HTML
Living Standard — Last Updated 10 September 2025
Living Standard — Last Updated 10 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.restore()CanvasRenderingContext2D/restore
Support in all current engines.
Pops the top state on the stack, restoring the context to that state.
context.reset()Resets the rendering context, which includes the backing buffer, the drawing state stack, path, and styles.
context.isContextLost()Returns true if the rendering context was lost. Context loss can occur due to driver crashes, running out of memory, etc. In these cases, the canvas loses its backing storage and takes steps to reset the rendering context to its default state.
The save() method
steps are to push a copy of the current drawing state onto the drawing state stack.
The restore()
method steps are to pop the top entry in the drawing state stack, and reset the drawing state it
describes. If there is no saved state, then the method must do nothing.
CanvasRenderingContext2D/reset
OffscreenCanvasRenderingContext2D#canvasrenderingcontext2d.reset
The reset()
method steps are to reset the rendering context to its default state.
To reset the rendering context to its default state:
Clear canvas's bitmap to transparent black.
Empty the list of subpaths in context's current default path.
Clear the context's drawing state stack.
Reset everything that drawing state consists of to their initial values.
CanvasRenderingContext2D/isContextLost
Support in one engine only.
The isContextLost() method steps are to return
this's context lost.
context.lineWidth [ = value ]CanvasRenderingContext2D/lineWidth
Support in all current engines.
styles.lineWidth [ = value ]Returns the current line width.
Can be set, to change the line width. Values that are not finite values greater than zero are ignored.
context.lineCap [ = value ]CanvasRenderingContext2D/lineCap
Support in all current engines.
styles.lineCap [ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
The possible line cap styles are "butt", "round", and "square". Other values are ignored.
context.lineJoin [ = value ]CanvasRenderingContext2D/lineJoin
Support in all current engines.
styles.lineJoin [ = value ]Returns the current line join style.
Can be set, to change the line join style.
The possible line join styles are "bevel", "round", and "miter". Other values are ignored.
context.miterLimit [ = value ]CanvasRenderingContext2D/miterLimit
Support in all current engines.
styles.miterLimit [ = value ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
context.setLineDash(segments)CanvasRenderingContext2D/setLineDash
Support in all current engines.
styles.setLineDash(segments)Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.
segments = context.getLineDash()CanvasRenderingContext2D/getLineDash
Support in all current engines.
segments = styles.getLineDash()Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).
context.lineDashOffsetCanvasRenderingContext2D/lineDashOffset
Support in all current engines.
styles.lineDashOffsetReturns the phase offset (in the same units as the line dash pattern).
Can be set, to change the phase offset. Values that are not finite values are ignored.
Objects that implement the CanvasPathDrawingStyles interface have attributes and
methods (defined in this section) that control how lines are treated by the object.
The lineWidth attribute gives the width of lines, in
coordinate space units. On getting, it must return the current value. On setting, zero, negative,
infinite, and NaN values must be ignored, leaving the value unchanged; other values must change
the current value to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineWidth attribute must initially have the value
1.0.
The lineCap attribute defines the type of endings that
UAs will place on the end of lines. The three valid values are "butt",
"round", and "square".
On getting, it must return the current value. On setting, the current value must be changed to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineCap attribute must initially have the value
"butt".
The lineJoin attribute defines the type of corners that
UAs will place where two lines meet. The three valid values are "bevel",
"round", and "miter".
On getting, it must return the current value. On setting, the current value must be changed to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineJoin attribute must initially have the value
"miter".
When the lineJoin attribute has the value "miter", strokes use the miter limit ratio to decide how to render joins. The
miter limit ratio can be explicitly set using the miterLimit
attribute. On getting, it must return the current value. On setting, zero, negative, infinite, and
NaN values must be ignored, leaving the value unchanged; other values must change the current
value to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
miterLimit attribute must initially have the value
10.0.
Each CanvasPathDrawingStyles object has a dash list, which is either
empty or consists of an even number of non-negative numbers. Initially, the dash list
must be empty.
The setLineDash(segments) method, when invoked, must run
these steps:
If any value in segments is not finite (e.g. an Infinity or a NaN value), or if any value is negative (less than zero), then return (without throwing an exception; user agents could show a message on a developer console, though, as that would be helpful for debugging).
If the number of elements in segments is odd, then let segments be the concatenation of two copies of segments.
Set the object's dash list to segments.
When the getLineDash() method is invoked, it must return a
sequence whose values are the values of the object's dash list, in the same
order.
It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching
ants" effect. The phase can be set using the lineDashOffset attribute. On getting, it must
return the current value. On setting, infinite and NaN values must be ignored, leaving the value
unchanged; other values must change the current value to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineDashOffset attribute must initially have
the value 0.0.
When a user agent is to trace a path, given an object style
that implements the CanvasPathDrawingStyles interface, it must run the following
algorithm. This algorithm returns a new path.
Let path be a copy of the path being traced.
Prune all zero-length line segments from path.
Remove from path any subpaths containing no lines (i.e. subpaths with just one point).
Replace each point in each subpath of path other than the first point and the last point of each subpath by a join that joins the line leading to that point to the line leading out of that point, such that the subpaths all consist of two points (a starting point with a line leading out of it, and an ending point with a line leading into it), one or more lines (connecting the points and the joins), and zero or more joins (each connecting one line to another), connected together such that each subpath is a series of one or more lines with a join between each one and a point on each end.
Add a straight closing line to each closed subpath in path connecting the last point and the first point of that subpath; change the last point to a join (from the previously last line to the newly added closing line), and change the first point to a join (from the newly added closing line to the first line).
If style's dash list is empty, then jump to the step labeled convert.
Let pattern width be the concatenation of all the entries of style's dash list, in coordinate space units.
For each subpath subpath in path, run the following substeps. These substeps mutate the subpaths in path in vivo.
Let subpath width be the length of all the lines of subpath, in coordinate space units.
Let offset be the value of style's lineDashOffset, in coordinate space
units.
While offset is greater than pattern width, decrement it by pattern width.
While offset is less than zero, increment it by pattern width.
Define L to be a linear coordinate line defined along all lines in subpath, such that the start of the first line in the subpath is defined as coordinate 0, and the end of the last line in the subpath is defined as coordinate subpath width.
Let position be 0 minus offset.
Let index be 0.
Let current state be off (the other states being on and zero-on).
Dash on: Let segment length be the value of style's dash list's indexth entry.
Increment position by segment length.
If position is greater than subpath width, then end these substeps for this subpath and start them again for the next subpath; if there are no more subpaths, then jump to the step labeled convert instead.
If segment length is nonzero, then let current state be on.
Increment index by one.
Dash off: Let segment length be the value of style's dash list's indexth entry.
Let start be the offset position on L.
Increment position by segment length.
If position is less than zero, then jump to the step labeled post-cut.
If start is less than zero, then let start be zero.
If position is greater than subpath width, then let end be the offset subpath width on L. Otherwise, let end be the offset position on L.
Jump to the first appropriate step:
Do nothing, just continue to the next step.
Cut the line on which end finds itself short at end and place a point there, cutting in two the subpath that it was in; remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.
The point has a directionality for the purposes of drawing line caps (see below). The directionality is the direction that the original line had at that point (i.e. when L was defined above).
Cut the line on which start finds itself into two at start and place a point there, cutting in two the subpath that it was in, and similarly cut the line on which end finds itself short at end and place a point there, cutting in two the subpath that it was in, and then remove all line segments, joins, points, and subpaths that are between start and end.
If start and end are the same point, then this results in just the line being cut in two and two points being inserted there, with nothing being removed, unless a join also happens to be at that point, in which case the join must be removed.
Post-cut: If position is greater than subpath width, then jump to the step labeled convert.
Increment index by one. If it is equal to the number of entries in style's dash list, then let index be 0.
Return to the step labeled dash on.
Convert: This is the step that converts the path to a new path that represents its stroke.
Create a new path that describes the edge of the areas
that would be covered if a straight line of length equal to style's
lineWidth was swept along each subpath in path while being kept at an angle such that the line is orthogonal to the path
being swept, replacing each point with the end cap necessary to satisfy style's lineCap attribute as
described previously and elaborated below, and replacing each join with the join necessary to
satisfy style's lineJoin
type, as defined below.
Caps: Each point has a flat edge perpendicular to the direction of the line
coming out of it. This is then augmented according to the value of style's lineCap. The "butt" value means that no additional line cap is added. The "round" value means that a semi-circle with the diameter equal to
style's lineWidth width must
additionally be placed on to the line coming out of each point. The "square" value means that a rectangle with the length of style's lineWidth width and the
width of half style's lineWidth width, placed flat against the edge
perpendicular to the direction of the line coming out of the point, must be added at each
point.
Points with no lines coming out of them must have two caps placed back-to-back as if it was really two points connected to each other by an infinitesimally short straight line in the direction of the point's directionality (as defined above).
Joins: In addition to the point where a join occurs, two additional points are relevant to each join, one for each line: the two corners found half the line width away from the join point, one perpendicular to each line, each on the side furthest from the other line.
A triangle connecting these two opposite corners with a straight line, with the third point
of the triangle being the join point, must be added at all joins. The lineJoin attribute controls whether anything else is
rendered. The three aforementioned values have the following meanings:
The "bevel" value means that this is all that is rendered at
joins.
The "round" value means that an arc connecting the two aforementioned
corners of the join, abutting (and not overlapping) the aforementioned triangle, with the
diameter equal to the line width and the origin at the point of the join, must be added at
joins.
The "miter" value means that a second triangle must (if it can given
the miter length) be added at the join, with one line being the line between the two
aforementioned corners, abutting the first triangle, and the other two being continuations of
the outside edges of the two joining lines, as long as required to intersect without going over
the miter length.
The miter length is the distance from the point where the join occurs to the intersection of
the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of
the miter length to half the line width. If the miter length would cause the miter limit ratio
(as set by style's miterLimit attribute) to be exceeded, then this second
triangle must not be added.
The subpaths in the newly created path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.
Return the newly created path.
context.lang [ = value ]styles.lang [ = value ]Returns the current language setting.
Can be set to a BCP 47 language tag, the empty string, or "inherit", to change the language used when
resolving fonts. "inherit" takes the language
from the canvas element's language, or the associated document
element when there is no canvas element.
The default is "inherit".
context.font [ = value ]Support in all current engines.
styles.font [ = value ]Returns the current font settings.
Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored. The default is "10px sans-serif".
Relative keywords and lengths are computed relative to the font of the canvas
element.
context.textAlign [ = value ]CanvasRenderingContext2D/textAlign
Support in all current engines.
styles.textAlign [ = value ]Returns the current text alignment settings.
Can be set, to change the alignment. The possible values are and their meanings are given
below. Other values are ignored. The default is "start".
context.textBaseline [ = value ]CanvasRenderingContext2D/textBaseline
Support in all current engines.
styles.textBaseline [ = value ]Returns the current baseline alignment settings.
Can be set, to change the baseline alignment. The possible values and their meanings are
given below. Other values are ignored. The default is "alphabetic".
context.direction [ = value ]CanvasRenderingContext2D/direction
Support in all current engines.
styles.direction [ = value ]Returns the current directionality.
Can be set, to change the directionality. The possible values and their meanings are given
below. Other values are ignored. The default is "inherit".
context.letterSpacing [ = value ]styles.letterSpacing [ = value ]Returns the current spacing between characters in the text.
Can be set, to change spacing between characters. Values that cannot be parsed as a CSS
<length> are ignored. The default is "0px".
context.fontKerning [ = value ]styles.fontKerning [ = value ]Returns the current font kerning settings.
Can be set, to change the font kerning. The possible values and their meanings are given
below. Other values are ignored. The default is "auto".
context.fontStretch [ = value ]styles.fontStretch [ = value ]Returns the current font stretch settings.
Can be set, to change the font stretch. The possible values and their meanings are given
below. Other values are ignored. The default is "normal".
context.fontVariantCaps [ = value ]styles.fontVariantCaps [ = value ]Returns the current font variant caps settings.
Can be set, to change the font variant caps. The possible values and their meanings are given
below. Other values are ignored. The default is "normal".
context.textRendering [ = value ]styles.textRendering [ = value ]Returns the current text rendering settings.
Can be set, to change the text rendering. The possible values and their meanings are given
below. Other values are ignored. The default is "auto".
context.wordSpacing [ = value ]styles.wordSpacing [ = value ]Returns the current spacing between words in the text.
Can be set, to change spacing between words. Values that cannot be parsed as a CSS
<length> are ignored. The default is "0px".
Objects that implement the CanvasTextDrawingStyles interface have attributes
(defined in this section) that control how text is laid out (rasterized or outlined) by the
object. Such objects can also have a font style source object. For
CanvasRenderingContext2D objects, this is the canvas element given by
the value of the context's canvas attribute. For
OffscreenCanvasRenderingContext2D objects, this is the associated
OffscreenCanvas object.
Font resolution for the font style source object requires a font
source. This is determined for a given object implementing
CanvasTextDrawingStyles by the following steps: [CSSFONTLOAD]
If object's font style source object is a canvas
element, return the element's node document.
Otherwise, object's font style source object is an
OffscreenCanvas object:
Let global be object's relevant global object.
If global is a Window object, then return global's
associated Document.
Assert: global implements
WorkerGlobalScope.
Return global.
This is an example of font resolution with a regular canvas element with ID c1.
const font = new FontFace( "MyCanvasFont" , "url(mycanvasfont.ttf)" );
documents. fonts. add( font);
const context = document. getElementById( "c1" ). getContext( "2d" );
document. fonts. ready. then( function () {
context. font = "64px MyCanvasFont" ;
context. fillText( "hello" , 0 , 0 );
}); In this example, the canvas will display text using mycanvasfont.ttf as
its font.
This is an example of how font resolution can happen using OffscreenCanvas.
Assuming a canvas element with ID c2 which is transferred to
a worker like so:
const offscreenCanvas = document. getElementById( "c2" ). transferControlToOffscreen();
worker. postMessage( offscreenCanvas, [ offscreenCanvas]); Then, in the worker:
self. onmessage = function ( ev) {
const transferredCanvas = ev. data;
const context = transferredCanvas. getContext( "2d" );
const font = new FontFace( "MyFont" , "url(myfont.ttf)" );
self. fonts. add( font);
self. fonts. ready. then( function () {
context. font = "64px MyFont" ;
context. fillText( "hello" , 0 , 0 );
});
}; In this example, the canvas will display a text using myfont.ttf.
Notice that the font is only loaded inside the worker, and not in the document context.
The font IDL attribute, on setting, must be parsed as a CSS <'font'> value (but
without supporting property-independent style sheet syntax like 'inherit'), and the resulting font
must be assigned to the context, with the 'line-height' component forced to 'normal',
with the 'font-size' component converted to CSS pixels,
and with system fonts being computed to explicit values. If the new value is syntactically
incorrect (including using property-independent style sheet syntax like 'inherit' or 'initial'),
then it must be ignored, without assigning a new font value. [CSS]
Font family names must be interpreted in the context of the font style source
object when the font is to be used; any fonts embedded using @font-face or loaded using FontFace objects that are visible to the
font style source object must therefore be available once they are loaded. (Each font style source
object has a font source, which determines what fonts are available.) If a font
is used before it is fully loaded, or if the font style source object does not have
that font in scope at the time the font is to be used, then it must be treated as if it was an
unknown font, falling back to another as described by the relevant CSS specifications.
[CSSFONTS] [CSSFONTLOAD]
On getting, the font attribute must return the serialized form of the current font of the context (with
no 'line-height' component). [CSSOM]
For example, after the following statement:
context. font = 'italic 400 12px/2 Unknown Font, sans-serif' ; ...the expression context.font would evaluate to the string "italic 12px "Unknown Font", sans-serif". The "400"
font-weight doesn't appear because that is the default value. The line-height doesn't appear
because it is forced to "normal", the default value.
When the object implementing the CanvasTextDrawingStyles interface is created, the
font of the context must be set to 10px sans-serif. When the 'font-size' component is
set to lengths using percentages, 'em' or 'ex' units, or the 'larger' or
'smaller' keywords, these must be interpreted relative to the computed value of the
'font-size' property of the font style source object at the time that
the attribute is set, if it is an element. When the 'font-weight' component is set to
the relative values 'bolder' and 'lighter', these must be interpreted relative to the
computed value of the 'font-weight' property of the font style
source object at the time that the attribute is set, if it is an element. If the computed values are undefined for a particular case (e.g. because
the font style source object is not an element or is not being
rendered), then the relative keywords must be interpreted relative to the normal-weight
10px sans-serif default.
The textAlign IDL attribute, on getting, must return
the current value. On setting, the current value must be changed to the new value. When the object
implementing the CanvasTextDrawingStyles interface is created, the textAlign attribute must initially have the value start.
The textBaseline IDL attribute, on getting, must
return the current value. On setting, the current value must be changed to the new value. When the
object implementing the CanvasTextDrawingStyles interface is created, the textBaseline attribute must initially have the value
alphabetic.
Objects that implement the CanvasTextDrawingStyles interface have an associated
language value used to localize font
rendering. Valid values are a BCP 47 language tag, the empty string, or "inherit" where the language comes from the
canvas element's language, or the associated document element when there
is no canvas element. Initially, the language must be "inherit".
The lang getter steps are to return
this's language.
The lang setter steps are to set this's
language to the given value.
The direction IDL attribute, on getting, must return
the current value. On setting, the current value must be changed to the new value. When the object
implementing the CanvasTextDrawingStyles interface is created, the direction attribute must initially have the value "inherit".
Objects that implement the CanvasTextDrawingStyles interface have attributes that
control the spacing between letters and words. Such objects have associated letter spacing and word spacing values, which are CSS
<length> values. Initially, both must be the result of parsing "0px" as a CSS
<length>.
CanvasRenderingContext2D/letterSpacing
Support in one engine only.
The letterSpacing getter steps are to return
the serialized form of this's letter spacing.
The letterSpacing setter steps are:
Let parsed be the result of parsing the given value as a CSS <length>.
If parsed is failure, then return.
Set this's letter spacing to parsed.
CanvasRenderingContext2D/wordSpacing
Support in one engine only.
The wordSpacing getter steps are to return the
serialized form of this's word spacing.
The wordSpacing setter steps are:
Let parsed be the result of parsing the given value as a CSS <length>.
If parsed is failure, then return.
Set this's word spacing to parsed.
CanvasRenderingContext2D/fontKerning
The fontKerning IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontKerning attribute must
initially have the value "auto".
CanvasRenderingContext2D/fontStretch
Support in one engine only.
The fontStretch IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontStretch attribute must
initially have the value "normal".
CanvasRenderingContext2D/fontVariantCaps
Support in one engine only.
The fontVariantCaps IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontVariantCaps attribute must
initially have the value "normal".
CanvasRenderingContext2D/textRendering
Support in one engine only.
The textRendering IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the textRendering attribute must
initially have the value "auto".
The textAlign attribute's allowed keywords are
as follows:
start
Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).
end
Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).
left
Align to the left.
right
Align to the right.
center
Align to the center.
The textBaseline
attribute's allowed keywords correspond to alignment points in the
font:
The keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
The direction attribute's allowed keywords are
as follows:
ltr
Treat input to the text preparation algorithm as left-to-right text.
rtl
Treat input to the text preparation algorithm as right-to-left text.
inherit
Use the process in the text preparation algorithm to obtain the text
direction from the canvas element, placeholder canvas element, or
document element.
The fontKerning attribute's allowed keywords
are as follows:
auto
Kerning is applied at the discretion of the user agent.
normal
Kerning is applied.
none
Kerning is not applied.
The fontStretch attribute's allowed keywords
are as follows:
ultra-condensed
Same as CSS 'font-stretch' 'ultra-condensed' setting.
extra-condensed
Same as CSS 'font-stretch' 'extra-condensed' setting.
condensed
Same as CSS 'font-stretch' 'condensed' setting.
semi-condensed
Same as CSS 'font-stretch' 'semi-condensed' setting.
normal
The default setting, where width of the glyphs is at 100%.
semi-expanded
Same as CSS 'font-stretch' 'semi-expanded' setting.
expanded
Same as CSS 'font-stretch' 'expanded' setting.
extra-expanded
Same as CSS 'font-stretch' 'extra-expanded' setting.
ultra-expanded
Same as CSS 'font-stretch' 'ultra-expanded' setting.
The fontVariantCaps attribute's allowed
keywords are as follows:
normal
None of the features listed below are enabled.
small-caps
Same as CSS 'font-variant-caps' 'small-caps' setting.
all-small-caps
Same as CSS 'font-variant-caps' 'all-small-caps' setting.
petite-caps
Same as CSS 'font-variant-caps' 'petite-caps' setting.
all-petite-caps
Same as CSS 'font-variant-caps' 'all-petite-caps' setting.
unicase
Same as CSS 'font-variant-caps' 'unicase' setting.
titling-caps
Same as CSS 'font-variant-caps' 'titling-caps' setting.
The textRendering attribute's allowed
keywords are as follows:
auto
Same as 'auto' in SVG text-rendering property.
optimizeSpeed
Same as 'optimizeSpeed' in SVG text-rendering property.
optimizeLegibility
Same as 'optimizeLegibility' in SVG text-rendering property.
geometricPrecision
Same as 'geometricPrecision' in SVG text-rendering property.
The text preparation algorithm is as follows. It takes as input a string text,
a CanvasTextDrawingStyles object target, and an optional length
maxWidth. It returns an array of glyph shapes, each positioned on a common coordinate
space, a physical alignment whose value is one of left, right, and
center, and an inline box. (Most callers of this algorithm ignore the
physical alignment and the inline box.)
If maxWidth was provided but is less than or equal to zero or equal to NaN, then return an empty array.
Replace all ASCII whitespace in text with U+0020 SPACE characters.
Let font be the current font of target, as given
by that object's font attribute.
Let language be the target's language.
If language is "inherit":
Let sourceObject be object's font style source object.
If sourceObject is a canvas
element, then set language to the sourceObject's
language.
Otherwise:
Assert: sourceObject is an OffscreenCanvas object.
Set language to the sourceObject's inherited language.
If language is the empty string, then set language to explicitly unknown.
Apply the appropriate step from the following list to determine the value of direction:
direction attribute has the value "ltr"direction attribute has the value "rtl"direction attribute has the value "inherit"Let sourceObject be object's font style source object.
If sourceObject is a canvas
element, then let direction be sourceObject's
directionality.
Otherwise:
Assert: sourceObject is an OffscreenCanvas object.
Let direction be sourceObject's inherited direction.
Form a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with the CSS content language set to language, and with its CSS properties set as follows:
| Property | Source |
|---|---|
| 'direction' | direction |
| 'font' | font |
| 'font-kerning' | target's fontKerning |
| 'font-stretch' | target's fontStretch |
| 'font-variant-caps' | target's fontVariantCaps |
| 'letter-spacing' | target's letter spacing |
| SVG text-rendering | target's textRendering |
| 'white-space' | 'pre' |
| 'word-spacing' | target's word spacing |
and with all other properties set to their initial values.
If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
The anchor point is a point on the inline box, and the physical
alignment is one of the values left, right, and center. These
variables are determined by the textAlign and
textBaseline values as follows:
Horizontal position:
textAlign is lefttextAlign is start and direction is
'ltr'textAlign is end and direction is 'rtl'textAlign is righttextAlign is end and direction is 'ltr'textAlign is start and direction is
'rtl'textAlign is centerVertical position:
textBaseline is toptextBaseline is hangingtextBaseline is middletextBaseline is alphabetictextBaseline is ideographictextBaseline is bottomLet result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin at the anchor point.
Return result, physical alignment, and the inline box.
Objects that implement the CanvasPath interface have a path. A path has a list of zero or
more subpaths. Each subpath consists of a list of one or more points, connected by straight or
curved line segments, and a flag indicating whether the subpath is closed or not. A
closed subpath is one where the last point of the subpath is connected to the first point of the
subpath by a straight line. Subpaths with only one point are ignored when painting the path.
Paths have a need new subpath flag. When this flag is set, certain APIs create a new subpath rather than extending the previous one. When a path is created, its need new subpath flag must be set.
When an object implementing the CanvasPath interface is created, its path must be initialized to zero subpaths.
context.moveTo(x, y)CanvasRenderingContext2D/moveTo
Support in all current engines.
path.moveTo(x, y)Creates a new subpath with the given point.
context.closePath()CanvasRenderingContext2D/closePath
Support in all current engines.
path.closePath()Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
context.lineTo(x, y)CanvasRenderingContext2D/lineTo
Support in all current engines.
path.lineTo(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
context.quadraticCurveTo(cpx, cpy, x, y)CanvasRenderingContext2D/quadraticCurveTo
Support in all current engines.
path.quadraticCurveTo(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
context.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)CanvasRenderingContext2D/bezierCurveTo
Support in all current engines.
path.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
context.arcTo(x1, y1, x2, y2, radius)CanvasRenderingContext2D/arcTo
Support in all current engines.
path.arcTo(x1, y1, x2, y2, radius)Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
context.arc(x, y, radius, startAngle, endAngle [, counterclockwise ])Support in all current engines.
path.arc(x, y, radius, startAngle, endAngle [, counterclockwise ])Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
context.ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise])CanvasRenderingContext2D/ellipse
Support in all current engines.
path.ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise])Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
context.rect(x, y, w, h)Support in all current engines.
path.rect(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
context.roundRect(x, y, w, h, radii)path.roundRect(x, y, w, h, radii)Adds a new closed subpath to the path representing the given rounded rectangle. radii is either a list of radii or a single radius representing the corners of the rectangle in pixels. If a list is provided, the number and order of these radii function in the same way as the CSS 'border-radius' property. A single radius behaves the same way as a list with a single element.
If w and h are both greater than or equal to 0, or if both are smaller than 0, then the path is drawn clockwise. Otherwise, it is drawn counterclockwise.
When w is negative, the rounded rectangle is flipped horizontally, which means that the radius values that normally apply to the left corners are used on the right and vice versa. Similarly, when h is negative, the rounded rect is flipped vertically.
When a value r in radii is a number, the corresponding corner(s) are drawn as circular arcs of radius r.
When a value r in radii is an object with { x, y
} properties, the corresponding corner(s) are drawn as elliptical arcs whose x
and y radii are equal to r.x and r.y, respectively.
When the sum of the radii of two corners of the same edge is greater than the length of the edge, all the radii of the rounded rectangle are scaled by a factor of length / (r1 + r2). If multiple edges have this property, the scale factor of the edge with the smallest scale factor is used. This is consistent with CSS behavior.
Throws a RangeError if radii is a list whose
size is not one, two, three, or four.
Throws a RangeError if a value in radii is a
negative number, or is an { x, y } object whose x
or y properties are negative numbers.
The following methods allow authors to manipulate the paths
of objects implementing the CanvasPath interface.
For objects implementing the CanvasDrawPath and CanvasTransform
interfaces, the points passed to the methods, and the resulting lines added to current
default path by these methods, must be transformed according to the current transformation matrix before being added to
the path.
The moveTo(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Create a new subpath with the specified point as its first (and only) point.
When the user agent is to ensure there is a subpath for a coordinate (x,
y) on a path, the user agent must check to see if
the path has its need new subpath flag set. If it
does, then the user agent must create a new subpath with the point (x, y) as
its first (and only) point, as if the moveTo() method
had been called, and must then unset the path's need new
subpath flag.
The closePath() method, when invoked, must do nothing
if the object's path has no subpaths. Otherwise, it must mark the last subpath as closed, create a
new subpath whose first point is the same as the previous subpath's first point, and finally add
this new subpath to the path.
If the last subpath had more than one point in its list of points, then this is equivalent to adding a straight line connecting the last point back to the first point of the last subpath, thus "closing" the subpath.
New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the object's path.
The lineTo(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
If the object's path has no subpaths, then ensure there is a subpath for (x, y).
Otherwise, connect the last point in the subpath to the given point (x, y) using a straight line, and then add the given point (x, y) to the subpath.
The quadraticCurveTo(cpx, cpy,
x, y) method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then return.
Ensure there is a subpath for (cpx, cpy).
Connect the last point in the subpath to the given point (x, y) using a quadratic Bézier curve with control point (cpx, cpy). [BEZIER]
Add the given point (x, y) to the subpath.
The bezierCurveTo(cp1x, cp1y,
cp2x, cp2y, x, y) method, when invoked,
must run these steps:
If any of the arguments are infinite or NaN, then return.
Ensure there is a subpath for (cp1x, cp1y).
Connect the last point in the subpath to the given point (x, y) using a cubic Bézier curve with control points (cp1x, cp1y) and (cp2x, cp2y). [BEZIER]
Add the point (x, y) to the subpath.
The arcTo(x1,
y1, x2, y2, radius) method, when invoked,
must run these steps:
If any of the arguments are infinite or NaN, then return.
Ensure there is a subpath for (x1, y1).
If radius is negative, then throw an "IndexSizeError"
DOMException.
Let the point (x0, y0) be the last point in the subpath, transformed by the inverse of the current transformation matrix (so that it is in the same coordinate system as the points passed to the method).
If the point (x0, y0) is equal to the point (x1, y1), or if the point (x1, y1) is equal to the point (x2, y2), or if radius is zero, then add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, let The Arc be the shortest arc given by circumference of the circle that has radius radius, and that has one point tangent to the half-infinite line that crosses the point (x0, y0) and ends at the point (x1, y1), and that has a different point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point (x2, y2). The points at which this circle touches these two lines are called the start and end tangent points respectively. Connect the point (x0, y0) to the start tangent point by a straight line, adding the start tangent point to the subpath, and then connect the start tangent point to the end tangent point by The Arc, adding the end tangent point to the subpath.
The arc(x,
y, radius, startAngle, endAngle,
counterclockwise) method, when invoked, must run the ellipse method
steps with this, x, y, radius, radius, 0,
startAngle, endAngle, and counterclockwise.
This makes it equivalent to ellipse()
except that both radii are equal and rotation is 0.
The ellipse(x,
y, radiusX, radiusY, rotation, startAngle,
endAngle, counterclockwise) method, when invoked, must run the
ellipse method steps with this, x, y, radiusX,
radiusY, rotation, startAngle, endAngle, and
counterclockwise.
The determine the point on an ellipse steps, given ellipse, and angle, are:
Let eccentricCircle be the circle that shares its origin with ellipse, with a radius equal to the semi-major axis of ellipse.
Let outerPoint be the point on eccentricCircle's circumference at angle measured in radians clockwise from ellipse's semi-major axis.
Let chord be the line perpendicular to ellipse's major axis between this axis and outerPoint.
Return the point on chord that crosses ellipse's circumference.
The ellipse method steps, given canvasPath, x, y, radiusX, radiusY, rotation, startAngle, endAngle, and counterclockwise, are:
If any of the arguments are infinite or NaN, then return.
If either radiusX or radiusY are negative, then throw an
"IndexSizeError" DOMException.
If canvasPath's path has any subpaths, then add a straight line from the last point in the subpath to the start point of the arc.
Add the start and end points of the arc to the subpath, and connect them with an arc. The arc and its start and end points are defined as follows:
Consider an ellipse that has its origin at (x, y), that has a major-axis radius radiusX and a minor-axis radius radiusY, and that is rotated about its origin such that its semi-major axis is inclined rotation radians clockwise from the x-axis.
If counterclockwise is false and endAngle − startAngle is greater than or equal to 2π, or, if counterclockwise is true and startAngle − endAngle is greater than or equal to 2π, then the arc is the whole circumference of this ellipse, and both the start point and the end point are the result of running the determine the point on an ellipse steps given this ellipse and startAngle.
Otherwise, the start point is the result of running the determine the point on an ellipse steps given this ellipse and startAngle, the end point is the result of running the determine the point on an ellipse steps given this ellipse and endAngle, and the arc is the path along the circumference of this ellipse from the start point to the end point, going counterclockwise if counterclockwise is true, and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles from zero, the arc can never cover an angle greater than 2π radians.
Even if the arc covers the entire circumference of the ellipse and there are no
other points in the subpath, the path is not closed unless the closePath() method is appropriately invoked.
The rect(x,
y, w, h) method, when invoked, must run these
steps:
If any of the arguments are infinite or NaN, then return.
Create a new subpath containing just the four points (x, y), (x+w, y), (x+w, y+h), (x, y+h), in that order, with those four points connected by straight lines.
Mark the subpath as closed.
Create a new subpath with the point (x, y) as the only point in the subpath.
CanvasRenderingContext2D/roundRect
Support in all current engines.
The roundRect(x, y, w,
h, radii) method steps are:
If any of x, y, w, or h are infinite or NaN, then return.
If radii is an unrestricted
double or DOMPointInit, then set radii to « radii
».
If radii is not a list of size one, two, three, or four, then throw a RangeError.
Let normalizedRadii be an empty list.
For each radius of radii:
If radius is a DOMPointInit:
If radius["x"] or
radius["y"] is infinite or NaN, then
return.
If radius["x"] or
radius["y"] is negative, then throw a RangeError.
Otherwise, append radius to normalizedRadii.
If radius is a unrestricted
double:
If radius is infinite or NaN, then return.
If radius is negative, then throw a RangeError.
Otherwise, append «[ "x" → radius,
"y" → radius ]» to
normalizedRadii.
Let upperLeft, upperRight, lowerRight, and lowerLeft be null.
If normalizedRadii's size is 4, then set upperLeft to normalizedRadii[0], set upperRight to normalizedRadii[1], set lowerRight to normalizedRadii[2], and set lowerLeft to normalizedRadii[3].
If normalizedRadii's size is 3, then set upperLeft to normalizedRadii[0], set upperRight and lowerLeft to normalizedRadii[1], and set lowerRight to normalizedRadii[2].
If normalizedRadii's size is 2, then set upperLeft and lowerRight to normalizedRadii[0] and set upperRight and lowerLeft to normalizedRadii[1].
If normalizedRadii's size is 1, then set upperLeft, upperRight, lowerRight, and lowerLeft to normalizedRadii[0].
Corner curves must not overlap. Scale all radii to prevent this:
Create a new subpath:
Move to the point (x + upperLeft["x"], y).
Draw a straight line to the point (x + w −
upperRight["x"], y).
Draw an arc to the point (x + w, y +
upperRight["y"]).
Draw a straight line to the point (x + w, y +
h − lowerRight["y"]).
Draw an arc to the point (x + w −
lowerRight["x"], y +
h).
Draw a straight line to the point (x + lowerLeft["x"], y + h).
Draw an arc to the point (x, y + h −
lowerLeft["y"]).
Draw a straight line to the point (x, y +
upperLeft["y"]).
Draw an arc to the point (x + upperLeft["x"], y).
Mark the subpath as closed.
Create a new subpath with the point (x, y) as the only point in the subpath.
This is designed to behave similarly to the CSS 'border-radius' property.
Path2D objectsSupport in all current engines.
Path2D objects can be used to declare paths that are then later used on
objects implementing the CanvasDrawPath interface. In addition to many of the APIs
described in earlier sections, Path2D objects have methods to combine paths, and to
add text to paths.
path = new Path2D()Support in all current engines.
Creates a new empty Path2D object.
path = new Path2D(path)When path is a Path2D object, returns a copy.
When path is a string, creates the path described by the argument, interpreted as SVG path data. [SVG]
path.addPath(path [, transform ])Support in all current engines.
Adds to the path the path given by the argument.
The Path2D(path) constructor, when invoked, must run these
steps:
Let output be a new Path2D object.
If path is not given, then return output.
If path is a Path2D object, then add all subpaths of
path to output and return output. (In other words, it returns a
copy of the argument.)
Let svgPath be the result of parsing and interpreting path according to SVG 2's rules for path data. [SVG]
The resulting path could be empty. SVG defines error handling rules for parsing and applying path data.
Let (x, y) be the last point in svgPath.
Add all the subpaths, if any, from svgPath to output.
Create a new subpath in output with (x, y) as the only point in the subpath.
Return output.
The addPath(path,
transform) method, when invoked on a Path2D object
a, must run these steps:
If the Path2D object path has no subpaths, then return.
Let matrix be the result of creating a DOMMatrix from the 2D dictionary
transform.
If one or more of matrix's m11 element, m12 element, m21 element, m22 element, m41 element, or m42 element are infinite or NaN, then return.
Create a copy of all the subpaths in path. Let c be this copy.
Transform all the coordinates and lines in c by the transform matrix matrix.
Let (x, y) be the last point in the last subpath of c.
Add all the subpaths in c to a.
Create a new subpath in a with (x, y) as the only point in the subpath.
Objects that implement the CanvasTransform interface have a current
transformation matrix, as well as methods (described in this section) to manipulate it. When
an object implementing the CanvasTransform interface is created, its transformation
matrix must be initialized to the identity matrix.
The current transformation matrix is applied to coordinates when creating the
current default path, and when painting text, shapes, and Path2D
objects, on objects implementing the CanvasTransform interface.
The transformations must be performed in reverse order.
For instance, if a scale transformation that doubles the width is applied to the canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.
context.scale(x, y)CanvasRenderingContext2D/scale
Support in all current engines.
Changes the current transformation matrix to apply a scaling transformation with the given characteristics.
context.rotate(angle)CanvasRenderingContext2D/rotate
Support in all current engines.
Changes the current transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.
context.translate(x, y)CanvasRenderingContext2D/translate
Support in all current engines.
Changes the current transformation matrix to apply a translation transformation with the given characteristics.
context.transform(a, b, c, d, e, f)CanvasRenderingContext2D/transform
Support in all current engines.
Changes the current transformation matrix to apply the matrix given by the arguments as described below.
matrix = context.getTransform()CanvasRenderingContext2D/getTransform
Support in all current engines.
Returns a copy of the current transformation matrix, as a newly created
DOMMatrix object.
context.setTransform(a, b, c, d, e, f)CanvasRenderingContext2D/setTransform
Support in all current engines.
Changes the current transformation matrix to the matrix given by the arguments as described below.
context.setTransform(transform)Changes the current transformation matrix to the matrix represented by
the passed DOMMatrix2DInit dictionary.
context.resetTransform()CanvasRenderingContext2D/resetTransform
Support in all current engines.
Changes the current transformation matrix to the identity matrix.
The scale(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Add the scaling transformation described by the arguments to the current transformation matrix. The x argument represents the scale factor in the horizontal direction and the y argument represents the scale factor in the vertical direction. The factors are multiples.
The rotate(angle) method, when invoked, must
run these steps:
If angle is infinite or NaN, then return.
Add the rotation transformation described by the argument to the current transformation matrix. The angle argument represents a clockwise rotation angle expressed in radians.
The translate(x, y) method, when
invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Add the translation transformation described by the arguments to the current transformation matrix. The x argument represents the translation distance in the horizontal direction and the y argument represents the translation distance in the vertical direction. The arguments are in coordinate space units.
The transform(a, b, c,
d, e, f) method, when invoked, must run these
steps:
If any of the arguments are infinite or NaN, then return.
Replace the current transformation matrix with the result of multiplying the current transformation matrix with the matrix described by:
| a | c | e |
| b | d | f |
| 0 | 0 | 1 |
The arguments a, b, c, d, e, and f are sometimes called m11, m12, m21, m22, dx, and dy or m11, m21, m12, m22, dx, and dy. Care ought to be taken in particular with the order of the second and third arguments (b and c) as their order varies from API to API and APIs sometimes use the notation m12/m21 and sometimes m21/m12 for those positions.
The getTransform() method, when invoked, must return
a newly created DOMMatrix representing a copy of the current transformation
matrix matrix of the context.
This returned object is not live, so updating it will not affect the
current transformation matrix, and updating the current transformation
matrix will not affect an already returned DOMMatrix.
The setTransform(a, b, c,
d, e, f) method, when invoked, must run these
steps:
If any of the arguments are infinite or NaN, then return.
Reset the current transformation matrix to the matrix described by:
| a | c | e |
| b | d | f |
| 0 | 0 | 1 |
The setTransform(transform)
method, when invoked, must run these steps:
Let matrix be the result of creating a DOMMatrix from the 2D dictionary
transform.
If one or more of matrix's m11 element, m12 element, m21 element, m22 element, m41 element, or m42 element are infinite or NaN, then return.
Reset the current transformation matrix to matrix.
The resetTransform() method, when invoked, must
reset the current transformation matrix to the identity matrix.
Given a matrix of the form created by the transform() and setTransform() methods, i.e.,
| a | c | e |
| b | d | f |
| 0 | 0 | 1 |
the resulting transformed coordinates after transform matrix multiplication will be
xnew = a x + c y + e
ynew = b x + d y + f
Some methods on the CanvasDrawImage and CanvasFillStrokeStyles
interfaces take the union type CanvasImageSource as an argument.
This union type allows objects implementing any of the following interfaces to be used as image sources:
HTMLOrSVGImageElement (img or SVG image
elements)HTMLVideoElement (video elements)HTMLCanvasElement (canvas elements)OffscreenCanvasImageBitmapVideoFrameAlthough not formally specified as such, SVG image
elements are expected to be implemented nearly identical to img elements. That is,
SVG image elements share the fundamental concepts and features of
img elements.
The ImageBitmap interface can be created from a number of other
image-representing types, including ImageData.
To check the usability of the image argument, where image
is a CanvasImageSource object, run these steps:
Switch on image:
HTMLOrSVGImageElementIf image's current request's state is broken, then
throw an "InvalidStateError" DOMException.
If image is not fully decodable, then return bad.
If image has a natural width or natural height (or both) equal to zero, then return bad.
HTMLVideoElementIf image's readyState
attribute is either HAVE_NOTHING or HAVE_METADATA, then return bad.
HTMLCanvasElementOffscreenCanvasIf image has either a horizontal dimension or a vertical dimension
equal to zero, then throw an "InvalidStateError"
DOMException.
ImageBitmapVideoFrameIf image's [[Detached]] internal slot value
is set to true, then throw an "InvalidStateError"
DOMException.
Return good.
When a CanvasImageSource object represents an HTMLOrSVGImageElement,
the element's image must be used as the source image.
Specifically, when a CanvasImageSource object represents an animated image in an
HTMLOrSVGImageElement, the user agent must use the default image of the animation
(the one that the format defines is to be used when animation is not supported or is disabled),
or, if there is no such image, the first frame of the animation, when rendering the image for
CanvasRenderingContext2D APIs.
When a CanvasImageSource object represents an HTMLVideoElement, then
the frame at the current playback position when the method with the argument is
invoked must be used as the source image when rendering the image for
CanvasRenderingContext2D APIs, and the source image's dimensions must be the natural width and natural height of the media resource
(i.e., after any aspect-ratio correction has been applied).
When a CanvasImageSource object represents an HTMLCanvasElement, the
element's bitmap must be used as the source image.
When a CanvasImageSource object represents an element that is being
rendered and that element has been resized, the original image data of the source image
must be used, not the image as it is rendered (e.g. width and
height attributes on the source element have no effect on how
the object is interpreted when rendering the image for CanvasRenderingContext2D
APIs).
When a CanvasImageSource object represents an ImageBitmap, the
object's bitmap image data must be used as the source image.
When a CanvasImageSource object represents an OffscreenCanvas, the
object's bitmap must be used as the source image.
When a CanvasImageSource object represents a VideoFrame, the object's
pixel data must be used as the source image, and the source image's dimensions must be the
object's [[display width]] and [[display height]].
An object image is not origin-clean if, switching on image's type:
HTMLOrSVGImageElement
image's current request's image data is CORS-cross-origin.
HTMLVideoElement
image's media data is CORS-cross-origin.
HTMLCanvasElement
ImageBitmap
OffscreenCanvas
image's bitmap's origin-clean flag is false.
context.fillStyle [ = value ]CanvasRenderingContext2D/fillStyle
Support in all current engines.
Returns the current style used for filling shapes.
Can be set, to change the fill style.
The style can be either a string containing a CSS color, or a CanvasGradient or
CanvasPattern object. Invalid values are ignored.
context.strokeStyle [ = value ]CanvasRenderingContext2D/strokeStyle
Support in all current engines.
Returns the current style used for stroking shapes.
Can be set, to change the stroke style.
The style can be either a string containing a CSS color, or a CanvasGradient or
CanvasPattern object. Invalid values are ignored.
Objects that implement the CanvasFillStrokeStyles interface have attributes and
methods (defined in this section) that control how shapes are treated by the object.
Such objects have associated fill
style and stroke style
values, which are either CSS colors, CanvasPatterns, or CanvasGradients.
Initially, both must be the result of parsing
the string "#000000".
When the value is a CSS color, it must not be affected by the transformation matrix when used to draw on bitmaps.
When set to a CanvasPattern or CanvasGradient object,
changes made to the object after the assignment do affect subsequent stroking or filling of
shapes.
The fillStyle getter steps are:
If this's fill style is a CSS color, then return the serialization of that color with HTML-compatible serialization requested.
Return this's fill style.
The fillStyle setter steps are:
If the given value is a string, then:
Let context be this's canvas attribute's value, if that is an element;
otherwise null.
Let parsedValue be the result of parsing the given value with context if non-null.
If parsedValue is failure, then return.
Set this's fill style to parsedValue.
Return.
If the given value is a CanvasPattern object that is marked as not origin-clean, then set
this's origin-clean flag to
false.
Set this's fill style to the given value.
The strokeStyle getter steps are:
If this's stroke style is a CSS color, then return the serialization of that color with HTML-compatible serialization requested.
Return this's stroke style.
The strokeStyle setter steps are:
If the given value is a string, then:
Let context be this's canvas attribute's value, if that is an element;
otherwise null.
Let parsedValue be the result of parsing the given value with context if non-null.
If parsedValue is failure, then return.
Set this's stroke style to parsedValue.
Return.
If the given value is a CanvasPattern object that is marked as not origin-clean, then set
this's origin-clean flag to
false.
Set this's stroke style to the given value.
There are three types of gradients, linear gradients, radial gradients, and conic gradients,
represented by objects implementing the opaque CanvasGradient interface.
Once a gradient has been created (see below), stops are placed along it to define how the colors are distributed along the gradient. 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.
gradient.addColorStop(offset, color)Support in all current engines.
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.
Throws an "IndexSizeError" DOMException if the offset
is out of range. Throws a "SyntaxError" DOMException if
the color cannot be parsed.
gradient = context.createLinearGradient(x0, y0, x1, y1)CanvasRenderingContext2D/createLinearGradient
Support in all current engines.
Returns a CanvasGradient object that represents a linear gradient that paints
along the line given by the coordinates represented by the arguments.
gradient = context.createRadialGradient(x0, y0, r0, x1, y1, r1)CanvasRenderingContext2D/createRadialGradient
Support in all current engines.
Returns a CanvasGradient object that represents a radial gradient that paints
along the cone given by the circles represented by the arguments.
If either of the radii are negative, throws an "IndexSizeError"
DOMException.
gradient = context.createConicGradient(startAngle, x, y)CanvasRenderingContext2D/createConicGradient
Support in all current engines.
Returns a CanvasGradient object that represents a conic gradient that paints
clockwise along the rotation around the center represented by the arguments.
The addColorStop(offset,
color) method on the CanvasGradient, when invoked, must run
these steps:
If the offset is less than 0 or greater than 1, then throw an
"IndexSizeError" DOMException.
Let parsed color be the result of parsing color.
No element is passed to the parser because CanvasGradient objects
are canvas-neutral — a CanvasGradient object created by one
canvas can be used by another, and there is therefore no way to know which is the
"element in question" at the time that the color is specified.
If parsed color is failure, throw a "SyntaxError"
DOMException.
Place a new stop on the gradient, at offset offset relative to the whole gradient, and with the color parsed color.
If multiple stops are added at the same offset on a gradient, then they must be placed in the order added, with the first one closest to the start of the gradient, and each subsequent one infinitesimally further along towards the end point (in effect causing all but the first and last stop added at each point to be ignored).
The createLinearGradient(x0, y0,
x1, y1) method takes four arguments that represent the start
point (x0, y0) and end point (x1, y1) of the gradient.
The method, when invoked, must return a linear CanvasGradient initialized with the
specified line.
Linear gradients must be rendered such that all points on a line perpendicular to the line that crosses the start and end points have the color at the point where those two lines cross (with the colors coming from the interpolation and extrapolation described above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.
If x0 = x1 and y0 = y1, then the linear gradient must paint nothing.
The createRadialGradient(x0, y0,
r0, x1, y1, r1) method takes six
arguments, the first three representing the start circle with origin (x0,
y0) and radius r0, and the last three representing the end circle with
origin (x1, y1) and radius r1. The values are in coordinate space
units. If either of r0 or r1 are negative, then an
"IndexSizeError" DOMException must be thrown. Otherwise,
the method, when invoked, must return a radial CanvasGradient initialized with the
two specified circles.
Radial gradients must be rendered by following these steps:
If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing. Return.
Let x(ω) = (x1-x0)ω + x0.
Let y(ω) = (y1-y0)ω + y0.
Let r(ω) = (r1-r0)ω + r0.
Let the color at ω be the color at that position on the gradient (with the colors coming from the interpolation and extrapolation described above).
For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the color at ω, but only painting on the parts of the bitmap that have not yet been painted on by earlier circles in this step for this rendering of the gradient.
This effectively creates a cone, touched by the two circles defined in the creation of the gradient, with the part of the cone before the start circle (0.0) using the 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).
The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.
The createConicGradient(startAngle,
x, y) method takes three arguments, the first argument,
startAngle, represents the angle in radians at which the gradient begins, and the last
two arguments, (x, y), represent the center of the gradient in CSS pixels. The method, when invoked, must return a conic
CanvasGradient initialized with the specified center and angle.
It follows the same rendering rule as CSS 'conic-gradient' and it is equivalent to CSS 'conic-gradient(from adjustedStartAnglerad at xpx ypx, angularColorStopList)'. Here:
adjustedStartAngle is given by startAngle + π/2;
angularColorStopList is given by the color stops that have been added to the
CanvasGradient using addColorStop(), with the color stop offsets
interpreted as percentages.
Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.
Patterns are represented by objects implementing the opaque CanvasPattern
interface.
pattern = context.createPattern(image, repetition)CanvasRenderingContext2D/createPattern
Support in all current engines.
Returns a CanvasPattern object that uses the given image and repeats in the
direction(s) given by the repetition argument.
The allowed values for repetition are repeat (both
directions), repeat-x (horizontal only), repeat-y
(vertical only), and no-repeat (neither). If the repetition
argument is empty, the value repeat is used.
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an "InvalidStateError" DOMException.
pattern.setTransform(transform)Support in all current engines.
Sets the transformation matrix that will be used when rendering the pattern during a fill or stroke painting operation.
The createPattern(image,
repetition) method, when invoked, must run these steps:
Let usability be the result of checking the usability of image.
If usability is bad, then return null.
Assert: usability is good.
If repetition is the empty string, then set it to "repeat".
If repetition is not identical to one of "repeat", "repeat-x", "repeat-y",
or "no-repeat", then throw a
"SyntaxError" DOMException.
Let pattern be a new CanvasPattern object with the image
image and the repetition behavior given by repetition.
If image is not origin-clean, then mark pattern as not origin-clean.
Return pattern.
Modifying the image used when creating a CanvasPattern object
after calling the createPattern() method must
not affect the pattern(s) rendered by the CanvasPattern object.
Patterns have a transformation matrix, which controls how the pattern is used when it is painted. Initially, a pattern's transformation matrix must be the identity matrix.
The setTransform(transform) method,
when invoked, must run these steps:
Let matrix be the result of creating a DOMMatrix from the 2D dictionary
transform.
If one or more of matrix's m11 element, m12 element, m21 element, m22 element, m41 element, or m42 element are infinite or NaN, then return.
Reset the pattern's transformation matrix to matrix.
When a pattern is to be rendered within an area, the user agent must run the following steps to determine what is rendered:
Create an infinite transparent black bitmap.
Place a copy of the image on the bitmap, anchored such that its top left corner is at the
origin of the coordinate space, with one coordinate space unit per CSS
pixel of the image, then place repeated copies of this image horizontally to the left and
right, if the repetition behavior is "repeat-x", or vertically up and
down, if the repetition behavior is "repeat-y", or in all four
directions all over the bitmap, if the repetition behavior is "repeat".
If the original image data is a bitmap image, then the value painted at a point in the area
of the repetitions is computed by filtering the original image data. When scaling up, if the
imageSmoothingEnabled attribute is
set to false, then the image must be rendered using nearest-neighbor interpolation. Otherwise,
the user agent may use any filtering algorithm (for example bilinear interpolation or
nearest-neighbor). User agents which support multiple filtering algorithms may use the value of
the imageSmoothingQuality attribute
to guide the choice of filtering algorithm. When such a filtering algorithm requires a pixel
value from outside the original image data, it must instead use the value from wrapping the
pixel's coordinates to the original image's dimensions. (That is, the filter uses 'repeat'
behavior, regardless of the value of the pattern's repetition behavior.)
Transform the resulting bitmap according to the pattern's transformation matrix.
Transform the resulting bitmap again, this time according to the current transformation matrix.
Replace any part of the image outside the area in which the pattern is to be rendered with transparent black.
The resulting bitmap is what is to be rendered, with the same origin and same scale.
If a radial gradient or repeated pattern is used when the transformation matrix is singular, then the resulting style must be transparent black (otherwise the gradient or pattern would be collapsed to a point or line, leaving the other pixels undefined). Linear gradients and solid colors always define all points even with singular transformation matrices.
Objects that implement the CanvasRect interface provide the following methods for
immediately drawing rectangles to the bitmap. The methods each take four arguments; the first two
give the x and y coordinates of the top left of the rectangle, and the
second two give the width w and height h of the rectangle, respectively.
The current transformation matrix must be applied to the following four coordinates, which form the path that must then be closed to get the specified rectangle: (x, y), (x+w, y), (x+w, y+h), (x, y+h).
Shapes are painted without affecting the current default path, and are subject to
the clipping region, and, with the exception of clearRect(), also shadow
effects, global alpha, and the
current compositing and blending operator.
context.