Abstract

This specification defines the 2D Context for the HTML canvas element.

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The W3C HTML Working Group is the W3C working group responsible for this specification's progress along the W3C Recommendation track. This specification is the 24 May 2011 Working Draft.

Work on this specification is also done at the WHATWG. The W3C HTML working group actively pursues convergence with the WHATWG, as required by the W3C HTML working group charter.

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Table of Contents

  1. 1 Conformance requirements
  2. 2 The canvas state
  3. 3 Transformations
  4. 4 Compositing
  5. 5 Colors and styles
  6. 6 Line styles
  7. 7 Shadows
  8. 8 Simple shapes (rectangles)
  9. 9 Complex shapes (paths)
  10. 10 Focus management
  11. 11 Text
  12. 12 Images
  13. 13 Pixel manipulation
  14. 14 Drawing model
  15. 15 Examples
  16. References
  17. Acknowledgements

1 Conformance requirements

This specification is an HTML specification. All the conformance requirements, conformance classes, definitions, dependencies, terminology, and typographical conventions described in the core HTML5 specification apply to this specification. [HTML5]

Interfaces are defined in terms of Web IDL. [WEBIDL]

This specification defines the 2d context type, whose API is implemented using the CanvasRenderingContext2D interface.

When the getContext() method of a canvas element is to return a new object for the contextId 2d, the user agent must return a new CanvasRenderingContext2D object. Any additional arguments are ignored.

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.

interface CanvasRenderingContext2D {

  // back-reference to the canvas
  readonly attribute 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 double x, in double y);
  void rotate(in double angle);
  void translate(in double x, in double y);
  void transform(in double a, in double b, in double c, in double d, in double e, in double f);
  void setTransform(in double a, in double b, in double c, in double d, in double e, in double f);
  // compositing
           attribute double globalAlpha; // (default 1.0)
           attribute DOMString globalCompositeOperation; // (default source-over)
  // colors and styles
           attribute any strokeStyle; // (default black)
           attribute any fillStyle; // (default black)
  CanvasGradient createLinearGradient(in double x0, in double y0, in double x1, in double y1);
  CanvasGradient createRadialGradient(in double x0, in double y0, in double r0, in double x1, in double y1, in double r1);
  CanvasPattern createPattern(in HTMLImageElement image, in DOMString repetition);
  CanvasPattern createPattern(in HTMLCanvasElement image, in DOMString repetition);
  CanvasPattern createPattern(in HTMLVideoElement image, in DOMString repetition);

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

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

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

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

  // Focus management
  boolean drawFocusRing(in Element element, in optional boolean canDrawCustom);

  // Caret and selection management
  long caretBlinkRate();
  boolean setCaretSelectionRect(in Element element, in double x, in double y, in double w, in double h);

  // text
           attribute DOMString font; // (default 10px sans-serif)
           attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start")
           attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
  void fillText(in DOMString text, in double x, in double y, in optional double maxWidth);
  void strokeText(in DOMString text, in double x, in double y, in optional double maxWidth);
  TextMetrics measureText(in DOMString text);

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

  // pixel manipulation
  ImageData createImageData(in double sw, in double sh);
  ImageData createImageData(in ImageData imagedata);
  ImageData getImageData(in double sx, in double sy, in double sw, in double sh);
  void putImageData(in ImageData imagedata, in double dx, in double dy, in optional double dirtyX, in double dirtyY, in double dirtyWidth, in double dirtyHeight);
};

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

interface CanvasPattern {
  // opaque object
};

interface TextMetrics {
  readonly attribute double width;
};

interface ImageData {
  readonly attribute unsigned long width;
  readonly attribute unsigned long height;
  readonly attribute CanvasPixelArray data;
};

interface CanvasPixelArray {
  readonly attribute unsigned long length;
  getter octet (in unsigned long index);
  setter void (in unsigned long index, in octet value);
};
context . canvas

Returns the canvas element.

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

Except where otherwise specified, for the 2D context interface, any method call with a numeric argument whose value is infinite or a NaN value must be ignored.

Whenever the CSS value currentColor is used as a color in this API, the "computed value of the 'color' property" for the purposes of determining the computed value of the currentColor keyword is the computed value of the 'color' property on the element in question at the time that the color is specified (e.g. when the appropriate attribute is set, or when the method is called; not when the color is rendered or otherwise used). If the computed value of the 'color' property is undefined for a particular case (e.g. because the element is not in a Document), then the "computed value of the 'color' property" for the purposes of determining the computed value of the currentColor keyword is fully opaque black. [CSSCOLOR]

In the case of addColorStop() on CanvasGradient, the "computed value of the 'color' property" for the purposes of determining the computed value of the currentColor keyword is always fully opaque black (there is no associated element). [CSSCOLOR]

This is 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.

2 The canvas state

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

The current path and the current 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.

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 must push a copy of the current drawing state onto the drawing state stack.

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

3 Transformations

The transformation matrix is applied to coordinates when creating shapes and paths.

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

context . scale(x, y)

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

context . rotate(angle)

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

context . translate(x, y)

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

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

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

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

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

The scale(x, y) method must add the scaling transformation described by the arguments to the 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 must add the rotation transformation described by the argument to the transformation matrix. The angle argument represents a clockwise rotation angle expressed in radians.

The translate(x, y) method must add the translation transformation described by the arguments to the 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 must replace the current transformation matrix with the result of multiplying the current transformation matrix with the matrix described by:

a c e
b d f
0 0 1

The arguments a, b, c, d, e, and f are sometimes called m11, m12, m21, m22, dx, and dy or m11, m21, m12, m22, dx, and dy. Care should 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 setTransform(a, b, c, d, e, f) method must reset the current transform to the identity matrix, and then invoke the transform(a, b, c, d, e, f) method with the same arguments.

4 Compositing

context . globalAlpha [ = value ]

Returns the current alpha value applied to rendering operations.

Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are ignored.

context . globalCompositeOperation [ = value ]

Returns the current composition operation, from the list below.

Can be set, to change the composition operation. Unknown values are ignored.

All drawing operations are affected by the global compositing attributes, globalAlpha and globalCompositeOperation.

The globalAlpha attribute gives an alpha value that is applied to shapes and images 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, including Infinity and Not-a-Number (NaN) values, the attribute must retain its previous value. When the context is created, the globalAlpha attribute must initially have the value 1.0.

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, A, is the shape or image being rendered, and the destination image, B, is the current state of the bitmap.

source-atop
A atop B. 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
A in B. Display the source image wherever both the source image and destination image are opaque. Display transparency elsewhere.
source-out
A out B. Display the source image wherever the source image is opaque and the destination image is transparent. Display transparency elsewhere.
source-over (default)
A over B. Display the source image wherever the source image is opaque. Display the destination image elsewhere.
destination-atop
B atop A. Same as source-atop but using the destination image instead of the source image and vice versa.
destination-in
B in A. Same as source-in but using the destination image instead of the source image and vice versa.
destination-out
B out A. Same as source-out but using the destination image instead of the source image and vice versa.
destination-over
B over A. Same as source-over but using the destination image instead of the source image and vice versa.
lighter
A plus B. Display the sum of the source image and destination image, with color values approaching 255 (100%) as a limit.
copy
A (B is ignored). Display the source image instead of the destination image.
xor
A xor B. Exclusive OR of the source image and destination image.
vendorName-operationName
Vendor-specific extensions to the list of composition operators should use this syntax.

The operators in the above list must be treated as described by the Porter-Duff operator given at the start of their description (e.g. A over B). They are to be applied as part of the drawing model, at which point the clipping region is also applied. (Without a clipping region, these operators act on the whole bitmap with every operation.) [PORTERDUFF]

These values are all case-sensitive — they must be used exactly as shown. User agents must not recognize values that are not a case-sensitive match for one of the values given above.

On setting, if the user agent does not recognize the specified value, it must be ignored, leaving the value of globalCompositeOperation unaffected.

When the context is created, the globalCompositeOperation attribute must initially have the value source-over.

5 Colors and styles

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.

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.

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 must be parsed as CSS <color> values and the color assigned, and CanvasGradient and CanvasPattern objects must be assigned themselves. [CSSCOLOR] If the value is a string but cannot be parsed as a CSS <color> value, or is neither a string, a CanvasGradient, nor a CanvasPattern, then it must be ignored, and the attribute must retain its previous value.

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 in the range 0-9 a-f (U+0030 to U+0039 and U+0061 to U+0066). 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 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, 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 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.

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


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 INDEX_SIZE_ERR exception if the offset is out of range. Throws a SYNTAX_ERR exception 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.

If any of the arguments are not finite numbers, throws a NOT_SUPPORTED_ERR exception.

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 any of the arguments are not finite numbers, throws a NOT_SUPPORTED_ERR exception. If either of the radii are negative, throws an INDEX_SIZE_ERR exception.

The addColorStop(offset, color) method on the CanvasGradient interface adds a new stop to a gradient. If the offset is less than 0, greater than 1, infinite, or NaN, then an INDEX_SIZE_ERR exception must be raised. If the color cannot be parsed as a CSS <color> value, then a SYNTAX_ERR exception must be raised. Otherwise, the gradient must have a new stop placed, at offset offset relative to the whole gradient, and with the color obtained by parsing color as a CSS <color> value. If multiple stops are added at the same offset on a gradient, 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. If any of the arguments to createLinearGradient() are infinite or NaN, the method must raise a NOT_SUPPORTED_ERR exception. Otherwise, the method 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 any of the arguments are infinite or NaN, a NOT_SUPPORTED_ERR exception must be raised. If either of r0 or r1 are negative, an INDEX_SIZE_ERR exception must be raised. Otherwise, the method 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. Abort these steps.

  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 canvas 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 points in the radial gradient must 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 or null, the value repeat is used.

If the first argument isn't an img, canvas, or video element, throws a TYPE_MISMATCH_ERR exception. If the image has no image data, throws an INVALID_STATE_ERR exception. If the second argument isn't one of the allowed values, throws a SYNTAX_ERR exception. If the image isn't yet fully decoded, then the method returns null.

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, HTMLCanvasElement, or HTMLVideoElement object). 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 unrecognized value is given, then the user agent must raise a SYNTAX_ERR exception. User agents must recognize the four values described above exactly (e.g. they must not do case folding). Except as specified below, the method must return a CanvasPattern object suitably initialized.

The image argument is an instance of either HTMLImageElement, HTMLCanvasElement, or HTMLVideoElement. If the image is null, the implementation must raise a TYPE_MISMATCH_ERR exception.

If the image argument is an HTMLImageElement object that is not fully decodable, or if the image argument is an HTMLVideoElement object whose readyState attribute is either HAVE_NOTHING or HAVE_METADATA, then the implementation must return null.

If the image argument is an HTMLCanvasElement object with either a horizontal dimension or a vertical dimension equal to zero, then the implementation must raise an INVALID_STATE_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 scaled by this process; one CSS pixel of the image must be painted on one coordinate space unit. Of course, patterns must actually be painted only where the stroking or filling effect requires that they be drawn, and are affected by the current transformation matrix.

If the original image data is a bitmap image, the value painted at a point in the area of the repetitions is computed by filtering the original image data. The user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from wrapping the pixel's coordinates to the original image's dimensions. (That is, the filter uses 'repeat' behavior, regardless of the value of repetition.)

When the createPattern() method is passed an animated image as its image argument, the user agent must use the poster frame of the animation, or, if there is no poster frame, the first frame of the animation.

When the image argument is an HTMLVideoElement, then the frame at the current playback position must be used as the source image, 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).

6 Line styles

context . 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 ]

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 ]

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 ]

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.

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 context 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. The butt value means that the end of each line has a flat edge perpendicular to the direction of the line (and that no additional line cap is added). The round value means that a semi-circle with the diameter equal to the width of the line must then be added on to the end of the line. The square value means that 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, must be added at the end of each line.

On getting, it must return the current value. On setting, if the new value is one of the literal strings butt, round, and square, then the current value must be changed to the new value; other values must 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 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, if the new value is one of the literal strings bevel, round, and miter, then the current value must be changed to the new value; other values must be ignored, leaving the value unchanged.

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


A join exists at any point in a subpath shared by two consecutive lines. When a subpath is closed, then a join also exists at its first point (equivalent to its last point) connecting the first and last lines in the subpath.

In addition to the point where the 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 filled triangle connecting these two opposite corners with a straight line, with the third point of the triangle being the join point, must be rendered 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 a filled 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 rendered at joins.

The miter value means that a second filled triangle must (if it can given the miter length) be rendered 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 to be exceeded, this second triangle must not be rendered.

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

7 Shadows

All drawing operations are affected by the four global shadow attributes.

context . shadowColor [ = value ]

Returns the current shadow color.

Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.

context . shadowOffsetX [ = value ]
context . shadowOffsetY [ = value ]

Returns the current shadow offset.

Can be set, to change the shadow offset. Values that are not finite numbers are ignored.

context . shadowBlur [ = value ]

Returns the current level of blur applied to shadows.

Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.

The shadowColor attribute sets the color of the shadow.

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

On getting, the serialization of the color must be returned.

On setting, the new value must be parsed as a CSS <color> value and the color assigned. If the value cannot be parsed as a CSS <color> value then it must be ignored, and the attribute must retain its previous value. [CSSCOLOR]

The shadowOffsetX and shadowOffsetY attributes specify the distance that the shadow will be offset in the positive horizontal and positive vertical distance respectively. Their values are in coordinate space units. They are not affected by the current transformation matrix.

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

On getting, they must return their current value. On setting, the attribute being set must be set to the new value, except if the value is infinite or NaN, in which case the new value must be ignored.

The shadowBlur attribute specifies the level of the blurring effect. (The units do not map to coordinate space units, and are not affected by the current transformation matrix.)

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

On getting, the attribute must return its current value. On setting the attribute must be set to the new value, except if the value is negative, infinite or NaN, in which case the new value must be ignored.

Shadows are only drawn if the opacity component of the alpha component of the color of shadowColor is non-zero and either the shadowBlur is non-zero, or the shadowOffsetX is non-zero, or the shadowOffsetY is non-zero.

When shadows are drawn, they must be rendered as follows:

  1. Let A be an infinite transparent black bitmap on which the source image for which a shadow is being created has been rendered.

  2. Let B be an infinite transparent black bitmap, with a coordinate space and an origin identical to A.

  3. Copy the alpha channel of A to B, offset by shadowOffsetX in the positive x direction, and shadowOffsetY in the positive y direction.

  4. If shadowBlur is greater than 0:

    1. Let σ be half the value of shadowBlur.

    2. Perform a 2D Gaussian Blur on B, using σ as the standard deviation.

    User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.

  5. Set the red, green, and blue components of every pixel in B to the red, green, and blue components (respectively) of the color of shadowColor.

  6. Multiply the alpha component of every pixel in B by the alpha component of the color of shadowColor.

  7. The shadow is in the bitmap B, and is rendered as part of the drawing model described below.

If the current composition operation is copy, shadows effectively won't render (since the shape will overwrite the shadow).

8 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 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 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 canvas in the given rectangle to transparent black.

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

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

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

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

The clearRect(x, y, w, h) method must clear the pixels in the specified rectangle that also intersect the current clipping region to a fully transparent black, erasing any previous image. If either height or width are zero, this method has no effect.

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

The strokeRect(x, y, w, h) method must stroke the specified rectangle's path using the strokeStyle, lineWidth, lineJoin, and (if appropriate) miterLimit attributes. If both height and width are zero, this method has no effect, since there is no path to stroke (it's a point). If only one of the two is zero, then the method will draw a line instead (the path for the outline is just a straight line along the non-zero dimension).

9 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.

context . beginPath()

Resets the current path.

context . moveTo(x, y)

Creates a new subpath with the given point.

context . 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)

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

context . 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)

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)

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 INDEX_SIZE_ERR exception if the given radius is negative.

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

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

Throws an INDEX_SIZE_ERR exception if the given radius is negative.

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

Adds a new closed subpath to the path, representing the given rectangle.

context . fill()

Fills the subpaths with the current fill style.

context . stroke()

Strokes the subpaths with the current stroke style.

context . clip()

Further constrains the clipping region to the given path.

context . isPointInPath(x, y)

Returns true if the given point is in the current path.

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

The points and lines added to the path by these methods must be transformed according to the current transformation matrix as they are added.

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.

When the user agent is to ensure there is a subpath for a coordinate (x, y), the user agent must check to see if the context has any subpaths, and if it does not, 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.

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 (possibly implied) 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 ensure there is a subpath for (x, y) 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 ensure there is a subpath for (cpx, cpy), and then must connect the last point in the subpath to the given point (x, y) using a quadratic Bézier curve with control point (cpx, cpy), and must then add the given point (x, y) to the subpath. [BEZIER]

The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) method must ensure there is a subpath for (cp1x, cp1y), and then must 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). Then, it must add the point (x, y) to the subpath. [BEZIER]


The arcTo(x1, y1, x2, y2, radius) method must first ensure there is a subpath for (x1, y1). Then, the behavior depends on the arguments and the last point in the subpath, as described below.

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

Let the point (x0, y0) be the last point in the subpath.

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 the radius radius is zero, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.

Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.

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


The arc(x, y, radius, startAngle, endAngle, anticlockwise) method 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 this circle's circumference, measured in radians clockwise from the positive x-axis, are the start and end points respectively.

If the anticlockwise argument is omitted or false and endAngle-startAngle is equal to or greater than , or, if the anticlockwise argument is true and startAngle-endAngle is equal to or greater than , then the arc is the whole circumference of this circle.

Otherwise, 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. Since the points are on the circle, as opposed to being simply angles from zero, the arc can never cover an angle greater than radians. If the two points are the same, or if the radius is zero, then the arc is defined as being of zero length in both directions.

Negative 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.

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

Thus, if two overlapping but otherwise independent subpaths have opposite windings, they cancel out and result in no fill. If they have the same winding, that area just gets painted once.

The stroke() method must calculate the strokes of all the subpaths of the current path, using the lineWidth, lineCap, lineJoin, and (if appropriate) miterLimit attributes, and then fill the combined stroke area using the strokeStyle attribute.

Since the subpaths are all stroked as one, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.

Paths, when filled or stroked, must be painted without affecting the current path, and must be subject to shadow effects, global alpha, the clipping region, and global composition operators. (Transformations affect the path when the path is created, not when it is painted, though the stroke style is still affected by the transformation during painting.)

Zero-length line segments must be pruned before stroking a path. Empty subpaths must be ignored.

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

When the context is initialized, the clipping region must be set to the 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 inside the current path as determined by the non-zero winding number rule; and must return false otherwise. Points on the path itself are considered to be inside the path. If either of the arguments is infinite or NaN, then the method must return false.

10 Focus management

When a canvas is interactive, authors should include focusable elements in the element's fallback content corresponding to each focusable part of the canvas.

To indicate which focusable part of the canvas is currently focused, authors should use the drawFocusRing() method, passing it the element for which a ring is being drawn. This method only draws the focus ring if the element is focused or is a descendant of the element with focus.

shouldDraw = context . drawFocusRing(element, [ canDrawCustom ])

If the given element is focused or a descendant of the element with focus, draws a focus ring around the current path, following the platform conventions for focus rings.

If the canDrawCustom argument is true, then the focus ring is only drawn if the user has configured his system to draw focus rings in a particular manner. (For example, high contrast focus rings.)

Returns true if the given element is focused, the canDrawCustom argument is true, and the user has not configured his system to draw focus rings in a particular manner. Otherwise, returns false.

When the method returns true, the author is expected to manually draw a focus ring.

The drawing path is used to form the focus ring provided that drawing path contains a closed path. The drawing path is used to form a best fit bounding rectangle in screen coordinates. The bounding rectangle and drawing path may be used to enhance accessibility properties [ARIA] for the targeted element.

The drawFocusRing(element, [canDrawCustom]) method, when invoked, must run the following steps:

  1. If the element is not focused or is not a descendant of the element with whose context the method is associated, then return false and abort these steps.

  2. If supporting an accessibility API, implementors may use the drawing path to form a best fit rectangle in screen coordinates and apply it to the bounding rectangle of the associated accessible object.

  3. If the user has requested the use of particular focus rings (e.g. high-contrast focus rings), or if the canDrawCustom argument is absent or false, then draw a focus ring of the appropriate style along the path, following platform conventions, return false, and abort these steps.

    The focus ring should not be subject to the shadow effects, the global alpha, or the global composition operators, but should be subject to the clipping region.

  4. Return true.

11 Caret and selection management

When a canvas implementation is used to render interactive content that contains a caret or selection, it is essential that all users be able to follow the current caret or selection position.

The setCaretSelectionRect() method should be used to indicate the location of the last rendered caret position or selection position on the canvas, passing it the canvas fallback element associated with the last drawn of either the caret position or the selection.

When drawing a blinking caret the author must adhere to the blink rates in systems that support this feature. User agents must provide the system caret blink rate to content authors. Default system caret blink rate settings are roughly once every 500 milliseconds.

To access the system caret blink rate in canvas use the caretBlinkRate() method

success = context . setCaretSelectionRect(element, x, y, w, h)

Returns true if the given element is focused or a document descendant of an element with focus. Otherwise, returns false.

rate = context . caretBlinkRate()

Returns the blink rate of the system in milliseconds if supported. Otherwise, returns -1 if it is unsupported by the system.

Screen magnifiers, used by low vision users, use this position to move the magnification point on the screen.

The setCaretSelectionRect(element, x, y, w, h) method, when invoked, must run the following steps:

  1. If element does not have selected content and is not focused return false and abort these steps.

  2. If the element is not a descendant of the element with whose context the method is associated, then return false and abort these step.

  3. Transform the given point (x, y), the width w, and the height h according to the current transformation matrix.

  4. If the user agent supports a platform accessibility API the user agent must use the element, transformed coordinates and transformed bounding box, and provide it through the supported accessibility API implementation.

  5. Return true.

If the user resizes or moves the user agent window the user agent report must reflect the revised (x, y, w, h) position (or rectangle) in the accessibility API mapping.

High blink rates may cause epileptic seizures in some users.

The caretBlinkRate() method, when invoked, must run the following steps:

  1. If the operating system supports a caret blink rate setting the user agent must return a long value in milliseconds.

  2. If the operating system does not support a caret blink rate setting the user agent must return a long value less than zero.

When the caret blink rate value returned:

Status: This example should be updated to remove compatibility code for non-compliant implementations:

This canvas element has a couple of checkboxes:

<canvas tabindex="-1" id="example" height="400" width="750" role="application">
 <!-- Canvas Subtree acts as Shadow DOM that the browser maps to the platform accessibility API -->

 <label id="labelA" for="showA"><input id="showA" role="checkbox" aria-labelledby="labelA" type="checkbox" /> Show "A"</label>
 <label id="labelB" for="showB"><input id="showB" role="checkbox" aria-labelledby="labelB" type="checkbox" /> Show "B"</label>

 <!-- ... -->
</canvas>
<script>
/*/
   Canvas is a low level API and requires manual management of all interactivity.
   The following example is quite verbose, and demonstrates the minimum of programming
   necessary to enable a simple checkbox element within canvas.
/*/
 function drawCheckbox(context, element, x, y, pathOnly) {
   context.save();
   context.font = '10px sans-serif';
   context.textAlign = 'left';
   context.textBaseline = 'middle';

   var label = document.getElementById(element.getAttribute('aria-labelledby'));
   var metrics = context.measureText(label.textContent);

   if(pathOnly) {
    var areaWidth = 15 + metrics.width;
    var areaHeight = 10;
    context.beginPath();
    context.rect(x-5, y-5, areaWidth, areaHeight);
    return;
   }

   context.beginPath();
   context.strokeStyle = 'black';
   context.rect(x-5, y-5, 10, 10);
   context.stroke();
   if (element.checked) {
    context.fillStyle = 'black';
    context.fill();
   }
   context.fillText(label.textContent, x+5, y);

  // Draw the Focus Ring
   var drawFocusManually = false;
   if (document.activeElement == element || document.activeElementFocus == element && document.activeElement == context.canvas) {
  try {
   drawFocusManually = !context.drawFocusRing(element);
  } catch(e) {
   drawFocusManually = true;
  }
   }
   if(drawFocusManually) {
  context.beginPath();
  context.rect(x-7, y-7, 12 + metrics.width+2, 14);
  context.strokeStyle = 'silver';
  context.stroke();
   }
   context.restore();
 }

 function drawBase() { /* ... */ }
 function drawAs() { /* ... */ }
 function drawBs() { /* ... */ }
 function redraw() {
   var canvas = document.getElementsByTagName('canvas')[0];
   var context = canvas.getContext('2d');
   var showA = document.getElementById('showA');
   var showB = document.getElementById('showB');

   context.clearRect(0, 0, canvas.width, canvas.height);
   drawCheckbox(context, showA, 20, 40);
   drawCheckbox(context, showB, 20, 60);
   drawBase();
   if (showA.checked) {
    drawAs();
   }
   if (showB.checked) {
    drawBs();
   }
 }

 function processMouseCoords(event,element) {
   var offsetLeft = 0, offsetTop = 0;
   while(element) {
     offsetLeft += element.offsetLeft >> 0;
     offsetTop += element.offsetTop >> 0;
     element = element.parentNode;
   }
   offsetLeft -= document.documentElement.scrollLeft;
   offsetTop -= document.documentElement.scrollTop;
   return { x: event.clientX - offsetLeft, y: event.clientY - offsetTop }
 }

 function processClick(event){
   var canvas = document.getElementById('example');
   var context = canvas.getContext('2d');
   var coords = processMouseCoords(event,canvas);
   var showA = document.getElementById('showA');
   var showB = document.getElementById('showB');
   var useRedraw = false;
   if(event.target.type == 'checkbox') {
    redraw();
    return;
   }

   drawCheckbox(context, showA, 20, 40, true);
   if (context.isPointInPath(coords.x, coords.y)) {
    showA.checked = !(showA.checked);
    showA.focus();
    document.activeElementFocus = showA;
    useRedraw = true;
   }

   drawCheckbox(context, showB, 20, 60, true);
   if (context.isPointInPath(coords.x, coords.y)) {
    showB.checked = !(showB.checked);
    document.activeElementFocus = showB;
    showB.focus();
    useRedraw = true;
   }

   if(!useRedraw &&
    (document.activeElementFocus != document.activeElement)) {
     document.activeElementFocus = document.activeElement;
     redraw();
   }

   if(useRedraw) redraw();
 }

    // Add the event listeners
 document.getElementById('showA').addEventListener('focus', redraw, true);
 document.getElementById('showB').addEventListener('focus', redraw, true);
 document.getElementById('showA').addEventListener('blur', redraw, true);
 document.getElementById('showB').addEventListener('blur', redraw, true);
 document.getElementById('example').addEventListener('change', redraw, true);
 document.getElementById('example').addEventListener('click', processClick, false);
 redraw();
</script>

The caret position is the rectangular left edge of the caret position within an element having focus. The selection position is the rectangular left or right edge of the highlighted position that moves in response to a users selection of content such as when dragging a pointing device during a content selection operation. The right edge is used when moving the selection position to the right and the left edge is used when moving the position to the left.

11 Text

context . 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 ]

Returns the current text alignment settings.

Can be set, to change the alignment. The possible values are start, end, left, right, and center. Other values are ignored. The default is start.

context . 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 . 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

Returns the advance width of the text that was passed to the measureText() method.

The font IDL attribute, on setting, must be parsed the same way as the 'font' property of CSS (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 names must be interpreted in the context of the canvas element's stylesheets; any fonts embedded using @font-face must therefore be available once they are loaded. (If a font is referenced before it is fully loaded, then it must be treated as if it was an unknown font, falling back to another as described by the relevant CSS specifications.) [CSSFONTS]

Only vector fonts should be used by the user agent; if a user agent were to use bitmap fonts then transformations would likely make the font look very ugly.

On getting, the font attribute must return the serialized form of the current font of the context (with no 'line-height' component). [CSSOM]

For example, after the following statement:

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

...the expression context.font would evaluate to the string "italic 12px "Unknown Font", sans-serif". The "400" font-weight doesn't appear because that is the default value. The line-height doesn't appear because it is forced to "normal", the default value.

When the context 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 corresponding canvas element at the time that the attribute is set. 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 corresponding canvas element at the time that the attribute is set. If the computed values are undefined for a particular case (e.g. because the canvas element is not in a Document), 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, if the value is one of start, end, left, right, or center, then the value must be changed to the new value. Otherwise, the new value must be ignored. When the context is created, the textAlign attribute must initially have the value start.

The textBaseline IDL attribute, on getting, must return the current value. On setting, if the value is one of top, hanging, middle, alphabetic, ideographic, or bottom, then the value must be changed to the new value. Otherwise, the new value must be ignored. When the context is created, the textBaseline attribute must initially have the value alphabetic.

The textBaseline attribute's allowed keywords correspond to alignment points in the font:

The top of the em square is roughly at the top of the glyphs in a font, the hanging baseline is where some glyphs like आ are anchored, the middle is half-way between the top of the em square and the bottom of the em square, the alphabetic baseline is where characters like Á, ÿ, f, and Ω are anchored, the ideographic baseline is where glyphs like 私 and 達 are anchored, and the bottom of the em square is roughly at the bottom of the glyphs in a font. The top and bottom of the bounding box can be far from these baselines, due to glyphs extending far outside the em square.

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 baseline
bottom
The bottom of the em square

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 called, the user agent must run the following steps:

  1. If maxWidth is present but less than or equal to zero, return without doing anything; abort these steps.

  2. Let font be the current font of the context, as given by the font attribute.

  3. Replace all the space characters in text with U+0020 SPACE characters.

  4. 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 and the 'direction' property of the inline box set to the directionality of the canvas element. [CSS]

  5. If the maxWidth argument was specified 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.

  6. Let the anchor point be a point on the inline box, determined by the textAlign and textBaseline values, as follows:

    Horizontal position:

    If textAlign is left
    If textAlign is start and the directionality of the canvas element is 'ltr'
    If textAlign is end and the directionality of the canvas element is 'rtl'
    Let the anchor point's horizontal position be the left edge of the inline box.
    If textAlign is right
    If textAlign is end and the directionality of the canvas element is 'ltr'
    If textAlign is start and the directionality of the canvas element is 'rtl'
    Let the anchor point's horizontal position be the right edge of the inline box.
    If textAlign is center
    Let the anchor point's horizontal position be half way between the left and right edges of the inline box.

    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 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.
  7. Paint the hypothetical inline box as the shape given by the text's glyphs, as transformed by the current transformation matrix, and anchored and sized so that before applying the current transformation matrix, the anchor point is at (x, y) and each CSS pixel is mapped to one coordinate space unit.

    For fillText() fillStyle must be applied to the glyphs and strokeStyle must be ignored. For strokeText() the reverse holds and strokeStyle must be applied to the glyph outlines and fillStyle must be ignored.

    Text is painted without affecting the current path, and is 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 replace all the space characters in text with U+0020 SPACE characters, and then must 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 element set to the current font of the context, as given by the font attribute, and must then return a new TextMetrics object with its width attribute set to the width of that inline box, in CSS pixels. [CSS]

The TextMetrics interface is used for the objects returned from measureText(). It has one attribute, width, which is set by the measureText() method.

Glyphs rendered using fillText() and strokeText() can spill out of the box given by the font size (the em square size) and the width returned by measureText() (the text width). This version of the specification does not provide a way to obtain the bounding box dimensions of the text. If the text is to be rendered and removed, care needs to be taken to replace the entire area of the canvas that the clipping region covers, not just the box given by the em square height and measured text width.

A future version of the 2D context API may provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.

12 Images

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

This method can be invoked with three different sets of arguments:

Each of those three can take either an HTMLImageElement, an HTMLCanvasElement, or an HTMLVideoElement for the image argument.

context . drawImage(image, dx, dy)
context . drawImage(image, dx, dy, dw, dh)
context . drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)

Draws the given image onto the canvas. The arguments are interpreted as follows:

The sx and sy parameters give the x and y coordinates of the source rectangle; the sw and sh arguments give the width and height of the source rectangle; the dx and dy give the x and y coordinates of the destination rectangle; and the dw and dh arguments give the width and height of the destination rectangle.

If the first argument isn't an img, canvas, or video element, throws a TYPE_MISMATCH_ERR exception. If the image has no image data, throws an INVALID_STATE_ERR exception. If the one of the source rectangle dimensions is zero, throws an INDEX_SIZE_ERR exception. If the image isn't yet fully decoded, then nothing is drawn.

If not specified, the dw and dh arguments must default to the values of sw and sh, interpreted such that one CSS pixel in the image is treated as one unit in the canvas coordinate space. If the sx, sy, sw, and sh arguments are omitted, they must default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively. If the image has no intrinsic dimensions, the concrete object size must be used instead, as determined using the CSS "Concrete Object Size Resolution" algorithm, with the specified size having neither a definite width nor height, nor any additional contraints, the object's intrinsic properties being those of the image argument, and the default object size being the size of the canvas element. [CSSIMAGES]

The image argument is an instance of either HTMLImageElement, HTMLCanvasElement, or HTMLVideoElement. If the image is null, the implementation must raise a TYPE_MISMATCH_ERR exception.

If the image argument is an HTMLImageElement object that is not fully decodable, or if the image argument is an HTMLVideoElement object whose readyState attribute is either HAVE_NOTHING or HAVE_METADATA, then the implementation must return without drawing anything.

If the image argument is an HTMLCanvasElement object with either a horizontal dimension or a vertical dimension equal to zero, then the implementation must raise an INVALID_STATE_ERR exception.

The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).

If one of the sw or sh arguments is zero, the implementation must raise an INDEX_SIZE_ERR exception.

The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).

When drawImage() is invoked, the region of the image specified by the source rectangle must be painted on the region of the canvas specified by the destination rectangle, after applying the current transformation matrix to the points of the destination rectangle.

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). The image data must be processed in the original direction, even if the dimensions given are negative.

This specification does not define the algorithm to use when scaling the image, if necessary.

When a canvas is drawn onto itself, the drawing model requires the source to be copied before the image is drawn back onto the canvas, so it is possible to copy parts of a canvas onto overlapping parts of itself.

If the original image data is a bitmap image, the value painted at a point in the destination rectangle is computed by filtering the original image data. The user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behavior.)

When the drawImage() method is passed an animated image as its image argument, the user agent must use the poster frame of the animation, or, if there is no poster frame, the first frame of the animation.

When the image argument is an HTMLVideoElement, then the frame at the current playback position must be used as the source image, 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).

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

13 Pixel manipulation

imagedata = context . createImageData(sw, sh)

Returns an ImageData object with the given dimensions in CSS pixels (which might map to a different number of actual device pixels exposed by the object itself). All the pixels in the returned object are transparent black.

imagedata = context . createImageData(imagedata)

Returns an ImageData object with the same dimensions as the argument. All the pixels in the returned object are transparent black.

Throws a NOT_SUPPORTED_ERR exception if the argument is null.

imagedata = context . getImageData(sx, sy, sw, sh)

Returns an ImageData object containing the image data for the given rectangle of the canvas.

Throws a NOT_SUPPORTED_ERR exception if any of the arguments are not finite. Throws an INDEX_SIZE_ERR exception if the either of the width or height arguments are zero.

imagedata . width
imagedata . height

Returns the actual dimensions of the data in the ImageData object, in device pixels.

imagedata . data

Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.

context . putImageData(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ])

Paints the data from the given ImageData object onto the canvas. If a dirty rectangle is provided, only the pixels from that rectangle are painted.

The globalAlpha and globalCompositeOperation attributes, as well as the shadow attributes, are ignored for the purposes of this method call; pixels in the canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.

If the first argument is null, throws a TYPE_MISMATCH_ERR exception. Throws a NOT_SUPPORTED_ERR exception if any of the other arguments are not finite.

The createImageData() method is used to instantiate new blank ImageData objects. When the method is invoked with two arguments sw and sh, it must return an ImageData object representing a rectangle with a width in CSS pixels equal to the absolute magnitude of sw and a height in CSS pixels equal to the absolute magnitude of sh. When invoked with a single imagedata argument, it must return an ImageData object representing a rectangle with the same dimensions as the ImageData object passed as the argument. The ImageData object returned must be filled with transparent black.

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 whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh), in canvas coordinate space units. Pixels outside the canvas must be returned as transparent black. Pixels must be returned as non-premultiplied alpha values.

If any of the arguments to createImageData() or getImageData() are infinite or NaN, or if the createImageData() method is invoked with only one argument but that argument is null, the method must instead raise a NOT_SUPPORTED_ERR exception. If either the sw or sh arguments are zero, the method must instead raise an INDEX_SIZE_ERR exception.

ImageData objects must be initialized so that their width attribute is set to w, the number of physical device pixels per row in the image data, their height attribute is set to h, the number of rows in the image data, and their data attribute is initialized to a CanvasPixelArray object holding the image data. At least one pixel's worth of image data must be returned.

The CanvasPixelArray object provides ordered, indexed access to the color components of each pixel of the image data. The data must be represented in left-to-right order, row by row top to bottom, starting with the top left, with each pixel's red, green, blue, and alpha components being given in that order for each pixel. Each component of each device pixel represented in this array must be in the range 0..255, representing the 8 bit value for that component. The components must be assigned consecutive indices starting with 0 for the top left pixel's red component.

The CanvasPixelArray object thus represents h×w×4 integers. The length attribute of a CanvasPixelArray object must return this number.

The object's supported property indices are the numbers in the range 0 .. h×w×4-1.

To determine the value of an indexed property index, the user agent must return the value of the indexth component in the array.

To set the value of an existing indexed property index to value value, the value of the indexth component in the array must be set to value.

The width and height (w and h) might be different from the sw and sh arguments to the above methods, e.g. if the canvas is backed by a high-resolution bitmap, or if the sw and sh arguments are negative.

The putImageData(imagedata, dx, dy, dirtyX, dirtyY, dirtyWidth, dirtyHeight) method writes data from ImageData structures back to the canvas.

If any of the arguments to the method are infinite or NaN, the method must raise a NOT_SUPPORTED_ERR exception.

If the first argument to the method is null, then the putImageData() method must raise a TYPE_MISMATCH_ERR exception.

When the last four arguments are omitted, they must be assumed to have the values 0, 0, the width member of the imagedata structure, and the height member of the imagedata structure, respectively.

When invoked with arguments that do not, per the last few paragraphs, cause an exception to be raised, the putImageData() method must act as follows:

  1. Let dxdevice be the x-coordinate of the device pixel in the underlying pixel data of the canvas corresponding to the dx coordinate in the canvas coordinate space.

    Let dydevice be the y-coordinate of the device pixel in the underlying pixel data of the canvas corresponding to the dy coordinate in the canvas coordinate space.

  2. If dirtyWidth is negative, let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal to the absolute magnitude of dirtyWidth.

    If dirtyHeight is negative, let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be equal to the absolute magnitude of dirtyHeight.

  3. If dirtyX is negative, let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.

    If dirtyY is negative, let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.

  4. If dirtyX+dirtyWidth is greater than the width attribute of the imagedata argument, let dirtyWidth be the value of that width attribute, minus the value of dirtyX.

    If dirtyY+dirtyHeight is greater than the height attribute of the imagedata argument, let dirtyHeight be the value of that height attribute, minus the value of dirtyY.

  5. If, after those changes, either dirtyWidth or dirtyHeight is negative or zero, stop these steps without affecting the canvas.

  6. Otherwise, for all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the four channels of the pixel with coordinate (x, y) in the imagedata data structure to the pixel with coordinate (dxdevice+x, dydevice+y) in the underlying pixel data of the canvas.

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), p, q);

...for any value of x, y, w, and h and where p is the smaller of x and the sum of x and w, and q is the smaller of y and the sum of y and h; and except that the following two calls:

context.createImageData(w, h);
context.getImageData(0, 0, w, h);

...must return ImageData objects with the same dimensions, for any value of w and h. In other words, while user agents may round the arguments of these methods so that they map to device pixel boundaries, any rounding performed must be performed consistently for all of the createImageData(), getImageData() and putImageData() operations.

Due to the lossy nature of converting to and from premultiplied alpha color values, pixels that have just been set using putImageData() might be returned to an equivalent getImageData() as different values.

The current path, transformation matrix, shadow attributes, global alpha, the clipping region, and global composition operator must not affect the getImageData() and putImageData() methods.

The data returned by getImageData() is at the resolution of the canvas backing store, which is likely to not be one device pixel to each CSS pixel if the display used is a high resolution display.

In the following example, the script generates an ImageData object so that it can draw onto it.

// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');

// create a blank slate
var data = context.createImageData(canvas.width, canvas.height);

// create some plasma
FillPlasma(data, 'green'); // green plasma

// add a cloud to the plasma
AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle

// paint the plasma+cloud on the canvas
context.putImageData(data, 0, 0);

// support methods
function FillPlasma(data, color) { ... }
function AddCloud(data, x, y) { ... }

Here is an example of using getImageData() and putImageData() to implement an edge detection filter.

<!DOCTYPE HTML>
<html>
 <head>
  <title>Edge detection demo</title>
  <script>
   var image = new Image();
   function init() {
     image.onload = demo;
     image.src = "image.jpeg";
   }
   function demo() {
     var canvas = document.getElementsByTagName('canvas')[0];
     var context = canvas.getContext('2d');

     // draw the image onto the canvas
     context.drawImage(image, 0, 0);

     // get the image data to manipulate
     var input = context.getImageData(0, 0, canvas.width, canvas.height);

     // get an empty slate to put the data into
     var output = context.createImageData(canvas.width, canvas.height);

     // alias some variables for convenience
     // notice that we are using input.width and input.height here
     // as they might not be the same as canvas.width and canvas.height
     // (in particular, they might be different on high-res displays)
     var w = input.width, h = input.height;
     var inputData = input.data;
     var outputData = output.data;

     // edge detection
     for (var y = 1; y < h-1; y += 1) {
       for (var x = 1; x < w-1; x += 1) {
         for (var c = 0; c < 3; c += 1) {
           var i = (y*w + x)*4 + c;
           outputData[i] = 127 + -inputData[i - w*4 - 4] -   inputData[i - w*4] - inputData[i - w*4 + 4] +
                                 -inputData[i - 4]       + 8*inputData[i]       - inputData[i + 4] +
                                 -inputData[i + w*4 - 4] -   inputData[i + w*4] - inputData[i + w*4 + 4];
         }
         outputData[(y*w + x)*4 + 3] = 255; // alpha
       }
     }

     // put the image data back after manipulation
     context.putImageData(output, 0, 0);
   }
  </script>
 </head>
 <body onload="init()">
  <canvas></canvas>
 </body>
</html>

14 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. Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honored, and the stroke must itself also be subjected to the current transformation matrix.

  2. When shadows are drawn, render the shadow from image A, using the current shadow styles, creating image B.

  3. When shadows are drawn, multiply the alpha component of every pixel in B by globalAlpha.

  4. When shadows are drawn, composite B within the clipping region over the current canvas bitmap using the current composition operator.

  5. Multiply the alpha component of every pixel in A by globalAlpha.

  6. Composite A within the clipping region over the current canvas bitmap using the current composition operator.

15 Examples

This section is non-normative.

Here is an example of a script that uses canvas to draw pretty glowing lines.

<canvas width="800" height="450"></canvas>
<script>

 var context = document.getElementsByTagName('canvas')[0].getContext('2d');

 var lastX = context.canvas.width * Math.random();
 var lastY = context.canvas.height * Math.random();
 var hue = 0;
 function line() {
   context.save();
   context.translate(context.canvas.width/2, context.canvas.height/2);
   context.scale(0.9, 0.9);
   context.translate(-context.canvas.width/2, -context.canvas.height/2);
   context.beginPath();
   context.lineWidth = 5 + Math.random() * 10;
   context.moveTo(lastX, lastY);
   lastX = context.canvas.width * Math.random();
   lastY = context.canvas.height * Math.random();
   context.bezierCurveTo(context.canvas.width * Math.random(),
                         context.canvas.height * Math.random(),
                         context.canvas.width * Math.random(),
                         context.canvas.height * Math.random(),
                         lastX, lastY);

   hue = hue + 10 * Math.random();
   context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)';
   context.shadowColor = 'white';
   context.shadowBlur = 10;
   context.stroke();
   context.restore();
 }
 setInterval(line, 50);

 function blank() {
   context.fillStyle = 'rgba(0,0,0,0.1)';
   context.fillRect(0, 0, context.canvas.width, context.canvas.height);
 }
 setInterval(blank, 40);

</script>

References

All references are normative unless marked "Non-normative".

[BEZIER]
Courbes à poles, P. de Casteljau. INPI, 1959.
[CSS]
Cascading Style Sheets Level 2 Revision 1, B. Bos, T. Çelik, I. Hickson, H. Lie. W3C.
[CSSCOLOR]
CSS Color Module Level 3, T. Çelik, C. Lilley, L. Baron. W3C.
[CSSFONTS]
CSS Fonts Module Level 3, J. Daggett. W3C.
[CSSIMAGES]
CSS Image Values and Replaced Content Module Level 3, E. Etemad, T. Atkins. W3C.
[CSSOM]
Cascading Style Sheets Object Model (CSSOM), A. van Kesteren. W3C.
[HTML5]
HTML5, I. Hickson. W3C.
[PORTERDUFF]
Compositing Digital Images, T. Porter, T. Duff. In Computer graphics, volume 18, number 3, pp. 253-259. ACM Press, July 1984.
[WEBIDL]
Web IDL, C. McCormack. W3C.

Acknowledgements

For a full list of acknowledgements, please see the HTML5 specification. [HTML5]