HTML
Living Standard — Last Updated 8 September 2025
Living Standard — Last Updated 8 September 2025
canvas elementPath2D objectsImageBitmap rendering contextOffscreenCanvas interfacecanvas elementscanvas elementSupport in all current engines.
Support in all current engines.
a elements, img elements with
usemap attributes, button elements,
input elements whose type attribute are in
the Checkbox or Radio Button states, input elements that are
buttons, and select elements with a multiple attribute or a display size greater than 1.width — Horizontal dimension
height — Vertical dimension
typedef (CanvasRenderingContext2D or ImageBitmapRenderingContext or WebGLRenderingContext or WebGL2RenderingContext or GPUCanvasContext ) RenderingContext ;
[Exposed =Window ]
interface HTMLCanvasElement : HTMLElement {
[HTMLConstructor ] constructor ();
[CEReactions ] attribute unsigned long width ;
[CEReactions ] attribute unsigned long height ;
RenderingContext ? getContext (DOMString contextId , optional any options = null );
USVString toDataURL (optional DOMString type = "image/png", optional any quality );
undefined toBlob (BlobCallback _callback , optional DOMString type = "image/png", optional any quality );
OffscreenCanvas transferControlToOffscreen ();
};
callback BlobCallback = undefined (Blob ? blob );The canvas element provides scripts with a resolution-dependent bitmap canvas,
which can be used for rendering graphs, game graphics, art, or other visual images on the fly.
Authors should not use the canvas element in a document when a more suitable
element is available. For example, it is inappropriate to use a canvas element to
render a page heading: if the desired presentation of the heading is graphically intense, it
should be marked up using appropriate elements (typically h1) and then styled using
CSS and supporting technologies such as shadow trees.
When authors use the canvas element, they must also provide content that, when
presented to the user, conveys essentially the same function or purpose as the
canvas's bitmap. This content may be placed as content of the canvas
element. The contents of the canvas element, if any, are the element's fallback
content.
In interactive visual media, if scripting is enabled for
the canvas element, and if support for canvas elements has been enabled,
then the canvas element represents embedded content
consisting of a dynamically created image, the element's bitmap.
In non-interactive, static, visual media, if the canvas element has been
previously associated with a rendering context (e.g. if the page was viewed in an interactive
visual medium and is now being printed, or if some script that ran during the page layout process
painted on the element), then the canvas element represents
embedded content with the element's current bitmap and size. Otherwise, the element
represents its fallback content instead.
In non-visual media, and in visual media if scripting is
disabled for the canvas element or if support for canvas elements
has been disabled, the canvas element represents its fallback
content instead.
When a canvas element represents embedded content, the
user can still focus descendants of the canvas element (in the fallback
content). When an element is focused, it is the target of keyboard interaction
events (even though the element itself is not visible). This allows authors to make an interactive
canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no
effect on mouse interaction events.) [UIEVENTS]
An element whose nearest canvas element ancestor is being rendered
and represents embedded content is an element that is being used as
relevant canvas fallback content.
The canvas element has two attributes to control the size of the element's bitmap:
width and height. These attributes,
when specified, must have values that are valid
non-negative integers. The rules for parsing non-negative
integers must be used to obtain their numeric
values. If an attribute is missing, or if parsing its value returns an error, then the
default value must be used instead. The width
attribute defaults to 300, and the height attribute
defaults to 150.
When setting the value of the width or height attribute, if the context mode of the canvas
element is set to placeholder, the
user agent must throw an "InvalidStateError" DOMException
and leave the attribute's value unchanged.
The natural dimensions of the canvas element when it
represents embedded content are equal to the dimensions of the
element's bitmap.
The user agent must use a square pixel density consisting of one pixel of image data per
coordinate space unit for the bitmaps of a canvas and its rendering contexts.
A canvas element can be sized arbitrarily by a style sheet, its
bitmap is then subject to the 'object-fit' CSS property.
The bitmaps of canvas elements, the bitmaps of ImageBitmap objects,
as well as some of the bitmaps of rendering contexts, such as those described in the sections on
the CanvasRenderingContext2D, OffscreenCanvasRenderingContext2D, and
ImageBitmapRenderingContext objects below, have an origin-clean flag, which can be set to true or false.
Initially, when the canvas element or ImageBitmap object is created, its
bitmap's origin-clean flag must be set to
true.
A canvas element can have a rendering context bound to it. Initially, it does not
have a bound rendering context. To keep track of whether it has a rendering context or not, and
what kind of rendering context it is, a canvas also has a canvas context mode, which is initially none but can be changed to either placeholder, 2d, bitmaprenderer, webgl, webgl2, or webgpu by algorithms defined in this specification.
When its canvas context mode is none, a canvas element has no rendering context,
and its bitmap must be transparent black with a natural width equal
to the numeric value of the element's width attribute and a natural height equal to
the numeric value of the element's height attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or
removed.
When its canvas context mode is placeholder, a canvas element has no
rendering context. It serves as a placeholder for an OffscreenCanvas object, and
the content of the canvas element is updated by the OffscreenCanvas
object's rendering context.
When a canvas element represents embedded content, it provides a
paint source whose width is the element's natural width, whose height
is the element's natural height, and whose appearance is the element's bitmap.
Whenever the width and height content attributes are set, removed, changed, or
redundantly set to the value they already have, then the user agent must perform the action
from the row of the following table that corresponds to the canvas element's context mode.
|
Action | |
|---|---|
|
Follow the steps to set bitmap
dimensions to the numeric values
of the | |
|
Follow the behavior defined in the WebGL specifications. [WEBGL] | |
|
Follow the behavior defined in WebGPU. [WEBGPU] | |
|
If the context's bitmap
mode is set to blank,
run the steps to set an | |
|
Do nothing. | |
|
Do nothing. |
Support in all current engines.
Support in all current engines.
The width
and height
IDL attributes must reflect the respective content attributes of the same name, with
the same defaults.
context = canvas.getContext(contextId [, options ])Support in all current engines.
Returns an object that exposes an API for drawing on the canvas. contextId
specifies the desired API: "2d", "bitmaprenderer", "webgl", "webgl2", or "webgpu". options is handled by that API.
This specification defines the "2d" and "bitmaprenderer" contexts below. The WebGL
specifications define the "webgl" and "webgl2" contexts. WebGPU defines the "webgpu" context. [WEBGL] [WEBGPU]
Returns null if contextId is not supported, or if the canvas has already been
initialized with another context type (e.g., trying to get a "2d" context after getting a "webgl" context).
The getContext(contextId, options)
method of the canvas element, when invoked, must run these steps:
If options is not an object, then set options to null.
Set options to the result of converting options to a JavaScript value.
Run the steps in the cell of the following table whose column header matches this
canvas element's canvas context
mode and whose row header matches contextId:
| none | 2d | bitmaprenderer | webgl or webgl2 | webgpu | placeholder | |
|---|---|---|---|---|---|---|
"2d"
|
| Return the same object as was returned the last time the method was invoked with this same first argument. | Return null. | Return null. | Return null. |
Throw an "InvalidStateError" DOMException.
|
"bitmaprenderer"
|
| Return null. | Return the same object as was returned the last time the method was invoked with this same first argument. | Return null. | Return null. |
Throw an "InvalidStateError" DOMException.
|
"webgl" or "webgl2", if the user agent supports the WebGL
feature in its current configuration
|
| Return null. | Return null. | Return the same object as was returned the last time the method was invoked with this same first argument. | Return null. |
Throw an "InvalidStateError" DOMException.
|
"webgpu", if the user
agent supports the WebGPU feature in its current configuration
|
| Return null. | Return null. | Return null. | Return the same object as was returned the last time the method was invoked with this same first argument. |
Throw an "InvalidStateError" DOMException.
|
| An unsupported value* | Return null. | Return null. | Return null. | Return null. | Return null. |
Throw an "InvalidStateError" DOMException.
|
* For example, the "webgl" or "webgl2" value in the case of a user agent having exhausted
the graphics hardware's abilities and having no software fallback implementation.
url = canvas.toDataURL([ type [, quality ] ])Support in all current engines.
Returns a data: URL for the image in the
canvas.
The first argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is "image/png"; that type is also used if the given type isn't
supported. The second argument applies if the type is an image format that supports variable
quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive
indicating the desired quality level for the resulting image.
When trying to use types other than "image/png", authors can check if the image
was really returned in the requested format by checking to see if the returned string starts
with one of the exact strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus the requested type was
not supported. (The one exception to this is if the canvas has either no height or no width, in
which case the result might simply be "data:,".)
canvas.toBlob(callback [, type [, quality ] ])Support in all current engines.
Creates a Blob object representing a file containing the image in the canvas,
and invokes a callback with a handle to that object.
The second argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is "image/png"; that type is also used if the given type isn't
supported. The third argument applies if the type is an image format that supports variable
quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive
indicating the desired quality level for the resulting image.
canvas.transferControlToOffscreen()HTMLCanvasElement/transferControlToOffscreen
Support in all current engines.
Returns a newly created OffscreenCanvas object that uses the
canvas element as a placeholder. Once the canvas element has become a
placeholder for an OffscreenCanvas object, its natural size can no longer be
changed, and it cannot have a rendering context. The content of the placeholder canvas is
updated on the OffscreenCanvas's relevant agent's event loop's update the rendering
steps.
The toDataURL(type, quality) method,
when invoked, must run these steps:
If this canvas element's bitmap's origin-clean flag is set to false, then throw a
"SecurityError" DOMException.
If this canvas element's bitmap has no pixels (i.e. either its horizontal
dimension or its vertical dimension is zero), then return the string "data:,". (This is the shortest data: URL; it represents the empty string in a text/plain resource.)
Let file be a
serialization of this canvas element's bitmap as a file, passing
type and quality if given.
If file is null, then return "data:,".
The toBlob(callback, type,
quality) method, when invoked, must run these steps:
If this canvas element's bitmap's origin-clean flag is set to false, then throw a
"SecurityError" DOMException.
Let result be null.
If this canvas element's bitmap has pixels (i.e., neither its horizontal
dimension nor its vertical dimension is zero), then set result to a copy of this
canvas element's bitmap.
Run these steps in parallel:
If result is non-null, then set result to a serialization of result as a file with type and quality if given.
Queue an element task on the
canvas blob serialization task
source given the canvas element to run these steps:
If result is non-null, then set result to a new
Blob object, created in the relevant
realm of this canvas element, representing result.
[FILEAPI]
Invoke callback with
« result » and "report".
The transferControlToOffscreen() method,
when invoked, must run these steps:
If this canvas element's context
mode is not set to none, throw an
"InvalidStateError" DOMException.
Let offscreenCanvas be a new OffscreenCanvas object with its
width and height equal to the values of the width
and height content attributes of this
canvas element.
Set the offscreenCanvas's placeholder canvas element
to a weak reference to this canvas element.
Set this canvas element's context
mode to placeholder.
Set the offscreenCanvas's inherited language to the
language of this canvas element.
Set the offscreenCanvas's inherited direction to the directionality of this canvas element.
Return offscreenCanvas.
Support in all current engines.
Support in all current engines.
Support in all current engines.
Support in all current engines.
typedef (HTMLImageElement or
SVGImageElement ) HTMLOrSVGImageElement ;
typedef (HTMLOrSVGImageElement or
HTMLVideoElement or
HTMLCanvasElement or
ImageBitmap or
OffscreenCanvas or
VideoFrame ) CanvasImageSource ;
enum PredefinedColorSpace { " srgb " , " display-p3 " };
enum CanvasColorType { " unorm8 " , " float16 " };
enum CanvasFillRule { " nonzero " , " evenodd " };
dictionary CanvasRenderingContext2DSettings {
boolean alpha = true ;
boolean desynchronized = false ;
PredefinedColorSpace colorSpace = "srgb";
CanvasColorType colorType = "unorm8";
boolean willReadFrequently = false ;
};
enum ImageSmoothingQuality { " low " , " medium " , " high " };
[Exposed =Window ]
interface CanvasRenderingContext2D {
// back-reference to the canvas
readonly attribute HTMLCanvasElement canvas ;
};
CanvasRenderingContext2D includes CanvasSettings ;
CanvasRenderingContext2D includes CanvasState ;
CanvasRenderingContext2D includes CanvasTransform ;
CanvasRenderingContext2D includes CanvasCompositing ;
CanvasRenderingContext2D includes CanvasImageSmoothing ;
CanvasRenderingContext2D includes CanvasFillStrokeStyles ;
CanvasRenderingContext2D includes CanvasShadowStyles ;
CanvasRenderingContext2D includes CanvasFilters ;
CanvasRenderingContext2D includes CanvasRect ;
CanvasRenderingContext2D includes CanvasDrawPath ;
CanvasRenderingContext2D includes CanvasUserInterface ;
CanvasRenderingContext2D includes CanvasText ;
CanvasRenderingContext2D includes CanvasDrawImage ;
CanvasRenderingContext2D includes CanvasImageData ;
CanvasRenderingContext2D includes CanvasPathDrawingStyles ;
CanvasRenderingContext2D includes CanvasTextDrawingStyles ;
CanvasRenderingContext2D includes CanvasPath ;
interface mixin CanvasSettings {
// settings
CanvasRenderingContext2DSettings getContextAttributes ();
};
interface mixin CanvasState {
// state
undefined save (); // push state on state stack
undefined restore (); // pop state stack and restore state
undefined reset (); // reset the rendering context to its default state
boolean isContextLost (); // return whether context is lost
};
interface mixin CanvasTransform {
// transformations (default transform is the identity matrix)
undefined scale (unrestricted double x , unrestricted double y );
undefined rotate (unrestricted double angle );
undefined translate (unrestricted double x , unrestricted double y );
undefined transform (unrestricted double a , unrestricted double b , unrestricted double c , unrestricted double d , unrestricted double e , unrestricted double f );
[NewObject ] DOMMatrix getTransform ();
undefined setTransform (unrestricted double a , unrestricted double b , unrestricted double c , unrestricted double d , unrestricted double e , unrestricted double f );
undefined setTransform (optional DOMMatrix2DInit transform = {});
undefined resetTransform ();
};
interface mixin CanvasCompositing {
// compositing
attribute unrestricted double globalAlpha ; // (default 1.0)
attribute DOMString globalCompositeOperation ; // (default "source-over")
};
interface mixin CanvasImageSmoothing {
// image smoothing
attribute boolean imageSmoothingEnabled ; // (default true)
attribute ImageSmoothingQuality imageSmoothingQuality ; // (default low)
};
interface mixin CanvasFillStrokeStyles {
// colors and styles (see also the CanvasPathDrawingStyles and CanvasTextDrawingStyles interfaces)
attribute (DOMString or CanvasGradient or CanvasPattern ) strokeStyle ; // (default black)
attribute (DOMString or CanvasGradient or CanvasPattern ) fillStyle ; // (default black)
CanvasGradient createLinearGradient (double x0 , double y0 , double x1 , double y1 );
CanvasGradient createRadialGradient (double x0 , double y0 , double r0 , double x1 , double y1 , double r1 );
CanvasGradient createConicGradient (double startAngle , double x , double y );
CanvasPattern ? createPattern (CanvasImageSource image , [LegacyNullToEmptyString ] DOMString repetition );
};
interface mixin CanvasShadowStyles {
// shadows
attribute unrestricted double shadowOffsetX ; // (default 0)
attribute unrestricted double shadowOffsetY ; // (default 0)
attribute unrestricted double shadowBlur ; // (default 0)
attribute DOMString shadowColor ; // (default transparent black)
};
interface mixin CanvasFilters {
// filters
attribute DOMString filter ; // (default "none")
};
interface mixin CanvasRect {
// rects
undefined clearRect (unrestricted double x , unrestricted double y , unrestricted double w , unrestricted double h );
undefined fillRect (unrestricted double x , unrestricted double y , unrestricted double w , unrestricted double h );
undefined strokeRect (unrestricted double x , unrestricted double y , unrestricted double w , unrestricted double h );
};
interface mixin CanvasDrawPath {
// path API (see also CanvasPath)
undefined beginPath ();
undefined fill (optional CanvasFillRule fillRule = "nonzero");
undefined fill (Path2D path , optional CanvasFillRule fillRule = "nonzero");
undefined stroke ();
undefined stroke (Path2D path );
undefined clip (optional CanvasFillRule fillRule = "nonzero");
undefined clip (Path2D path , optional CanvasFillRule fillRule = "nonzero");
boolean isPointInPath (unrestricted double x , unrestricted double y , optional CanvasFillRule fillRule = "nonzero");
boolean isPointInPath (Path2D path , unrestricted double x , unrestricted double y , optional CanvasFillRule fillRule = "nonzero");
boolean isPointInStroke (unrestricted double x , unrestricted double y );
boolean isPointInStroke (Path2D path , unrestricted double x , unrestricted double y );
};
interface mixin CanvasUserInterface {
undefined drawFocusIfNeeded (Element element );
undefined drawFocusIfNeeded (Path2D path , Element element );
};
interface mixin CanvasText {
// text (see also the CanvasPathDrawingStyles and CanvasTextDrawingStyles interfaces)
undefined fillText (DOMString text , unrestricted double x , unrestricted double y , optional unrestricted double maxWidth );
undefined strokeText (DOMString text , unrestricted double x , unrestricted double y , optional unrestricted double maxWidth );
TextMetrics measureText (DOMString text );
};
interface mixin CanvasDrawImage {
// drawing images
undefined drawImage (CanvasImageSource image , unrestricted double dx , unrestricted double dy );
undefined drawImage (CanvasImageSource image , unrestricted double dx , unrestricted double dy , unrestricted double dw , unrestricted double dh );
undefined drawImage (CanvasImageSource image , unrestricted double sx , unrestricted double sy , unrestricted double sw , unrestricted double sh , unrestricted double dx , unrestricted double dy , unrestricted double dw , unrestricted double dh );
};
interface mixin CanvasImageData {
// pixel manipulation
ImageData createImageData ([EnforceRange ] long sw , [EnforceRange ] long sh , optional ImageDataSettings settings = {});
ImageData createImageData (ImageData imageData );
ImageData getImageData ([EnforceRange ] long sx , [EnforceRange ] long sy , [EnforceRange ] long sw , [EnforceRange ] long sh , optional ImageDataSettings settings = {});
undefined putImageData (ImageData imageData , [EnforceRange ] long dx , [EnforceRange ] long dy );
undefined putImageData (ImageData imageData , [EnforceRange ] long dx , [EnforceRange ] long dy , [EnforceRange ] long dirtyX , [EnforceRange ] long dirtyY , [EnforceRange ] long dirtyWidth , [EnforceRange ] long dirtyHeight );
};
enum CanvasLineCap { "butt" , "round" , "square" };
enum CanvasLineJoin { "round" , "bevel" , "miter" };
enum CanvasTextAlign { " start " , " end " , " left " , " right " , " center " };
enum CanvasTextBaseline { " top " , " hanging " , " middle " , " alphabetic " , " ideographic " , " bottom " };
enum CanvasDirection { " ltr " , " rtl " , " inherit " };
enum CanvasFontKerning { " auto " , " normal " , " none " };
enum CanvasFontStretch { " ultra-condensed " , " extra-condensed " , " condensed " , " semi-condensed " , " normal " , " semi-expanded " , " expanded " , " extra-expanded " , " ultra-expanded " };
enum CanvasFontVariantCaps { " normal " , " small-caps " , " all-small-caps " , " petite-caps " , " all-petite-caps " , " unicase " , " titling-caps " };
enum CanvasTextRendering { " auto " , " optimizeSpeed " , " optimizeLegibility " , " geometricPrecision " };
interface mixin CanvasPathDrawingStyles {
// line caps/joins
attribute unrestricted double lineWidth ; // (default 1)
attribute CanvasLineCap lineCap ; // (default "butt")
attribute CanvasLineJoin lineJoin ; // (default "miter")
attribute unrestricted double miterLimit ; // (default 10)
// dashed lines
undefined setLineDash (sequence <unrestricted double > segments ); // default empty
sequence <unrestricted double > getLineDash ();
attribute unrestricted double lineDashOffset ;
};
interface mixin CanvasTextDrawingStyles {
// text
attribute DOMString lang ; // (default: "inherit")
attribute DOMString font ; // (default 10px sans-serif)
attribute CanvasTextAlign textAlign ; // (default: "start")
attribute CanvasTextBaseline textBaseline ; // (default: "alphabetic")
attribute CanvasDirection direction ; // (default: "inherit")
attribute DOMString letterSpacing ; // (default: "0px")
attribute CanvasFontKerning fontKerning ; // (default: "auto")
attribute CanvasFontStretch fontStretch ; // (default: "normal")
attribute CanvasFontVariantCaps fontVariantCaps ; // (default: "normal")
attribute CanvasTextRendering textRendering ; // (default: "auto")
attribute DOMString wordSpacing ; // (default: "0px")
};
interface mixin CanvasPath {
// shared path API methods
undefined closePath ();
undefined moveTo (unrestricted double x , unrestricted double y );
undefined lineTo (unrestricted double x , unrestricted double y );
undefined quadraticCurveTo (unrestricted double cpx , unrestricted double cpy , unrestricted double x , unrestricted double y );
undefined bezierCurveTo (unrestricted double cp1x , unrestricted double cp1y , unrestricted double cp2x , unrestricted double cp2y , unrestricted double x , unrestricted double y );
undefined arcTo (unrestricted double x1 , unrestricted double y1 , unrestricted double x2 , unrestricted double y2 , unrestricted double radius );
undefined rect (unrestricted double x , unrestricted double y , unrestricted double w , unrestricted double h );
undefined roundRect (unrestricted double x , unrestricted double y , unrestricted double w , unrestricted double h , optional (unrestricted double or DOMPointInit or sequence <(unrestricted double or DOMPointInit )>) radii = 0);
undefined arc (unrestricted double x , unrestricted double y , unrestricted double radius , unrestricted double startAngle , unrestricted double endAngle , optional boolean counterclockwise = false );
undefined ellipse (unrestricted double x , unrestricted double y , unrestricted double radiusX , unrestricted double radiusY , unrestricted double rotation , unrestricted double startAngle , unrestricted double endAngle , optional boolean counterclockwise = false );
};
[Exposed =(Window ,Worker )]
interface CanvasGradient {
// opaque object
undefined addColorStop (double offset , DOMString color );
};
[Exposed =(Window ,Worker )]
interface CanvasPattern {
// opaque object
undefined setTransform (optional DOMMatrix2DInit transform = {});
};
[Exposed =(Window ,Worker )]
interface TextMetrics {
// x-direction
readonly attribute double width ; // advance width
readonly attribute double actualBoundingBoxLeft ;
readonly attribute double actualBoundingBoxRight ;
// y-direction
readonly attribute double fontBoundingBoxAscent ;
readonly attribute double fontBoundingBoxDescent ;
readonly attribute double actualBoundingBoxAscent ;
readonly attribute double actualBoundingBoxDescent ;
readonly attribute double emHeightAscent ;
readonly attribute double emHeightDescent ;
readonly attribute double hangingBaseline ;
readonly attribute double alphabeticBaseline ;
readonly attribute double ideographicBaseline ;
};
[Exposed =(Window ,Worker )]
interface Path2D {
constructor (optional (Path2D or DOMString ) path );
undefined addPath (Path2D path , optional DOMMatrix2DInit transform = {});
};
Path2D includes CanvasPath ;To maintain compatibility with existing web content, user agents need to
enumerate methods defined in CanvasUserInterface immediately after the stroke() method on CanvasRenderingContext2D
objects.
context = canvas.getContext('2d' [, { [ alpha: true ] [, desynchronized: false ] [, colorSpace: 'srgb'] [, willReadFrequently: false ]} ])Returns a CanvasRenderingContext2D object that is permanently bound to a
particular canvas element.
If the alpha member is
false, then the context is forced to always be opaque.
If the desynchronized member is
true, then the context might be desynchronized.
The colorSpace member
specifies the color space of the rendering
context.
The colorType member
specifies the color type of the rendering
context.
If the willReadFrequently
member is true, then the context is marked for readback optimization.
context.canvasCanvasRenderingContext2D/canvas
Support in all current engines.
Returns the canvas element.
attributes = context.getContextAttributes()Returns an object whose:
alpha member is true if the context has an alpha
component that is not 1.0; otherwise false.desynchronized
member is true if the context can be desynchronized.colorSpace member is
a string indicating the context's color
space.colorType member is
a string indicating the context's color
type.willReadFrequently
member is true if the context is marked for readback
optimization.The CanvasRenderingContext2D 2D rendering context represents a flat linear
Cartesian surface whose origin (0,0) is at the top left corner, with the coordinate space having
x values increasing when going right, and y values increasing when going
down. The x-coordinate of the right-most edge is equal to the width of the rendering
context's output bitmap in CSS pixels; similarly, the
y-coordinate of the bottom-most edge is equal to the height of the rendering context's
output bitmap in CSS pixels.
The size of the coordinate space does not necessarily represent the size of the actual bitmaps that the user agent will use internally or during rendering. On high-definition displays, for instance, the user agent may internally use bitmaps with four device pixels per unit in the coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can similarly be implemented using oversampling with bitmaps of a higher resolution than the final image on the display.
Using CSS pixels to describe the size of a rendering context's output bitmap does not mean that when rendered the canvas will cover an equivalent area in CSS pixels. CSS pixels are reused for ease of integration with CSS features, such as text layout.
In other words, the canvas element below's rendering context has a 200x200
output bitmap (which internally uses CSS pixels as a
unit for ease of integration with CSS) and is rendered as 100x100 CSS
pixels:
< canvas width = 200 height = 200 style = width:100px;height:100px > The 2D context creation algorithm, which is passed a target (a
canvas element) and options, consists of running these steps:
Let settings be the result of converting options to the dictionary type
CanvasRenderingContext2DSettings. (This can throw an exception.)
Let context be a new CanvasRenderingContext2D object.
Initialize context's canvas
attribute to point to target.
Set context's output bitmap to the same bitmap as target's bitmap (so that they are shared).
Set bitmap dimensions to
the numeric values of target's width and height
content attributes.
Run the canvas settings output bitmap initialization algorithm, given context and settings.
Return context.
When the user agent is to set bitmap dimensions to width and height, it must run these steps:
Resize the output bitmap to the new width and height.
Let canvas be the canvas element to which the rendering
context's canvas attribute was initialized.
If the numeric value of
canvas's width content
attribute differs from width, then set canvas's width content attribute to the shortest possible string
representing width as a valid non-negative integer.
If the numeric value of
canvas's height content
attribute differs from height, then set canvas's height content attribute to the shortest possible string
representing height as a valid non-negative integer.
Only one square appears to be drawn in the following example:
// canvas is a reference to a <canvas> element
var context = canvas. getContext( '2d' );
context. fillRect( 0 , 0 , 50 , 50 );
canvas. setAttribute( 'width' , '300' ); // clears the canvas
context. fillRect( 0 , 100 , 50 , 50 );
canvas. width = canvas. width; // clears the canvas
context. fillRect( 100 , 0 , 50 , 50 ); // only this square remains The canvas attribute must return the value it was
initialized to when the object was created.
The PredefinedColorSpace enumeration is used to specify the color space of the canvas's backing store.
The "srgb" value indicates the 'srgb'
color space.
The "display-p3" value indicates the
'display-p3' color space.
The algorithm for converting between color spaces can be found in the Converting Colors section of CSS Color. [CSSCOLOR]
The CanvasColorType enumeration is used to specify the color type of the canvas's backing store.
The "unorm8" value indicates that the type
for all color components is 8-bit unsigned normalized.
The "float16" value indicates that the type
for all color components is 16-bit floating point.
The CanvasFillRule enumeration is used to select the fill rule
algorithm by which to determine if a point is inside or outside a path.
The "nonzero" value indicates the nonzero winding
rule, wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path going in one direction is equal to the
number of times it crosses the path going in the other direction.
The "evenodd" value indicates the even-odd rule,
wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path is even.
If a point is not outside a shape, it is inside the shape.
The ImageSmoothingQuality enumeration is used to express a preference for the
interpolation quality to use when smoothing images.
The "low" value indicates a preference
for a low level of image interpolation quality. Low-quality image interpolation may be more
computationally efficient than higher settings.
The "medium" value indicates a
preference for a medium level of image interpolation quality.
The "high" value indicates a preference
for a high level of image interpolation quality. High-quality image interpolation may be more
computationally expensive than lower settings.
Bilinear scaling is an example of a relatively fast, lower-quality image-smoothing algorithm. Bicubic or Lanczos scaling are examples of image-smoothing algorithms that produce higher-quality output. This specification does not mandate that specific interpolation algorithms be used.
This section is non-normative.
The output bitmap, when it is not directly displayed by the user agent,
implementations can, instead of updating this bitmap, merely remember the sequence of drawing
operations that have been applied to it until such time as the bitmap's actual data is needed
(for example because of a call to drawImage(), or
the createImageBitmap() factory method). In many
cases, this will be more memory efficient.
The bitmap of a canvas element is the one bitmap that's pretty much always going
to be needed in practice. The output bitmap of a rendering context, when it has one,
is always just an alias to a canvas element's bitmap.
Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its natural size, or to enable double buffering so that graphics updates, like page scrolling for example, can be processed concurrently while canvas draw commands are being executed.
A CanvasSettings object has an output bitmap that is initialized when
the object is created.
The output bitmap has an origin-clean flag, which can be set to true or false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.
The CanvasSettings object also has an alpha boolean. When a CanvasSettings object's
alpha is false, then its alpha component must be fixed
to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel must
be silently ignored.
Thus, the bitmap of such a context starts off as opaque black instead
of transparent black; clearRect()
always results in opaque black pixels, every fourth byte from getImageData() is always 255, the putImageData() method effectively ignores every fourth
byte in its input, and so on. However, the alpha component of styles and images drawn onto the
canvas are still honoured up to the point where they would impact the output bitmap's
alpha component; for instance, drawing a 50% transparent white square on a freshly created
output bitmap with its alpha set to false
will result in a fully-opaque gray square.
The CanvasSettings object also has a desynchronized boolean. When a
CanvasSettings object's desynchronized is true, then the user agent may
optimize the rendering of the canvas to reduce the latency, as measured from input events to
rasterization, by desynchronizing the canvas paint cycle from the event loop, bypassing the
ordinary user agent rendering algorithm, or both. Insofar as this mode involves bypassing the
usual paint mechanisms, rasterization, or both, it might introduce visible tearing artifacts.
The user agent usually renders on a buffer which is not being displayed, quickly swapping it and the one being scanned out for presentation; the former buffer is called back buffer and the latter front buffer. A popular technique for reducing latency is called front buffer rendering, also known as single buffer rendering, where rendering happens in parallel and racily with the scanning out process. This technique reduces the latency at the price of potentially introducing tearing artifacts and can be used to implement in total or part of the desynchronized boolean. [MULTIPLEBUFFERING]
The desynchronized boolean can be useful when implementing certain kinds of applications, such as drawing applications, where the latency between input and rasterization is critical.
The CanvasSettings object also has a will read frequently boolean. When a
CanvasSettings object's will read
frequently is true, the user agent may optimize the canvas for readback operations.
On most devices the user agent needs to decide whether to store the canvas's
output bitmap on the GPU (this is also called "hardware accelerated"), or on the CPU
(also called "software"). Most rendering operations are more performant for accelerated canvases,
with the major exception being readback with getImageData(), toDataURL(), or toBlob(). CanvasSettings objects with will read frequently equal to true tell the
user agent that the webpage is likely to perform many readback operations and that it is
advantageous to use a software canvas.
The CanvasSettings object also has a color space setting of type
PredefinedColorSpace. The CanvasSettings object's color space indicates the color space for the
output bitmap.
The CanvasSettings object also has a color type setting of type
CanvasColorType. The CanvasSettings object's color type indicates the data type of the
color and alpha components of the pixels of the output bitmap.
To initialize a CanvasSettings output
bitmap, given a CanvasSettings context and a
CanvasRenderingContext2DSettings settings:
Set context's alpha to
settings["alpha"].
Set context's desynchronized to settings["desynchronized"].
Set context's color space to
settings["colorSpace"].
Set context's color type to
settings["colorType"].
Set context's will read
frequently to settings["willReadFrequently"].
The getContextAttributes() method
steps are to return «[ "alpha" →
this's alpha, "desynchronized" →
this's desynchronized, "colorSpace" → this's
color space, "colorType" → this's
color type, "willReadFrequently" →
this's will read frequently
]».
Objects that implement the CanvasState interface maintain a stack of drawing
states. Drawing states consist of:
The current transformation matrix.
The current clipping region.
The current letter spacing, word spacing, fill style, stroke style, filter, global alpha, compositing and blending operator, and shadow color.
The current values of the following attributes: lineWidth, lineCap, lineJoin, miterLimit, lineDashOffset, shadowOffsetX, shadowOffsetY, shadowBlur, lang, font, textAlign, textBaseline, direction, fontKerning, fontStretch, fontVariantCaps, textRendering, imageSmoothingEnabled, imageSmoothingQuality.
The current dash list.
The rendering context's bitmaps are not part of the drawing state, as they
depend on whether and how the rendering context is bound to a canvas element.
Objects that implement the CanvasState mixin have a context lost boolean, that is initialized to false
when the object is created. The context lost
value is updated in the context lost steps.
context.save()Support in all current engines.
Pushes the current state onto the stack.
context.