canvas elementPath2D objectsImageBitmap 渲染上下文OffscreenCanvas interfacecanvas 元素的安全问题canvas 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 dimensionheight — Vertical dimensiontypedef (CanvasRenderingContext2D or ImageBitmapRenderingContext or WebGLRenderingContext or WebGL2RenderingContext) 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 intrinsic dimensions of the canvas element when it
represents embedded content are equal to the dimensions of the
element's bitmap.
The user agent must use a square pixel density consisting of one pixel of image data per
coordinate space unit for the bitmaps of a canvas and its rendering contexts.
A canvas element can be sized arbitrarily by a style sheet, its
bitmap is then subject to the 'object-fit' CSS property.
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 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, or webgl2 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 an intrinsic width equal
to the numeric value of the element's width attribute and an intrinsic height equal to
the numeric value of the element's height attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or
removed.
When 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 calling the commit() method of the OffscreenCanvas
object's rendering context.
When a canvas element represents embedded content, it provides a
paint source whose width is the element's intrinsic width, whose height
is the element's intrinsic height, and whose appearance is the element's bitmap.
Whenever the width and height content attributes are set, removed, changed, or
redundantly set to the value they already have, 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] | |
|
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.
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", or "webgl2". options is handled by that API.
This specification defines the "2d" and "bitmaprenderer" contexts below. The WebGL
specifications define the "webgl" and "webgl2" contexts. [WEBGL]
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 | placeholder | |
|---|---|---|---|---|---|
"2d"
|
Follow the 2D context creation algorithm defined in the section below, passing
it this canvas element and options, to obtain a
CanvasRenderingContext2D object; if this does not throw an exception, then set
this canvas element's context
mode to 2d, and return the
CanvasRenderingContext2D object.
| 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.
|
"bitmaprenderer"
|
Follow the ImageBitmapRenderingContext creation algorithm defined
in the section below, passing it this canvas element and options, to
obtain an ImageBitmapRenderingContext object; then set this canvas
element's context mode to bitmaprenderer, and return the
ImageBitmapRenderingContext object.
| 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.
|
"webgl" or "webgl2", if the user agent supports the WebGL
feature in its current configuration
|
Follow the instructions given in the WebGL specifications' Context Creation sections
to obtain a WebGLRenderingContext, WebGL2RenderingContext, or
null; if the returned value is null, then return null; otherwise, set this
canvas element's context mode
to webgl or webgl2, and return the
WebGLRenderingContext or WebGL2RenderingContext object. [WEBGL]
| 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. |
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.
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:,".)
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.
transferControlToOffscreen()HTMLCanvasElement/transferControlToOffscreen
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 intrinsic size can no longer be
changed, and it cannot have a rendering context. The content of the placeholder canvas is
updated by calling the commit() method of the
OffscreenCanvas object's rendering context.
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 ».
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 placeholder canvas
element of offscreenCanvas to be a weak reference to this canvas
element.
Set this canvas element's context
mode to placeholder.
Return offscreenCanvas.
Support in all current engines.
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) CanvasImageSource;
enum CanvasFillRule { "nonzero", "evenodd" };
dictionary CanvasRenderingContext2DSettings {
boolean alpha = true;
boolean desynchronized = false;
};
enum ImageSmoothingQuality { "low", "medium", "high" };
[Exposed=Window]
interface CanvasRenderingContext2D {
// back-reference to the canvas
readonly attribute HTMLCanvasElement canvas;
CanvasRenderingContext2DSettings getContextAttributes();
};
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 CanvasState {
// state
undefined save(); // push state on state stack
undefined restore(); // pop state stack and restore state
};
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);
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);
undefined scrollPathIntoView();
undefined scrollPathIntoView(Path2D path);
};
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(long sw, long sh);
ImageData createImageData(ImageData imagedata);
ImageData getImageData(long sx, long sy, long sw, long sh);
undefined putImageData(ImageData imagedata, long dx, long dy);
undefined putImageData(ImageData imagedata, long dx, long dy, long dirtyX, long dirtyY, long dirtyWidth, 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" };
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 font; // (default 10px sans-serif)
attribute CanvasTextAlign textAlign; // (default: "start")
attribute CanvasTextBaseline textBaseline; // (default: "alphabetic")
attribute CanvasDirection direction; // (default: "inherit")
};
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 arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = 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 anticlockwise = 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),
Serializable]
interface ImageData {
constructor(unsigned long sw, unsigned long sh);
constructor(Uint8ClampedArray data, unsigned long sw, optional unsigned long sh);
readonly attribute unsigned long width;
readonly attribute unsigned long height;
readonly attribute Uint8ClampedArray data;
};
[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.
getContext('2d' [, { [ alpha: true ] [, desynchronized: 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.
canvasCanvasRenderingContext2D/canvas
Support in all current engines.
Returns the canvas element.
getContextAttributes()Returns an object whose:
alpha member is true if the context has an alpha
channel, or false if it was forced to be opaque.desynchronized
member is true if the context can be desynchronized.A CanvasRenderingContext2D 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 CanvasRenderingContext2D object also has an alpha boolean. When a
CanvasRenderingContext2D object's alpha is false, then its alpha channel must be fixed to 1.0
(fully opaque) for all pixels, and attempts to change the alpha component of any pixel must be
silently ignored.
Thus, 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 channel; 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 CanvasRenderingContext2D object also has a desynchronized boolean. When a
CanvasRenderingContext2D 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 getContextAttributes() method,
when invoked, must return a CanvasRenderingContext2DSettings dictionary containing
the following members:
desynchronized, set to this context's
desynchronized.
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.
Set context's alpha to
settings's alpha.
Set context's desynchronized to settings's desynchronized.
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 and clear it to transparent black.
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 remainsThe canvas attribute must return the value it was
initialized to when the object was created.
The CanvasFillRule enumeration is used to select the fill rule
algorithm by which to determine if a point is inside or outside a path.
The value "nonzero" value
indicates the 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.
输出位图，当没有被用户代理直接直接显示时，
实现中可以只记录已经应用的绘图操作序列，而不更新整个位图，直到需要位图的实际数据时。
（例如调用了 drawImage() 或
createImageBitmap() 工厂方法）
很多情况下这样可以提高内存的使用效率。
canvas 元素的位图在实践中总是需要的，
渲染上下文的 输出位图（如果有的话）
只是 canvas 元素的位图的别名。
有时也会需要额外的位图，比如为了使 canvas 在绘制大小与其 固有尺寸 不同时更快， 或者为了实现双缓冲来使得绘制命令继续执行的同时图像可以持续地更新（比如在页面滚动时）。
实现 CanvasState 接口的对象维护一个绘图状态栈。
绘图状态 这样组成：
strokeStyle, fillStyle, globalAlpha, lineWidth, lineCap, lineJoin, miterLimit, lineDashOffset, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, filter, globalCompositeOperation, font, textAlign, textBaseline, direction, imageSmoothingEnabled, imageSmoothingQuality。当前默认路径
和渲染上下文的位图不属于绘图状态。
当前默认路径 是持久的，只能使用
beginPath() 方法重置。
位图取决于渲染上下文是否（以及如何）绑定到 canvas 元素。
save()Support in all current engines.
把当前状态推入栈中。
restore()CanvasRenderingContext2D/restore
Support in all current engines.
把栈顶部状态弹出并恢复到这个状态。
save() 方法被调用时，
必须把当前绘图状态拷贝一份然后推入到绘图状态栈。
restore() 方法被调用时，
必须把绘图状态栈顶的项目弹出，按它的描述重置绘图状态。
如果没有保存的状态，这个方法必须什么都不做。
当用户代理要 重置渲染上下文到默认状态 时， 它必须清除绘图状态栈，并把构成 绘图状态 的所有东西设置回初始值。
lineWidth [ = value ]CanvasRenderingContext2D/lineWidth
Support in all current engines.
lineWidth [ = value ]Returns the current line width.
Can be set, to change the line width. Values that are not finite values greater than zero are ignored.
lineCap [ = value ]CanvasRenderingContext2D/lineCap
Support in all current engines.
lineCap [ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
The possible line cap styles are "butt", "round", and "square". Other values are ignored.
lineJoin [ = value ]CanvasRenderingContext2D/lineJoin
Support in all current engines.
lineJoin [ = value ]Returns the current line join style.
Can be set, to change the line join style.
The possible line join styles are "bevel", "round", and "miter". Other values are ignored.
miterLimit [ = value ]CanvasRenderingContext2D/miterLimit
Support in all current engines.
miterLimit [ = value ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
setLineDash(segments)CanvasRenderingContext2D/setLineDash
Support in all current engines.
setLineDash(segments)Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.
getLineDash()CanvasRenderingContext2D/getLineDash
Support in all current engines.
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).
lineDashOffsetCanvasRenderingContext2D/lineDashOffset
Support in all current engines.
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() method, when
invoked, must run these steps:
Let a be the argument.
If any value in a is not finite (e.g. an Infinity or a NaN value), or if any value is negative (less than zero), then 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 a is odd, then let a be the concatenation of two copies of a.
Let the object's dash list be a.
When the getLineDash() method is
invoked, it must return a sequence whose values are the values of the object's dash
list, in the same order.
It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching
ants" effect. The phase can be set using the lineDashOffset attribute. On getting, it must
return the current value. On setting, infinite and NaN values must be ignored, leaving the value
unchanged; other values must change the current value to the new value.
When the object implementing the 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 zero minus offset.
Let index be 0.
Let current state be off (the other states being on and zero-on).
Dash on: Let segment length be the value of 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.
If segment length is greater than zero, then let positioned-at-on-dash be false.
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.
font [ = value ]Support in all current engines.
font [ = value ]Returns the current font settings.
Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.
Relative keywords and lengths are computed relative to the font of the canvas
element.
textAlign [ = value ]CanvasRenderingContext2D/textAlign
Support in all current engines.
textAlign [ = value ]Returns the current text alignment settings.
Can be set, to change the alignment. The possible values are and their meanings are given
below. Other values are ignored. The default is "start".
textBaseline [ = value ]CanvasRenderingContext2D/textBaseline
Support in all current engines.
textBaseline [ = value ]Returns the current baseline alignment settings.
Can be set, to change the baseline alignment. The possible values and their meanings are
given below. Other values are ignored. The default is "alphabetic".
direction [ = value ]CanvasRenderingContext2D/direction
direction [ = value ]Returns the current directionality.
Can be set, to change the directionality. The possible values and their meanings are given
below. Other values are ignored. The default is "inherit".
Objects that implement the 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.postMesage(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.
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".
The textAlign attribute's allowed keywords are
as follows:
start
Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).
end
Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).
left
Align to the left.
right
Align to the right.
center
Align to the center.
The textBaseline
attribute's allowed keywords correspond to alignment points in the
font:
The keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
The direction attribute's allowed keywords are
as follows:
ltr
Treat input to the text preparation algorithm as left-to-right text.
rtl
Treat input to the text preparation algorithm as right-to-left text.
inherit
Default to the directionality of the canvas element or Document
as appropriate.
The text preparation algorithm is as follows. It takes as input a string text
, a 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.
Apply the appropriate step from the following list to determine the value of direction:
direction attribute has the value "ltr"direction attribute has the value "rtl"Document with a non-null document elementForm a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]
If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
The anchor point is a point on the inline box, and the physical
alignment is one of the values left, right, and center. These
variables are determined by the textAlign and
textBaseline values as follows:
Horizontal position:
textAlign is 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 is 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.
moveTo(x, y)CanvasRenderingContext2D/moveTo
Support in all current engines.
moveTo(x, y)Creates a new subpath with the given point.
closePath()CanvasRenderingContext2D/closePath
Support in all current engines.
closePath()Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
lineTo(x, y)CanvasRenderingContext2D/lineTo
Support in all current engines.
lineTo(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
quadraticCurveTo(cpx, cpy, x, y)CanvasRenderingContext2D/quadraticCurveTo
Support in all current engines.
quadraticCurveTo(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)CanvasRenderingContext2D/bezierCurveTo
Support in all current engines.
bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
arcTo(x1, y1, x2, y2, radius)CanvasRenderingContext2D/arcTo
Support in all current engines.
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.
arc(x, y, radius, startAngle, endAngle [, anticlockwise ] )Support in all current engines.
arc(x, y, radius, startAngle, endAngle [, anticlockwise ] )Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )CanvasRenderingContext2D/ellipse
Support in all current engines.
ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
rect(x, y, w, h)Support in all current engines.
rect(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
The following methods allow authors to manipulate the paths
of objects implementing the 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,
anticlockwise) method, when invoked, must run the ellipse method
steps with this, x, y, radius, radius, 0,
startAngle, endAngle, and anticlockwise.
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, anticlockwise) method, when invoked, must run the
ellipse method steps with this, x, y, radiusX,
radiusY, rotation, startAngle, endAngle, and
anticlockwise.
The ellipse method steps, given canvasPath, x, y, radiusX, radiusY, rotation, startAngle, endAngle, and anticlockwise, 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 anticlockwise is false and endAngle-startAngle is equal to or greater than 2π, or, if anticlockwise is true and startAngle-endAngle is equal to or greater than 2π, then the arc is the whole circumference of this ellipse, and the point at startAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, acts as both the start point and the end point.
Otherwise, the points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start and end points respectively, and the arc is the path along the circumference of this ellipse from the start point to the end point, going anti-clockwise if anticlockwise 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.
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.
Path2D()Support in all current engines.
Creates a new empty Path2D object.
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]
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(b,
transform) method, when invoked on a Path2D object
a, must run these steps:
If the Path2D object b 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 b. Let this copy be known as c.
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.
scale(x, y)CanvasRenderingContext2D/scale
Support in all current engines.
Changes the current transformation matrix to apply a scaling transformation with the given characteristics.
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.
translate(x, y)CanvasRenderingContext2D/translate
Support in all current engines.
Changes the current transformation matrix to apply a translation transformation with the given characteristics.
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.
getTransform()CanvasRenderingContext2D/getTransform
Support in all current engines.
Returns a copy of the current transformation matrix, as a newly created
DOMMatrix object.
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.
setTransform(transform)Changes the current transformation matrix to the matrix represented by
the passed DOMMatrix2DInit dictionary.
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 identity matrix.
Invoke the transform(a, b, c,
d, e, f) method with the same arguments.
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)ImageBitmapAlthough 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 an intrinsic width or intrinsic 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.
ImageBitmapIf 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 intrinsic width and intrinsic height of the media resource
(i.e. after any aspect-ratio correction has been applied).
When a CanvasImageSource object represents an HTMLCanvasElement, the
element's bitmap must be used as the source image.
When a CanvasImageSource object represents 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.
An object image is not origin-clean if, switching on image:
HTMLOrSVGImageElement
HTMLVideoElement
image's origin is not same origin with entry settings object's origin.
HTMLCanvasElement
ImageBitMap
image's bitmap's origin-clean flag is false.
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.
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.
The fillStyle attribute represents the
color or style to use inside shapes, and the strokeStyle attribute represents the color
or style to use for the lines around the shapes.
Both attributes can be either strings, CanvasGradients, or
CanvasPatterns. On setting, strings must be parsed with this canvas element and the color assigned, and
CanvasGradient and CanvasPattern objects must be assigned themselves. If
parsing the value results in failure, then it must be ignored, and the attribute must retain its
previous value. If the new value is a CanvasPattern object that is marked as not origin-clean, then the
CanvasRenderingContext2D's origin-clean flag must be set to false.
When set to a CanvasPattern or CanvasGradient object, the assignment
is live, meaning that changes made to the object after the assignment do affect
subsequent stroking or filling of shapes.
On getting, if the value is a color, then the serialization of the color must be returned. Otherwise, if it is not a color but a
CanvasGradient or CanvasPattern, then the respective object must be
returned. (Such objects are opaque and therefore only useful for assigning to other attributes or
for comparison to other gradients or patterns.)
The serialization of a color for a color
value is a string, computed as follows: if it has alpha equal to 1.0, then the string is a
lowercase six-digit hex value, prefixed with a "#" character (U+0023 NUMBER SIGN), with the first
two digits representing the red component, the next two digits representing the green component,
and the last two digits representing the blue component, the digits being ASCII lower hex digits. Otherwise, the color value has alpha less than
1.0, and the string is the color value in the CSS rgba()
functional-notation format: the literal string "rgba" (U+0072 U+0067 U+0062
U+0061) followed by a U+0028 LEFT PARENTHESIS, a base-ten integer in the range 0-255 representing
the red component (using ASCII digits in the shortest form possible), a literal
U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer
for the blue component, another comma and space, a U+0030 DIGIT ZERO, if the alpha value is
greater than zero then a U+002E FULL STOP (representing the decimal point), if the alpha value is
greater than zero then one or more ASCII digits representing the fractional part of
the alpha, and finally a U+0029
RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with
the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same
alpha value.
When the context is created, the fillStyle and strokeStyle attributes
must initially have the string value #000000.
When the value is a color, it must not be affected by the transformation matrix when used to draw on bitmaps.
There are two types of gradients, linear gradients and radial gradients, both represented by
objects implementing the opaque CanvasGradient interface.
Once a gradient has been created (see below), stops are placed along it to define how the 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.
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.
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.
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 exception.
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.
Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.
Patterns are represented by objects implementing the opaque CanvasPattern
interface.
createPattern(image, repetition)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.
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 result is bad, then return null.
Assert: result 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 global composition operators.
clearRect(x, y, w, h)CanvasRenderingContext2D/clearRect
Support in all current engines.
Clears all pixels on the bitmap in the given rectangle to transparent black.
fillRect(x, y, w, h)CanvasRenderingContext2D/fillRect
Support in all current engines.
Paints the given rectangle onto the bitmap, using the current fill style.
strokeRect(x, y, w, h)CanvasRenderingContext2D/strokeRect
Support in all current engines.
Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.
The clearRect(x, y,
w, h) method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then return.
Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.
Clear the pixels in pixels to a transparent black, erasing any previous image.
If either height or width are zero, this method has no effect, since the set of pixels would be empty.
The fillRect(x, y,
w, h) method, when invoked, must must run these steps:
If any of the arguments are infinite or NaN, then return.
If either w or h are zero, then return.
Paint the specified rectangular area using the fillStyle.
The strokeRect(x, y,
w, h) method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then return.
Take the result of tracing the path described below,
using the CanvasPathDrawingStyles interface's line styles, and fill it with the
strokeStyle.
If both w and h are zero, the path has a single subpath with just one point (x, y), and no lines, and this method thus has no effect (the trace a path algorithm returns an empty path in that case).
If just one of either w or h is zero, then the path has a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.
Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w, y+h), and (x, y+h), connected to each other in that order by straight lines.
Support in all current engines.
fillText(text, x, y [, maxWidth ] )CanvasRenderingContext2D/fillText
Support in all current engines.
strokeText(text, x, y [, maxWidth ] )CanvasRenderingContext2D/strokeText
Support in all current engines.
Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.
measureText(text)CanvasRenderingContext2D/measureText
Support in all current engines.
Returns a TextMetrics object with the metrics of the given text in the current
font.
widthSupport in all current engines.
actualBoundingBoxLeftTextMetrics/actualBoundingBoxLeft
Support in all current engines.
actualBoundingBoxRightTextMetrics/actualBoundingBoxRight
Support in all current engines.
fontBoundingBoxAscentTextMetrics/fontBoundingBoxAscent
Support in all current engines.
fontBoundingBoxDescentTextMetrics/fontBoundingBoxDescent
Support in all current engines.
actualBoundingBoxAscentTextMetrics/actualBoundingBoxAscent
Support in all current engines.
actualBoundingBoxDescentTextMetrics/actualBoundingBoxDescent
Support in all current engines.
emHeightAscentSupport in all current engines.
emHeightDescentSupport in all current engines.
hangingBaselinealphabeticBaselineTextMetrics/alphabeticBaseline
ideographicBaselineTextMetrics/ideographicBaseline
Returns the measurement described below.
Objects that implement the CanvasText interface provide the following methods for
rendering text.
The fillText() and strokeText() methods take three or four arguments,
text, x, y, and optionally maxWidth, and render the
given text at the given (x, y) coordinates ensuring that the text
isn't wider than maxWidth if specified, using the current font, textAlign, and textBaseline values. Specifically, when the methods
are invoked, the user agent must run these steps:
If any of the arguments are infinite or NaN, then return.
Run the text preparation algorithm, passing it text, the object
implementing the CanvasText interface, and, if the maxWidth argument was
provided, that argument. Let glyphs be the result.
Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.
Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.
For fillText(), fillStyle must be applied to the shapes and strokeStyle must be ignored. For strokeText(), the reverse holds: strokeStyle must be applied to the result of tracing the shapes using the object implementing the
CanvasText interface for the line styles, and fillStyle must be ignored.
These shapes are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.
The measureText() method takes one
argument, text. When the method is invoked, the user agent must run the text
preparation algorithm, passing it text and the object implementing the
CanvasText interface, and then using the returned inline box must return
a new TextMetrics object with members behaving as described in the following list:
[CSS]
width
attributeThe width of that inline box, in CSS pixels. (The text's advance width.)
actualBoundingBoxLeft attributeThe distance parallel to the baseline from the alignment point given by the textAlign attribute to the left side of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going left from the given alignment point.
The sum of this value and the next (actualBoundingBoxRight) can be wider than
the width of the inline box (width), in
particular with slanted fonts where characters overhang their advance width.
actualBoundingBoxRight attributeThe distance parallel to the baseline from the alignment point given by the textAlign attribute to the right side of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going right from the given alignment point.
fontBoundingBoxAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the ascent
metric of the first available font, in CSS
pixels; positive numbers indicating a distance going up from the given baseline.
This value and the next are useful when rendering a background that have to have
a consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent attribute (and
its corresponding attribute for the descent) are useful when drawing a bounding box around
specific text.
fontBoundingBoxDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the descent
metric of the first available font, in CSS pixels;
positive numbers indicating a distance going down from the given baseline.
actualBoundingBoxAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the top of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going up from the given baseline.
This number can vary greatly based on the input text, even if the first font
specified covers all the characters in the input. For example, the actualBoundingBoxAscent of a lowercase
"o" from an alphabetic baseline would be less than that of an uppercase "F". The
value can easily be negative; for example, the distance from the top of the em box (textBaseline value "top") to the top of the bounding rectangle when
the given text is just a single comma "," would likely (unless the font is
quite unusual) be negative.
actualBoundingBoxDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the bottom of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going down from the given baseline.
emHeightAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the highest top of the em
squares in the inline box, in CSS pixels; positive numbers
indicating that the given baseline is below the top of that em square (so this value will usually
be positive). Zero if the given baseline is the top of that em square; half the font size if the
given baseline is the middle of that em square.
emHeightDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the lowest bottom of the em
squares in the inline box, in CSS pixels; positive numbers
indicating that the given baseline is above the bottom of that em square. (Zero if the given baseline
is the bottom of that em square.)
hangingBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the hanging
baseline of the inline box, in CSS pixels;
positive numbers indicating that the given baseline is below the hanging baseline.
(Zero if the given baseline is the hanging baseline.)
alphabeticBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the alphabetic
baseline of the inline box, in CSS pixels;
positive numbers indicating that the given baseline is below the alphabetic
baseline. (Zero if the given baseline is the alphabetic baseline.)
ideographicBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the ideographic-under
baseline of the inline box, in CSS pixels;
positive numbers indicating that the given baseline is below the ideographic-under
baseline. (Zero if the given baseline is the ideographic-under
baseline.)
Glyphs rendered using fillText() and
strokeText() can spill out of the box given by the
font size (the em square size) and the width returned by measureText() (the text width). Authors are encouraged
to use the bounding box values described above if this is an issue.
A future version of the 2D context API might provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.
Objects that implement the CanvasDrawPath interface have a current default
path. There is only one current default path, it is not part of the
drawing state. The current default path is a path, as described above.
beginPath()CanvasRenderingContext2D/beginPath
Support in all current engines.
Resets the current default path.
fill( [ fillRule ] )Support in all current engines.
fill(path [, fillRule ] )Fills the subpaths of the current default path or the given path with the current fill style, obeying the given fill rule.
stroke()CanvasRenderingContext2D/stroke
Support in all current engines.
stroke(path)Strokes the subpaths of the current default path or the given path with the current stroke style.
clip( [ fillRule ] )Support in all current engines.
clip(path [, fillRule ] )Further constrains the clipping region to the current default path or the given path, using the given fill rule to determine what points are in the path.
isPointInPath(x, y [, fillRule ] )CanvasRenderingContext2D/isPointInPath
Support in all current engines.
isPointInPath(path, x, y [, fillRule ] )Returns true if the given point is in the current default path or the given path, using the given fill rule to determine what points are in the path.
isPointInStroke(x, y)CanvasRenderingContext2D/isPointInStroke
Support in all current engines.
isPointInStroke(path, x, y)Returns true if the given point would be in the region covered by the stroke of the current default path or the given path, given the current stroke style.
The beginPath() method, when invoked,
must empty the list of subpaths in the context's current default path so that it
once again has zero subpaths.
Where the following method definitions use the term intended path, it means the
Path2D argument, if one was provided, or the current default path
otherwise.
When the intended path is a Path2D object, the coordinates and lines of its
subpaths must be transformed according to the
current transformation matrix on the object implementing the
CanvasTransform interface when used by these methods (without affecting the
Path2D object itself). When the intended path is the current default
path, it is not affected by the transform. (This is because transformations already affect
the current default path when it is constructed, so applying it when it is painted as
well would result in a double transformation.)
The fill()
method, when invoked, must fill all the subpaths of the intended path, using fillStyle, and using the fill rule indicated
by the fillRule argument. Open subpaths must be implicitly closed when being filled
(without affecting the actual subpaths).
The stroke()
method, when invoked, must trace the intended path, using this
CanvasPathDrawingStyles object for the line styles, and then fill the resulting path
using the strokeStyle attribute, using the nonzero winding rule.
As a result of how the algorithm to trace a path is defined, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.
The stroke style is affected by the transformation during painting, even if the intended path is the current default path.
Paths, when filled or stroked, must be painted without affecting the current default
path or any Path2D objects, and must be subject to shadow effects, global
alpha, the clipping region, and global composition operators. (The effect
of transformations is described above and varies based on which path is being used.)
The clip()
method, when invoked, must create a new clipping region by calculating the intersection
of the current clipping region and the area described by the intended path, using the fill
rule indicated by the fillRule argument. Open subpaths must be implicitly closed
when computing the clipping region, without affecting the actual subpaths. The new clipping region
replaces the current clipping region.
When the context is initialized, the clipping region must be set to the largest infinite surface (i.e. by default, no clipping occurs).
The isPointInPath() method, when
invoked, must return true if the point given by the x and y coordinates
passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the
current transformation, is inside the intended path as determined by the fill rule
indicated by the fillRule argument; and must return false otherwise. Open subpaths must
be implicitly closed when computing the area inside the path, without affecting the actual
subpaths. Points on the path itself must be considered to be inside the path. If either of the
arguments are infinite or NaN, then the method must return false.
The isPointInStroke() method,
when invoked, must return true if the point given by the x and y coordinates
passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the
current transformation, is inside the path that results from tracing the intended path, using the nonzero winding rule, and using the
CanvasPathDrawingStyles interface for the line styles; and must return false
otherwise. Points on the resulting path must be considered to be inside the path. If either of the
arguments are infinite or NaN, then the method must return false.
This canvas element has a couple of checkboxes. The path-related commands are
highlighted:
<canvas height=400 width=750>
<label><input type=checkbox id=showA> Show As</label>
<label><input type=checkbox id=showB> Show Bs</label>
<!-- ... -->
</canvas>
<script>
function drawCheckbox(context, element, x, y, paint) {
context.save();
context.font = '10px sans-serif';
context.textAlign = 'left';
context.textBaseline = 'middle';
var metrics = context.measureText(element.labels[0].textContent);
if (paint) {
context.beginPath();
context.strokeStyle = 'black';
context.rect(x-5, y-5, 10, 10);
context.stroke();
if (element.checked) {
context.fillStyle = 'black';
context.fill();
}
context.fillText(element.labels[0].textContent, x+5, y);
}
context.beginPath();
context.rect(x-7, y-7, 12 + metrics.width+2, 14);
context.drawFocusIfNeeded(element);
context.restore();
}
function drawBase() { /* ... */ }
function drawAs() { /* ... */ }
function drawBs() { /* ... */ }
function redraw() {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
drawCheckbox(context, document.getElementById('showA'), 20, 40, true);
drawCheckbox(context, document.getElementById('showB'), 20, 60, true);
drawBase();
if (document.getElementById('showA').checked)
drawAs();
if (document.getElementById('showB').checked)
drawBs();
}
function processClick(event) {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
var x = event.clientX;
var y = event.clientY;
var node = event.target;
while (node) {
x -= node.offsetLeft - node.scrollLeft;
y -= node.offsetTop - node.scrollTop;
node = node.offsetParent;
}
drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
if (context.isPointInPath(x, y))
document.getElementById('showA').checked = !(document.getElementById('showA').checked);
drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
if (context.isPointInPath(x, y))
document.getElementById('showB').checked = !(document.getElementById('showB').checked);
redraw();
}
document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false);
redraw();
</script>drawFocusIfNeeded(element)CanvasRenderingContext2D/drawFocusIfNeeded
Support in all current engines.
drawFocusIfNeeded(path, element)If the given element is focused, draws a focus ring around the current default path or the given path, following the platform conventions for focus rings.
scrollPathIntoView()CanvasRenderingContext2D/scrollPathIntoView
Support in one engine only.
scrollPathIntoView(path)Scrolls the current default path or the given path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.
CanvasUserInterface interface provide the following
methods to control drawing focus rings and scrolling paths into view.
The drawFocusIfNeeded(element)
method, when invoked, must run these steps:
If element is not focused or is not a descendant of the element with whose context the method is associated, then return.
Draw a focus ring of the appropriate style along the intended path, following platform conventions.
Some platforms only draw focus rings around elements that have been focused from
the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus
rings around some elements at all unless relevant accessibility features are enabled. This API
is intended to follow these conventions. User agents that implement distinctions based on the
manner in which the element was focused are encouraged to classify focus driven by the focus() method based on the kind of user interaction event from which
the call was triggered (if any).
The focus ring should not be subject to the shadow effects, the
global alpha, the global composition operators, or any of
the members in the CanvasFillStrokeStyles, CanvasPathDrawingStyles,
CanvasTextDrawingStyles interfaces, but should be subject to the
clipping region. (The effect of transformations is described above and varies based
on which path is being used.)
Inform the user that the focus is at the location given by the intended path. User agents may wait until the next time the event loop reaches its update the rendering step to optionally inform the user.
User agents should not implicitly close open subpaths in the intended path when drawing the focus ring.
This might be a moot point, however. For example, if the focus ring is drawn as an axis-aligned bounding rectangle around the points in the intended path, then whether the subpaths are closed or not has no effect. This specification intentionally does not specify precisely how focus rings are to be drawn: user agents are expected to honor their platform's native conventions.
The scrollPathIntoView()
method, when invoked, must run these steps:
Let specifiedRectangle be the rectangle of the bounding box of the intended path.
Let notionalChild be a hypothetical element that is a rendered child of the
canvas element whose dimensions are those of specifiedRectangle.
Scroll notionalChild into view with behavior set to "auto", block set to "start", and inline set to "nearest".
Optionally, inform the user that the caret or selection (or both) cover specifiedRectangle of the canvas. The user agent may wait until the next time the event loop reaches its update the rendering step to optionally inform the user.
"Inform the user", as used in this section, does not imply any persistent state change. It could mean, for instance, calling a system accessibility API to notify assistive technologies such as magnification tools so that the user's magnifier moves to the given area of the canvas. However, it does not associate the path with the element, or provide a region for tactile feedback, etc.
Objects that implement the CanvasDrawImage interface have the drawImage method to draw images.
This method can be invoked with three different sets of arguments:
drawImage(image, dx, dy)
drawImage(image, dx, dy, dw, dh)
drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
drawImage(image, dx, dy)CanvasRenderingContext2D/drawImage
Support in all current engines.
drawImage(image, dx, dy, dw, dh)drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)Draws the given image onto the canvas. The arguments are interpreted as follows:
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an "InvalidStateError" DOMException.
When the drawImage() method is invoked, the user
agent must run these steps:
If any of the arguments are infinite or NaN, then return.
Let usability be the result of checking the usability of image.
If usability is bad, then return (without drawing anything).
Establish the source and destination rectangles as follows:
If not specified, the dw and dh arguments must default to the values of sw and sh, interpreted such that one CSS pixel in the image is treated as one unit in the output bitmap's coordinate space. If the sx, sy, sw, and sh arguments are omitted, then they must default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively. If the image has no intrinsic dimensions, then the concrete object size must be used instead, as determined using the CSS "Concrete Object Size Resolution" algorithm, with the specified size having neither a definite width nor height, nor any additional constraints, the object's intrinsic properties being those of the image argument, and the default object size being the size of the output bitmap. [CSSIMAGES]
The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).
The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).
When the source rectangle is outside the source image, the source rectangle must be clipped to the source image and the destination rectangle must be clipped in the same proportion.
When the destination rectangle is outside the destination image (the output bitmap), the pixels that land outside the output bitmap are discarded, as if the destination was an infinite canvas whose rendering was clipped to the dimensions of the output bitmap.
If one of the sw or sh arguments is zero, then return. Nothing is painted.
Paint the region of the image argument specified by the source rectangle on the region of the rendering context's output bitmap specified by the destination rectangle, after applying the current transformation matrix to the destination rectangle.
The image data must be processed in the original direction, even if the dimensions given are negative.
When scaling up, if the imageSmoothingEnabled attribute is set to
true, the user agent should attempt to apply a smoothing algorithm to the image data when it is
scaled. User agents which support multiple filtering algorithms may use the value of the imageSmoothingQuality attribute to guide
the choice of filtering algorithm when the imageSmoothingEnabled attribute is set to
true. Otherwise, the image must be rendered using nearest-neighbor interpolation.
This specification does not define the precise algorithm to use when scaling an
image down, or when scaling an image up when the imageSmoothingEnabled attribute is set to
true.
When a canvas element is drawn onto itself, the drawing
model requires the source to be copied before the image is drawn, so it is possible to
copy parts of a canvas element onto overlapping parts of itself.
If the original image data is a bitmap image, then the value painted at a point in the destination rectangle is computed by filtering the original image data. The user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behavior.) When the filtering algorithm requires a pixel value from outside the source rectangle but inside the original image data, then the value from the original image data must be used.
Thus, scaling an image in parts or in whole will have the same effect. This does
mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in
the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is
surrounded by a border of transparent black, or by copying sprites to be scaled
into temporary canvas elements and drawing the scaled sprites from there.
Images are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.
If image is not origin-clean, then set the
CanvasRenderingContext2D's origin-clean flag to false.
ImageData(sw, sh)Support in all current engines.
createImageData(sw, sh)CanvasRenderingContext2D/createImageData
Support in all current engines.
Returns an ImageData object with the given dimensions. All the pixels in the
returned object are transparent black.
Throws an "IndexSizeError" DOMException if either of
the width or height arguments are zero.
createImageData(imagedata)Returns an ImageData object with the same dimensions as the argument. All the
pixels in the returned object are transparent black.
ImageData(data, sw [, sh ] )Returns an ImageData object using the data provided in the Uint8ClampedArray argument, interpreted using the given
dimensions.
As each pixel in the data is represented by four numbers, the length of the data needs to be a multiple of four times the given width. If the height is provided as well, then the length needs to be exactly the width times the height times 4.
Throws an "IndexSizeError" DOMException if the given
data and dimensions can't be interpreted consistently, or if either dimension is zero.
getImageData(sx, sy, sw, sh)CanvasRenderingContext2D/getImageData
Support in all current engines.
Returns an ImageData object containing the image data for the given rectangle of
the bitmap.
Throws an "IndexSizeError" DOMException if the either
of the width or height arguments are zero.
widthSupport in all current engines.
heightSupport in all current engines.
Returns the actual dimensions of the data in the ImageData object, in
pixels.
dataSupport in all current engines.
Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.
putImageData(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ] )CanvasRenderingContext2D/putImageData
Support in all current engines.
Paints the data from the given ImageData object onto the bitmap. If a dirty
rectangle is provided, only the pixels from that rectangle are painted.
The globalAlpha and globalCompositeOperation attributes, as
well as the shadow attributes, are ignored for the purposes of this method call; pixels in the
canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.
Throws an "InvalidStateError" DOMException if the
imagedata object's data attribute value's
[[ViewedArrayBuffer]] internal slot is detached.
Objects that implement the CanvasImageData interface provide the following methods
for reading and writing pixel data to the bitmap.
The ImageData()
constructors and the createImageData() methods are used to
instantiate new ImageData objects.
When the ImageData() constructor is invoked with two
numeric arguments sw and sh, it must run these steps:
If one or both of sw and sh are zero, then throw an
"IndexSizeError" DOMException.
Create an ImageData object with parameter
pixelsPerRow set to sw, and rows set to sh.
Initialize the image data of the newly created ImageData object to
transparent black.
Return the newly created ImageData object.
When the ImageData() constructor is invoked with its first
argument being an Uint8ClampedArray source
and its second and optional third arguments being numeric arguments sw and
sh, it must run these steps:
Let length be the number of bytes in source.
If length is not a nonzero integral multiple of four, then throw an
"InvalidStateError" DOMException.
Let length be length divided by four.
If length is not an integral multiple of sw, then throw an
"IndexSizeError" DOMException.
At this step, the length is guaranteed to be greater than zero (otherwise the second step above would have aborted the steps), so if sw is zero, this step will throw the exception and return.
Let height be length divided by sw.
If the sh argument was not omitted, and its value is not equal to
height, then throw an "IndexSizeError"
DOMException.
Create an ImageData object, with parameter pixelsPerRow
set to sw, rows set to sh, and using source.
Return the newly created ImageData object.
The resulting object's data is not a copy of source, it's
the actual Uint8ClampedArray object passed as the
first argument to the constructor.
When the createImageData() method is
invoked with two numeric arguments sw and sh, it must create an
ImageData object, with parameter pixelsPerRow set to the
absolute magnitude of sw, and parameter rows set to the absolute magnitude
of sh. Initialize the image data of the new ImageData object to
transparent black. If both sw and sh are nonzero, then return
the new ImageData object. If one or both of sw and sh are
zero, then throw an "IndexSizeError" DOMException
instead.
When the createImageData() method is
invoked with a single imagedata argument, it must create an
ImageData object, with parameter pixelsPerRow set to the value of
the width attribute of the ImageData
object passed as the argument, and the rows parameter set to the value of the
height attribute.
Initialize the image data of the new ImageData object to transparent
black. Return the newly created ImageData object.
The getImageData(sx,
sy, sw, sh) method, when invoked, must,
if either the sw or sh arguments are zero, throw an
"IndexSizeError" DOMException; otherwise,
if the CanvasRenderingContext2D's origin-clean flag is set to false, it must throw a
"SecurityError" DOMException;
otherwise, it must create an ImageData object, with parameter
pixelsPerRow set to sw, and parameter rows set to sh.
Set the pixel values of the image data of the newly created ImageData object to
represent the output bitmap for the area of that bitmap denoted by the rectangle
whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx,
sy+sh), in the bitmap's coordinate space units.
Pixels outside the output bitmap must be set to transparent black. Pixel
values must not be premultiplied by alpha.
When the user agent is required to create an ImageData object, given a
positive integer number of rows rows, a positive integer number of pixels per row
pixelsPerRow, and an optional Uint8ClampedArray source, it must run these
steps:
Let imageData be a new uninitialized ImageData object.
If source is specified, then assign the data attribute of imageData to
source.
If source is not specified, then initialize the data attribute of imageData to a new Uint8ClampedArray object. The Uint8ClampedArray object must use a new Canvas
Pixel ArrayBuffer for its storage, and must have a
zero start offset and a length equal to the length of its storage, in bytes. The Canvas
Pixel ArrayBuffer must have the correct size to
store rows × pixelsPerRow pixels.
If the Canvas Pixel ArrayBuffer cannot be
allocated, then rethrow the RangeError thrown by JavaScript,
and return.
Initialize the width attribute of
imageData to pixelsPerRow.
Initialize the height attribute of
imageData to rows.
Return imageData.
ImageData objects are serializable objects. Their serialization
steps, given value and serialized, are:
Set serialized.[[Data]] to the sub-serialization of the value of
value's data attribute.
Set serialized.[[Width]] to the value of value's width attribute.
Set serialized.[[Height]] to the value of value's height attribute.
Their deserialization steps, given serialized and value, are:
Initialize value's data attribute
to the sub-deserialization of serialized.[[Data]].
Initialize value's width attribute
to serialized.[[Width]].
Initialize value's height attribute
to serialized.[[Height]].
A Canvas Pixel ArrayBuffer is an ArrayBuffer whose data is represented in left-to-right order, row
by row top to bottom, starting with the top left, with each pixel's red, green, blue, and alpha
components being given in that order for each pixel. Each component of each pixel represented in
this array must be in the range 0..255, representing the 8 bit value for that component. The
components must be assigned consecutive indices starting with 0 for the top left pixel's red
component.
The putImageData() method writes
data from ImageData structures back to the rendering context's output
bitmap. Its arguments are: imagedata, dx, dy,
dirtyX, dirtyY, dirtyWidth, and dirtyHeight.
When the last four arguments to this method are omitted, they must be assumed to have the
values 0, 0, the width member of the imagedata structure, and the height
member of the imagedata structure, respectively.
The method, when invoked, must act as follows:
Let buffer be imagedata's data attribute value's [[ViewedArrayBuffer]] internal
slot.
If IsDetachedBuffer(buffer) is true, then throw an
"InvalidStateError" DOMException.
If dirtyWidth is negative, then let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal to the absolute magnitude of dirtyWidth.
If dirtyHeight is negative, then let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be equal to the absolute magnitude of dirtyHeight.
If dirtyX is negative, then let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.
If dirtyY is negative, then let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.
If dirtyX+dirtyWidth is greater than the width attribute of the imagedata argument, then
let dirtyWidth be the value of that width
attribute, minus the value of dirtyX.
If dirtyY+dirtyHeight is greater than the height attribute of the imagedata argument, then
let dirtyHeight be the value of that height attribute, minus the value of dirtyY.
If, after those changes, either dirtyWidth or dirtyHeight are negative or zero, then return without affecting any bitmaps.
For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the
four channels of the pixel with coordinate (x, y) in
the imagedata data structure's Canvas Pixel
ArrayBuffer to the pixel with coordinate (dx+x, dy+y)
in the rendering context's output bitmap.
Due to the lossy nature of converting to and from premultiplied alpha color
values, pixels that have just been set using putImageData() might be returned to an equivalent
getImageData() as different values.
The current path, transformation matrix, shadow attributes, global alpha, the clipping region, and global composition operator must not affect the methods described in this section.
In the following example, the script generates an ImageData object so that it can
draw onto it.
// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');
// create a blank slate
var data = context.createImageData(canvas.width, canvas.height);
// create some plasma
FillPlasma(data, 'green'); // green plasma
// add a cloud to the plasma
AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle
// paint the plasma+cloud on the canvas
context.putImageData(data, 0, 0);
// support methods
function FillPlasma(data, color) { ... }
function AddCloud(data, x, y) { ... }Here is an example of using getImageData() and putImageData() to implement an edge detection
filter.
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>Edge detection demo</title>
<script>
var image = new Image();
function init() {
image.onload = demo;
image.src = "image.jpeg";
}
function demo() {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
// draw the image onto the canvas
context.drawImage(image, 0, 0);
// get the image data to manipulate
var input = context.getImageData(0, 0, canvas.width, canvas.height);
// get an empty slate to put the data into
var output = context.createImageData(canvas.width, canvas.height);
// alias some variables for convenience
// In this case input.width and input.height
// match canvas.width and canvas.height
// but we'll use the former to keep the code generic.
var w = input.width, h = input.height;
var inputData = input.data;
var outputData = output.data;
// edge detection
for (var y = 1; y < h-1; y += 1) {
for (var x = 1; x < w-1; x += 1) {
for (var c = 0; c < 3; c += 1) {
var i = (y*w + x)*4 + c;
outputData[i] = 127 + -inputData[i - w*4 - 4] - inputData[i - w*4] - inputData[i - w*4 + 4] +
-inputData[i - 4] + 8*inputData[i] - inputData[i + 4] +
-inputData[i + w*4 - 4] - inputData[i + w*4] - inputData[i + w*4 + 4];
}
outputData[(y*w + x)*4 + 3] = 255; // alpha
}
}
// put the image data back after manipulation
context.putImageData(output, 0, 0);
}
</script>
</head>
<body onload="init()">
<canvas></canvas>
</body>
</html>globalAlpha [ = value ]CanvasRenderingContext2D/globalAlpha
Support in all current engines.
返回当前应用于渲染操作的 alpha 值。
可以设置来改变 alpha 值。在 0.0 .. 1.0 之外的值会被忽略。
globalCompositeOperation [ = value ]CanvasRenderingContext2D/globalCompositeOperation
Support in all current engines.
返回当前合成操作，来自 Compositing 和 Blending 规范定义的值。[COMPOSITE].
可以设置来改变合成操作。未知值会被忽略。
在实现了 CanvasCompositing 接口的对象上的所有绘图操作都会被全局合成属性影响：
globalAlpha 和
globalCompositeOperation。
globalAlpha 属性给出了应用于形状和图像在合成到
输出位图 上之前的 alpha 值。这个值必须在 0.0 （完全透明） 到 1.0 （不透明）之间。
如果尝试把这个值设置到范围外，包括 Infinity 和 Not-a-Number (NaN) 值，该属性必须保持之前的值。
当创建上下文时，必须把 globalAlpha 属性设置初始值 1.0。
globalCompositeOperation 属性
设置了 当前合成操作，
它控制着给定 globalAlpha
和当前转换矩阵，形状和图像如何绘制到 输出位图。
可能的值定义在 Compositing 和 Blending 规范，也包括 source-over
和 copy 值。
[COMPOSITE]
这些值是大小写敏感的 — 它们必须完全按照定义的方式使用。用户代理不得识别非 大小写敏感匹配的 Compositing 和 Blending 规范给出的值。[COMPOSITE]
在设置时，如果用户代理不识别指定的值，就必须忽略它，让
globalCompositeOperation 的值不受影响。
否则，必须把该属性的值设置为给定的新值。
当创建上下文时，必须把 globalCompositeOperation
属性设为初始值 source-over。
imageSmoothingEnabled [ = value ]CanvasRenderingContext2D/imageSmoothingEnabled
Support in all current engines.
Returns whether pattern fills and the drawImage() method will attempt to smooth images if
their pixels don't line up exactly with the display, when scaling images up.
Can be set, to change whether images are smoothed (true) or not (false).
imageSmoothingQuality [ = value ]CanvasRenderingContext2D/imageSmoothingQuality
Returns the current image-smoothing-quality preference.
Can be set, to change the preferred quality of image smoothing. The possible values are
"low", "medium" and "high". Unknown values are ignored.
Objects that implement the CanvasImageSmoothing interface have attributes that
control how image smoothing is performed.
The imageSmoothingEnabled
attribute, on getting, must return the last value it was set to. On setting, it must be set to the
new value. When the object implementing the CanvasImageSmoothing interface is
created, the attribute must be set to true.
The imageSmoothingQuality
attribute, on getting, must return the last value it was set to. On setting, it must be set to the
new value. When the object implementing the CanvasImageSmoothing interface is
created, the attribute must be set to "low".
All drawing operations on an object which implements the CanvasShadowStyles
interface are affected by the four global shadow attributes.
shadowColor [ = value ]CanvasRenderingContext2D/shadowColor
Support in all current engines.
Returns the current shadow color.
Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.
shadowOffsetX [ = value ]CanvasRenderingContext2D/shadowOffsetX
Support in all current engines.
shadowOffsetY [ = value ]CanvasRenderingContext2D/shadowOffsetY
Support in all current engines.
Returns the current shadow offset.
Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
shadowBlur [ = value ]CanvasRenderingContext2D/shadowBlur
Support in all current engines.
Returns the current level of blur applied to shadows.
Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.
The shadowColor attribute sets the
color of the shadow.
When the context is created, the shadowColor
attribute initially must be transparent black.
On getting, the serialization of the color must be returned.
On setting, the new value must be parsed
with this canvas element and the color assigned. If parsing the value results in
failure then it must be ignored, and the attribute must retain its previous value. [CSSCOLOR]
The shadowOffsetX and shadowOffsetY attributes specify the distance
that the shadow will be offset in the positive horizontal and positive vertical distance
respectively. Their values are in coordinate space units. They are not affected by the current
transformation matrix.
When the context is created, the shadow offset attributes must initially have the value 0.
On getting, they must return their current value. On setting, the attribute being set must be set to the new value, except if the value is infinite or NaN, in which case the new value must be ignored.
The shadowBlur attribute specifies
the level of the blurring effect. (The units do not map to coordinate space units, and are not
affected by the current transformation matrix.)
When the context is created, the shadowBlur
attribute must initially have the value 0.
On getting, the attribute must return its current value. On setting the attribute must be set to the new value, except if the value is negative, infinite or NaN, in which case the new value must be ignored.
Shadows are only drawn if the opacity component of
the alpha component of the color of shadowColor is
nonzero and either the shadowBlur is nonzero, or
the shadowOffsetX is nonzero, or the shadowOffsetY is nonzero.
When shadows are drawn, they must be rendered as follows:
Let A be an infinite transparent black bitmap on which the source image for which a shadow is being created has been rendered.
Let B be an infinite transparent black bitmap, with a coordinate space and an origin identical to A.
Copy the alpha channel of A to B, offset by shadowOffsetX in the positive x
direction, and shadowOffsetY in the positive
y direction.
If shadowBlur is greater than 0:
Let σ be half the value of shadowBlur.
Perform a 2D Gaussian Blur on B, using σ as the standard deviation.
User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.
Set the red, green, and blue components of every pixel in B to the
red, green, and blue components (respectively) of the color of shadowColor.
Multiply the alpha component of every pixel in B by the alpha
component of the color of shadowColor.
The shadow is in the bitmap B, and is rendered as part of the drawing model described below.
If the current composition operation is copy, then shadows
effectively won't render (since the shape will overwrite the shadow).
对实现了 CanvasFilters 接口的对象的所有绘图操作
都会被全局 滤镜 属性影响。
filter [ = value ]CanvasRenderingContext2D/filter
返回当前滤镜。
可以设置它来改变滤镜。不能解析为 <filter-function-list> 的值会被忽略。
在读取 filter 属性时，
必须返回上一个成功设置的值。不得重新序列化它的值。
在设置时，如果新的值是 'none' （不是空字符串、 null 或 undefined），
那么必须禁用这个上下文的滤镜。
否则，必须把这个值解析为 <filter-function-list> 值。
如果该值不能解析为一个 <filter-function-list> 值，
其中独立于属性的样式表语法如 'inherit' 或 'initial' 被认为是非法值并被忽略，
而且该属性的值必须保持之前的值。当创建实现了 CanvasFilters 接口的对象时，
必须把该属性的值设为 'none'。
<filter-function-list> 值包括一个或更多滤镜函数或 SVG 滤镜的引用组成的序列。 滤镜的输入被用作该列表中第一个滤镜的输入。后续项取前一项的输出作为输入。 [FILTERS]
解释
filter 属性的值使用的坐标时，
一像素相当于一个 SVG 用户空间单位，一个 canvas 坐标空间单位。
滤镜坐标不会受 当前转换矩阵 影响。
当前转换矩阵只影响滤镜的输入。滤镜作用于 输出位图
的坐标空间。
如果 filter 属性的值使用
百分比或 'em' 或 'ex' 单位定义了长度，那么必须相对于设置该属性时
字体样式源对象 的 'font-size' 属性的
计算后的值 来解释该长度。
如果 计算后的值 是 undefined
（比如由于 字体样式源对象 不是元素或没有
被渲染），
那么必须相对于 font 属性的默认值来解释相对关键字。
不支持 'larger' 和 'smaller' 关键字。
如果 filter 属性的值是同一文档中
SVG 滤镜的引用，而且该 SVG 滤镜发生了改变，那么改变后的滤镜将会用作下一次绘图操作。
如果 filter 属性的值是外部资源文档中的
SVG 滤镜的引用，而且在调用绘图操作时该外部文档还没加载完成，那么执行该绘制操作时不得使用滤镜。
This section is non-normative.
因为在外部定义的滤镜完成载入之前，绘制使用的滤镜值都是 'none'，
因此作者可能希望在进行绘图操作前确定滤镜是否已经载入。
一种实现方式是在同一个页面的其他地方加一个元素来载入外部定义的滤镜，
（比如 SVG use 元素），
然后等待 load 事件的派发。
When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):
Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honored, and the stroke must itself also be subjected to the current transformation matrix.
When the filter attribute is set to a value other than 'none' and all the
externally-defined filters it references, if any, are in documents that are currently loaded,
then use image A as the input to the filter, creating image B. Otherwise, let
B be an alias for A.
When shadows are drawn, render the shadow from image B, using the current shadow styles, creating image C.
When shadows are drawn, multiply the alpha component of every pixel in C by globalAlpha.
When shadows are drawn, composite C within the clipping region over the current output bitmap using the current composition operator.
Multiply the alpha component of every pixel in B by globalAlpha.
Composite B within the clipping region over the current output bitmap using the current composition operator.
When compositing onto the output bitmap, pixels that would fall outside of the output bitmap must be discarded.
如果画布可交互，作者应该对画布的每个可聚焦部分，都在该元素的 fallback 内容中包含一个可聚焦元素， 像 上述例子 中那样。
当渲染焦点环时，为了确保焦点环的外观和原生焦点环相同，作者应该使用 drawFocusIfNeeded() 方法，
把需要绘制焦点环的元素传给它。该方法只在元素 获得焦点
时才绘制焦点环。这样就可以在每次绘制元素时简单地调用它，而不需要先检查元素是否已经获得焦点。
除了绘制焦点环以外，作者还应该在画布中的元素获得焦点时调用 scrollPathIntoView() 方法，
以确保它在屏幕上可见（如果可以的话）。
作者应该避免使用 canvas 元素
实现文本编辑控件，这样做有很多缺点：
这是巨大的工作量，而且强烈建议作者避免做任何这些事情，不如直接使用
input 元素， textarea 元素，或者
contenteditable 属性。
This section is non-normative.
这是一个使用 canvas 绘图 漂亮的发光线 的脚本示例。
<canvas width="800" height="450"></canvas>
<script>
var context = document.getElementsByTagName('canvas')[0].getContext('2d');
var lastX = context.canvas.width * Math.random();
var lastY = context.canvas.height * Math.random();
var hue = 0;
function line() {
context.save();
context.translate(context.canvas.width/2, context.canvas.height/2);
context.scale(0.9, 0.9);
context.translate(-context.canvas.width/2, -context.canvas.height/2);
context.beginPath();
context.lineWidth = 5 + Math.random() * 10;
context.moveTo(lastX, lastY);
lastX = context.canvas.width * Math.random();
lastY = context.canvas.height * Math.random();
context.bezierCurveTo(context.canvas.width * Math.random(),
context.canvas.height * Math.random(),
context.canvas.width * Math.random(),
context.canvas.height * Math.random(),
lastX, lastY);
hue = hue + 10 * Math.random();
context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)';
context.shadowColor = 'white';
context.shadowBlur = 10;
context.stroke();
context.restore();
}
setInterval(line, 50);
function blank() {
context.fillStyle = 'rgba(0,0,0,0.1)';
context.fillRect(0, 0, context.canvas.width, context.canvas.height);
}
setInterval(blank, 40);
</script>
基于雪碧图的游戏经常使用 canvas 的 2D 渲染上下文。下面的例子可以作为演示：
这是该例子的源码：
<!DOCTYPE HTML>
<html lang="en">
<meta charset="utf-8">
<title>Blue Robot Demo</title>
<style>
html { overflow: hidden; min-height: 200px; min-width: 380px; }
body { height: 200px; position: relative; margin: 8px; }
.buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; }
</style>
<canvas width="380" height="200"></canvas>
<script>
var Landscape = function (context, width, height) {
this.offset = 0;
this.width = width;
this.advance = function (dx) {
this.offset += dx;
};
this.horizon = height * 0.7;
// This creates the sky gradient (from a darker blue to white at the bottom)
this.sky = context.createLinearGradient(0, 0, 0, this.horizon);
this.sky.addColorStop(0.0, 'rgb(55,121,179)');
this.sky.addColorStop(0.7, 'rgb(121,194,245)');
this.sky.addColorStop(1.0, 'rgb(164,200,214)');
// this creates the grass gradient (from a darker green to a lighter green)
this.earth = context.createLinearGradient(0, this.horizon, 0, height);
this.earth.addColorStop(0.0, 'rgb(81,140,20)');
this.earth.addColorStop(1.0, 'rgb(123,177,57)');
this.paintBackground = function (context, width, height) {
// first, paint the sky and grass rectangles
context.fillStyle = this.sky;
context.fillRect(0, 0, width, this.horizon);
context.fillStyle = this.earth;
context.fillRect(0, this.horizon, width, height-this.horizon);
// then, draw the cloudy banner
// we make it cloudy by having the draw text off the top of the
// canvas, and just having the blurred shadow shown on the canvas
context.save();
context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0);
context.shadowColor = 'white';
context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas
context.shadowBlur = '5';
context.fillStyle = 'white';
context.textAlign = 'left';
context.textBaseline = 'top';
context.font = '20px sans-serif';
context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas
context.restore();
// then, draw the background tree
context.save();
context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0);
context.beginPath();
context.fillStyle = 'rgb(143,89,2)';
context.lineStyle = 'rgb(10,10,10)';
context.lineWidth = 2;
context.rect(0, this.horizon+5, 10, -50); // trunk
context.fill();
context.stroke();
context.beginPath();
context.fillStyle = 'rgb(78,154,6)';
context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves
context.fill();
context.stroke();
context.restore();
};
this.paintForeground = function (context, width, height) {
// draw the box that goes in front
context.save();
context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0);
context.beginPath();
context.rect(0, this.horizon - 5, 25, 25);
context.fillStyle = 'rgb(220,154,94)';
context.lineStyle = 'rgb(10,10,10)';
context.lineWidth = 2;
context.fill();
context.stroke();
context.restore();
};
};
</script>
<script>
var BlueRobot = function () {
this.sprites = new Image();
this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells
this.targetMode = 'idle';
this.walk = function () {
this.targetMode = 'walk';
};
this.stop = function () {
this.targetMode = 'idle';
};
this.frameIndex = {
'idle': [0], // first cell is the idle frame
'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6
'stop': [7], // last cell is the stopping animation
};
this.mode = 'idle';
this.frame = 0; // index into frameIndex
this.tick = function () {
// this advances the frame and the robot
// the return value is how many pixels the robot has moved
this.frame += 1;
if (this.frame >= this.frameIndex[this.mode].length) {
// we've reached the end of this animation cycle
this.frame = 0;
if (this.mode != this.targetMode) {
// switch to next cycle
if (this.mode == 'walk') {
// we need to stop walking before we decide what to do next
this.mode = 'stop';
} else if (this.mode == 'stop') {
if (this.targetMode == 'walk')
this.mode = 'walk';
else
this.mode = 'idle';
} else if (this.mode == 'idle') {
if (this.targetMode == 'walk')
this.mode = 'walk';
}
}
}
if (this.mode == 'walk')
return 8;
return 0;
},
this.paint = function (context, x, y) {
if (!this.sprites.complete) return;
// draw the right frame out of the sprite sheet onto the canvas
// we assume each frame is as high as the sprite sheet
// the x,y coordinates give the position of the bottom center of the sprite
context.drawImage(this.sprites,
this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height,
x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height);
};
};
</script>
<script>
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
var landscape = new Landscape(context, canvas.width, canvas.height);
var blueRobot = new BlueRobot();
// paint when the browser wants us to, using requestAnimationFrame()
function paint() {
context.clearRect(0, 0, canvas.width, canvas.height);
landscape.paintBackground(context, canvas.width, canvas.height);
blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1);
landscape.paintForeground(context, canvas.width, canvas.height);
requestAnimationFrame(paint);
}
paint();
// but tick every 100ms, so that we don't slow down when we don't paint
setInterval(function () {
var dx = blueRobot.tick();
landscape.advance(dx);
}, 100);
</script>
<p class="buttons">
<input type=button value="Walk" onclick="blueRobot.walk()">
<input type=button value="Stop" onclick="blueRobot.stop()">
<footer>
<small> Blue Robot Player Sprite by <a href="https://johncolburn.deviantart.com/">JohnColburn</a>.
Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small>
<small> This work is itself licensed under a <a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">Creative
Commons Attribution-ShareAlike 3.0 Unported License</a>.</small>
</footer>
ImageBitmap 渲染上下文ImageBitmapRenderingContext is a performance-oriented interface that provides a
low overhead method for displaying the contents of ImageBitmap objects. It uses
transfer semantics to reduce overall memory consumption. It also streamlines performance by
avoiding intermediate compositing, unlike the drawImage() method of
CanvasRenderingContext2D.
Using an img element as an intermediate for getting an image resource into a
canvas, for example, would result in two copies of the decoded image existing in memory at the
same time: the img element's copy, and the one in the canvas's backing store. This
memory cost can be prohibitive when dealing with extremely large images. This can be avoided by
using ImageBitmapRenderingContext.
Using ImageBitmapRenderingContext, here is how to transcode an image to the JPEG
format in a memory- and CPU-efficient way:
createImageBitmap(inputImageBlob).then(image => {
const canvas = document.createElement('canvas');
const context = canvas.getContext('bitmaprenderer');
context.transferFromImageBitmap(image);
canvas.toBlob(outputJPEGBlob => {
// Do something with outputJPEGBlob.
}, 'image/jpeg');
});ImageBitmapRenderingContext interfaceSupport in all current engines.
[Exposed=(Window,Worker)]
interface ImageBitmapRenderingContext {
readonly attribute (HTMLCanvasElement or OffscreenCanvas) canvas;
undefined transferFromImageBitmap(ImageBitmap? bitmap);
};
dictionary ImageBitmapRenderingContextSettings {
boolean alpha = true;
};getContext('bitmaprenderer' [, { [ alpha: false ] } ] )Returns an ImageBitmapRenderingContext object that is permanently bound to a
particular canvas element.
If the alpha setting is
provided and set to false, then the canvas is forced to always be opaque.
canvasReturns the canvas element that the context is bound to.
transferFromImageBitmap(imageBitmap)ImageBitmapRenderingContext/transferFromImageBitmap
Support in all current engines.
Transfers the underlying bitmap data
from imageBitmap to context, and the bitmap becomes the contents of the
canvas element to which context is bound.
transferFromImageBitmap(null)Replaces contents of the canvas element to which context
is bound with a transparent black bitmap whose size corresponds to the width and height
content attributes of the canvas element.
The canvas attribute
must return the value it was initialized to when the object was created.
An ImageBitmapRenderingContext object has an output bitmap, which is a
reference to bitmap data.
An ImageBitmapRenderingContext object has a bitmap mode, which can be set to
valid or blank. A value of valid indicates that the context's
output bitmap refers to
bitmap data that was acquired via transferFromImageBitmap().
A value blank indicates that the
context's output
bitmap is a default transparent bitmap.
An ImageBitmapRenderingContext object also has an alpha flag, which can be set to true or
false. When an ImageBitmapRenderingContext object has its alpha flag set to false, the contents
of the canvas element to which the context is bound are obtained by
compositing the context's output bitmap onto an
opaque black bitmap of the same size using the source-over composite operation. If
the alpha flag is set to true,
then the output bitmap is
used as the contents of the canvas element to which the context is bound. [COMPOSITE]
The step of compositing over an opaque black bitmap ought to be elided whenever equivalent results can be obtained more efficiently by other means.
When a user agent is required to set an ImageBitmapRenderingContext's output
bitmap, with a context argument that is an
ImageBitmapRenderingContext object and an optional argument bitmap that
refers to bitmap data, it must run these
steps:
If a bitmap argument was not provided, then:
Set context's bitmap mode to blank.
Let canvas be the canvas element to which context
is bound.
Set context's output bitmap to be
transparent black with an intrinsic width equal to the numeric value of canvas's width attribute and an intrinsic height equal
to the numeric value of canvas's
height attribute, those values being interpreted
in CSS pixels.
Set the output bitmap's origin-clean flag to true.
If a bitmap argument was provided, then:
Set context's bitmap mode to valid.
Set context's output bitmap to refer to the same underlying bitmap data as bitmap, without making a copy.
The origin-clean flag of bitmap is included in the bitmap data to be referenced by context's output bitmap.
The ImageBitmapRenderingContext creation algorithm, which is passed a
target and options, consists of running these steps:
Let settings be the result of converting options to the dictionary type
ImageBitmapRenderingContextSettings. (This can throw an exception.)
Let context be a new ImageBitmapRenderingContext 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).
Run the steps to set an ImageBitmapRenderingContext's output
bitmap with context.
Initialize context's alpha flag to true.
Process each of the members of settings as follows:
alphaReturn context.
The transferFromImageBitmap(imageBitmap)
method, when invoked, must run these steps:
Let bitmapContext be the ImageBitmapRenderingContext object on
which the transferFromImageBitmap()
method was called.
If imageBitmap is null, then run the steps to set an ImageBitmapRenderingContext's output bitmap, with bitmapContext as the context argument and no bitmap argument, then return.
If the value of imageBitmap's [[Detached]] internal slot is set to
true, then throw an "InvalidStateError" DOMException.
Run the steps to set an ImageBitmapRenderingContext's output
bitmap, with the context argument equal to bitmapContext, and the
bitmap argument referring to imageBitmap's underlying bitmap data.
Set the value of imageBitmap's [[Detached]] internal slot to true.
Unset imageBitmap's bitmap data.
OffscreenCanvas interfaceSupport in one engine only.
typedef (OffscreenCanvasRenderingContext2D or ImageBitmapRenderingContext or WebGLRenderingContext or WebGL2RenderingContext) OffscreenRenderingContext;
dictionary ImageEncodeOptions {
DOMString type = "image/png";
unrestricted double quality;
};
enum OffscreenRenderingContextId { "2d", "bitmaprenderer", "webgl", "webgl2" };
[Exposed=(Window,Worker), Transferable]
interface OffscreenCanvas : EventTarget {
constructor([EnforceRange] unsigned long long width, [EnforceRange] unsigned long long height);
attribute [EnforceRange] unsigned long long width;
attribute [EnforceRange] unsigned long long height;
OffscreenRenderingContext? getContext(OffscreenRenderingContextId contextId, optional any options = null);
ImageBitmap transferToImageBitmap();
Promise<Blob> convertToBlob(optional ImageEncodeOptions options = {});
};OffscreenCanvas is an EventTarget so that WebGL can
fire webglcontextlost and webglcontextrestored
events at it. [WEBGL]
OffscreenCanvas objects are used to create rendering contexts, much like an
HTMLCanvasElement, but with no connection to the DOM. This makes it possible to
use canvas rendering contexts in workers.
Support in one engine only.
An OffscreenCanvas object may hold a weak reference to a placeholder canvas element, which is
typically in the DOM, whose embedded content is provided by the OffscreenCanvas
object. The bitmap of the OffscreenCanvas object is pushed to the placeholder canvas element by
calling the commit() method of the
OffscreenCanvas object's rendering context. All rendering context types that
can be created by an OffscreenCanvas object must implement a commit() method. The exact behavior of the commit
method (e.g. whether it copies or transfers bitmaps) may vary, as defined by the rendering
contexts' respective specifications. Only the 2D context for offscreen canvases is defined
in this specification.
OffscreenCanvas(width,
height)OffscreenCanvas/OffscreenCanvas
Support in one engine only.
Returns a new OffscreenCanvas object that is not linked to a placeholder canvas element, and
whose bitmap's size is determined by the width and height arguments.
getContext(contextId [,
options ] )Support in one engine only.
Returns an object that exposes an API for drawing on the OffscreenCanvas
object. contextId specifies the desired API: "2d", "bitmaprenderer", "webgl", or "webgl2". options is handled by that
API.
This specification defines the "2d" context below,
which is similar but distinct from the "2d"
context that is created from a canvas element. The WebGL specifications define the
"webgl" and "webgl2" contexts. [WEBGL]
Returns null if the canvas has already been initialized with another context type (e.g.,
trying to get a "2d" context after getting a
"webgl" context).
An OffscreenCanvas object has an internal bitmap that is initialized when the object
is created. The width and height of the bitmap are
equal to the values of the width and height attributes of the OffscreenCanvas
object. Initially, all the bitmap's pixels are transparent black.
An OffscreenCanvas object 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, an OffscreenCanvas object also
has a context mode, which is initially none but can be changed to either 2d, bitmaprenderer, webgl, webgl2, or detached by algorithms defined in this
specification.
The constructor OffscreenCanvas(width,
height), when invoked, must create a new OffscreenCanvas
object with its bitmap initialized to a rectangular
array of transparent black pixels of the dimensions specified by width and
height; and its width and height attributes initialized to width and
height respectively.
OffscreenCanvas objects are transferable. Their transfer steps, given value and
dataHolder, are as follows:
If value's context mode is
not equal to none, then throw an
"InvalidStateError" DOMException.
Set value's context mode to detached.
Let width and height be the dimensions of value's bitmap.
Unset value's bitmap.
Set dataHolder.[[Width]] to width and dataHolder.[[Height]] to height.
Set dataHolder.[[PlaceholderCanvas]] to be a weak reference to
value's placeholder canvas
element, if value has one, or null if it does not.
Their transfer-receiving steps, given dataHolder and value, are:
Initialize value's bitmap to a rectangular array of transparent black pixels with width given by dataHolder.[[Width]] and height given by dataHolder.[[Height]].
If dataHolder.[[PlaceholderCanvas]] is not null, set value's placeholder canvas element to
dataHolder.[[PlaceholderCanvas]] (while maintaining the weak reference
semantics).
The getContext(contextId,
options) method of an OffscreenCanvas object, 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
OffscreenCanvas object's context
mode and whose row header matches contextId:
| none | 2d | bitmaprenderer | webgl or webgl2 | detached | |
|---|---|---|---|---|---|
"2d"
|
Follow the offscreen 2D context
creation algorithm defined in the section below, passing it this
OffscreenCanvas object and options, to obtain an
OffscreenCanvasRenderingContext2D object; if this does not throw an exception,
then set this OffscreenCanvas object's context mode to 2d, and return the new
OffscreenCanvasRenderingContext2D object.
| 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.
|
"bitmaprenderer"
|
Follow the ImageBitmapRenderingContext creation algorithm
defined in the section above, passing it this
OffscreenCanvas object and options,
to obtain an ImageBitmapRenderingcontext object; if this does not throw
an exception, then set this OffscreenCanvas object's context mode to bitmaprenderer, and return the new
ImageBitmapRenderingcontext object.
| 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.
|
"webgl" or "webgl2"
|
Follow the instructions given in the WebGL specifications' Context Creation sections
to obtain either a WebGLRenderingContext, WebGL2RenderingContext,
or null; if the returned value is null, then return null; otherwise, set this
OffscreenCanvas object's context
mode to webgl or webgl2, and return the
WebGLRenderingContext or WebGL2RenderingContext object. [WEBGL]
| Return null. | Return null. | Return the same value as was returned the last time the method was invoked with this same first argument. |
Throw an "InvalidStateError" DOMException.
|
width [
= value ]Support in one engine only.
height [
= value ]Support in one engine only.
These attributes return the dimensions of the OffscreenCanvas object's bitmap.
They can be set, to replace the bitmap with a new, transparent black bitmap of the specified dimensions (effectively resizing it).
If either the width or height attributes of an
OffscreenCanvas object are set (to a new value or to the same value as before) and
the OffscreenCanvas object's context
mode is 2d, then replace the
OffscreenCanvas object's bitmap with a
new transparent black bitmap and reset the rendering context to its default
state. The new bitmap's dimensions are equal to the new values of the width and height attributes.
The resizing behavior for "webgl" and "webgl2" contexts is defined in the WebGL
specifications. [WEBGL]
If an OffscreenCanvas object whose dimensions were changed has
a placeholder canvas element, then
the placeholder canvas element's
intrinsic size will only be updated via the commit() method of the OffscreenCanvas
object's rendering context.
convertToBlob( [options] )Support in one engine only.
Returns a promise that will fulfill with a new Blob object representing a file
containing the image in the OffscreenCanvas object.
The argument, if provided, is a dictionary that controls the encoding options of the image
file to be created. The type
field specifies the file format and has a default value of "image/png"; that type
is also used if the requested type isn't supported. If the image format supports variable
quality (such as "image/jpeg"), then the quality field is a number in the range 0.0
to 1.0 inclusive indicating the desired quality level for the resulting image.
transferToImageBitmap()OffscreenCanvas/transferToImageBitmap
Support in one engine only.
Returns a newly created ImageBitmap object with the image in the
OffscreenCanvas object. The image in the OffscreenCanvas object is
replaced with a new blank image.
The convertToBlob(options) method,
when invoked, must run the following steps:
If the value of this OffscreenCanvas object's [[Detached]]
internal slot is set to true, then return a promise rejected with an
"InvalidStateError" DOMException.
If this OffscreenCanvas object's context mode is 2d and the rendering context's bitmap's origin-clean flag is set to false, then return a
promise rejected with a "SecurityError" DOMException.
If this OffscreenCanvas object's bitmap has no pixels (i.e., either its
horizontal dimension or its vertical dimension is zero) then return a promise rejected with an
"IndexSizeError" DOMException.
Let bitmap be a copy of this OffscreenCanvas object's bitmap.
Let result be a new promise object.
Run these steps in parallel:
Let file be a
serialization of bitmap as a file, with options's type and quality if present.
Queue an element task on the canvas blob serialization task
source given the canvas element to run these steps:
If file is null, then reject result with an
"EncodingError" DOMException.
Otherwise, resolve result with a new Blob object, created in
the relevant Realm of this
OffscreenCanvas object, representing file. [FILEAPI]
Return result.
The transferToImageBitmap() method,
when invoked, must run the following steps:
If the value of this OffscreenCanvas object's [[Detached]]
internal slot is set to true, then throw an "InvalidStateError"
DOMException.
If this OffscreenCanvas object's context mode is set to none, then throw an
"InvalidStateError" DOMException.
Let image be a newly created ImageBitmap object that references
the same underlying bitmap data as this OffscreenCanvas object's bitmap.
Set this OffscreenCanvas object's bitmap to reference a newly created bitmap of the same
dimensions as the previous bitmap, and with its pixels initialized to transparent
black, or opaque black if the rendering context's alpha flag is set to false.
This means that if the rendering context of this OffscreenCanvas is
a WebGLRenderingContext, the value of preserveDrawingBuffer will have no effect. [WEBGL]
Return image.
[Exposed=(Window,Worker)]
interface OffscreenCanvasRenderingContext2D {
undefined commit();
readonly attribute OffscreenCanvas canvas;
};
OffscreenCanvasRenderingContext2D includes CanvasState;
OffscreenCanvasRenderingContext2D includes CanvasTransform;
OffscreenCanvasRenderingContext2D includes CanvasCompositing;
OffscreenCanvasRenderingContext2D includes CanvasImageSmoothing;
OffscreenCanvasRenderingContext2D includes CanvasFillStrokeStyles;
OffscreenCanvasRenderingContext2D includes CanvasShadowStyles;
OffscreenCanvasRenderingContext2D includes CanvasFilters;
OffscreenCanvasRenderingContext2D includes CanvasRect;
OffscreenCanvasRenderingContext2D includes CanvasDrawPath;
OffscreenCanvasRenderingContext2D includes CanvasText;
OffscreenCanvasRenderingContext2D includes CanvasDrawImage;
OffscreenCanvasRenderingContext2D includes CanvasImageData;
OffscreenCanvasRenderingContext2D includes CanvasPathDrawingStyles;
OffscreenCanvasRenderingContext2D includes CanvasTextDrawingStyles;
OffscreenCanvasRenderingContext2D includes CanvasPath;The OffscreenCanvasRenderingContext2D object is a rendering context for drawing to
the bitmap of an OffscreenCanvas object.
It is similar to the CanvasRenderingContext2D object, with the following
differences:
there is no support for user interface features;
its canvas attribute refers to an
OffscreenCanvas object rather than a canvas element;
it has a commit() method for pushing the
rendered image to the context's OffscreenCanvas object's placeholder canvas element.
An OffscreenCanvasRenderingContext2D object has a bitmap that is initialized when the object is
created.
The 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.
An OffscreenCanvasRenderingContext2D object also has an alpha flag, which can be set to true or false. Initially,
when the context is created, its alpha flag must be set to true. When an
OffscreenCanvasRenderingContext2D object has its alpha flag set to false, then its alpha channel must be
fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel
must be silently ignored.
An OffscreenCanvasRenderingContext2D object has an associated
OffscreenCanvas object, which is the OffscreenCanvas object
from which the OffscreenCanvasRenderingContext2D object was created.
commit()Copies the rendering context's bitmap to
the bitmap of the placeholder canvas
element of the associated OffscreenCanvas object. The copy
operation is synchronous. Calling this method is not needed for the transfer, since it happens
automatically during the event loop execution.
canvasReturns the associated OffscreenCanvas object.
The offscreen 2D context creation algorithm, which is passed a
target (an OffscreenCanvas object) and optionally some arguments,
consists of running the following steps:
If the algorithm was passed some arguments, let arg be the first such argument. Otherwise, let arg be undefined.
Let settings be the result of converting options to the dictionary type
CanvasRenderingContext2DSettings. (This can throw an exception.).
Let context be a new OffscreenCanvasRenderingContext2D
object.
Set context's associated OffscreenCanvas object to
target.
Process each of the members of settings as follows:
Set context's bitmap to a newly
created bitmap with the dimensions specified by the width and height attributes of target, and set
target's bitmap to the same bitmap (so that they are shared).
If context's alpha flag is set to true, initialize all the pixels of context's bitmap to transparent black. Otherwise, initialize the pixels to opaque black.
Return context.
The commit() method, when invoked,
must run the following steps:
If this OffscreenCanvasRenderingContext2D's associated
OffscreenCanvas object does not have a placeholder canvas element, then
return.
Let image be a copy of this OffscreenCanvasRenderingContext2D's
bitmap, including the value of its origin-clean flag.
Queue an element task on the DOM manipulation task source given
the placeholder canvas element
to set the placeholder canvas
element's output bitmap to be a reference to image.
If image has different dimensions than the bitmap previously
referenced as the placeholder canvas
element's output bitmap, then this task will result in a change in
the placeholder canvas
element's intrinsic size, which can affect
document layout.
Implementations are encouraged to short-circuit the graphics update steps of
the window event loop for the purposes of updating the contents of a placeholder canvas element to the
display. This could mean, for example, that the commit() method can copy the bitmap contents directly
to a graphics buffer that is mapped to the physical display location of the placeholder canvas element. This or
similar short-circuiting approaches can significantly reduce display latency, especially in cases
where the commit() method is invoked from a
worker event loop and the window event loop of the placeholder canvas element is busy.
However, such shortcuts can not have any script-observable side-effects. This means that the
committed bitmap still needs to be sent to the placeholder canvas element, in case the
element is used as a CanvasImageSource, as an ImageBitmapSource, or in
case toDataURL() or toBlob() are called on it.
The canvas attribute, on getting,
must return this OffscreenCanvasRenderingContext2D's associated
OffscreenCanvas object.
The canvas APIs must perform color correction at only two points: when rendering
images with their own gamma correction and color space information onto a bitmap, to convert the
image to the color space used by the bitmaps (e.g. using the 2D Context's drawImage() method with an
HTMLOrSVGImageElement object), and when rendering the actual canvas bitmap to the
output device.
Thus, in the 2D context, colors used to draw shapes onto the canvas will exactly
match colors obtained through the getImageData() method.
The toDataURL() method, when invoked, must not
include color space information in the resources they return. Where the output format allows it,
the color of pixels in resources created by toDataURL()
must match those returned by the getImageData()
method.
In user agents that support CSS, the color space used by a canvas element must
match the color space used for processing any colors for that element in CSS.
The gamma correction and color space information of images must be handled in such a way that
an image rendered directly using an img element would use the same colors as one
painted on a canvas element that is then itself rendered. Furthermore, the rendering
of images that have no color correction information (such as those returned by the toDataURL() method) must be rendered with no color
correction.
Thus, in the 2D context, calling the drawImage() method to render the output of the toDataURL() method to the canvas, given the appropriate
dimensions, has no visible effect.
When a user agent is to create a serialization of the bitmap as a file, given a type and an optional quality, it must create an image file in the format given by type. If an error occurs during the creation of the image file (e.g. an internal encoder error), then the result of the serialization is null. [PNG]
The image file's pixel data must be the bitmap's pixel data scaled to one image pixel per coordinate space unit, and if the file format used supports encoding resolution metadata, the resolution must be given as 96dpi (one image pixel per CSS pixel).
If type is supplied, then it must be interpreted as a MIME type giving the format to use. If the type has any parameters, then it must be treated as not supported.
For example, the value "image/png" would mean to generate a PNG
image, the value "image/jpeg" would mean to generate a JPEG image, and the value
"image/svg+xml" would mean to generate an SVG image (which would require that the
user agent track how the bitmap was generated, an unlikely, though potentially awesome,
feature).
User agents must support PNG ("image/png"). User agents may support other types.
If the user agent does not support the requested type, then it must create the file using the PNG
format. [PNG]
User agents must convert the provided type to ASCII lowercase before establishing if they support that type.
For image types that do not support an alpha channel, the serialized image must be the bitmap image composited onto an opaque black background using the source-over operator.
If type is an image format that supports variable quality (such as
"image/jpeg"), quality is given, and type is not
"image/png", then, if Type(quality) is
Number, and quality is in the range 0.0 to 1.0 inclusive, the user agent must treat
quality as the desired quality level. Otherwise, the user agent must use its default
quality value, as if the quality argument had not been given.
The use of type-testing here, instead of simply declaring quality as
a Web IDL double, is a historical artifact.
Different implementations can have slightly different interpretations of "quality". When the quality is not specified, an implementation-specific default is used that represents a reasonable compromise between compression ratio, image quality, and encoding time.
canvas 元素的安全问题This section is non-normative.
如果一个 域 的脚本可以访问另一个（非 同域 的） 域的图片信息（例如读取像素）就可能发生 信息泄露。
为了减轻这个问题，
canvas 元素和 ImageBitmap 对象使用的位图定义有一个标志，
来表示它们是否 origin-clean。
所有位图开始时 origin-clean 都设为 true。
使用到跨域图片时该标志被设为 false。
toDataURL(), toBlob(), 和 getImageData() 方法会检查这个标志，
并且会抛出一个 "SecurityError" DOMException，
以保证不泄漏跨域数据。
origin-clean 标志的值会通过
createImageBitmap() 从一个
源 canvas 元素的位图传播到新的 ImageBitmap 对象。
反过来，如果源 ImageBitmap 对象的位图的
origin-clean 标志为 false，
目标 canvas 元素的位图在 drawImage
时会把它的
origin-clean 标志设为 false。
在某些情况下这个标志会被重置；例如绑定有 CanvasRenderingContext2D
的 canvas 元素的
width 或 height
内容属性的值改变时，该位图会被清空且它的
origin-clean
标志会被重置。
在 ImageBitmapRenderingContext 中，
当 ImageBitmap 对象通过
transferFromImageBitmap()
转移到 canvas 时，它的
origin-clean 标志会被传播。