Web Applications 1.0

3.14.11. The canvas element

Strictly inline-level embedded content.

Contexts in which this element may be used:

As the only embedded content child of a figure element.

Where strictly inline-level content is allowed.

Content model:

Inline-level content.

Element-specific attributes:

height

width

Predefined classes that apply to this element:

None.

DOM interface:

interface HTMLCanvasElement : HTMLElement {
         attribute long width; size.width size.attributes size.attributes.type.get size.attributes.type.set 
         attribute long height; size.height size.attributes size.attributes.type.get size.attributes.type.set 

  DOMString toDataURL();
  DOMString toDataURL(in DOMString type);

  DOMObject getContext(in DOMString contextID);
};

Shouldn't allow inline-level content to be the content model when the parent's content model is strictly inline only.

The canvas element represents a resolution-dependent bitmap canvas, which can be used for rendering graphs, game graphics, 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 XBL.

When authors use the canvas element, they should also provide content that, when presented to the user, conveys essentially the same function or purpose as the bitmap canvas. This content may be placed as content of the canvas element. The contents of the canvas element, if any, are the element's fallback content.

In interactive visual media with scripting enabled, the canvas element is an embedded element with a dynamically created image.

In non-interactive, static, visual media, if the canvas element has been previously painted on (e.g. if the page was viewed in an interactive visual medium and is now being printed, or if some script that ran during the page layout process painted on the element), then the canvas element must be treated as embedded content with the current image and size. Otherwise, the element's fallback content must be used instead.

In non-visual media, and in visual media with scripting disabled, the canvas element's fallback content must be used instead.

The canvas element has two attributes to control the size of the coordinate space: height and width. 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 size.nonnegativeinteger size.attributes.parse.whitespace size.attributes.parse.nonnumber size.attributes.parse.zero size.attributes.parse.negative size.attributes.parse.floatsuffix.zero size.attributes.parse.floatsuffix.nonzero size.attributes.parse.badsuffix . If an attribute is missing size.missing size.attributes.default size.attributes.removed , or if parsing its value returns an error size.error size.attributes.parse.nonnumber size.attributes.parse.negative , then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150 size.default size.attributes.default .

The intrinsic dimensions of the canvas element equal the size of the coordinate space, with the numbers interpreted in CSS pixels. However, the element can be sized arbitrarily by a style sheet. During rendering, the image is scaled to fit this layout size size.css size.attributes.style .

The size of the coordinate space does not necessarily represent the size of the actual bitmap that the user agent will use internally or during rendering. On high-definition displays, for instance, the user agent may internally use a bitmap with two device pixels per unit in the coordinate space, so that the rendering remains at high quality throughout.

The canvas must initially be fully transparent black initial.colour initial.colour .

If the width and height attributes are dynamically modified, the bitmap and any associated contexts must be cleared back to their initial state initial.reset initial.reset.different initial.reset.same and reinitialised with the newly specified coordinate space dimensions.

The width and height DOM attributes must reflect the content attributes of the same name size.reflect size.attributes.reflect.1 size.attributes.reflect.2 .

To draw on the canvas, authors must first obtain a reference to a context using the getContext method of the canvas element.

This specification only defines one context, with the name "2d". If getContext() is called with that exact string, then the UA must return a reference to an object implementing CanvasRenderingContext2D context.2d 2d.getcontext.exists . Other specifications may define their own contexts, which would return different objects.

Vendors may also define experimental contexts using the syntax vendorname-context, for example, moz-3d.

When the UA is passed an empty string context.empty context.empty or a string specifying a context that it does not support context.unrecognised context.unrecognised.badname context.unrecognised.badsuffix context.unrecognised.nullsuffix context.unrecognised.unicode , then it must return null. String comparisons should be literal and case-sensitive context.casesensitive context.casesensitive .

A future version of this specification will probably define a 3d context (probably based on the OpenGL ES API).

The toDataURL() method must, when called with no arguments, return a data: URI containing a representation of the image as a PNG file toDataURL.noarguments toDataURL.default toDataURL.primarycolours . [PNG].

The toDataURL(type) method (when called with one or more arguments) must return a data: URI containing a representation of the image in the format given by type. The possible values are MIME types with no parameters, for example image/png, image/jpeg, or even maybe image/svg+xml if the implementation actually keeps enough information to reliably render an SVG image from the canvas.

Only support for image/png is required toDataURL.png toDataURL.png toDataURL.complexcolours . User agents may support other types. If the user agent does not support the requested type, it must return the image using the PNG format toDataURL.unrecognised toDataURL.unrecognised .

User agents must convert the provided type to lower case before establishing if they support that type and before creating the data: URL toDataURL.lowercase toDataURL.lowercase .

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 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.

Arguments other than the type must be ignored, and must not cause the user agent to raise an exception (as would normally occur if a method was called with the wrong number of arguments) toDataURL.arguments toDataURL.arguments.1 toDataURL.arguments.2 toDataURL.arguments.3 . A future version of this specification will probably allow extra parameters to be passed to toDataURL() to allow authors to more carefully control compression settings, image metadata, etc.

Security: To prevent information leakage, the toDataURL() toDataURL.security toDataURL.security.domain toDataURL.security.propagate toDataURL.security.pattern toDataURL.security.subdomain toDataURL.security.superdomain and getImageData() getImageData.security methods should raise a security exception if the canvas ever had images painted on it that originate from a domain other than the domain of the script that painted the images onto the canvas.

3.14.11.1. The 2D context

When the getContext() method of a canvas element is invoked with 2d as the argument, a CanvasRenderingContext2D object is returned.

There is only one CanvasRenderingContext2D object per canvas, so calling the getContext() method with the 2d argument a second time must return the same object context.2d.unique 2d.getcontext.unique 2d.getcontext.shared .

The 2D context represents a flat 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 2d.coordinatespace 2d.coordinatespace .

interface CanvasRenderingContext2D {

  // back-reference to the canvas readonly attribute 2d.canvas.attribute 2d.canvas.readonly 
  HTMLCanvasElement canvas;

  // state
  void save(); // push state on state stack
  void restore(); // pop state stack and restore state

  // transformations (default transform is the identity matrix)
  void scale(in float x, in float y);
  void rotate(in float angle);
  void translate(in float x, in float y);
  void transform(in float m11, in float m12, in float m21, in float m22, in float dx, in float dy);
  void setTransform(in float m11, in float m12, in float m21, in float m22, in float dx, in float dy);

  // compositing
           attribute float globalAlpha; // (default 1.0)
           attribute DOMString globalCompositeOperation; // (default over)

  // colors and styles
           attribute DOMObject strokeStyle; // (default black)
           attribute DOMObject fillStyle; // (default black)
  CanvasGradient createLinearGradient(in float x0, in float y0, in float x1, in float y1);
  CanvasGradient createRadialGradient(in float x0, in float y0, in float r0, in float x1, in float y1, in float r1);
  CanvasPattern createPattern(in HTMLImageElement image, DOMString repetition);
  CanvasPattern createPattern(in HTMLCanvasElement image, DOMString repetition);

  // line caps/joins
           attribute float lineWidth; // (default 1)
           attribute DOMString lineCap; // "butt", "round", "square" (default "butt")
           attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter")
           attribute float miterLimit; // (default 10)

  // shadows
           attribute float shadowOffsetX; // (default 0)
           attribute float shadowOffsetY; // (default 0)
           attribute float shadowBlur; // (default 0)
           attribute DOMString shadowColor; // (default black)

  // rects
  void clearRect(in float x, in float y, in float w, in float h);
  void fillRect(in float x, in float y, in float w, in float h);
  void strokeRect(in float x, in float y, in float w, in float h);

  // path API
  void beginPath();
  void closePath();
  void moveTo(in float x, in float y);
  void lineTo(in float x, in float y);
  void quadraticCurveTo(in float cpx, in float cpy, in float x, in float y);
  void bezierCurveTo(in float cp1x, in float cp1y, in float cp2x, in float cp2y, in float x, in float y);
  void arcTo(in float x1, in float y1, in float x2, in float y2, in float radius);
  void rect(in float x, in float y, in float w, in float h);
  void arc(in float x, in float y, in float radius, in float startAngle, in float endAngle, in boolean anticlockwise);
  void fill();
  void stroke();
  void clip();
  boolean isPointInPath(in float x, in float y);

  // drawing images
  void drawImage(in HTMLImageElement image, in float dx, in float dy);
  void drawImage(in HTMLImageElement image, in float dx, in float dy, in float dw, in float dh);
  void drawImage(in HTMLImageElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh);
  void drawImage(in HTMLCanvasElement image, in float dx, in float dy);
  void drawImage(in HTMLCanvasElement image, in float dx, in float dy, in float dw, in float dh);
  void drawImage(in HTMLCanvasElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh);

  // pixel manipulation
  ImageData getImageData(in float sx, in float sy, in float sw, in float sh);
  void putImageData(in ImageData image, in float dx, in float dy);

  // drawing text is not supported in this version of the API
  // (there is no way to predict what metrics the fonts will have,
  // which makes fonts very hard to use for painting)

};

interface CanvasGradient {
  // opaque object
  void addColorStop(in float offset, in DOMString color);
};

interface CanvasPattern {
  // opaque object
};

interface ImageData {
  readonly attribute long int width;
  readonly attribute long int height;
  readonly attribute int[] data;
};

The canvas attribute must return the canvas element that the context paints on 2d.canvas 2d.canvas.reference .

3.14.11.1.1. The canvas state

Each context maintains a stack of drawing states. Drawing states consist of:

• The current transformation matrix 2d.state.transformation 2d.state.saverestore.transformation .
• The current clip region 2d.state.clip 2d.state.saverestore.clip .
• The current values of the following attributes: strokeStyle 2d.state.strokeStyle 2d.state.saverestore.strokeStyle , fillStyle 2d.state.fillStyle 2d.state.saverestore.fillStyle , globalAlpha 2d.state.globalAlpha 2d.state.saverestore.globalAlpha , lineWidth 2d.state.lineWidth 2d.state.saverestore.lineWidth , lineCap 2d.state.lineCap 2d.state.saverestore.lineCap , lineJoin 2d.state.lineJoin 2d.state.saverestore.lineJoin , miterLimit 2d.state.miterLimit 2d.state.saverestore.miterLimit , shadowOffsetX 2d.state.shadowOffsetX 2d.state.saverestore.shadowOffsetX , shadowOffsetY 2d.state.shadowOffsetY 2d.state.saverestore.shadowOffsetY , shadowBlur 2d.state.shadowBlur 2d.state.saverestore.shadowBlur , shadowColor 2d.state.shadowColor 2d.state.saverestore.shadowColor , globalCompositeOperation 2d.state.globalCompositeOperation 2d.state.saverestore.globalCompositeOperation .

The current path 2d.state.path 2d.state.saverestore.path and the current bitmap 2d.state.bitmap 2d.state.saverestore.bitmap are not part of the drawing state. The current path is persistent, and can only be reset using the beginPath() method. The current bitmap is a property of the canvas, not the context.

The save() method pushes a copy of the current drawing state onto the drawing state stack 2d.state.save 2d.state.saverestore.stack 2d.state.saverestore.stackdepth .

The restore() method pops the top entry in the drawing state stack, and resets the drawing state it describes 2d.state.restore 2d.state.saverestore.stack 2d.state.saverestore.stackdepth . If there is no saved state, the method does nothing 2d.state.restore.underflow 2d.state.saverestore.underflow .

3.14.11.1.2. Transformations

The transformation matrix is applied to all drawing operations prior to their being rendered. It is also applied when creating the clip region.

When the context is created, the transformation matrix must initially be the identity transform. It may then be adjusted using the three transformation methods.

The transformations must be performed in reverse order. For instance, if a scale transformation that doubles the width is applied, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square 2d.transformation.order 2d.transformation.order .

The scale(x, y) method must add the scaling transformation described by the arguments to the transformation matrix 2d.transformation.scale 2d.transformation.scale.basic 2d.transformation.scale.zero 2d.transformation.scale.negative 2d.transformation.scale.large 2d.transformation.scale.infinity 2d.transformation.scale.neginfinity 2d.transformation.scale.nan . 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 2d.transformation.scale.multiple 2d.transformation.scale.multiple .

The rotate(angle) method must add the rotation transformation described by the argument to the transformation matrix 2d.transformation.rotate 2d.transformation.rotate.zero 2d.transformation.rotate.wrap 2d.transformation.rotate.wrapnegative 2d.transformation.rotate.infinity 2d.transformation.rotate.neginfinity 2d.transformation.rotate.nan . The angle argument represents a clockwise rotation angle 2d.transformation.rotate.direction 2d.transformation.rotate.direction expressed in radians 2d.transformation.rotate.radians 2d.transformation.rotate.radians .

The translate(x, y) method must add the translation transformation described by the arguments to the transformation matrix 2d.transformation.translate 2d.transformation.translate.basic 2d.transformation.translate.infinity 2d.transformation.translate.nan . 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(m11, m12, m21, m22, dx, dy) method must multiply 2d.transformation.transform.multiply 2d.transformation.transform.multiply the current transformation matrix with the matrix described by 2d.transformation.transform 2d.transformation.transform.identity 2d.transformation.transform.skewed 2d.transformation.transform.infinity :

The setTransform(m11, m12, m21, m22, dx, dy) method must reset the current transform to the identity matrix 2d.transformation.setTransform.identity 2d.transformation.setTransform.multiple , and then invoke the transform(m11, m12, m21, m22, dx, dy) method with the same arguments 2d.transformation.setTransform 2d.transformation.setTransform.skewed .

3.14.11.1.3. Compositing

All drawing operations are affected by the global compositing attributes, globalAlpha and globalCompositeOperation 2d.composite.operation 2d.composite.operation.get .

The globalAlpha attribute gives an alpha value that is applied to shapes 2d.composite.globalAlpha.shape 2d.composite.globalAlpha.fill and images 2d.composite.globalAlpha.image 2d.composite.globalAlpha.image 2d.composite.globalAlpha.canvas before they are composited onto the canvas. The value must be in the range from 0.0 (fully transparent) to 1.0 (no additional transparency). If an attempt is made to set the attribute to a value outside this range, the attribute must retain its previous value 2d.composite.globalAlpha.range 2d.composite.globalAlpha.range 2d.composite.globalAlpha.invalid . When the context is created, the globalAlpha attribute must initially have the value 1.0 2d.composite.globalAlpha.default 2d.composite.globalAlpha.default .

The globalCompositeOperation attribute sets how shapes and images are drawn onto the existing bitmap, once they have had globalAlpha and the current transformation matrix applied. It must be set to a value from the following list. In the descriptions below, the source image is the shape or image being rendered, and the destination image is the current state of the bitmap.

The source-* descriptions below don't define what should happen with semi-transparent regions.

source-atop 2d.composite.source-atop 2d.composite.solid.source-atop 2d.composite.transparent.source-atop 2d.composite.image.source-atop

Display the source image wherever both images are opaque. Display the destination image wherever the destination image is opaque but the source image is transparent. Display transparency elsewhere.

source-in 2d.composite.source-in 2d.composite.solid.source-in 2d.composite.transparent.source-in 2d.composite.uncovered.fill.source-in 2d.composite.uncovered.image.source-in 2d.composite.uncovered.pattern.source-in 2d.composite.image.source-in

Display the source image wherever both the source image and destination image are opaque. Display transparency elsewhere.

source-out 2d.composite.source-out 2d.composite.solid.source-out 2d.composite.transparent.source-out 2d.composite.uncovered.fill.source-out 2d.composite.uncovered.image.source-out 2d.composite.uncovered.pattern.source-out 2d.composite.image.source-out

Display the source image wherever the source image is opaque and the destination image is transparent. Display transparency elsewhere.

source-over 2d.composite.source-over 2d.composite.solid.source-over 2d.composite.transparent.source-over 2d.composite.image.source-over (default)

Display the source image wherever the source image is opaque. Display the destination image elsewhere.

destination-atop 2d.composite.destination-atop 2d.composite.solid.destination-atop 2d.composite.transparent.destination-atop 2d.composite.uncovered.fill.destination-atop 2d.composite.uncovered.image.destination-atop 2d.composite.uncovered.pattern.destination-atop 2d.composite.image.destination-atop

Same as source-atop but using the destination image instead of the source image and vice versa.

destination-in 2d.composite.destination-in 2d.composite.solid.destination-in 2d.composite.transparent.destination-in 2d.composite.uncovered.fill.destination-in 2d.composite.uncovered.image.destination-in 2d.composite.uncovered.pattern.destination-in 2d.composite.image.destination-in

Same as source-in but using the destination image instead of the source image and vice versa.

destination-out 2d.composite.destination-out 2d.composite.solid.destination-out 2d.composite.transparent.destination-out 2d.composite.image.destination-out

Same as source-out but using the destination image instead of the source image and vice versa.

destination-over 2d.composite.destination-over 2d.composite.solid.destination-over 2d.composite.transparent.destination-over 2d.composite.image.destination-over

Same as source-over but using the destination image instead of the source image and vice versa.

darker 2d.composite.darker

Display the sum of the source image and destination images, with color values approaching 0 as a limit.

lighter 2d.composite.lighter 2d.composite.solid.lighter 2d.composite.transparent.lighter 2d.composite.image.lighter

Display the sum of the source image and destination image, with color values approaching 1 as a limit.

copy 2d.composite.copy 2d.composite.solid.copy 2d.composite.transparent.copy 2d.composite.uncovered.fill.copy 2d.composite.uncovered.image.copy 2d.composite.uncovered.pattern.copy 2d.composite.image.copy

Display the source image instead of the destination image.

xor 2d.composite.xor 2d.composite.solid.xor 2d.composite.transparent.xor 2d.composite.image.xor

Exclusive OR of the source and destination images.

vendorName-operationName

Vendor-specific extensions to the list of composition operators should use this syntax.

These values are all case-sensitive 2d.composite.operation.casesensitive 2d.composite.operation.casesensitive — they must be used exactly as shown. User agents must only recognise values that exactly match the values given above 2d.composite.operation.exact 2d.composite.operation.nullsuffix .

On setting, if the user agent does not recognise the specified value, it must be ignored, leaving the value of globalCompositeOperation unaffected 2d.composite.operation.unrecognised 2d.composite.operation.unrecognised .

When the context is created, the globalCompositeOperation attribute must initially have the value source-over 2d.composite.operation.default 2d.composite.operation.default .

3.14.11.1.4. Colors and styles

The strokeStyle attribute represents the color or style to use for the lines around shapes, and the fillStyle attribute represents the color or style to use inside the shapes.

Both attributes can be either strings, CanvasGradients, or CanvasPatterns. On setting, strings should be parsed as CSS <color> values and the color assigned 2d.colours.parse 2d.fillStyle.parse.html4 2d.fillStyle.parse.hex-3 2d.fillStyle.parse.hex-6 2d.fillStyle.parse.rgb-num 2d.fillStyle.parse.rgb-clamp 2d.fillStyle.parse.rgb-percent 2d.fillStyle.parse.rgba-num 2d.fillStyle.parse.rgba-percent 2d.fillStyle.parse.transparent-1 2d.fillStyle.parse.transparent-2 2d.fillStyle.parse.hsl-1 2d.fillStyle.parse.hsl-2 2d.fillStyle.parse.hsla 2d.fillStyle.parse.svg-1 2d.fillStyle.parse.svg-2 , and CanvasGradient and CanvasPattern objects must be assigned themselves. [CSS3COLOR] If the value is a string but is not a valid color 2d.colours.invalidstring 2d.fillStyle.invalidstring , or is neither a string, a CanvasGradient, nor a CanvasPattern 2d.colours.invalidtype 2d.fillStyle.invalidtype , then it must be ignored, and the attribute must retain its previous value.

On getting, if the value is a color, then: if it has alpha equal to 1.0, then the color must be returned as an uppercase six-digit hex value 2d.colours.getcolour.solid 2d.fillStyle.get.solid , prefixed with a "#" character (U+0023 NUMBER SIGN), with the first two digits representing the red component, the next two digits representing the green component, and the last two digits representing the blue component, the digits being in the range 0-9 A-F (U+0030 to U+0039 and U+0041 to U+0046). If the value has alpha less than 1.0, then the value must instead be returned 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 digits 0-9, U+0030 to U+0039, in the shortest form possible), a literal U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer for the blue component, another comma and space, a U+0030 DIGIT ZERO, a U+002E FULL STOP (representing the decimal point), one or more digits in the range 0-9 (U+0030 to U+0039) representing the fractional part of the alpha value, and finally a U+0029 RIGHT PARENTHESIS 2d.colours.getcolour.transparent 2d.fillStyle.get.semitransparent 2d.fillStyle.get.transparent .

Otherwise, if it is not a color but a CanvasGradient 2d.gradient.object 2d.gradient.object.compare or CanvasPattern 2d.pattern.object , then an object supporting those interfaces must be returned. Such objects are opaque and therefore only useful for assigning to other attributes or for comparison to other gradients or patterns.

When the context is created, the strokeStyle and fillStyle attributes must initially have the string value #000000 2d.colours.default 2d.fillStyle.default 2d.strokeStyle.default .

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 must be placed along it to define how the colors are distributed along the gradient. Between each such stop, the colors and the alpha component must be interpolated over the RGBA space to find the color to use at that offset 2d.gradient.interpolate 2d.gradient.interpolate.solid 2d.gradient.interpolate.colour 2d.gradient.interpolate.alpha 2d.gradient.interpolate.colouralpha 2d.gradient.interpolate.vertical 2d.gradient.interpolate.multiple 2d.gradient.interpolate.overlap . Immediately before the 0 offset and immediately after the 1 offset, transparent black stops are be assumed 2d.gradient.ends 2d.gradient.interpolate.ends .

The addColorStop(offset, color) method on the CanvasGradient interface adds a new stop to a gradient. If the offset is less than 0 or greater than 1 then an INDEX_SIZE_ERR exception must be raised 2d.gradient.invalidoffset 2d.gradient.object.invalidoffset . If the color cannot be parsed as a CSS color, then a SYNTAX_ERR exception must be raised 2d.gradient.invalidcolour 2d.gradient.object.invalidcolour . Otherwise, the gradient must be updated with the new stop information 2d.gradient.update 2d.gradient.object.update .

The createLinearGradient(x0, y0, x1, y1) method takes four arguments, representing the start point (x0, y0) and end point (x1, y1) of the gradient, in coordinate space units, and must return a linear CanvasGradient initialised with that line.

Linear gradients must be rendered such that at the starting point on the canvas the color at offset 0 is used, that at the ending point the color at offset 1 is used, that all points on a line perpendicular to the line between the start and end points have the color at the point where those two lines cross (interpolation happening as described above), and that any points beyond the start or end points are a transparent black 2d.gradient.outside 2d.gradient.interpolate.outside 2d.gradient.interpolate.outside2 .

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. The method must return a radial CanvasGradient initialised with those two circles.

Radial gradients must be rendered such that a cone is created from the two circles, so that at the circumference of the starting circle the color at offset 0 is used, that at the circumference around the ending circle the color at offset 1 is used, that the circumference of a circle drawn a certain fraction of the way along the line between the two origins with a radius the same fraction of the way between the two radii has the color at that offset (interpolation happening as described above), that the end circle appear to be above the start circle when the end circle is not completely enclosed by the start circle, that the end circle be filled by the color at offset 1, and that any points not described by the gradient are a transparent black.

If a gradient has no stops defined, then the gradient must be treated as a solid transparent black. Gradients are, naturally, only painted where the stroking or filling effect requires that they be drawn.

Support for actually painting gradients is optional. Instead of painting the gradients, user agents may instead just paint the first stop's color. However, createLinearGradient() and createRadialGradient() must always return objects when passed valid arguments.

Patterns are represented by objects implementing the opaque CanvasPattern interface.

To create objects of this type, the createPattern(image, repetition) method is used. The first argument gives the image to use as the pattern (either an HTMLImageElement or an HTMLCanvasElement). Modifying this image after calling the createPattern() method must not affect the pattern. The second argument must be a string with one of the following values: repeat, repeat-x, repeat-y, no-repeat. If the empty string or null is specified, repeat must be assumed. If an unrecognised value is given, then the user agent must raise a SYNTAX_ERR exception. User agents must recognise the four values described above exactly (e.g. they must not do case folding). The method must return a CanvasPattern object suitably initialised.

The image argument must be an instance of an HTMLImageElement or HTMLCanvasElement. If the image is of the wrong type, the implementation must raise a TYPE_MISMATCH_ERR exception.

Patterns must be painted so that the top left of the first image is anchored at the origin of the coordinate space, and images are then repeated horizontally to the left and right (if the repeat-x string was specified) or vertically up and down (if the repeat-y string was specified) or in all four directions all over the canvas (if the repeat string was specified). The images are not be scaled by this process; one CSS pixel of the image must be painted on one coordinate space unit. Of course, patterns must only actually painted where the stroking or filling effect requires that they be drawn, and are affected by the current transformation matrix.

Support for patterns is optional. If the user agent doesn't support patterns, then createPattern() must return null.

3.14.11.1.5. Line styles

The lineWidth attribute gives the default width of lines, in coordinate space units. On setting, zero and negative values must be ignored, leaving the value unchanged.

When the context is created, the lineWidth attribute must initially have the value 1.0.

The lineCap attribute defines the type of endings that UAs shall place on the end of lines. The three valid values are butt, round, and square. The butt value means that the end of each line is a flat edge perpendicular to the direction of the line. The round value means that a semi-circle with the diameter equal to the width of the line is then added on to the end of the line. The square value means that at the end of each line is a rectangle with the length of the line width and the width of half the line width, placed flat against the edge perpendicular to the direction of the line. On setting, any other value than the literal strings butt, round, and square must be ignored, leaving the value unchanged.

When the context is created, the lineCap attribute must initially have the value butt.

The lineJoin attribute defines the type of corners that that UAs will place where two lines meet. The three valid values are round, bevel, and miter.

On setting, any other value than the literal strings round, bevel and miter must be ignored, leaving the value unchanged.

When the context is created, the lineJoin attribute must initially have the value miter.

The round value means that a filled arc connecting the corners on the outside of the join, with the diameter equal to the line width, and the origin at the point where the inside edges of the lines touch, must be rendered at joins. The bevel value means that a filled triangle connecting those two corners with a straight line, the third point of the triangle being the point where the lines touch on the inside of the join, must be rendered at joins. The miter value means that a filled four- or five-sided polygon must be placed at the join, with two of the lines being the perpendicular edges of the joining lines, 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 limit.

The miter length is the distance from the point where the lines touch on the inside of the join 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 the line width. If the miter limit would be exceeded, then a fifth line must be added to the polygon, connecting the two outside lines, such that the distance from the inside point of the join to the point in the middle of this fifth line is the maximum allowed value for the miter length.

The miter limit ratio can be explicitly set using the miterLimit attribute. On setting, zero and negative values must be ignored, leaving the value unchanged.

When the context is created, the miterLimit attribute must initially have the value 10.0.

3.14.11.1.6. Shadows

All drawing operations are affected by the four global shadow attributes. Shadows form part of the source image during composition.

The shadowColor attribute sets the color of the shadow.

When the context is created, the shadowColor attribute initially must be fully-transparent black.

The shadowOffsetX and shadowOffsetY attributes specify the distance that the shadow will be offset in the positive horizontal and positive vertical distance respectively. Their values are in coordinate space units.

When the context is created, the shadow offset attributes initially have the value 0.

The shadowBlur attribute specifies the number of coordinate space units that the blurring is to cover. On setting, negative numbers must be ignored, leaving the attribute unmodified.

When the context is created, the shadowBlur attribute must initially have the value 0.

Support for shadows is optional. When they are supported, then, when shadows are drawn, they must be rendered using the specified color, offset, and blur radius.

3.14.11.1.7. Simple shapes (rectangles)

There are three methods that immediately draw rectangles to the bitmap. They 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 and height of the rectangle, respectively.

Shapes are painted without affecting the current path, and are subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.

Negative values for width and height must cause the implementation to raise an INDEX_SIZE_ERR exception.

The clearRect() method must clear the pixels in the specified rectangle to a fully transparent black, erasing any previous image. If either height or width are zero, this method has no effect.

The fillRect() method must paint the specified rectangular area using the fillStyle. If either height or width are zero, this method has no effect.

The strokeRect() method must draw a rectangular outline of the specified size using the strokeStyle, lineWidth, lineJoin, and (if appropriate) miterLimit attributes. What should happen with zero heights or widths?

3.14.11.1.8. Complex shapes (paths)

The context always has a current path. There is only one current path, it is not part of the drawing state.

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 lines, and a flag indicating whether the subpath is closed or not. A closed subpath is one where the last point of the subpath is connected to the first point of the subpath by a straight line. Subpaths with fewer than two points are ignored when painting the path.

Initially, the context's path must have zero subpaths.

The beginPath() method must empty the list of subpaths so that the context once again has zero subpaths.

The moveTo(x, y) method must create a new subpath with the specified point as its first (and only) point.

The closePath() method must do nothing if the context has no subpaths. Otherwise, it must mark the last subpath as closed, create a new subpath whose first point is the same as the previous subpath's first point, and finally add this new subpath to the path. (If the last subpath had more than one point in its list of points, then this is equivalent to adding a straight line connecting the last point back to the first point, thus "closing" the shape, and then repeating the last moveTo() call.)

New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the context's paths.

The lineTo(x, y) method must do nothing if the context has no subpaths. Otherwise, it must connect the last point in the subpath to the given point (x, y) using a straight line, and must then add the given point (x, y) to the subpath.

The quadraticCurveTo(cpx, cpy, x, y) method must do nothing if the context has no subpaths. Otherwise it must connect the last point in the subpath to the given point (x, y) by a quadratic curve with control point (cpx, cpy), and must then add the given point (x, y) to the subpath.

The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) method must do nothing if the context has no subpaths. Otherwise, it must connect the last point in the subpath to the given point (x, y) using a bezier curve with control points (cp1x, cp1y) and (cp2x, cp2y). Then, it must add the point (x, y) to the subpath.

The arcTo(x1, y1, x2, y2, radius) method must do nothing if the context has no subpaths. If the context does have a subpath, then the behaviour depends on the arguments and the last point in the subpath.

Let the point (x0, y0) be the last point in the subpath. Let The Arc be the shortest arc given by circumference of the circle that has one point tangent to the line defined by the points (x0, y0) and (x1, y1), another point tangent to the line defined by the points (x1, y1) and (x2, y2), and that has radius radius. The points at which this circle touches these two lines are called the start and end tangent points respectively.

If the point (x2, y2) is on the line defined by the points (x0, y0) and (x1, y1) then the method must do nothing, as no arc would satisfy the above constraints.

Otherwise, the method must connect the point (x0, y0) to the start tangent point by a straight line, then connect the start tangent point to the end tangent point by The Arc, and finally add the start and end tangent points to the subpath.

Negative or zero values for radius must cause the implementation to raise an INDEX_SIZE_ERR exception.

The arc(x, y, radius, startAngle, endAngle, anticlockwise) method draws an arc. If the context has any subpaths, then the method must add a straight line from the last point in the subpath to the start point of the arc. In any case, it must draw the arc between the start point of the arc and the end point of the arc, and add the start and end points of the arc to the subpath. The arc and its start and end points are defined as follows:

Consider a circle that has its origin at (x, y) and that has radius radius. The points at startAngle and endAngle along the circle's circumference, measured in radians clockwise from the positive x-axis, are the start and end points respectively. The arc is the path along the circumference of this circle from the start point to the end point, going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise.

Negative or zero values for radius must cause the implementation to raise an INDEX_SIZE_ERR exception.

The rect(x, y, w, h) method must create a new subpath containing just the four points (x, y), (x+w, y), (x+w, y+h), (x, y+h), with those four points connected by straight lines, and must then mark the subpath as closed. It must then create a new subpath with the point (x, y) as the only point in the subpath.

Negative values for w and h must cause the implementation to raise an INDEX_SIZE_ERR exception.

The fill() method must fill each subpath of the current path in turn, using fillStyle, and using the non-zero winding number rule. Open subpaths must be implicitly closed when being filled (without affecting the actual subpaths).

The stroke() method must stroke each subpath of the current path in turn, using the strokeStyle, lineWidth, lineJoin, and (if appropriate) miterLimit attributes.

Paths, when filled or stroked, must be painted without affecting the current path, and must be subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.

The transformation is applied to the path when it is drawn, not when the path is constructed. Thus, a single path can be constructed and then drawn according to different transformations without recreating the path.

The clip() method must create a new clipping path by calculating the intersection of the current clipping path and the area described by the current path (after applying the current transformation), using the non-zero winding number rule. Open subpaths must be implicitly closed when computing the clipping path, without affecting the actual subpaths.

When the context is created, the initial clipping path is the rectangle with the top left corner at (0,0) and the width and height of the coordinate space.

The isPointInPath(x, y) method must return true if the point given by the x and y coordinates passed to the method, when treated as coordinates in the canvas' coordinate space unaffected by the current transformation, is within the area of the canvas that is inside the current path; and must return false otherwise.

3.14.11.1.9. Images

To draw images onto the canvas, the drawImage method can be used.

This method is overloaded with three variants: drawImage(image, dx, dy), drawImage(image, dx, dy, dw, dh), and drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh). (Actually it is overloaded with six; each of those three can take either an HTMLImageElement or an HTMLCanvasElement for the image argument.) If not specified, the dw and dh arguments default to the values of sw and sh, interpreted such that one CSS pixel in the image is treated as one unit in the canvas coordinate space. If the sx, sy, sw, and sh arguments are omitted, they default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively.

The image argument must be an instance of an HTMLImageElement or HTMLCanvasElement. If the image is of the wrong type, the implementation must raise a TYPE_MISMATCH_ERR exception. If one of the sy, sw, sw, and sh arguments is outside the size of the image, or if one of the dw and dh arguments is negative, the implementation must raise an INDEX_SIZE_ERR exception.

When drawImage() is invoked, the specified region of the image specified by the source rectangle (sx, sy, sw, sh) must be painted on the region of the canvas specified by the destination rectangle (dx, dy, dw, dh).

Images are painted without affecting the current path, and are subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.

3.14.11.1.10. Pixel manipulation

The getImageData(sx, sy, sw, sh) method must return an ImageData object representing the underlying pixel data for the area of the canvas denoted by the rectangle which has one corner at the (sx, sy) coordinate, and that has width sw and height sh. Pixels outside the canvas must be returned as transparent black.

ImageData objects must be initialised so that their height attribute is set to h, the number of rows in the image data, their width attribute is set to w, the number of physical device pixels per row in the image data, and the data attribute is initialised to an array of h×w×4 integers. The pixels must be represented in this array in left-to-right order, row by row, starting at the top left, with each pixel's red, green, blue, and alpha components being given in that order. Each component of each device pixel represented in this array must be in the range 0..255, representing the 8 bit value for that component.

The putImageData(image, dx, dy) method must take the given ImageData structure, and draw it at the specified location dx,dy in the canvas coordinate space, mapping each pixel represented by the ImageData structure into one device pixel.

The handling of pixel rounding when the specified coordinates do not exactly map to the device coordinate space is not defined by this specification, except that the following must result in no visible changes to the rendering:

context.putImageData(context.getImageData(x, y, w, h), x, y);

...for any value of x and y. In other words, while user agents may round the arguments of the two methods so that they map to device pixel boundaries, any rounding performed must be performed consistently for both the getImageData() and putImageData() operations.

The current transformation matrix must not affect the getImageData() and putImageData() methods.

3.14.11.1.11. Drawing model

When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):

1. The coordinates are transformed by the current transformation matrix.
2. The shape or image is rendered, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honoured.
3. The shadow is rendered from image A, using the current shadow styles, creating image B.
4. Image A is composited over image B creating the source image.
5. The source image has its alpha adjusted by globalAlpha.
6. Within the clip region (as affected by the current transformation matrix), the source image is composited over the current canvas bitmap using the current composition operator.