HTML

Living Standard — Last Updated 7 April 2021

1. 1. 1. 4.12.5 The canvas element
         1. 4.12.5.1 The 2D rendering context
            1. 4.12.5.1.1 Implementation notes
            2. 4.12.5.1.2 The canvas state
            3. 4.12.5.1.3 Line styles
            4. 4.12.5.1.4 Text styles
            5. 4.12.5.1.5 Building paths
            6. 4.12.5.1.6 Path2D objects
            7. 4.12.5.1.7 Transformations
            8. 4.12.5.1.8 Image sources for 2D rendering contexts
            9. 4.12.5.1.9 Fill and stroke styles
            10. 4.12.5.1.10 Drawing rectangles to the bitmap
            11. 4.12.5.1.11 Drawing text to the bitmap
            12. 4.12.5.1.12 Drawing paths to the canvas
            13. 4.12.5.1.13 Drawing focus rings and scrolling paths into view
            14. 4.12.5.1.14 Drawing images
            15. 4.12.5.1.15 Pixel manipulation
            16. 4.12.5.1.16 Compositing
            17. 4.12.5.1.17 Image smoothing
            18. 4.12.5.1.18 Shadows
            19. 4.12.5.1.19 Filters
            20. 4.12.5.1.20 Working with externally-defined SVG filters
            21. 4.12.5.1.21 Drawing model
            22. 4.12.5.1.22 Best practices
            23. 4.12.5.1.23 Examples
         2. 4.12.5.2 The ImageBitmap rendering context
            1. 4.12.5.2.1 Introduction
            2. 4.12.5.2.2 The ImageBitmapRenderingContext interface
         3. 4.12.5.3 The OffscreenCanvas interface
            1. 4.12.5.3.1 The offscreen 2D rendering context
         4. 4.12.5.4 Color spaces and color correction
         5. 4.12.5.5 Serializing bitmaps to a file
         6. 4.12.5.6 Security with canvas elements

4.12.5 The canvas element

Categories:

Flow content.

Phrasing content.

Embedded content.

Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

Transparent, but with no interactive content descendants except for 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.

Content attributes:

Global attributes

width

height

Accessibility considerations:

For authors.

For implementers.

DOM interface:

typedef (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.

The width and height IDL attributes must reflect the respective content attributes of the same name, with the same defaults.

context = canvas . getContext(contextId [, options ] )

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:

1. If options is not an object, then set options to null.

2. Set options to the result of converting options to a JavaScript value.

3. 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.

url = canvas . toDataURL( [ type [, quality ] ] )

Returns a data: URL for the image in the canvas.

The first argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is "image/png"; that type is also used if the given type isn't supported. The second argument applies if the type is an image format that supports variable quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive indicating the desired quality level for the resulting image.

When trying to use types other than "image/png", authors can check if the image was really returned in the requested format by checking to see if the returned string starts with one of the exact strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus the requested type was not supported. (The one exception to this is if the canvas has either no height or no width, in which case the result might simply be "data:,".)

canvas . toBlob(callback [, type [, quality ] ] )

Creates a Blob object representing a file containing the image in the canvas, and invokes a callback with a handle to that object.

The second argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is "image/png"; that type is also used if the given type isn't supported. The third argument applies if the type is an image format that supports variable quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive indicating the desired quality level for the resulting image.

canvas . transferControlToOffscreen()

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:

1. If this canvas element's bitmap's origin-clean flag is set to false, then throw a "SecurityError" DOMException.

2. 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.)

3. Let file be a serialization of this canvas element's bitmap as a file, passing type and quality if given.

4. If file is null then return "data:,".

5. Return a data: URL representing file. [RFC2397]

The toBlob(callback, type, quality) method, when invoked, must run these steps:

1. If this canvas element's bitmap's origin-clean flag is set to false, then throw a "SecurityError" DOMException.

2. Let result be null.

3. 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.

4. Run these steps in parallel:

   1. If result is non-null, then set result to a serialization of result as a file with type and quality if given.

   2. Queue an element task on the canvas blob serialization task source given the canvas element to run these steps:

      1. 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]

      2. Invoke callback with « result ».

The transferControlToOffscreen() method, when invoked, must run these steps:

1. If this canvas element's context mode is not set to none, throw an "InvalidStateError" DOMException.

2. 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.

3. Set the placeholder canvas element of offscreenCanvas to be a weak reference to this canvas element.

4. Set this canvas element's context mode to placeholder.

5. Return offscreenCanvas.

4.12.5.1 The 2D rendering context

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([EnforceRange] long sw, [EnforceRange] long sh);
  ImageData createImageData(ImageData imagedata);
  ImageData getImageData([EnforceRange] long sx, [EnforceRange] long sy, [EnforceRange] long sw, [EnforceRange] long sh);
  undefined putImageData(ImageData imagedata, [EnforceRange] long dx, [EnforceRange] long dy);
  undefined putImageData(ImageData imagedata, [EnforceRange] long dx, [EnforceRange] long dy, [EnforceRange] long dirtyX, [EnforceRange] long dirtyY, [EnforceRange] long dirtyWidth, [EnforceRange] long dirtyHeight);
};

enum CanvasLineCap { "butt", "round", "square" };
enum CanvasLineJoin { "round", "bevel", "miter" };
enum CanvasTextAlign { "start", "end", "left", "right", "center" };
enum CanvasTextBaseline { "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" };
enum CanvasDirection { "ltr", "rtl", "inherit" };

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 counterclockwise = false); 
  undefined ellipse(unrestricted double x, unrestricted double y, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation, unrestricted double startAngle, unrestricted double endAngle, optional boolean counterclockwise = false); 
};

[Exposed=(Window,Worker)]
interface CanvasGradient {
  // opaque object
  undefined addColorStop(double offset, DOMString color);
};

[Exposed=(Window,Worker)]
interface CanvasPattern {
  // opaque object
  undefined setTransform(optional DOMMatrix2DInit transform = {});
};

[Exposed=(Window,Worker)]
interface TextMetrics {
  // x-direction
  readonly attribute double width; // advance width
  readonly attribute double actualBoundingBoxLeft;
  readonly attribute double actualBoundingBoxRight;

  // y-direction
  readonly attribute double fontBoundingBoxAscent;
  readonly attribute double fontBoundingBoxDescent;
  readonly attribute double actualBoundingBoxAscent;
  readonly attribute double actualBoundingBoxDescent;
  readonly attribute double emHeightAscent;
  readonly attribute double emHeightDescent;
  readonly attribute double hangingBaseline;
  readonly attribute double alphabeticBaseline;
  readonly attribute double ideographicBaseline;
};

[Exposed=(Window,Worker),
 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.

context = canvas . 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.

context . canvas

Returns the canvas element.

attributes = canvas . 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:

• alpha, set to this context's alpha.

• 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.

<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:

1. Let settings be the result of converting options to the dictionary type CanvasRenderingContext2DSettings. (This can throw an exception.).

2. Let context be a new CanvasRenderingContext2D object.

3. Initialize context's canvas attribute to point to target.

4. Set context's output bitmap to the same bitmap as target's bitmap (so that they are shared).

5. Set bitmap dimensions to the numeric values of target's width and height content attributes.

6. Set context's alpha to settings's alpha.

7. Set context's desynchronized to settings's desynchronized.

8. Return context.

When the user agent is to set bitmap dimensions to width and height, it must run these steps:

1. Reset the rendering context to its default state.

2. Resize the output bitmap to the new width and height and clear it to transparent black.

3. Let canvas be the canvas element to which the rendering context's canvas attribute was initialized.

4. 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.

5. 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.

// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');
context.fillRect(0,0,50,50);
canvas.setAttribute('width', '300'); // clears the canvas
context.fillRect(0,100,50,50);
canvas.width = canvas.width; // clears the canvas
context.fillRect(100,0,50,50); // only this square remains

The canvas attribute must return the value it was initialized to when the object was created.

The 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.

4.12.5.1.1 Implementation notes

This section is non-normative.

The output bitmap, when it is not directly displayed by the user agent, implementations can, instead of updating this bitmap, merely remember the sequence of drawing operations that have been applied to it until such time as the bitmap's actual data is needed (for example because of a call to drawImage(), or the createImageBitmap() factory method). In many cases, this will be more memory efficient.

The bitmap of a canvas element is the one bitmap that's pretty much always going to be needed in practice. The output bitmap of a rendering context, when it has one, is always just an alias to a canvas element's bitmap.

Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its intrinsic size, or to enable double buffering so that graphics updates, like page scrolling for example, can be processed concurrently while canvas draw commands are being executed.

4.12.5.1.2 The canvas state

Objects that implement the CanvasState interface maintain a stack of drawing states. Drawing states consist of:

• The current transformation matrix.
• The current clipping region.
• The current values of the following attributes: strokeStyle, fillStyle, globalAlpha, lineWidth, lineCap, lineJoin, miterLimit, lineDashOffset, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, filter, globalCompositeOperation, font, textAlign, textBaseline, direction, imageSmoothingEnabled, imageSmoothingQuality.
• The current dash list.

The current default path and the rendering context's bitmaps are not part of the drawing state. The current default path is persistent, and can only be reset using the beginPath() method. The bitmaps depend on whether and how the rendering context is bound to a canvas element.

context . save()

Pushes the current state onto the stack.

context . restore()

Pops the top state on the stack, restoring the context to that state.

The save() method, when invoked, must push a copy of the current drawing state onto the drawing state stack.

The restore() method, when invoked, must pop the top entry in the drawing state stack, and reset the drawing state it describes. If there is no saved state, then the method must do nothing.

When the user agent is to reset the rendering context to its default state, it must clear the drawing state stack and everything that drawing state consists of to initial values.

4.12.5.1.3 Line styles

context . lineWidth [ = value ]

styles . lineWidth [ = value ]

Returns the current line width.

Can be set, to change the line width. Values that are not finite values greater than zero are ignored.

context . lineCap [ = value ]

styles . lineCap [ = value ]

Returns the current line cap style.

Can be set, to change the line cap style.

The possible line cap styles are "butt", "round", and "square". Other values are ignored.

context . lineJoin [ = value ]

styles . lineJoin [ = value ]

Returns the current line join style.

Can be set, to change the line join style.

The possible line join styles are "bevel", "round", and "miter". Other values are ignored.

context . miterLimit [ = value ]

styles . miterLimit [ = value ]

Returns the current miter limit ratio.

Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.

context . setLineDash(segments)

styles . setLineDash(segments)

Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.

segments = context . getLineDash()

segments = styles . getLineDash()

Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).

context . lineDashOffset

styles . lineDashOffset

Returns the phase offset (in the same units as the line dash pattern).

Can be set, to change the phase offset. Values that are not finite values are ignored.

Objects that implement the 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:

1. Let a be the argument.

2. 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).

3. If the number of elements in a is odd, then let a be the concatenation of two copies of a.

4. 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.

1. Let path be a copy of the path being traced.

2. Prune all zero-length line segments from path.

3. Remove from path any subpaths containing no lines (i.e. subpaths with just one point).

4. 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.

5. 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).

6. If style's dash list is empty, then jump to the step labeled convert.

7. Let pattern width be the concatenation of all the entries of style's dash list, in coordinate space units.

8. For each subpath subpath in path, run the following substeps. These substeps mutate the subpaths in path in vivo.

   1. Let subpath width be the length of all the lines of subpath, in coordinate space units.

   2. Let offset be the value of style's lineDashOffset, in coordinate space units.

   3. While offset is greater than pattern width, decrement it by pattern width.

      While offset is less than zero, increment it by pattern width.

   4. 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.

   5. Let position be zero minus offset.

   6. Let index be 0.

   7. Let current state be off (the other states being on and zero-on).

   8. Dash on: Let segment length be the value of style's dash list's indexth entry.

   9. Increment position by segment length.

   10. 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.

   11. If segment length is nonzero, then let current state be on.

   12. Increment index by one.

   13. Dash off: Let segment length be the value of style's dash list's indexth entry.

   14. Let start be the offset position on L.

   15. Increment position by segment length.

   16. If position is less than zero, then jump to the step labeled post-cut.

   17. If start is less than zero, then let start be zero.

   18. 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.

   19. Jump to the first appropriate step:

       If segment length is zero and current state is off

       Do nothing, just continue to the next step.

       If current state is off

       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).

       Otherwise

       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.

   20. Post-cut: If position is greater than subpath width, then jump to the step labeled convert.

   21. If segment length is greater than zero, then let positioned-at-on-dash be false.

   22. Increment index by one. If it is equal to the number of entries in style's dash list, then let index be 0.

   23. Return to the step labeled dash on.

9. 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.

10. Return the newly created path.

4.12.5.1.4 Text styles

context . font [ = value ]

styles . font [ = value ]

Returns the current font settings.

Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.

Relative keywords and lengths are computed relative to the font of the canvas element.

context . textAlign [ = value ]

styles . textAlign [ = value ]

Returns the current text alignment settings.

Can be set, to change the alignment. The possible values are and their meanings are given below. Other values are ignored. The default is "start".

context . textBaseline [ = value ]

styles . textBaseline [ = value ]

Returns the current baseline alignment settings.

Can be set, to change the baseline alignment. The possible values and their meanings are given below. Other values are ignored. The default is "alphabetic".

context . direction [ = value ]

styles . direction [ = value ]

Returns the current directionality.

Can be set, to change the directionality. The possible values and their meanings are given below. Other values are ignored. The default is "inherit".

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]

1. If object's font style source object is a canvas element, return the element's node document.

2. Otherwise, object's font style source object is an OffscreenCanvas object:

   1. Let global be object's relevant global object.

   2. If global is a Window object, then return global's associated Document.

   3. Assert: global implements WorkerGlobalScope.

   4. Return global.

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);
});

const offscreenCanvas = document.getElementById("c2").transferControlToOffscreen();
worker.postMessage(offscreenCanvas, [offscreenCanvas]);

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);
  });
};

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]

context.font = 'italic 400 12px/2 Unknown Font, sans-serif';

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

The top of the em square

hanging

The hanging baseline

middle

The middle of the em square

alphabetic

The alphabetic baseline

ideographic

The ideographic-under baseline

bottom

The bottom of the em square

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.)

1. If maxWidth was provided but is less than or equal to zero or equal to NaN, then return an empty array.

2. Replace all ASCII whitespace in text with U+0020 SPACE characters.

3. Let font be the current font of target, as given by that object's font attribute.

4. Apply the appropriate step from the following list to determine the value of direction:

   If the target object's direction attribute has the value "ltr"

   Let direction be 'ltr'.

   If the target object's direction attribute has the value "rtl"

   Let direction be 'rtl'.

   If the target object's font style source object is an element

   Let direction be the directionality of the target object's font style source object.

   If the target object's font style source object is a Document with a non-null document element

   Let direction be the directionality of the target object's font style source object's document element.

   Otherwise

   Let direction be 'ltr'.

5. Form 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]

6. 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.

7. 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:

   If textAlign is left

   If textAlign is start and direction is 'ltr'

   If textAlign is end and direction is 'rtl'

   Let the anchor point's horizontal position be the left edge of the inline box, and let physical alignment be left.

   If textAlign is right

   If textAlign is end and direction is 'ltr'

   If textAlign is start and direction is 'rtl'

   Let the anchor point's horizontal position be the right edge of the inline box, and let physical alignment be right.

   If textAlign is center

   Let the anchor point's horizontal position be half way between the left and right edges of the inline box, and let physical alignment be center.

   Vertical position:

   If textBaseline is top

   Let the anchor point's vertical position be the top of the em box of the first available font of the inline box.

   If textBaseline is hanging

   Let the anchor point's vertical position be the hanging baseline of the first available font of the inline box.

   If textBaseline is middle

   Let the anchor point's vertical position be half way between the bottom and the top of the em box of the first available font of the inline box.

   If textBaseline is alphabetic

   Let the anchor point's vertical position be the alphabetic baseline of the first available font of the inline box.

   If textBaseline is ideographic

   Let the anchor point's vertical position be the ideographic-under baseline of the first available font of the inline box.

   If textBaseline is bottom

   Let the anchor point's vertical position be the bottom of the em box of the first available font of the inline box.

8. Let result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin is at the anchor point.

9. Return result, physical alignment, and the inline box.

4.12.5.1.5 Building paths

Objects that implement the CanvasPath interface have a path. A path has a list of zero or more subpaths. Each subpath consists of a list of one or more points, connected by straight or curved line segments, and a flag indicating whether the subpath is closed or not. A closed subpath is one where the last point of the subpath is connected to the first point of the subpath by a straight line. Subpaths with only one point are ignored when painting the path.

Paths have a need new subpath flag. When this flag is set, certain APIs create a new subpath rather than extending the previous one. When a path is created, its need new subpath flag must be set.

When an object implementing the CanvasPath interface is created, its path must be initialized to zero subpaths.

context . moveTo(x, y)

path . moveTo(x, y)

Creates a new subpath with the given point.

context . closePath()

path . closePath()

Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.

context . lineTo(x, y)

path . lineTo(x, y)

Adds the given point to the current subpath, connected to the previous one by a straight line.

context . quadraticCurveTo(cpx, cpy, x, y)

path . quadraticCurveTo(cpx, cpy, x, y)

Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.

context . bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

path . bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.

context . arcTo(x1, y1, x2, y2, radius)

path . arcTo(x1, y1, x2, y2, radius)

Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context . arc(x, y, radius, startAngle, endAngle [, counterclockwise ] )

path . arc(x, y, radius, startAngle, endAngle [, counterclockwise ] )

Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context . ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise] )

path . ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise] )

Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context . rect(x, y, w, h)

path . 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:

1. If either of the arguments are infinite or NaN, then return.

2. 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:

1. If either of the arguments are infinite or NaN, then return.

2. If the object's path has no subpaths, then ensure there is a subpath for (x, y).

3. 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:

1. If any of the arguments are infinite or NaN, then return.

2. Ensure there is a subpath for (cpx, cpy)

3. Connect the last point in the subpath to the given point (x, y) using a quadratic Bézier curve with control point (cpx, cpy). [BEZIER]

4. Add the given point (x, y) to the subpath.

The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) method, when invoked, must run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. Ensure there is a subpath for (cp1x, cp1y).

3. 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]

4. Add the point (x, y) to the subpath.

The arcTo(x1, y1, x2, y2, radius) method, when invoked, must run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. Ensure there is a subpath for (x1, y1).

3. If radius is negative, then throw an "IndexSizeError" DOMException.

4. 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).

5. 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.

6. 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.

7. Otherwise, let The Arc be the shortest arc given by circumference of the circle that has radius radius, and that has one point tangent to the half-infinite line that crosses the point (x0, y0) and ends at the point (x1, y1), and that has a different point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point (x2, y2). The points at which this circle touches these two lines are called the start and end tangent points respectively. Connect the point (x0, y0) to the start tangent point by a straight line, adding the start tangent point to the subpath, and then connect the start tangent point to the end tangent point by The Arc, adding the end tangent point to the subpath.

The arc(x, y, radius, startAngle, endAngle, counterclockwise) method, when invoked, must run the ellipse method steps with this, x, y, radius, radius, 0, startAngle, endAngle, and counterclockwise.

This makes it equivalent to ellipse() except that both radii are equal and rotation is 0.

The ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, counterclockwise) method, when invoked, must run the ellipse method steps with this, x, y, radiusX, radiusY, rotation, startAngle, endAngle, and counterclockwise.

The ellipse method steps, given canvasPath, x, y, radiusX, radiusY, rotation, startAngle, endAngle, and counterclockwise, are:

1. If any of the arguments are infinite or NaN, then return.

2. If either radiusX or radiusY are negative, then throw an "IndexSizeError" DOMException.

3. 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.

4. Add the start and end points of the arc to the subpath, and connect them with an arc. The arc and its start and end points are defined as follows:

   Consider an ellipse that has its origin at (x, y), that has a major-axis radius radiusX and a minor-axis radius radiusY, and that is rotated about its origin such that its semi-major axis is inclined rotation radians clockwise from the x-axis.

   If counterclockwise is false and endAngle-startAngle is equal to or greater than 2π, or, if counterclockwise 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 counterclockwise if counterclockwise is true, and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles from zero, the arc can never cover an angle greater than 2π radians.

   Even if the arc covers the entire circumference of the ellipse and there are no other points in the subpath, the path is not closed unless the closePath() method is appropriately invoked.

The rect(x, y, w, h) method, when invoked, must run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. 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.

3. Mark the subpath as closed.

4. Create a new subpath with the point (x, y) as the only point in the subpath.

4.12.5.1.6 Path2D objects

Path2D objects can be used to declare paths that are then later used on objects implementing the CanvasDrawPath interface. In addition to many of the APIs described in earlier sections, Path2D objects have methods to combine paths, and to add text to paths.

path = new Path2D()

Creates a new empty Path2D object.

path = new Path2D(path)

When path is a Path2D object, returns a copy.

When path is a string, creates the path described by the argument, interpreted as SVG path data. [SVG]

path . addPath(path [, transform ] )

Adds to the path the path given by the argument.

The Path2D(path) constructor, when invoked, must run these steps:

1. Let output be a new Path2D object.

2. If path is not given, then return output.

3. 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.)

4. 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.

5. Let (x, y) be the last point in svgPath.

6. Add all the subpaths, if any, from svgPath to output.

7. Create a new subpath in output with (x, y) as the only point in the subpath.

8. Return output.

The addPath(b, transform) method, when invoked on a Path2D object a, must run these steps:

1. If the Path2D object b has no subpaths, then return.

2. Let matrix be the result of creating a DOMMatrix from the 2D dictionary transform.

3. 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.

4. Create a copy of all the subpaths in b. Let this copy be known as c.

5. Transform all the coordinates and lines in c by the transform matrix matrix.

6. Let (x, y) be the last point in the last subpath of c.

7. Add all the subpaths in c to a.

8. Create a new subpath in a with (x, y) as the only point in the subpath.

4.12.5.1.7 Transformations

Objects that implement the CanvasTransform interface have a current transformation matrix, as well as methods (described in this section) to manipulate it. When an object implementing the CanvasTransform interface is created, its transformation matrix must be initialized to the identity matrix.

The current transformation matrix is applied to coordinates when creating the current default path, and when painting text, shapes, and Path2D objects, on objects implementing the CanvasTransform interface.

The transformations must be performed in reverse order.

For instance, if a scale transformation that doubles the width is applied to the canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.

context . scale(x, y)

Changes the current transformation matrix to apply a scaling transformation with the given characteristics.

context . rotate(angle)

Changes the current transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.

context . translate(x, y)

Changes the current transformation matrix to apply a translation transformation with the given characteristics.

context . transform(a, b, c, d, e, f)

Changes the current transformation matrix to apply the matrix given by the arguments as described below.

matrix = context . getTransform()

Returns a copy of the current transformation matrix, as a newly created DOMMatrix object.

context . setTransform(a, b, c, d, e, f)

Changes the current transformation matrix to the matrix given by the arguments as described below.

context . setTransform(transform)

Changes the current transformation matrix to the matrix represented by the passed DOMMatrix2DInit dictionary.

context . resetTransform()

Changes the current transformation matrix to the identity matrix.

The scale(x, y) method, when invoked, must run these steps:

1. If either of the arguments are infinite or NaN, then return.

2. 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:

1. If angle is infinite or NaN, then return.

2. 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:

1. If either of the arguments are infinite or NaN, then return.

2. 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:

1. If any of the arguments are infinite or NaN, then return.

2. Replace the current transformation matrix with the result of multiplying the current transformation matrix with the matrix described by:

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:

1. If any of the arguments are infinite or NaN, then return.

2. Reset the current transformation matrix to the identity matrix.

3. Invoke the transform(a, b, c, d, e, f) method with the same arguments.

The setTransform(transform) method, when invoked, must run these steps:

1. Let matrix be the result of creating a DOMMatrix from the 2D dictionary transform.

2. 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.

3. Reset the current transformation matrix to matrix.

The resetTransform() method, when invoked, must reset the current transformation matrix to the identity matrix.

4.12.5.1.8 Image sources for 2D rendering contexts

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)
• ImageBitmap

Although 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:

1. Switch on image:

   HTMLOrSVGImageElement

   If 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.

   HTMLVideoElement

   If image's readyState attribute is either HAVE_NOTHING or HAVE_METADATA, then return bad.

   HTMLCanvasElement

   OffscreenCanvas

   If image has either a horizontal dimension or a vertical dimension equal to zero, then throw an "InvalidStateError" DOMException.

   ImageBitmap

   If image's [[Detached]] internal slot value is set to true, then throw an "InvalidStateError" DOMException.

2. 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.

4.12.5.1.9 Fill and stroke styles

context . fillStyle [ = value ]

Returns the current style used for filling shapes.

Can be set, to change the fill style.

The style can be either a string containing a CSS color, or a CanvasGradient or CanvasPattern object. Invalid values are ignored.

context . strokeStyle [ = value ]

Returns the current style used for stroking shapes.

Can be set, to change the stroke style.

The style can be either a string containing a CSS 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.

gradient . addColorStop(offset, color)

Adds a color stop with the given color to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.

Throws an "IndexSizeError" DOMException if the offset is out of range. Throws a "SyntaxError" DOMException if the color cannot be parsed.

gradient = context . createLinearGradient(x0, y0, x1, y1)

Returns a CanvasGradient object that represents a linear gradient that paints along the line given by the coordinates represented by the arguments.

gradient = context . createRadialGradient(x0, y0, r0, x1, y1, r1)

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:

1. If the offset is less than 0 or greater than 1, then throw an "IndexSizeError" DOMException.

2. 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.

3. If parsed color is failure, throw a "SyntaxError" DOMException.

4. 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:

1. If x₀ = x₁ and y₀ = y₁ and r₀ = r₁, then the radial gradient must paint nothing. Return.

2. 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).

3. 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.

pattern = context . createPattern(image, repetition)

Returns a CanvasPattern object that uses the given image and repeats in the direction(s) given by the repetition argument.

The allowed values for repetition are repeat (both directions), repeat-x (horizontal only), repeat-y (vertical only), and no-repeat (neither). If the repetition argument is empty, the value repeat is used.

If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no data, throws an "InvalidStateError" DOMException.

pattern . setTransform(transform)

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:

1. Let usability be the result of checking the usability of image.

2. If usability is bad, then return null.

3. Assert: usability is good.

4. If repetition is the empty string, then set it to "repeat".

5. If repetition is not identical to one of "repeat", "repeat-x", "repeat-y", or "no-repeat", then throw a "SyntaxError" DOMException.

6. Let pattern be a new CanvasPattern object with the image image and the repetition behavior given by repetition.

7. If image is not origin-clean, then mark pattern as not origin-clean.

8. 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:

1. Let matrix be the result of creating a DOMMatrix from the 2D dictionary transform.

2. 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.

3. 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:

1. Create an infinite transparent black bitmap.

2. 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.)

3. Transform the resulting bitmap according to the pattern's transformation matrix.

4. Transform the resulting bitmap again, this time according to the current transformation matrix.

5. Replace any part of the image outside the area in which the pattern is to be rendered with transparent black.

6. 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.

4.12.5.1.10 Drawing rectangles to the bitmap

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.

context . clearRect(x, y, w, h)

Clears all pixels on the bitmap in the given rectangle to transparent black.

context . fillRect(x, y, w, h)

Paints the given rectangle onto the bitmap, using the current fill style.

context . strokeRect(x, y, w, h)

Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.

The clearRect(x, y, w, h) method, when invoked, must run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.

3. 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 run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. If either w or h are zero, then return.

3. Paint the specified rectangular area using the fillStyle.

The strokeRect(x, y, w, h) method, when invoked, must run these steps:

1. If any of the arguments are infinite or NaN, then return.

2. 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.

4.12.5.1.11 Drawing text to the bitmap

context . fillText(text, x, y [, maxWidth ] )

context . strokeText(text, x, y [, maxWidth ] )

Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.

metrics = context . measureText(text)

Returns a TextMetrics object with the metrics of the given text in the current font.

metrics . width

metrics . actualBoundingBoxLeft

metrics . actualBoundingBoxRight

metrics . fontBoundingBoxAscent

metrics . fontBoundingBoxDescent

metrics . actualBoundingBoxAscent

metrics . actualBoundingBoxDescent

metrics . emHeightAscent

metrics . emHeightDescent

metrics . hangingBaseline

metrics . alphabeticBaseline

metrics . 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:

1. If any of the arguments are infinite or NaN, then return.

2. 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.

3. Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.

4. 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 attribute

✔MDN

TextMetrics/width

Support in all current engines.

Firefox1.5+Safari3.1+Chrome4+

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Opera9+Edge79+

---

Edge (Legacy)12+Internet Explorer9+

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Firefox Android31+Safari iOS3.2+Chrome AndroidYesWebView AndroidYesSamsung InternetYesOpera AndroidYes

The width of that inline box, in CSS pixels. (The text's advance width.)

actualBoundingBoxLeft attribute

✔MDN

TextMetrics/actualBoundingBoxLeft

Support in all current engines.

Firefox74+SafariYesChrome77+

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Opera64+Edge79+

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Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android77+WebView Android77+Samsung Internet12.0+Opera Android55+

The 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 attribute

✔MDN

TextMetrics/actualBoundingBoxRight

Support in all current engines.

Firefox74+SafariYesChrome77+

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Opera64+Edge79+

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android77+WebView Android77+Samsung Internet12.0+Opera Android55+

The 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 attribute

✔MDN

TextMetrics/fontBoundingBoxAscent

Support in all current engines.

Firefox🔰 74+SafariYesChrome87+

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Opera73+Edge87+

---

Edge (Legacy)NoInternet ExplorerNo

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Firefox AndroidNoSafari iOSYesChrome Android87+WebView Android87+Samsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/fontBoundingBoxDescent

Support in all current engines.

Firefox🔰 74+SafariYesChrome87+

---

Opera73+Edge87+

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSNoChrome Android87+WebView Android87+Samsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/actualBoundingBoxAscent

Support in all current engines.

Firefox74+SafariYesChrome77+

---

Opera64+Edge79+

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android77+WebView Android77+Samsung Internet12.0+Opera Android55+

The 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 attribute

✔MDN

TextMetrics/actualBoundingBoxDescent

Support in all current engines.

Firefox74+SafariYesChrome77+

---

Opera64+Edge79+

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android77+WebView Android77+Samsung Internet12.0+Opera Android55+

The 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 attribute

✔MDN

TextMetrics/emHeightAscent

Support in all current engines.

Firefox🔰 74+SafariYesChrome🔰 Yes

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OperaNoEdge🔰 Yes

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android🔰 YesWebView AndroidNoSamsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/emHeightDescent

Support in all current engines.

Firefox🔰 74+SafariYesChrome🔰 Yes

---

OperaNoEdge🔰 Yes

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android🔰 YesWebView AndroidNoSamsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/hangingBaseline

Support in all current engines.

Firefox🔰 74+SafariYesChrome🔰 Yes

---

OperaNoEdge🔰 Yes

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android🔰 YesWebView AndroidNoSamsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/alphabeticBaseline

Support in all current engines.

Firefox🔰 74+SafariYesChrome🔰 Yes

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OperaNoEdge🔰 Yes

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android🔰 YesWebView AndroidNoSamsung InternetNoOpera AndroidNo

The 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 attribute

✔MDN

TextMetrics/ideographicBaseline

Support in all current engines.

Firefox🔰 74+SafariYesChrome🔰 Yes

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OperaNoEdge🔰 Yes

---

Edge (Legacy)NoInternet ExplorerNo

---

Firefox AndroidNoSafari iOSYesChrome Android🔰 YesWebView AndroidNoSamsung InternetNoOpera AndroidNo

The 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.)