2d-measureText"><a href="#dom-context-2d-measuretext">measureText()</a></code> method of
the 2D context of a <code><a href="#the-canvas-element">canvas</a></code> element ends up using a font
that has an <a href="#origin">origin</a> that is not the <a href="#same-origin" title="same
origin">same</a> as that of the <code><a href="#document">Document</a></code> object that
owns the <code><a href="#the-canvas-element">canvas</a></code> element, the method must raise a
<code><a href="#security_err">SECURITY_ERR</a></code> exception.</p>
<p class="note">Even resetting the canvas state by changing its
<code title="attr-canvas-width"><a href="#attr-canvas-width">width</a></code> or <code title="attr-canvas-height"><a href="#attr-canvas-height">height</a></code> attributes doesn't reset
the <i>origin-clean</i> flag.</p>
</div>
</body></html>
|
