Abstract

This specification defines the 2D Context for the HTML canvas element. The 2D Context provides objects, methods, and properties to draw and manipulate graphics on a canvas drawing surface.

Status of This document

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This document was published by the HTML Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 01 September 2014. All feedback is welcome.

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. There are various ways to follow this work at the WHATWG:

The following features are at risk and may be removed due to lack of implementation.

This specification is an extension to the HTML5 language. All normative content in the HTML5 specification, unless specifically overridden by this specification, is intended to be the basis for this specification.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

  1. 1 Conformance requirements
  2. 2 The canvas state
  3. 3 DrawingStyle objects
  4. 4 Line styles
  5. 5 Text styles
  6. 6 Building paths
  7. 7 Path objects
  8. 8 Transformations
  9. 9 Fill and stroke styles
  10. 10 Drawing rectangles to the canvas
  11. 11 Drawing text to the canvas
  12. 12 Drawing paths to the canvas
  13. 13 Drawing images to the canvas
  14. 14 Hit regions
  15. 15 Pixel manipulation
  16. 16 Compositing
  17. 17 Shadows
  18. 18 Drawing model
  19. 19 Best practices
  20. 20 Examples
  21. References

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(unrestricted double x, unrestricted double y);
  void rotate(unrestricted double angle);
  void translate(unrestricted double x, unrestricted double y);
  void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
  void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);

  // compositing
           attribute unrestricted double globalAlpha; // (default 1.0)
           attribute DOMString globalCompositeOperation; // (default source-over)

  // colors and styles (see also the CanvasDrawingStyles interface)
           attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black)
           attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black)
  CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1);
  CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1);
  CanvasPattern createPattern((HTMLImageElement or HTMLCanvasElement or HTMLVideoElement) image, [TreatNullAs=Emptytring]DOMString repetition);

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

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

  // path API (see also CanvasPathMethods)
  void beginPath();
  void fill();
  void fill(Path path);
  void stroke();
  void stroke(Path path);
  void drawSystemFocusRing(Element element);
  void drawSystemFocusRing(Path path, Element element);
  boolean drawCustomFocusRing(Element element);
  boolean drawCustomFocusRing(Path path, Element element);
  void scrollPathIntoView();
  void scrollPathIntoView(Path path);
  void clip();
  void clip(Path path);
  boolean isPointInPath(unrestricted double x, unrestricted double y);
  boolean isPointInPath(Path path, unrestricted double x, unrestricted double y);

  // text (see also the CanvasDrawingStyles interface)
  void fillText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
  void strokeText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
  TextMetrics measureText(DOMString text);

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

  // hit regions
  void addHitRegion(HitRegionOptions options);
  void removeHitRegion(HitRegionOptions options);

  // pixel manipulation
  ImageData createImageData(unrestricted double sw, unrestricted double sh);
  ImageData createImageData(ImageData imagedata);
  ImageData getImageData(double sx, double sy, double sw, double sh);
  void putImageData(ImageData imagedata, double dx, double dy, double dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight);
  void putImageData(ImageData imagedata, double dx, double dy);
};
CanvasRenderingContext2D implements CanvasDrawingStyles;
CanvasRenderingContext2D implements CanvasPathMethods;

[NoInterfaceObject]
interface CanvasDrawingStyles {
  // line caps/joins
           attribute unrestricted double lineWidth; // (default 1)
           attribute DOMString lineCap; // "butt", "round", "square" (default "butt")
           attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter")
           attribute unrestricted double miterLimit; // (default 10)

  // dashed lines
  void setLineDash(sequence<unrestricted double> segments); // default empty
  sequence<unrestricted double> getLineDash();
           attribute unrestricted double lineDashOffset; 


  // 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")
};

[NoInterfaceObject]
interface CanvasPathMethods {
  // shared path API methods
  void closePath();
  void moveTo(unrestricted double x, unrestricted double y);
  void lineTo(unrestricted double x, unrestricted double y);
  void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy, unrestricted double x, unrestricted double y);
  void bezierCurveTo(unrestricted double cp1x, unrestricted double cp1y, unrestricted double cp2x, unrestricted double cp2y, unrestricted double x, unrestricted double y);
  void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radius); 
  void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
  void arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false); 
  void ellipse(unrestricted double x, unrestricted double y, unrestricted double radiusX, unrestricted double radiusY,  unrestricted double rotation, unrestricted double startAngle, unrestricted double endAngle, boolean anticlockwise); 
  };

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

interface CanvasPattern {
  // opaque object
};

interface TextMetrics {
  // x-direction
  readonly attribute double width;
  readonly attribute double actualBoundingBoxLeft;
  readonly attribute double actualBoundingBoxRight;

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

dictionary HitRegionOptions {
  Path? path = null;
  DOMString id = "";
  DOMString? parentID = null;
  DOMString cursor = "inherit"; 
  // for control-backed regions:
  Element? control = null;
  // for unbacked regions:
  DOMString? label = null;
  DOMString? role = null;
};

interface ImageData {
  readonly attribute unsigned long width;
  readonly attribute unsigned long height;
  readonly attribute Uint8ClampedArray data;
};

[Constructor(optional Element scope)]
interface DrawingStyle { };
DrawingStyle implements CanvasDrawingStyles;

[Constructor]
interface Path {
  void addPath(Path path, SVGMatrix? transformation);
  void addPathByStrokingPath(Path path, CanvasDrawingStyles styles, SVGMatrix? transformation);
  void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
  void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
  void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path path, optional unrestricted double maxWidth);
  void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path path, optional unrestricted double maxWidth);

};
Path implements CanvasPathMethods;
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.

Similar concerns exist with font-related properties; the rules for those are described in detail in the relevant section below.

2 The canvas state

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

The current default path and the current bitmap are not part of the drawing state. The current default 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 DrawingStyle objects

All the line styles (line width, caps, joins, and dash patterns) and text styles (fonts) described in the next two sections apply to CanvasRenderingContext2D objects and to DrawingStyle objects. This section defines the constructor used to obtain a DrawingStyle object. This object is then used by methods on Path objects to control how text and paths are rasterised and stroked.

styles = new DrawingStyle([ element ])

Creates a new DrawingStyle object, optionally using a specific element for resolving relative keywords and sizes in font specifications.

Each DrawingStyle object has a styles scope node.

The DrawingStyle() constructor, when invoked, must return a newly created DrawingStyle object. If the constructor was passed an argument, then the DrawingStyle object's styles scope node is that element. Otherwise, the object's styles scope node is the Document object of the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found.

4 Line styles

context . lineWidth [ = value ]
styles . lineWidth [ = value ]

Returns the current line width.

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

context . lineCap [ = value ]
styles . lineCap [ = value ]

Returns the current line cap style.

Can be set, to change the line cap style.

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

context . lineJoin [ = value ]
styles . lineJoin [ = value ]

Returns the current line join style.

Can be set, to change the line join style.

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

context . miterLimit [ = value ]
styles . miterLimit [ = value ]

Returns the current miter limit ratio.

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

context . setLineDash(segments)
styles . setLineDash(segments)

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

segments = context . getLineDash()
segments = styles . getLineDash()

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

context . lineDashOffset
styles . lineDashOffset

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

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

Objects that implement the CanvasDrawingStyles interface have attributes and methods (defined in this section) that control how lines are treated by the object.

The lineWidth attribute gives the width of lines, in coordinate space units. On getting, it must return the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the CanvasDrawingStyles interface is created, the lineWidth attribute must initially have the value 1.0.


The lineCap attribute defines the type of endings that UAs will place on the end of lines. The three valid values are butt, round, and square.

On getting, it must return the current value. On setting, 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 object implementing the CanvasDrawingStyles interface is created, the lineCap attribute must initially have the value butt.


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

On getting, it must return the current value. On setting, 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 object implementing the CanvasDrawingStyles interface is created, the lineJoin attribute must initially have the value miter.


When the lineJoin attribute has the value miter, strokes use the miter limit ratio to decide how to render joins. The miter limit ratio can be explicitly set using the miterLimit attribute. On getting, it must return the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the CanvasDrawingStyles interface is created, the miterLimit attribute must initially have the value 10.0.



Each CanvasDrawingStyles object has a dash list, which is either empty or consists of an even number of positive non-zero numbers. Initially, the dash list must be empty.

When the setLineDash() method is invoked, it must run the following steps:

  1. Let a be a copy of the array provided as the argument.

  2. If any value in the array is not finite (e.g. an Infinity or a NaN value), or if any value is negative (less than zero), then abort these steps (without throwing an exception; user agents could show a message on a developer console, though, as that would be helpful for debugging).

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

  4. Let the object's dash list be a.

When the getLineDash() method is invoked, it must return a newly created array whose values are the values of the object's dash list, in the same order.

It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching ants" effect. The phase can be set using the lineDashOffset attribute. On getting, it must return the current value. On setting, infinite and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the CanvasDrawingStyles interface is created, the lineDashOffset attribute must initially have the value 0.0.


When a user agent is to trace a path, given an object style that implements the CanvasDrawingStyles interface, it must run the following algorithm. This algorithm returns a new path.

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

  2. Remove from path any subpaths containing no lines (i.e. empty subpaths with zero points, and subpaths with just one point).

  3. Replace each point in each subpath of path other than the first point and the last point of each subpath by a join that joins the line leading to that point to the line leading out of that point, such that the subpaths all consist of two points (a starting point with a line leading out of it, and an ending point with a line leading into it), one or more lines (connecting the points and the joins), and zero or more joins (each connecting one line to another), connected together such that each subpath is a series of one or more lines with a join between each one and a point on each end.

  4. Add a straight closing line to each closed subpath in path connecting the last point and the first point of that subpath; change the last point to a join (from the previously last line to the newly added closing line), and change the first point to a join (from the newly added closing line to the first line).

  5. If the styles dash list is empty, jump to the step labeled joins.

  6. Let width be the aggregate length of all the lines of all the subpaths in path, in coordinate space units.

  7. Let offset be the value of the styles lineDashOffset, in coordinate space units.

  8. While offset is greater than width, decrement it by width.

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

  9. Offset subpath: If offset is non-zero, add a new subpath at the start of path consisting of two points connected by a line whose length is offset coordinate space units. (This path is temporary and will be removed in the joins step below. Its purpose is to offset the dash pattern.)

  10. Define L to be a linear coordinate line defined along all lines in all the subpaths in path, such that the start of the first line in the first subpath is defined as coordinate 0, and the end of the last line in the last subpath is defined as coordinate width.

  11. Let position be 0.

  12. Let index be 0.

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

  14. Dash On: Let segment length be the value of the styles dash list's indexth entry.

  15. Increment position by segment length.

  16. If position is greater than width, then jump to the step labeled joins.

  17. If segment length is non-zero, let current state be on.

  18. Increment index by one.

  19. Dash Off: Let segment length be the value of the styles dash list's indexth entry.

  20. Let start be the offset position on L.

  21. Increment position by segment length.

  22. If position is greater than width, then let end be the offset width on L. Otherwise, let end be the offset position on L.

  23. Jump to the first appropriate step:

    If segment length is zero and current state is off

    Do nothing, just continue to the next step.

    If current state is off

    Cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two; remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.

    The point has a directionality for the purposes of drawing line caps (see below). The directionality is the direction that the original line had at that point (i.e. when L was defined above).

    Otherwise

    Cut the line on which start finds itself into two at start and place a point there, cutting the subpath that it was in in two, and similarly cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two, and then remove all line segments, joins, points, and subpaths that are between start and end.

    If start and end are the same point, then this results in just the line being cut in two and two points being inserted there, with nothing being removed, unless a join also happens to be at that point, in which case the join must be removed.

  24. If position is greater than width, then jump to the step labeled joins.

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

  26. Increment index by one. If it is equal to the number of entries in the styles dash list, then let index be 0.

  27. Return to the step labeled dash on.

  28. Joins: Remove from path any subpath that originally formed part of the subpath added in the offset subpath step above.

  29. Create a new path that describes the result of inflating the paths in path perpendicular to the direction of the path to the styles lineWidth width, replacing each point with the end cap necessary to satisfy the styles lineCap attribute as described previously and elaborated below, and replacing each join with the join necessary to satisfy the styles lineJoin type, as defined below.

    Caps: Each point has a flat edge perpendicular to the direction of the line coming out of it. This is them augmented according to the value of the styles lineCap. The butt value means that no additional line cap is added. The round value means that a semi-circle with the diameter equal to the styles lineWidth width must additionally be placed on to the line coming out of each point. The square value means that a rectangle with the length of the styles lineWidth width and the width of half the styles lineWidth width, placed flat against the edge perpendicular to the direction of the line coming out of the point, must be added at each point.

    Points with no lines coming out of them must have two caps placed back-to-back as if it was really two points connected to each other by an infinitesimally short straight line in the direction of the point's directionality (as defined above).

    Joins: In addition to the point where a join occurs, two additional points are relevant to each join, one for each line: the two corners found half the line width away from the join point, one perpendicular to each line, each on the side furthest from the other line.

    A filled triangle connecting these two opposite corners with a straight line, with the third point of the triangle being the join point, must be added at all joins. The lineJoin attribute controls whether anything else is rendered. The three aforementioned values have the following meanings:

    The bevel value means that this is all that is rendered at joins.

    The round value means that 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 added at joins.

    The miter value means that a second filled triangle must (if it can given the miter length) be added at the join, with one line being the line between the two aforementioned corners, abutting the first triangle, and the other two being continuations of the outside edges of the two joining lines, as long as required to intersect without going over the miter length.

    The miter length is the distance from the point where the join occurs to the intersection of the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of the miter length to half the line width. If the miter length would cause the miter limit ratio (as set by the style miterLimit attribute) to be exceeded, this second triangle must not be added.

    Subpaths in the newly created path must wind clockwise, regardless of the direction of paths in path.

  30. Return the newly created path.

5 Text styles

context . font [ = value ]
styles . font [ = value ]

Returns the current font settings.

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

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

context . textAlign [ = value ]
styles . textAlign [ = value ]

Returns the current text alignment settings.

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

context . textBaseline [ = value ]
styles . textBaseline [ = value ]

Returns the current baseline alignment settings.

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

Objects that implement the CanvasDrawingStyles interface have attributes (defined in this section) that control how text is laid out (rasterized or outlined) by the object. Such objects also have a font style source node. For CanvasRenderingContext2D objects, this is the canvas element. For DrawingStyle objects, it's the styles scope node.

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 font style source node's stylesheets when the font is to be used; any fonts embedded using @font-face that are visible to that element must therefore be available once they are loaded. (If a reference font is used before it is fully loaded, or if the font style source node does not have that font in scope at the time the font is to be used, then it must be treated as if it was an unknown font, falling back to another as described by the relevant CSS specifications.) [CSSFONTS]

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 object implementing the CanvasDrawingStyles interface is created, the font of the context must be set to 10px sans-serif. When the 'font-size' component is set to lengths using percentages, 'em' or 'ex' units, or the 'larger' or 'smaller' keywords, these must be interpreted relative to the computed value of the 'font-size' property of the font style source node at the time that the attribute is set, if that is an element. When the 'font-weight' component is set to the relative values 'bolder' and 'lighter', these must be interpreted relative to the computed value of the 'font-weight' property of the font style source node at the time that the attribute is set, if that is an element. If the computed values are undefined for a particular case (e.g. because the font style source node is not an element or 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 object implementing the CanvasDrawingStyles interface is created, the textAlign attribute must initially have the value start.

The textBaseline IDL attribute, on getting, must return the current value. On setting, 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 object implementing the CanvasDrawingStyles interface 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 text preparation algorithm is as follows. It takes as input a string text, a CanvasDrawingStyles object target, and an optional length maxWidth. It returns an array of glyph shapes, each positioned on a common coordinate space, and a physical alignment whose value is one of left, right, and center. (Most callers of this algorithm ignore the physical alignment.)

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

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

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

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

    If the target object's font style source node is an element
    Let direction be the directionality of the target object's font style source node.
    If the target object's font style source node is a Document and that Document has a root element child
    Let direction be the directionality of the target object's font style source node's root element child.
    If the target object's font style source node is a Document and that Document has no root element child
    Let direction be 'ltr'.
  5. Form a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]

  6. If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.

  7. The anchor point is a point on the inline box, and the physical alignment is one of the values left, right, and center. These variables are determined by the textAlign and textBaseline values as follows:

    Horizontal position:

    If textAlign is left
    If textAlign is start and direction is 'ltr'
    If textAlign is end and direction is 'rtl'
    Let the anchor point's horizontal position be the left edge of the inline box, and let physical alignment be left.
    If textAlign is right
    If textAlign is end and direction is 'ltr'
    If textAlign is start and direction is 'rtl'
    Let the anchor point's horizontal position be the right edge of the inline box, and let physical alignment be right.
    If textAlign is center
    Let the anchor point's horizontal position be half way between the left and right edges of the inline box, and let physical alignment be center.

    Vertical position:

    If textBaseline is top
    Let the anchor point's vertical position be the top of the em box of the first available font of the inline box.
    If textBaseline is hanging
    Let the anchor point's vertical position be the hanging baseline of the first available font of the inline box.
    If textBaseline is middle
    Let the anchor point's vertical position be half way between the bottom and the top of the em box of the first available font of the inline box.
    If textBaseline is alphabetic
    Let the anchor point's vertical position be the alphabetic baseline of the first available font of the inline box.
    If textBaseline is ideographic
    Let the anchor point's vertical position be the ideographic baseline of the first available font of the inline box.
    If textBaseline is bottom
    Let the anchor point's vertical position be the bottom of the em box of the first available font of the inline box.
  8. Let result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin is at the anchor point.

  9. Return result, and, for callers that need it, physical alignment as the alignment value.

6 Building paths

Each object implementing the CanvasPathMethods interface has a path. A path has a list of zero or more subpaths. Each subpath consists of a list of one or more points, connected by straight or curved 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.

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

context . moveTo(x, y)
path . moveTo(x, y)

Creates a new subpath with the given point.

context . closePath()
path . closePath()

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

context . lineTo(x, y)
path . lineTo(x, y)

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

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

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

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

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

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

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

If two radii are provided, the first controls the width of the arc's ellipse, and the second controls the height. If only one is provided, or if they are the same, the arc is from a circle. In the case of an ellipse, the rotation argument controls the clockwise inclination of the ellipse relative to the x-axis.

Throws an IndexSizeError exception if the given radius is negative.

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

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

Throws an IndexSizeError exception if the given radius is negative.

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

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

The following methods allow authors to manipulate the paths of objects implementing the CanvasPathMethods interface.

For CanvasRenderingContext2D objects, the points and lines added to current default path by these methods must be transformed according to the current transformation matrix before they are added to the path.

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) on a path, the user agent must check to see if the path 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 object's path has no subpaths. Otherwise, it must mark the last subpath as closed, create a new subpath whose first point is the same as the previous subpath's first point, and finally add this new subpath to the path.

If the last subpath had more than one point in its list of points, then this is equivalent to adding a straight line connecting the last point back to the first point, 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 object's path.

The lineTo(x, y) method must ensure there is a subpath for (x, y) if the object's path 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 throw an IndexSizeError 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 ellipse that has radius radiusX on the major axis and radius radiusY on the minor axis, and whose semi-major axis is rotated rotation radians clockwise from the positive x-axis, 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 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 ellipse 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) and ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise) methods draw arcs.

The arc() method is equivalent to the ellipse() method in the case where the two radii are equal. When the arc() method is invoked, it must act as if the ellipse() method had been invoked with the radiusX and radiusY arguments set to the value of the radius argument, the rotation argument set to zero, and the other arguments set to the same values as their identically named arguments on the arc() method.

When the ellipse() method is invoked, it must proceed as follows. First, if the object's path 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. Then, it must add the start and end points of the arc to the subpath, and connect them with an arc. The arc and its start and end points are defined as follows:

Consider an ellipse that has its origin at (x, y), that has a major-axis radius radiusX and a minor-axis radius radiusY, and that is rotated about its origin such that its semi-major axis is inclined rotation radians clockwise from the x-axis. The points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start and end points respectively.

If the anticlockwise argument 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 ellipse.

Otherwise, the arc is the path along the circumference of this ellipse 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 ellipse, as opposed to being simply angles from zero, the arc can never cover an angle greater than radians.

Negative values for radiusX or radiusY must cause the implementation to throw an IndexSizeError exception.


context . ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)
path . ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)

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

Throws an IndexSizeError exception if the given radius is negative.

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.

7 Path objects

Path objects can be used to declare paths that are then later used on CanvasRenderingContext2D objects. In addition to many of the APIs described in earlier sections, Path objects have methods to combine paths, and to add text to paths.

path = new Path()

Creates a new empty Path object.

path . addPath(path, transform)
path . addPathByStrokingPath(path, styles, transform)

Adds to the path the path given by the argument.

In the case of the stroking variants, the line styles are taken from the styles argument, which can be either a DrawingStyle object or a CanvasRenderingContext2D object.

path . addText(text, styles, transform, x, y [, maxWidth ])
path . addText(text, styles, transform, path [, maxWidth ])
path . addPathByStrokingText(text, styles, transform, x, y [, maxWidth ])
path . addPathByStrokingText(text, styles, transform, path [, maxWidth ])

Adds to the path a series of subpaths corresponding to the given text. If the arguments give a coordinate, the text is drawn horizontally at the given coordinates. If the arguments give a path, the text is drawn along the path. If a maximum width is provided, the text will be scaled to fit that width if necessary.

The font, and in the case of the stroking variants, the line styles, are taken from the styles argument, which can be either a DrawingStyle object or a CanvasRenderingContext2D object.

The Path() constructor, when invoked, must return a newly created Path object.


The addPath(b, transform) method, when invoked on a Path object a, must run the following steps:

  1. If the Path object b has no subpaths, abort these steps.

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

  3. Transform all the coordinates and lines in c by the transform matrix transform, if it is not null.

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

  5. Add all the subpaths in c to a.

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


The addPathByStrokingPath(b, styles, transform) method, when invoked on a Path object a, must run the following steps:

  1. If the Path object b has no subpaths, abort these steps.

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

  3. Transform all the coordinates and lines in c by transformation matrix transform, if it is not null.

  4. Let a new list of subpaths d be the result of tracing c, using the styles argument for the line styles.

  5. Let (x, y) be the last point in the last subpath of d.

  6. Add all the subpaths in d to a.

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


The addText() and addPathByStrokingText() methods each come in two variants: one rendering text at a given coordinate, and one rendering text along a given path. In both cases, the methods take a CanvasDrawingStyles object argument for the text and (if appropriate) line styles to use, an SVGMatrix object transform (which can be null), and a maximum width can optionally be provided.

When one of the addText() and addPathByStrokingText() variants that take as argument an (x, y) coordinate is invoked, the method must run the following algorithm:

  1. Run the text preparation algorithm, passing it text, the CanvasDrawingStyles object argument, and, if the maxWidth argument was provided, that argument. Let glyphs be the result.

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

  3. Let glyph subpaths be a path describing the shapes given in glyphs, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit in glyph subpaths. Subpaths in glyph subpaths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.

  4. If the method is addPathByStrokingText(), replace glyph subpaths by the result of tracing glyph subpaths, using the CanvasDrawingStyles object argument for the line styles.

  5. Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.

  6. Let (xfinal, yfinal) be the last point in the last subpath of glyph subpaths.

  7. Add all the subpaths in glyph subpaths to the Path object.

  8. Create a new subpath in the Path object with (xfinal, yfinal) as the only point in the subpath.

When one of the addText() and addPathByStrokingText() variants that take as argument a Path object is invoked, the method must run the following algorithm:

  1. Let target be the Path object on which the method was invoked.

  2. Let path be the Path object that was provided in the method's arguments.

  3. Run the text preparation algorithm, passing it text, the CanvasDrawingStyles object argument, and, if the maxWidth argument was provided, that argument. Let glyphs be the resulting array, and physical alignment be the resulting alignment value.

  4. Let width be the aggregate length of all the subpaths in path, including the distances from the last point of each closed subpath to the first point of that subpath.

  5. Define L to be a linear coordinate line for of all the subpaths in path, with additional lines drawn between the last point and the first point of each closed subpath, such that the first point of the first subpath is defined as point 0, and the last point of the last subpath, if the last subpath is not closed, or the second occurrence first point of that subpath, if it is closed, is defined as point width.

  6. Let offset be determined according to the appropriate step below:

    If physical alignment is left
    Let offset be zero.
    If physical alignment is right
    Let offset be width.
    If physical alignment is center
    Let offset be half of width.
  7. Move all the shapes in glyphs to the right by offset CSS pixels.

  8. For each glyph glyph in the glyphs array, run these substeps:

    1. Let dx be the x-coordinate of the horizontal center of the bounding box of the shape described by glyph, in CSS pixels.

    2. If dx is negative or greater than width, skip the remainder of these substeps for this glyph.

    3. Recast dx to coordinate spaces units in path. (This just changes the dimensionality of dx, not its numeric value.)

    4. Find the point p on path (or implied closing lines in path) that corresponds to the position dx on the coordinate line L.

    5. Let θ be the clockwise angle from the positive x-axis to the side of the line that is tangential to path at the point p that is going in the same direction as the line at point p.

    6. Rotate the shape described by glyph clockwise by θ about the point that is at the dx coordinate horizontally and the zero coordinate vertically.

    7. Let (x, y) be the coordinate of the point p.

    8. Move the shape described by glyph to the right by x and down by y.

    9. Let glyph subpaths be a list of subpaths describing the shape given in glyph, with each CSS pixel in the coordinate space of glyph mapped to one coordinate space unit in glyph subpaths. Subpaths in glyph subpaths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.

    10. If the method is addPathByStrokingText(), replace glyph subpaths by the result of tracing glyph subpaths, using the CanvasDrawingStyles object argument for the line styles.

    11. Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.

    12. Let (xfinal, yfinal) be the last point in the last subpath of glyph subpaths. (This coordinate is only used if this is the last glyph processed.)

    13. Add all the subpaths in glyph subpaths to target.

  9. Create a new subpath in the Path object with (xfinal, yfinal) as the only point in the subpath.

8 Transformations

Each CanvasRenderingContext2D object has a current transformation matrix, as well as methods (described in this section) to manipulate it. When a CanvasRenderingContext2D object is created, its transformation matrix must be initialized to the identity transform.

The transformation matrix is applied to coordinates when creating the current default path, and when painting text, shapes, and Path objects, on CanvasRenderingContext2D objects.

Most of the API uses SVGMatrix objects rather than this API. This API remains mostly for historical reasons.

The transformations must be performed in reverse order.

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

context . scale(x, y)

Changes the 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.

9 Fill and stroke styles

context . fillStyle [ = value ]

Returns the current style used for filling shapes.

Can be set, to change the fill style.

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

context . strokeStyle [ = value ]

Returns the current style used for stroking shapes.

Can be set, to change the stroke style.

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

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

Both attributes can be either strings, CanvasGradients, or CanvasPatterns. On setting, strings must be parsed 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, and finally a U+0029 RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same alpha value.

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

When the value is a color, it must not be affected by the transformation matrix when used to draw on the canvas.


There are two types of gradients, linear gradients and radial gradients, both represented by objects implementing the opaque CanvasGradient interface.

Once a gradient has been created (see below), stops are placed along it to define how the colors are distributed along the gradient. The color of the gradient at each stop is the color specified for that stop. Between each such stop, the colors and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the color to use at that offset. Before the first stop, the color must be the color of the first stop. After the last stop, the color must be the color of the last stop. When there are no stops, the gradient is transparent black.

gradient . addColorStop(offset, color)

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

Throws an IndexSizeError exception if the offset is out of range. Throws a SyntaxError 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.

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

Returns a CanvasGradient object that represents a radial gradient that paints along the cone given by the circles represented by the arguments.

If either of the radii are negative, throws an IndexSizeError exception.

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 IndexSizeError exception must be thrown. If the color cannot be parsed as a CSS <color> value, then a SyntaxError exception must be thrown. 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. 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 either of r0 or r1 are negative, an IndexSizeError exception must be thrown. 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 resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.

Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.


Patterns are represented by objects implementing the opaque CanvasPattern interface.

pattern = context . createPattern(image, repetition)

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

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

If the image has no image data, throws an InvalidStateError exception. If the second argument isn't one of the allowed values, throws a SyntaxError 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 second argument is empty, the value repeat must be assumed. If an unrecognized value is given, then the user agent must throw a SyntaxError 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 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 throw an InvalidStateError 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, to create the repeated pattern that is used for rendering. The images are not scaled by this process; one CSS pixel of the image must be painted on one coordinate space unit in generating the repeated pattern. When rendered, however, patterns must actually be painted only where the stroking or filling effect requires that they be drawn, and the repeated pattern must be affected by the current transformation matrix. Pixels not covered by the repeating pattern (if the repeat string was not specified) must be transparent black.

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).


If a radial gradient or repeated pattern is used when the transformation matrix is singular, the resulting style must be transparent black (otherwise the gradient or pattern would be collapsed to a point or line, leaving the other pixels undefined). Linear gradients and solid colors always define all points even with singular tranformation matrices.

10 Drawing rectangles to the canvas

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 default path, and are subject to the clipping region, and, with the exception of clearRect(), also shadow effects, global alpha, and global composition operators.

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

Clears all pixels on the 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 run the following steps:

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

  2. Clear the pixels in pixels to a fully transparent black, erasing any previous image.

  3. Clear regions that cover the pixels in pixels in the canvas element.

If either height or width are zero, this method has no effect, since the set of pixels would be empty.

The fillRect(x, y, w, h) method 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 take the result of tracing the path described below, using the CanvasRenderingContext2D object's line styles, and fill it with the strokeStyle.

If both w and h are zero, the path has a single subpath with just one point (x, y), and no lines, and this method thus has no effect (the trace a path algorithm returns an empty path in that case).

If just one of either w or h is zero, then the path has a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.

Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w, y+h), and (x, y+h), connected to each other in that order by straight lines.

11 Drawing text to the canvas

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

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

metrics = context . measureText(text)

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

metrics . width
metrics . actualBoundingBoxLeft
metrics . actualBoundingBoxRight
metrics . fontBoundingBoxAscent
metrics . fontBoundingBoxDescent
metrics . actualBoundingBoxAscent
metrics . actualBoundingBoxDescent
metrics . emHeightAscent
metrics . emHeightDescent
metrics . hangingBaseline
metrics . alphabeticBaseline
metrics . ideographicBaseline

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

The CanvasRenderingContext2D interface provides the following methods for rendering text directly to the canvas.

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. Run the text preparation algorithm, passing it text, the CanvasRenderingContext2D object, and, if the maxWidth argument was provided, that argument. Let glyphs be the result.

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

  3. Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.

    For fillText(), fillStyle must be applied to the shapes and strokeStyle must be ignored. For strokeText(), the reverse holds: strokeStyle must be applied to the result of tracing the shapes using the CanvasRenderingContext2D object for the line styles, and fillStyle must be ignored.

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

The measureText() method takes one argument, text. When the method is invoked, the user agent must 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 'white-space' property of the inline element set to 'pre' and the 'font' property of the inline element set to the current font of the context as given by the font attribute, and must then create a new TextMetrics object with its width attribute set to the width of that inline box, in CSS pixels.

actualBoundingBoxLeft attribute

The distance parallel to the baseline from the alignment point given by the textAlign attribute to the left side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going left from the given alignment point.

The sum of this value and the next (actualBoundingBoxRight) can be wider than the width of the inline box (width), in particular with slanted fonts where characters overhang their advance width.

actualBoundingBoxRight attribute

The distance parallel to the baseline from the alignment point given by the textAlign attribute to the right side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going right from the given alignment point.

fontBoundingBoxAscent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the top of the highest bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers indicating a distance going up from the given baseline.

This value and the next are useful when rendering a background that must have a consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent attribute (and its corresponding attribute for the descent) are useful when drawing a bounding box around specific text.

fontBoundingBoxDescent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the bottom of the lowest bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers indicating a distance going down from the given baseline.

actualBoundingBoxAscent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the top of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going up from the given baseline.

This number can vary greatly based on the input text, even if the first font specified covers all the characters in the input. For example, the actualBoundingBoxAscent of a lowercase "o" from an alphabetic baseline would be less than that of an uppercase "F". The value can easily be negative; for example, the distance from the top of the em box (textBaseline value "top") to the top of the bounding rectangle when the given text is just a single comma "," would likely (unless the font is quite unusual) be negative.

actualBoundingBoxDescent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the bottom of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going down from the given baseline.

emHeightAscent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the top of the em square in the line box, in CSS pixels; positive numbers indicating that the given baseline is below the top of the em square (so this value will usually be positive). Zero if the given baseline is the top of the em square; half the font size if the given baseline is the middle of the em square.

emHeightDescent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the bottom of the em square in the line box, in CSS pixels; positive numbers indicating that the given baseline is below the bottom of the em square (so this value will usually be negative). (Zero if the given baseline is the top of the em square.)

hangingBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the hanging baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the hanging baseline. (Zero if the given baseline is the hanging baseline.)

alphabeticBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the alphabetic baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the alphabetic baseline. (Zero if the given baseline is the alphabetic baseline.)

ideographicBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the ideographic baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the ideographic baseline. (Zero if the given baseline is the ideographic baseline.)

If doing these measurements requires using a font that has an origin that is not the same as that of the Document object that owns the canvas element (even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font), then the method must throw a SecurityError exception. Otherwise, it must return the new TextMetrics object. [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 Drawing paths to the canvas

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

context . beginPath()

Resets the current default path.

context . fill()
context . fill(path)

Fills the subpaths of the current default path or the given path with the current fill style.

context . stroke()
context . stroke(path)

Strokes the subpaths of the current default path or the given path with the current stroke style.

context . drawSystemFocusRing(element)
context . drawSystemFocusRing(path, element)

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

shouldDraw = context . drawCustomFocusRing(element)
shouldDraw = context . drawCustomFocusRing(path, element)

If the given element is focused, and the user has configured his system to draw focus rings in a particular manner (for example, high contrast focus rings), draws a focus ring around the current default path or the given path and returns false.

Otherwise, returns true if the given element is focused, and false otherwise. This can thus be used to determine when to draw a focus ring (see the example below).

context . scrollPathIntoView()
context . scrollPathIntoView(path)

Scrolls the current default path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.

context . clip()
context . clip(path)

Further constrains the clipping region to the current default path.

context . isPointInPath(x, y)
context . isPointInPath(path, x, y)

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

The beginPath() method must empty the list of subpaths in the context's current default path so that the it once again has zero subpaths.

Where the following method definitions use the term intended path, it means the Path argument, if one was provided, or the current default path otherwise.

When the intended path is a Path object, the coordinates and lines of its subpaths must be transformed according to the CanvasRenderingContext2D object's current transformation matrix when used by these methods (without affecting the Path object itself). When the intended path is the current default path, it is not affected by the transform. (This is because transformations already affect the current default path when it is constructed, so applying it when it is painted as well would result in a double transformation.)

The fill() method must fill all the subpaths of the intended 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 trace the intended path, using the CanvasRenderingContext2D object for the line styles, and then fill the combined stroke area using the strokeStyle attribute.

As a result of how the algorithm to trace a path is defined, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.

The stroke style is affected by the transformation during painting, even if the intended path is the current default path.

Paths, when filled or stroked, must be painted without affecting the current default path or any Path objects, and must be subject to shadow effects, global alpha, the clipping region, and global composition operators. (The effect of transformations is described above and varies based on which path is being used.)

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


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

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

  2. If the user has requested the use of particular focus rings (e.g. high-contrast focus rings), or if the element would have a focus ring drawn around it, then draw a focus ring of the appropriate style along the intended path, following platform conventions.

    Some platforms only draw focus rings around elements that have been focused from the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus rings around some elements at all unless relevant accessibility features are enabled. This API is intended to follow these conventions. User agents that implement distinctions based on the manner in which the element was focused are encouraged to classify focus driven by the focus() method based on the kind of user interaction event from which the call was triggered (if any).

    The focus ring should not be subject to the shadow effects, the global alpha, or the global composition operators, but should be subject to the clipping region. (The effect of transformations is described above and varies based on which path is being used.)

  3. Optionally, inform the user that the focus is at the location given by the intended path. User agents may wait until the next time the event loop reaches its "update the rendering" step to optionally inform the user.

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

  1. If 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. Let result be true.

  3. If the user has requested the use of particular focus rings (e.g. high-contrast focus rings), then draw a focus ring of the appropriate style along the intended path, and set result to false.

    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. (The effect of transformations is described above and varies based on which path is being used.)

  4. Optionally, inform the user that the focus is at the location given by the intended path. User agents may wait until the next time the event loop reaches its "update the rendering" step to optionally inform the user.

  5. Return result.

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

  1. Let the specified rectangle be the rectangle of the bounding box of the intended path.

  2. Let notional child be a hypothetical element that is a rendered child of the canvas element whose dimensions are those of the specified rectangle.

  3. Scroll notional child into view with the align to top flag set.

  4. Optionally, inform the user that the caret and/or selection cover the specified rectangle of the canvas. User agents may wait until the next time the event loop reaches its "update the rendering" step to optionally inform the user.

"Inform the user", as used in this section, could mean calling a system accessibility API, which would notify assistive technologies such as magnification tools. To properly drive magnification based on a focus change, a system accessibility API driving a screen magnifier needs the bounds for the newly focused object. The methods above are intended to enable this by allowing the user agent to report the bounding box of the path used to render the focus ring as the bounds of the element element passed as an argument, if that element is focused, and the bounding box of the area to which the user agent is scrolling as the bounding box of the current selection.


The clip() method must create a new clipping region by calculating the intersection of the current clipping region and the area described by the intended path, using the 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() 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 intended path as determined by the non-zero winding number rule; and must return false otherwise. Points on the path itself must be considered to be inside the path. If either of the arguments is infinite or NaN, then the method must return false.


This canvas element has a couple of checkboxes. The path-related commands are highlighted:

<canvas height=400 width=750>
 <label><input type=checkbox id=showA> Show As</label>
 <label><input type=checkbox id=showB> Show Bs</label>
 <!-- ... -->
</canvas>
<script>
 function drawCheckbox(context, element, x, y, paint) {
   context.save();
   context.font = '10px sans-serif';
   context.textAlign = 'left';
   context.textBaseline = 'middle';
   var metrics = context.measureText(element.labels[0].textContent);
   if (paint) {
     context.beginPath();
     context.strokeStyle = 'black';
     context.rect(x-5, y-5, 10, 10);
     context.stroke();
     if (element.checked) {
       context.fillStyle = 'black';
       context.fill();
     }
     context.fillText(element.labels[0].textContent, x+5, y);
   }
   context.beginPath();
   context.rect(x-7, y-7, 12 + metrics.width+2, 14);
   if (paint && context.drawCustomFocusRing(element)) {
     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');
   context.clearRect(0, 0, canvas.width, canvas.height);
   drawCheckbox(context, document.getElementById('showA'), 20, 40, true);
   drawCheckbox(context, document.getElementById('showB'), 20, 60, true);
   drawBase();
   if (document.getElementById('showA').checked)
     drawAs();
   if (document.getElementById('showB').checked)
     drawBs();
 }
 function processClick(event) {
   var canvas = document.getElementsByTagName('canvas')[0];
   var context = canvas.getContext('2d');
   var x = event.clientX;
   var y = event.clientY;
   var node = event.target;
   while (node) {
     x -= node.offsetLeft - node.scrollLeft;
     y -= node.offsetTop - node.scrollTop;
     node = node.offsetParent;
   }
   drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
   if (context.isPointInPath(x, y))
     document.getElementById('showA').checked = !(document.getElementById('showA').checked);
   drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
   if (context.isPointInPath(x, y))
     document.getElementById('showB').checked = !(document.getElementById('showB').checked);
   redraw();
 }
 document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false);
 redraw();
</script>

13 Drawing images to the canvas

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 TypeMismatchError exception. If the image has no image data, throws an InvalidStateError exception. If the one of the source rectangle dimensions is zero, throws an IndexSizeError 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 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 throw an InvalidStateError 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 throw an IndexSizeError 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 the source rectangle is outside the source image, it must be clipped to the source image, and the destination rectangle must be clipped in the same proportion.

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.

14 Hit regions

Each canvas element whose primary context is a CanvasRenderingContext2D object must have a hit region list associated with its bitmap.

The hit region list is a list of hit regions.

Each hit region consists of the following information:

context . addHitRegion(options)

Adds a hit region to the canvas bitmap. The argument is an object with the following members:

path (default null)
A Path object that describes the pixels that form part of the region. If this member is not provided or is set to null, the current default path is used instead.
id (default empty string)
The ID to use for this region. This is used in MouseEvent events on the canvas (event.region) and as a way to reference this region in later calls to addHitRegion().
parentID (default null)
The ID of the parent region, for purposes of navigation by accessibility tools and for cursor fallback.
cursor (default "inherit")
The cursor to use when the mouse is over this region. The value "inherit" means to use the cursor for the parent region (as specified by the parentID member), if any, or to use the canvas element's cursor if the region has no parent.
control (default null)
An element (that is a descendant of the canvas) to which events are to be routed, and which accessibility tools are to use as a surrogate for describing and interacting with this region.
label (default null)
A text label for accessibility tools to use as a description of this region, if there is no control.
role (default null)
An ARIA role for accessibility tools to use to determine how to represent this region, if there is no control.

Hit regions can be used for a variety of purposes:

context . removeHitRegion(options)

Removes a hit region (and all its ancestors) from the canvas bitmap. The argument is the ID of a region added using addHitRegion().

The pixels that were covered by this region and its descendants are effectively cleared by this operation, leaving the regions non-interactive. In particular, regions that occupied the same pixels before the removed regions were added, overlapping them, do not resume their previous role.

A hit region A is an ancestor region of a hit region B if B has a parent and its parent is either A or another hit region for which A is an ancestor region.

The region identified by the ID ID in a canvas element ancestor is the value returned by the following algorithm (which can return a hit region or nothing):

  1. If ID is null, return nothing and abort these steps.

  2. Let list be the hit region list associated with ancestor's bitmap.

  3. If there is a hit region in list whose ID is a case-sensitive match for ID, then return that hit region and abort these steps.

  4. Otherwise, return nothing.

The region representing the control control for a canvas element ancestor is the value returned by the following algorithm (which can return a hit region or nothing):

  1. If control is not a descendant of ancestor, then return nothing and abort these steps.

  2. Let list be the hit region list associated with ancestor's bitmap.

  3. If there is a hit region in list whose control is control, then return that hit region and abort these steps.

  4. Otherwise, return nothing.

The control represented by a region region for a canvas element ancestor is the value returned by the following algorithm (which can return an element or nothing):

  1. If region has no control, return nothing and abort these steps.

  2. Let control be region's control.

  3. If control is not a descendant of ancestor, then return nothing and abort these steps.

  4. Otherwise, return control.

The cursor for a hit region region of a canvas element ancestor is the value returned by the following algorithm:

  1. Loop: If region has a cursor specification other than "inherit", then return that hit region's cursor specification and abort these steps.

  2. If region has a parent, then let region be that hit region's parent, and return to the step labeled loop.

  3. Otherwise, return the used value of the 'cursor' property for the canvas element. [CSSUI]

The region for a pixel pixel on a canvas element ancestor is the value returned by the following algorithm (which can return a hit region or nothing):

  1. Let list be the hit region list associated with ancestor's bitmap.

  2. If there is a hit region in list whose set of pixels contains pixel, then return that hit region and abort these steps.

  3. Otherwise, return nothing.

To clear regions that cover the pixels pixels on a canvas element ancestor, the user agent must run the following steps:

  1. Let list be the hit region list associated with ancestor's bitmap.

  2. Remove all pixels in pixels from the set of pixels of each hit region in list.

  3. Garbage-collect the regions of ancestor.

To garbage-collect the regions of a canvas element ancestor, the user agent must run the following steps:

  1. Let list be the hit region list associated with ancestor's bitmap.

  2. Loop: Let victim be the first hit region in list to have an empty set of pixels and a zero child count, if any. If there is no such hit region, abort these steps.

  3. If victim has a parent, then decrement that hit region's child count by one.

  4. Remove victim from list.

  5. Jump back to the step labeled loop.

Adding a new region and calling clearRect() are the two ways this clearing algorithm can be invoked. Resizing the canvas also clears the hit region list (since it clears the canvas back to its initial state).


When the addHitRegion() method is invoked, the user agent must run the following steps:

  1. Let arguments be the dictionary object provided as the method's argument.

  2. If the arguments object's path member is not null, let source path be the path member's value. Otherwise, let it be the CanvasRenderingContext2D object's current default path.

  3. Transform all the coordinates and lines in source path by the current transform matrix, if the arguments object's path member is not null.

  4. If the arguments object's id member is the empty string, let it be null instead.

  5. If the arguments object's id member is not null, then let previous region for this ID be the region identified by the ID given by the id member's value in this canvas element, if any. If the id member is null or no such region currently exists, let previous region for this ID be null.

  6. If the arguments object's parent member is the empty string, let it be null instead.

  7. If the arguments object's parent member is not null, then let parent region be the region identified by the ID given by the parent member's value in this canvas element, if any. If the parent member is null or no such region currently exists, let parent region be null.

  8. If the arguments object's label member is the empty string, let it be null instead.

  9. If any of the following conditions are met, throw a NotSupportedError exception and abort these steps.

    • The arguments object's control and label members are both non-null.
    • The arguments object's control and role members are both non-null.
    • The arguments object's role member's value is the empty string, and the label member's value is either null or the empty string.
    • The path source path describes a shape with no pixels.
    • The arguments object's control member is not null and is not an element that is a descendant of this canvas element.
    • The parent region is not null but has a control.
    • The previous region for this ID is the same hit region as the parent region.
    • The previous region for this ID is an ancestor region of the parent region.
  10. If the parent member is not null but parent region is null, then throw a NotFoundError exception and abort these steps.

  11. If any of the following conditions are met, throw a SyntaxError exception and abort these steps.

    • The arguments object's cursor member is not null but is neither an ASCII case-insensitive match for the string "inherit", nor a valid CSS 'cursor' property value. [CSSUI]
    • The arguments object's role member is not null but its value is not an ordered set of unique space-separated tokens whose tokens are all case-sensitive matches for names of non-abstract WAI-ARIA roles. [ARIA]
  12. Let region be a newly created hit region, with its information configured as follows:

    Hit region's set of pixels

    The pixels contained in source path.

    Hit region's bounding circumference

    A user-agent-defined shape that wraps the pixels contained in source path. (In the simplest case, this can just be the bounding rectangle; this specification allows it to be any shape in order to allow other interfaces.)

    Hit region's ID

    If the arguments object's id member is not null: the value of the id member. Otherwise, region has no id.

    Hit region's parent

    If parent region is not null: parent region. Otherwise, region has no parent.

    Hit region's child count

    Initially zero.

    Hit region's cursor specification

    If parent region is not null: parent region. Otherwise, region has no parent.

    Hit region's control

    If the arguments object's control member is not null: the value of the control member. Otherwise, region has no control.

    Hit region's label

    If the arguments object's label member is not null: the value of the label member. Otherwise, region has no label.

    Hit region's ARIA role

    If the arguments object's role member is not null: the value of the role member (which might be the empty string). Otherwise, if the arguments object's label member is not null: the empty string. Otherwise, region has no ARIA role.

  13. If the arguments object's cursor member is not null, then act as if a CSS rule for the canvas element setting its 'cursor' property had been seen, whose value was the hit region's cursor specification.

    For example, if the user agent prefetches cursor values, this would cause that to happen in response to an appropriately-formed addHitRegion() call.

  14. If the arguments object's control member is not null, then let previous region for the control be the region representing the control given by the control member's value for this canvas element, if any. If the control member is null or no such region currently exists, let previous region for the control be null.

  15. If there is a previous region with this control, remove it from the canvas element's hit region list; then, if it had a parent region, decrement that hit region's child count by one.

  16. If there is a previous region with this ID, remove it, and all hit regions for which it is an ancestor region, from the canvas element's hit region list; then, if it had a parent region, decrement that hit region's child count by one.

  17. If there is a parent region, increment its hit region's child count by one.

  18. Clear regions that cover the pixels in region's set of pixels on this canvas element.

  19. Add region to the canvas element's hit region list.

If the hitregion is interactive or requires the use of WAI-ARIA states or properties (in addition to role or label), authors must not use an unbacked region description (as there is no method to attach WAI-ARIA states or properties to an unbacked region description and unbacked region are unable to receive focus and therefore cannot be in a focused state).

In cases where the region is functioning as an interactive control it must have a control. It is recommended that the control() references an html interactive element unless html does not provide the desired features. In which case WAI-ARIA role, states and properties may be used as allowed in html.

Examples of regions where a WAI-ARIA role requires keyboard focus or the use of WAI-ARIA states and properties other than label, are a button and a checkbox. A button has a role="button" and it is required to be focusable using the keyboard. A checkbox has a role ="checkbox", requires an aria-checked state and it is required to be focusable using the keyboard. [ARIA]

When the removeHitRegion() method is invoked, the user agent must run the following steps:

  1. Let region be the region identified by the ID given by the method's argument in this canvas element, if any. If no such region currently exists, abort these steps.

  2. Remove region, and all hit regions for which it is an ancestor region, from the canvas element's hit region list; then, if it had a parent region, decrement that hit region's child count by one.

  3. Garbage-collect the regions of ancestor.


The MouseEvent interface is extended to support hit regions:

partial interface MouseEvent {
  readonly attribute DOMString? region;
};

partial dictionary MouseEventInit {
  DOMString? region;
};
event . region

If the mouse was over a hit region, then this returns the hit region's ID, if it has one.

Otherwise, returns null.

The region attribute on MouseEvent objects must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents the hit region's ID if the mouse was over a hit region when the event was fired.

When a MouseEvent is to be fired at a canvas element by the user agent in response to a pointing device action, the user agent must instead follow these steps. If these steps say to act as normal, that means that the event must be fired as it would have had these requirements not been applied.

  1. If the pointing device is not indicating a pixel on the canvas, act as normal and abort these steps.

  2. Let pixel be the pixel indicated by the pointing device.

  3. Let region be the hit region that is the region for the pixel pixel on this canvas element, if any.

  4. If there is no region, then act as normal and abort these steps.

  5. Let id be the region's ID, if any.

  6. If there is an id, then initialize the event object's region attribute to id.

  7. Let control be the region's control, if any.

  8. If there is a control, then target the event object at control instead of the canvas element.

  9. Continue dispatching the event, but with the updated event object and target as given in the above steps.


When a user's pointing device cursor is positioned over a canvas element, user agents should render the pointing device cursor according to the cursor specification described by the cursor for the hit region that is the region for the pixel that the pointing device designates.


User agents are encouraged to make use of the information present in a canvas element's hit region list to improve the accessibility of canvas elements.

Each hit region should be handled in a fashion equivalent to a node in a virtual DOM tree rooted at the canvas element. The hierarchy of this virtual DOM tree must match the hierarchy of the hit regions, as described by the parent of each region. Regions without a parent must be treated as children of the canvas element for the purpose of this virtual DOM tree. For each node in such a DOM tree, the hit region's bounding circumference gives the region of the screen to use when representing the node (if appropriate).

The semantics of a hit region for the purposes of this virtual DOM tree are those of the hit region's control, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.

For the purposes of accessibility tools, when an element C is a descendant of a canvas element and there is a region representing the control C for that canvas element, then the element's position relative to the document should be presented as if it was that region in the canvas element's virtual DOM tree.

The semantics of a hit region for the purposes of this virtual DOM tree are those of the hit region's control, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.

Thus, for instance, a user agent on a touch-screen device could provide haptic feedback when the user croses over a hit region's bounding circumference, and then read the hit region's label to the user. Similarly, a desktop user agent with a virtual accessibility focus separate from the keyboard input focus could allow the user to navigate through the hit regions, using the virtual DOM tree described above to enable hierarchical navigation. When an interactive control inside the canvas element is focused, if the control has a corresponding region, then that hit region's bounding circumference could be used to determine what area of the display to magnify.

15 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.

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

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

Throws an IndexSizeError exception if the either of the width or height arguments are zero.

The data will be returned with one pixel of image data for each coordinate space unit on the canvas (ignoring transforms).

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.

Throws a NotSupportedError exception if any of the 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, if the canvas element's origin-clean flag is set to false, throw a SecurityError exception; otherwise, it 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, the method must instead throw a NotSupportedError exception. If either the sw or sh arguments are zero, the method must instead throw an IndexSizeError 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 Uint8ClampedArray object. The Uint8ClampedArray object must use a Canvas Pixel ArrayBuffer for its storage, and must have a zero start offset and a length equal to the length of its storage, in bytes. The Canvas Pixel ArrayBuffer must contain the image data. At least one pixel's worth of image data must be returned. [TYPEDARRAY]

A Canvas Pixel ArrayBuffer is an ArrayBuffer that whose data is represented in left-to-right order, row by row top to bottom, starting with the top left, with each pixel's red, green, blue, and alpha components being given in that order for each pixel. Each component of each 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. [TYPEDARRAY]

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 throw a NotSupportedError 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 thrown, 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.

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

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.

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>

16 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.

17 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.

It is likely that this will change: browser vendors have indicated an interest in changing the processing model for shadows such that they only draw when the composition operator is "source-over" (the default). Read more...

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).

18 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.

19 Best practices

This section is non-normative.

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

To indicate which focusable part of the canvas is currently focused, authors should use the drawSystemFocusRing() method, passing it the element for which a ring is being drawn. This method only draws the focus ring if the element is focused, so that it can simply be called whenever drawing the element, without checking whether the element is focused or not first.

Authors should avoid implementing text editing controls using the canvas element. Doing so has a large number of disadvantages:

This is a huge amount of work, and authors are most strongly encouraged to avoid doing any of it by instead using the input element, the textarea element, or the contenteditable attribute.

20 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".

[2DCONTEXT]
HTML Canvas 2D Context, I. Hickson. W3C.
[ABNF]
Augmented BNF for Syntax Specifications: ABNF, D. Crocker, P. Overell. IETF.
[ABOUT]
The 'about' URI scheme, L. Hunt, M. Yevstifeyev. IETF. Work in progress.
[AES128CTR]
Advanced Encryption Standard (AES). NIST.
[ARIA]
Accessible Rich Internet Applications (WAI-ARIA), J. Craig, M. Cooper, L. Pappas, R. Schwerdtfeger, L. Seeman. W3C.
[ARIAIMPL]
WAI-ARIA 1.0 User Agent Implementation Guide, A. Snow-Weaver, M. Cooper. W3C.
[ATAG]
(Non-normative) Authoring Tool Accessibility Guidelines (ATAG) 2.0, J. Richards, J. Spellman, J. Treviranus. W3C.
[ATOM]
(Non-normative) The Atom Syndication Format, M. Nottingham, R. Sayre. IETF.
[BCP47]
Tags for Identifying Languages; Matching of Language Tags, A. Phillips, M. Davis. IETF.
[BECSS]
Behavioral Extensions to CSS, I. Hickson. W3C.
[BEZIER]
Courbes à poles, P. de Casteljau. INPI, 1959.
[BIDI]
UAX #9: Unicode Bidirectional Algorithm, M. Davis. Unicode Consortium.
[BIG5]
(Non-normative) Chinese Coded Character Set in Computer. Institute for Information Industry, March 1984.
[BOCU1]
(Non-normative) UTN #6: BOCU-1: MIME-Compatible Unicode Compression, M. Scherer, M. Davis. Unicode Consortium.
[CESU8]
(Non-normative) UTR #26: Compatibility Encoding Scheme For UTF-16: 8-BIT (CESU-8), T. Phipps. Unicode Consortium.
[CHARMOD]
(Non-normative) Character Model for the World Wide Web 1.0: Fundamentals, M. Dürst, F. Yergeau, R. Ishida, M. Wolf, T. Texin. W3C.
[CLDR]
Unicode Common Locale Data Repository. Unicode.
[COMPUTABLE]
(Non-normative) On computable numbers, with an application to the Entscheidungsproblem, A. Turing. In Proceedings of the London Mathematical Society, series 2, volume 42, pages 230-265. London Mathematical Society, 1937.
[COOKIES]
HTTP State Management Mechanism, A. Barth. IETF.
[CORS]
Cross-Origin Resource Sharing, A. van Kesteren. W3C.
[CP50220]
CP50220, Y. Naruse. IANA.
[CP51932]
CP51932, Y. Naruse. IANA.
[CSS]
Cascading Style Sheets Level 2 Revision 1, B. Bos, T. Çelik, I. Hickson, H. Lie. W3C.
[CSSATTR]
CSS Styling Attribute Syntax, T. Çelik, E. Etemad. 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.
[CSSOMVIEW]
CSSOM View Module, A. van Kesteren. W3C.
[CSSRUBY]
CSS3 Ruby Module, R. Ishida. W3C.
[CSSUI]
CSS3 Basic User Interface Module, T. Çelik. W3C.
[CSSVALUES]
CSS3 Values and Units, H. Lie, C. Lilley. W3C.
[DASH]
Dynamic adaptive streaming over HTTP (DASH). ISO.
[DOM2HTML]
(Non-normative) Document Object Model (DOM) Level 2 HTML Specification, J. Stenback, P. Le Hegaret, A. Le Hors. W3C.
[DOMCORE]
Web DOM Core, A. van Kesteren. W3C.
[DOMEVENTS]
Document Object Model (DOM) Level 3 Events Specification, T. Leithead. W3C.
[DOMPARSING]
DOM Parsing and Serialization, Ms2ger. html5.org.
[DOT]
(Non-normative) The DOT Language. Graphviz.
[E163]
Recommendation E.163 — Numbering Plan for The International Telephone Service, CCITT Blue Book, Fascicle II.2, pp. 128-134, November 1988.
[ECMA262]
ECMAScript Language Specification. ECMA.
[EDITING]
HTML Editing APIs, A. Gregor.
[ECMA357]
(Non-normative) ECMAScript for XML (E4X) Specification. ECMA.
[EUCKR]
Hangul Unix Environment. Korea Industrial Standards Association. Ref. No. KS C 5861-1992.
[EUCJP]
Definition and Notes of Japanese EUC. UI-OSF-USLP. In English in the abridged translation of the UI-OSF Application Platform Profile for Japanese Environment, Appendix C.
[EVENTSOURCE]
Server-Sent Events, I. Hickson. W3C.
[FILEAPI]
File API, A. Ranganathan. W3C.
[FILESYSTEMAPI]
File API: Directories and System, E. Uhrhane. W3C.
[GBK]
Chinese Internal Code Specification. Chinese IT Standardization Technical Committee.
[GRAPHICS]
(Non-normative) Computer Graphics: Principles and Practice in C, Second Edition, J. Foley, A. van Dam, S. Feiner, J. Hughes. Addison-Wesley. ISBN 0-201-84840-6.
[GREGORIAN]
(Non-normative) Inter Gravissimas, A. Lilius, C. Clavius. Gregory XIII Papal Bulls, February 1582.
[HATOM]
(Non-normative) hAtom, D Janes. Microformats.
[HMAC]
The Keyed-Hash Message Authentication Code (HMAC). NIST.
[HPAAIG]
HTML to Platform Accessibility APIs Implementation Guide. W3C.
[HTML4]
(Non-normative) HTML 4.01 Specification, D. Raggett, A. Le Hors, I. Jacobs. W3C.
[HTML]
HTML, I. Hickson. WHATWG.
[HTML5]
HTML5, I. Hickson. W3C.
[HTMLALTTECHS]
(Non-normative) HTML5: Techniques for providing useful text alternatives, S. Faulkner. W3C.
[HTMLDIFF]
(Non-normative) HTML5 differences from HTML4, A. van Kesteren. W3C.
[HTTP]
Hypertext Transfer Protocol — HTTP/1.1, R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. IETF.
[IANACHARSET]
Character Sets. IANA.
[IANALINKTYPE]
Link Relations. IANA.
[IANAPERMHEADERS]
Permanent Message Header Field Names. IANA.
[ICE]
Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols, J. Rosenberg. IETF.
[IEEE754]
IEEE Standard for Floating-Point Arithmetic (IEEE 754). IEEE. ISBN 978-0-7381-5753-5.
[ISO8601]
(Non-normative) ISO8601: Data elements and interchange formats — Information interchange — Representation of dates and times. ISO.
[ISO885911]
ISO-8859-11: Information technology — 8-bit single-byte coded graphic character sets — Part 11: Latin/Thai alphabet. ISO.
[JPEG]
JPEG File Interchange Format, E. Hamilton.
[JSON]
The application/json Media Type for JavaScript Object Notation (JSON), D. Crockford. IETF.
[JSURL]
The 'javascript' resource identifier scheme, B. Höhrmann. IETF. Work in progress.
[MAILTO]
(Non-normative) The 'mailto' URI scheme, M. Duerst, L. Masinter, J. Zawinski. IETF.
[MATHML]
Mathematical Markup Language (MathML), D. Carlisle, P. Ion, R. Miner, N. Poppelier. W3C.
[MEDIAFRAG]
Media Fragments URI, R. Troncy, E. Mannens, S. Pfeiffer, D. Van Deursen. W3C.
[MFREL]
Microformats Wiki: existing rel values. Microformats.
[MIMESNIFF]
MIME Sniffing, A. Barth, I. Hickson. WHATWG.
[MQ]
Media Queries, H. Lie, T. Çelik, D. Glazman, A. van Kesteren. W3C.
[NPAPI]
(Non-normative) Gecko Plugin API Reference. Mozilla.
[NPN]
Transport Layer Security (TLS) Next Protocol Negotiation Extension, A. Langley. IETF. Work in progress.
[OGGROLE]
SkeletonHeaders. Xiph.Org.
[OPENSEARCH]
Autodiscovery in HTML/XHTML. In OpenSearch 1.1 Draft 4, Section 4.6.2. OpenSearch.org.
[ORIGIN]
The Web Origin Concept, A. Barth. IETF.
[PAGEVIS]
(Non-normative) Page Visibility, J. Mann, A. Jain. W3C.
[PDF]
(Non-normative) Document management — Portable document format — Part 1: PDF. ISO.
[PINGBACK]
Pingback 1.0, S. Langridge, I. Hickson.
[PNG]
Portable Network Graphics (PNG) Specification, D. Duce. W3C.
[POLYGLOT]
(Non-normative) Polyglot Markup: HTML-Compatible XHTML Documents, E. Graff. W3C.
[PORTERDUFF]
Compositing Digital Images, T. Porter, T. Duff. In Computer graphics, volume 18, number 3, pp. 253-259. ACM Press, July 1984.
[PPUTF8]
(Non-normative) The Properties and Promises of UTF-8, M. Dürst. University of Zürich. In Proceedings of the 11th International Unicode Conference.
[PROGRESS]
Progress Events, A. van Kesteren. W3C.
[PSL]
Public Suffix List. Mozilla Foundation.
[RFC1034]
Domain Names - Concepts and Facilities, P. Mockapetris. IETF, November 1987.
[RFC1321]
The MD5 Message-Digest Algorithm, R. Rivest. IETF.
[RFC1345]
Character Mnemonics and Character Sets, K. Simonsen. IETF.
[RFC1468]
Japanese Character Encoding for Internet Messages, J. Murai, M. Crispin, E. van der Poel. IETF.
[RFC1494]
(Non-normative) Equivalences between 1988 X.400 and RFC-822 Message Bodies, H. Alvestrand, S. Thompson. IETF.
[RFC1554]
ISO-2022-JP-2: Multilingual Extension of ISO-2022-JP, M. Ohta, K. Handa. IETF.
[RFC1557]
Korean Character Encoding for Internet Messages, U. Choi, K. Chon, H. Park. IETF.
[RFC1842]
ASCII Printable Characters-Based Chinese Character Encoding for Internet Messages, Y. Wei, Y. Zhang, J. Li, J. Ding, Y. Jiang. IETF.
[RFC1922]
Chinese Character Encoding for Internet Messages, HF. Zhu, DY. Hu, ZG. Wang, TC. Kao, WCH. Chang, M. Crispin. IETF.
[RFC2045]
Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, N. Freed, N. Borenstein. IETF.
[RFC2046]
Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types, N. Freed, N. Borenstein. IETF.
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels, S. Bradner. IETF.
[RFC2237]
Japanese Character Encoding for Internet Messages, K. Tamaru. IETF.
[RFC2246]
The TLS Protocol Version 1.0, T. Dierks, C. Allen. IETF.
[RFC2313]
PKCS #1: RSA Encryption, B. Kaliski. IETF.
[RFC2318]
The text/css Media Type, H. Lie, B. Bos, C. Lilley. IETF.
[RFC2388]
Returning Values from Forms: multipart/form-data, L. Masinter. IETF.
[RFC2397]
The "data" URL scheme, L. Masinter. IETF.
[RFC2445]
Internet Calendaring and Scheduling Core Object Specification (iCalendar), F. Dawson, D. Stenerson. IETF.
[RFC2483]
URI Resolution Services Necessary for URN Resolution, M. Mealling, R. Daniel. IETF.
[RFC2781]
UTF-16, an encoding of ISO 10646, P. Hoffman, F. Yergeau. IETF.
[RFC3676]
The Text/Plain Format and DelSp Parameters, R. Gellens. IETF.
[RFC2806]
(Non-normative) URLs for Telephone Calls, A. Vaha-Sipila. IETF.
[RFC3023]
XML Media Types, M. Murata, S. St. Laurent, D. Kohn. IETF.
[RFC3279]
Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile, W. Polk, R. Housley, L. Bassham. IETF.
[RFC3490]
Internationalizing Domain Names in Applications (IDNA), P. Faltstrom, P. Hoffman, A. Costello. IETF.
[RFC3629]
UTF-8, a transformation format of ISO 10646, F. Yergeau. IETF.
[RFC3864]
Registration Procedures for Message Header Fields, G. Klyne, M. Nottingham, J. Mogul. IETF.
[RFC3986]
Uniform Resource Identifier (URI): Generic Syntax, T. Berners-Lee, R. Fielding, L. Masinter. IETF.
[RFC3987]
Internationalized Resource Identifiers (IRIs), M. Dürst, M. Suignard. IETF.
[RFC4281]
The Codecs Parameter for "Bucket" Media Types, R. Gellens, D. Singer, P. Frojdh. IETF.
[RFC4329]
(Non-normative) Scripting Media Types, B. Höhrmann. IETF.
[RFC4366]
Transport Layer Security (TLS) Extensions, S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, T. Wright. IETF.
[RFC4395]
Guidelines and Registration Procedures for New URI Schemes, T. Hansen, T. Hardie, L. Masinter. IETF.
[RFC4648]
The Base16, Base32, and Base64 Data Encodings, S. Josefsson. IETF.
[RFC4770]
vCard Extensions for Instant Messaging (IM), C. Jennings, J. Reschke. IETF.
[RFC5280]
Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile, D. Cooper, S. Santesson, S. Farrell, S. Boeyen, R. Housley, W. Polk. IETF.
[RFC5322]
Internet Message Format, P. Resnick. IETF.
[RFC5724]
URI Scheme for Global System for Mobile Communications (GSM) Short Message Service (SMS), E. Wilde, A. Vaha-Sipila. IETF.
[RFC6266]
Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP), J. Reschke. IETF.
[RFC6350]
vCard Format Specification, S. Perreault. IETF.
[SCSU]
(Non-normative) UTR #6: A Standard Compression Scheme For Unicode, M. Wolf, K. Whistler, C. Wicksteed, M. Davis, A. Freytag, M. Scherer. Unicode Consortium.
[SDP]
SDP: Session Description Protocol, M. Handley, V. Jacobson, C. Perkins. IETF.
[SDPLABEL]
The Session Description Protocol (SDP) Label Attribute, O. Levin, G. Camarillo. IETF.
[SDPOFFERANSWER]
An Offer/Answer Model with the Session Description Protocol (SDP), J. Rosenberg, H. Schulzrinne. IETF.
[SELECTORS]
Selectors, E. Etemad, T. Çelik, D. Glazman, I. Hickson, P. Linss, J. Williams. W3C.
[SHA1]
Secure Hash Standard. NIST.
[SHIFTJIS]
JIS X0208: 7-bit and 8-bit double byte coded KANJI sets for information interchange. Japanese Industrial Standards Committee.
[SRGB]
IEC 61966-2-1: Multimedia systems and equipment — Colour measurement and management — Part 2-1: Colour management — Default RGB colour space — sRGB. IEC.
[STUN]
Session Traversal Utilities for NAT (STUN), J. Rosenberg, R. Mahy, P. Matthews, D. Wing. IETF.
[SVG]
Scalable Vector Graphics (SVG) Tiny 1.2 Specification, O. Andersson, R. Berjon, E. Dahlström, A. Emmons, J. Ferraiolo, A. Grasso, V. Hardy, S. Hayman, D. Jackson, C. Lilley, C. McCormack, A. Neumann, C. Northway, A. Quint, N. Ramani, D. Schepers, A. Shellshear. W3C.
[TIS620]
UDC 681.3.04:003.62. Thai Industrial Standards Institute, Ministry of Industry, Royal Thai Government. ISBN 974-606-153-4.
[TURN]
Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN), R. Mahy, P. Matthews, J. Rosenberg. IETF.
[TYPEDARRAY]
Typed Array Specification, D. Herman, K. Russell. Khronos.
[UAAG]
(Non-normative) User Agent Accessibility Guidelines (UAAG) 2.0, J. Allan, K. Ford, J. Richards, J. Spellman. W3C.
[UNDO]
UndoManager and DOM Transaction, R. Niwa.
[UNICODE]
The Unicode Standard. Unicode Consortium.
[UNIVCHARDET]
(Non-normative) A composite approach to language/encoding detection, S. Li, K. Momoi. Netscape. In Proceedings of the 19th International Unicode Conference.
[UTF7]
UTF-7: A Mail-Safe Transformation Format of Unicode, D. Goldsmith, M. Davis. IETF.
[UTF8DET]
(Non-normative) Multilingual form encoding, M. Dürst. W3C.
[UTR36]
(Non-normative) UTR #36: Unicode Security Considerations, M. Davis, M. Suignard. Unicode Consortium.
[WCAG]
(Non-normative) Web Content Accessibility Guidelines (WCAG) 2.0, B. Caldwell, M. Cooper, L. Reid, G. Vanderheiden. W3C.
[WEBADDRESSES]
Web addresses in HTML5, D. Connolly, C. Sperberg-McQueen.
[WEBGL]
WebGL Specification, C. Marrin. Khronos Group.
[WEBIDL]
Web IDL, C. McCormack. W3C.
Web Linking, M. Nottingham. IETF.
[WEBMCG]
WebM Container Guidelines. The WebM Project.
[WEBSOCKET]
The WebSocket API, I. Hickson. W3C.
[WEBSTORAGE]
Web Storage, I. Hickson. W3C.
[WEBVTT]
WebVTT, I. Hickson. W3C.
[WEBWORKERS]
Web Workers, I. Hickson. W3C.
[WHATWGBLOG]
The WHATWG Blog. WHATWG.
[WHATWGWIKI]
The WHATWG Wiki. WHATWG.
[WIN1252]
Windows 1252. Microsoft.
[WIN1254]
Windows 1254. Microsoft.
[WIN31J]
Windows Codepage 932. Microsoft.
[WIN874]
Windows 874. Microsoft.
[WIN949]
Windows Codepage 949. Microsoft.
[WSP]
The WebSocket protocol, I. Fette. IETF.
[X121]
Recommendation X.121 — International Numbering Plan for Public Data Networks, CCITT Blue Book, Fascicle VIII.3, pp. 317-332.
[X690]
Recommendation X.690 — Information Technology — ASN.1 Encoding Rules — Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER), and Distinguished Encoding Rules (DER). International Telecommunication Union.
[XFN]
XFN 1.1 profile, T. Çelik, M. Mullenweg, E. Meyer. GMPG.
[XHR]
XMLHttpRequest, A. van Kesteren. W3C.
[XHTML1]
XHTML(TM) 1.0 The Extensible HyperText Markup Language (Second Edition). W3C.
[XHTMLMOD]
Modularization of XHTML(TM), M. Altheim, F. Boumphrey, S. Dooley, S. McCarron, S. Schnitzenbaumer, T. Wugofski. W3C.
[XML]
Extensible Markup Language, T. Bray, J. Paoli, C. Sperberg-McQueen, E. Maler, F. Yergeau. W3C.
[XMLBASE]
XML Base, J. Marsh, R. Tobin. W3C.
[XMLNS]
Namespaces in XML, T. Bray, D. Hollander, A. Layman, R. Tobin. W3C.
[XPATH10]
XML Path Language (XPath) Version 1.0, J. Clark, S. DeRose. W3C.
[XSLT10]
(Non-normative) XSL Transformations (XSLT) Version 1.0, J. Clark. W3C.
[RFC2109]
HTTP State Management Mechanism, D. Kristol, L. Montulli. IETF.
[RFC2616]
Hypertext Transfer Protocol — HTTP/1.1, R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. IETF.
[RFC2965]
HTTP State Management Mechanism, D. Kristol, L. Montulli. IETF.
[RFC5234]
Augmented BNF for Syntax Specifications: ABNF, D. Crocker, P. Overell. IETF.