Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
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.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
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The bulk of the text of this specification is also available in the WHATWG Web Applications 1.0 specification, under a license that permits reuse of the specification text.
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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 17 June 2013. 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:
svn checkout http://svn.whatwg.org/webapps/
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.
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DrawingStyle
objectsPath
objectsThis 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;
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.
Each context maintains a stack of drawing states. Drawing states consist of:
strokeStyle
, fillStyle
, globalAlpha
, lineWidth
, lineCap
, lineJoin
, miterLimit
, shadowOffsetX
, shadowOffsetY
, shadowBlur
, shadowColor
, globalCompositeOperation
, font
, textAlign
, textBaseline
.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.
save
()Pushes the current state onto the stack.
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.
DrawingStyle
objectsAll 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.
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.
lineWidth
[ = value ]lineWidth
[ = value ]Returns the current line width.
Can be set, to change the line width. Values that are not finite values greater than zero are ignored.
lineCap
[ = value ]lineCap
[ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
The possible line cap styles are butt
,
round
, and square
. Other values are
ignored.
lineJoin
[ = value ]lineJoin
[ = value ]Returns the current line join style.
Can be set, to change the line join style.
The possible line join styles are bevel
,
round
, and miter
. Other values are
ignored.
miterLimit
[ = value ]miterLimit
[ = value ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
setLineDash
(segments)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.
getLineDash
()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).
lineDashOffset
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:
Let a be a copy of the array provided as the argument.
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).
If the number of elements in a is odd, then let a be the concatentation of two copies of a.
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.
Let path be a copy of the path being traced.
Remove from path any subpaths containing no lines (i.e. empty subpaths with zero points, and subpaths with just one point).
Replace each point in each subpath of path other than the first point and the last point of each subpath by a join that joins the line leading to that point to the line leading out of that point, such that the subpaths all consist of two points (a starting point with a line leading out of it, and an ending point with a line leading into it), one or more lines (connecting the points and the joins), and zero or more joins (each connecting one line to another), connected together such that each subpath is a series of one or more lines with a join between each one and a point on each end.
Add a straight closing line to each closed subpath in path connecting the last point and the first point of that subpath; change the last point to a join (from the previously last line to the newly added closing line), and change the first point to a join (from the newly added closing line to the first line).
If the styles dash list is empty, jump to the step labeled joins.
Let width be the aggregate length of all the lines of all the subpaths in path, in coordinate space units.
Let offset be the value of the styles lineDashOffset
, in
coordinate space units.
While offset is greater than width, decrement it by width.
While offset is less than width, increment it by width.
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.)
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.
Let position be 0.
Let index be 0.
Let current state be off (the other states being on and zero-on).
Dash On: Let segment length be the value of the styles dash list's indexth entry.
Increment position by segment length.
If position is greater than width, then jump to the step labeled joins.
If segment length is non-zero, let current state be on.
Increment index by one.
Dash Off: Let segment length be the value of the styles dash list's indexth entry.
Let start be the offset position on L.
Increment position by segment length.
If position is greater than width, then let end be the offset width on L. Otherwise, let end be the offset position on L.
Jump to the first appropriate step:
Do nothing, just continue to the next step.
Cut the line on which end finds itself short at end and place a point there, cutting 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).
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.
If position is greater than width, then jump to the step labeled joins.
If segment length is greater than zero, let positioned-at-on-dash be false.
Increment index by one. If it is equal to the number of entries in the styles dash list, then let index be 0.
Return to the step labeled dash on.
Joins: Remove from path any subpath that originally formed part of the subpath added in the offset subpath step above.
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.
Return the newly created path.
font
[ = value ]font
[ = value ]Returns the current font settings.
Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.
Relative keywords and lengths are computed relative to the font
of the canvas
element.
textAlign
[ = value ]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
.
textBaseline
[ = value ]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 keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
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.)
If maxWidth was provided but is less than or equal to zero, return an empty array.
Replace all the space characters in text with U+0020 SPACE characters.
Let font be the current font of target, as given by that object's font
attribute.
Apply the appropriate step from the following list to determine the value of direction:
Document
and that
Document
has a root element childDocument
and that
Document
has no root element childForm a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]
If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
The anchor point is a point on the inline
box, and the physical alignment is one of the
values left, right, and center. These
variables are determined by the textAlign
and textBaseline
values as
follows:
Horizontal position:
textAlign
is
left
textAlign
is
start
and direction is
'ltr'textAlign
is
end
and direction is
'rtl'textAlign
is
right
textAlign
is
end
and direction is
'ltr'textAlign
is
start
and direction is
'rtl'textAlign
is
center
Vertical position:
textBaseline
is top
textBaseline
is hanging
textBaseline
is middle
textBaseline
is alphabetic
textBaseline
is ideographic
textBaseline
is bottom
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.
Return result, and, for callers that need it, physical alignment as the alignment value.
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.
moveTo
(x, y)moveTo
(x, y)Creates a new subpath with the given point.
closePath
()closePath
()Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
lineTo
(x, y)lineTo
(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
quadraticCurveTo
(cpx, cpy, x, y)quadraticCurveTo
(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
arcTo
(x1, y1, x2, y2, radius)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.
arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )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.
rect
(x, y, w, h)rect
(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
ellipse
(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)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 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 2π, or, if the anticlockwise argument is true and startAngle-endAngle is equal to or greater than 2π, 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 2π radians.
Negative values for radiusX or radiusY must cause the implementation to throw an
IndexSizeError
exception.
The rect(x, y, w, h)
method must create a new subpath
containing just the four points (x, y), (x+w,
y), (x+w, y+h),
(x, y+h), with those four points connected by straight
lines, and must then mark the subpath as closed. It must then create
a new subpath with the point (x, y) as the only point in the subpath.
Path
objectsPath
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
()Creates a new empty Path
object.
addPath
(path, transform)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.
addText
(text, styles, transform, x, y [, maxWidth ])addText
(text, styles, transform, path [, maxWidth ])addPathByStrokingText
(text, styles, transform, x, y [, maxWidth ])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:
If the Path
object b has
no subpaths, abort these steps.
Create a copy of all the subpaths in b. Let this copy be known as c.
Transform all the coordinates and lines in c by the transform matrix transform, if it is not null.
Let (x, y) be the last point in the last subpath of c.
Add all the subpaths in c to a.
Create a new subpath in a with (x, y) as the only point in the subpath.
The addPathByStrokingPath(b, styles, transform)
method, when invoked on a
Path
object a, must run the
following steps:
If the Path
object b has
no subpaths, abort these steps.
Create a copy of all the subpaths in b. Let this copy be known as c.
Transform all the coordinates and lines in c by transformation matrix transform, if it is not null.
Let a new list of subpaths d be the result of tracing c, using the styles argument for the line styles.
Let (x, y) be the last point in the last subpath of d.
Add all the subpaths in d to a.
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:
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.
Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.
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.
If the method is addPathByStrokingText()
,
replace glyph subpaths by the result of tracing glyph
subpaths, using the CanvasDrawingStyles
object
argument for the line styles.
Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.
Let (xfinal, yfinal) be the last point in the last subpath of glyph subpaths.
Add all the subpaths in glyph
subpaths to the Path
object.
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:
Let target be the Path
object on which the method was invoked.
Let path be the Path
object
that was provided in the method's arguments.
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.
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.
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.
Let offset be determined according to the appropriate step below:
Move all the shapes in glyphs to the right by offset CSS pixels.
For each glyph glyph in the glyphs array, run these substeps:
Let dx be the x-coordinate of the horizontal center of the bounding box of the shape described by glyph, in CSS pixels.
If dx is negative or greater than width, skip the remainder of these substeps for this glyph.
Recast dx to coordinate spaces units in path. (This just changes the dimensionality of dx, not its numeric value.)
Find the point p on path (or implied closing lines in path) that corresponds to the position dx on the coordinate line L.
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.
Rotate the shape described by glyph clockwise by θ about the point that is at the dx coordinate horizontally and the zero coordinate vertically.
Let (x, y) be the coordinate of the point p.
Move the shape described by glyph to the right by x and down by y.
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.
If the method is addPathByStrokingText()
,
replace glyph subpaths by the result of tracing glyph
subpaths, using the CanvasDrawingStyles
object
argument for the line styles.
Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.
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.)
Add all the subpaths in glyph subpaths to target.
Create a new subpath in the Path
object with
(xfinal, yfinal) as the only point in the
subpath.
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.
scale
(x, y)Changes the transformation matrix to apply a scaling transformation with the given characteristics.
rotate
(angle)Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.
translate
(x, y)Changes the transformation matrix to apply a translation transformation with the given characteristics.
transform
(a, b, c, d, e, f)Changes the transformation matrix to apply the matrix given by the arguments as described below.
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.
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.
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,
CanvasGradient
s, or CanvasPattern
s. 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.
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.
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.
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:
If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing. Abort these steps.
Let x(ω) = (x1-x0)ω + x0
Let y(ω) = (y1-y0)ω + y0
Let r(ω) = (r1-r0)ω + r0
Let the color at ω be the color at that position on the gradient (with the colors coming from the interpolation and extrapolation described above).
For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the color at ω, but only painting on the parts of the 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.
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.
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.
clearRect
(x, y, w, h)Clears all pixels on the canvas in the given rectangle to transparent black.
fillRect
(x, y, w, h)Paints the given rectangle onto the canvas, using the current fill style.
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:
Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.
Clear the pixels in pixels to a fully transparent black, erasing any previous image.
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.
fillText
(text, x, y [, maxWidth ] )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.
measureText
(text)Returns a TextMetrics
object with the metrics of the given text in the current font.
width
actualBoundingBoxLeft
actualBoundingBoxRight
fontBoundingBoxAscent
fontBoundingBoxDescent
actualBoundingBoxAscent
actualBoundingBoxDescent
emHeightAscent
emHeightDescent
hangingBaseline
alphabeticBaseline
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:
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.
Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.
Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.
For fillText()
,
fillStyle
must be
applied to the shapes and strokeStyle
must be
ignored. For strokeText()
, the reverse
holds: strokeStyle
must be applied to the result of tracing the shapes using the
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 run the text preparation algorithm, pass
a new TextMetrics object with its attributes set as described in the following list.
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]
width
attributeThe width of that inline box, in CSS pixels. (The text's advance width.)
actualBoundingBoxLeft
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the left side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going left from the given alignment point.
The sum of this value and the next (actualBoundingBoxRight
) can be wider than the width of the inline box (width
), in particular with slanted fonts where characters overhang their advance width.
actualBoundingBoxRight
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the right side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going right from the given alignment point.
fontBoundingBoxAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the 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
attributeThe 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
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the top of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going up from the given baseline.
This number can vary greatly based on the input text, even if the first font specified covers all the characters in the input. For example, the actualBoundingBoxAscent
of a lowercase "o" from an alphabetic baseline would be less than that of an uppercase "F". The value can easily be negative; for example, the distance from the top of the em box (textBaseline
value "top
") to the top of the bounding rectangle when the given text is just a single comma ",
" would likely (unless the font is quite unusual) be negative.
actualBoundingBoxDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the bottom of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going down from the given baseline.
emHeightAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the 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
attributeThe 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
attributeThe 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
attributeThe 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
attributeThe 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.)
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.
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.
beginPath
()Resets the current default path.
fill
()fill
(path)Fills the subpaths of the current default path or the given path with the current fill style.
stroke
()stroke
(path)Strokes the subpaths of the current default path or the given path with the current stroke style.
drawSystemFocusRing
(element)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.
drawCustomFocusRing
(element)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).
scrollPathIntoView
()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.
clip
()clip
(path)Further constrains the clipping region to the current default path.
isPointInPath
(x, y)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:
If element is not focused or is not a descendant of the element with whose context the method is associated, then abort these steps.
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.)
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:
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.
Let result be true.
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.)
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.
Return result.
The scrollPathIntoView()
method, when invoked, must run the following steps:
Let the specified rectangle be the rectangle of the bounding box of the intended path.
Let notional child be a hypothetical
element that is a rendered child of the canvas
element
whose dimensions are those of the specified
rectangle.
Scroll notional child into view with the align to top flag set.
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>
To draw images onto the canvas, the drawImage
method
can be used.
This method can be invoked with three different sets of arguments:
drawImage(image, dx, dy)
drawImage(image, dx, dy, dw, dh)
drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
Each of those three can take either an
HTMLImageElement
, an HTMLCanvasElement
, or
an HTMLVideoElement
for the image
argument.
drawImage
(image, dx, dy)drawImage
(image, dx, dy, dw, dh)drawImage
(image, sx, sy, sw, sh, dx, dy, dw, dh)Draws the given image onto the canvas. The arguments are interpreted as follows:
If the 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.
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:
A set of
pixels on the canvas
element's bitmap for which
this region is responsible.
A bounding
circumference on the canvas
element's bitmap
that surrounds the hit region's set of pixels as they
stood when it was created.
Optionally, a non-empty string representing an ID for distinguishing the region from others.
Optionally, a reference to another region that acts as the parent for this one.
A count of regions that have this one as their parent, known as the hit region's child count.
A cursor
specification, in the form of either a CSS cursor value, or
the string "inherit
" meaning that the cursor
of the hit region's parent, if any, or of the
canvas
element, if not, is to be used
instead.
Optionally, either a control, or an unbacked region description.
A control is just a
reference to an Element
node, to which, in certain
conditions, the user agent will route events, and from which the
user agent will determine the state of the hit region for the
purposes of accessibility tools.
An unbacked region description consists of the following:
Optionally, a label.
An ARIA role, which, if the unbacked region description also has a label, could be the empty string.
addHitRegion
(options)Adds a hit region to the canvas bitmap. The argument is an object with the following members:
path
(default null)
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)
MouseEvent
events on the canvas
(event.region
) and as a way
to reference this region in later calls to addHitRegion()
.parentID
(default null)
cursor
(default "inherit
")
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)
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)
role
(default null)
Hit regions can be used for a variety of purposes:
button
element.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):
If ID is null, return nothing and abort these steps.
Let list be the hit region list associated with ancestor's bitmap.
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.
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):
If control is not a descendant of ancestor, then return nothing and abort these steps.
Let list be the hit region list associated with ancestor's bitmap.
If there is a hit region in list whose control is control, then return that hit region and abort these steps.
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):
If region has no control, return nothing and abort these steps.
Let control be region's control.
If control is not a descendant of ancestor, then return nothing and abort these steps.
Otherwise, return control.
The cursor for a hit region region of a canvas
element ancestor is the value returned by the following
algorithm:
Loop: If region has a cursor
specification other than "inherit
",
then return that hit region's cursor specification
and abort these steps.
If region has a parent, then let region be that hit region's parent, and return to the step labeled loop.
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):
Let list be the hit region list associated with ancestor's bitmap.
If there is a hit region in list whose set of pixels contains pixel, then return that hit region and abort these steps.
Otherwise, return nothing.
To clear regions that cover the pixels pixels on a canvas
element ancestor, the user agent must run the following
steps:
Let list be the hit region list associated with ancestor's bitmap.
Remove all pixels in pixels from the set of pixels of each hit region in list.
Garbage-collect the regions of ancestor.
To garbage-collect the regions of a
canvas
element ancestor, the user
agent must run the following steps:
Let list be the hit region list associated with ancestor's bitmap.
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.
If victim has a parent, then decrement that hit region's child count by one.
Remove victim from list.
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:
Let arguments be the dictionary object provided as the method's argument.
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.
Transform all the coordinates and lines in source path by the current transform matrix, if the
arguments object's path
member is not
null.
If the arguments object's id
member is the
empty string, let it be null instead.
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.
If the arguments object's parent
member is the
empty string, let it be null instead.
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.
If the arguments object's label
member is the
empty string, let it be null instead.
If any of the following conditions are met, throw a
NotSupportedError
exception and abort these
steps.
control
and label
members are both
non-null.control
and role
members are both
non-null.role
member's value is
the empty string, and the label
member's value is
either null or the empty string.control
member is not
null and is not an element that is a descendant of this
canvas
element.If the parent
member is not
null but parent region is null, then throw a
NotFoundError
exception and abort these
steps.
If any of the following conditions are met, throw a
SyntaxError
exception and abort these
steps.
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]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]Let region be a newly created hit region, with its information configured as follows:
The pixels contained in source path.
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.)
If the arguments object's id
member is not null: the
value of the id
member. Otherwise, region has no id.
If parent region is not null: parent region. Otherwise, region has no parent.
Initially zero.
If parent region is not null: parent region. Otherwise, region has no parent.
If the arguments object's control
member is not
null: the value of the control
member.
Otherwise, region has no control.
If the arguments object's label
member is not
null: the value of the label
member.
Otherwise, region has no label.
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.
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.
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.
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.
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.
If there is a parent region, increment its hit region's child count by one.
Clear regions that cover the pixels in region's set of pixels on this canvas
element.
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:
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.
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.
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; };
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.
If the pointing device is not indicating a pixel on the
canvas
, act as normal and abort these steps.
Let pixel be the pixel indicated by the pointing device.
Let region be the hit
region that is the
region for the pixel pixel on this
canvas
element, if any.
If there is no region, then act as normal and abort these steps.
Let id be the region's ID, if any.
If there is an id, then initialize the
event object's region
attribute to id.
Let control be the region's control, if any.
If there is a control, then target the
event object at control instead of the
canvas
element.
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.
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.
createImageData
(imagedata)Returns an ImageData
object with the same
dimensions as the argument. All the pixels in the returned object
are transparent black.
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).
width
height
Returns the actual dimensions of the data in the ImageData
object, in device pixels.
data
Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.
putImageData
(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ])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:
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.
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.
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.
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.
If, after those changes, either dirtyWidth or dirtyHeight is negative or zero, stop these steps without affecting the canvas.
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>
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.
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
source-in
source-out
source-over
(default)destination-atop
source-atop
but using the
destination image instead of the source image and vice versa.destination-in
source-in
but using the destination
image instead of the source image and vice versa.destination-out
source-out
but using the destination
image instead of the source image and vice versa.destination-over
source-over
but using the
destination image instead of the source image and vice versa.lighter
copy
xor
vendorName-operationName
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
.
All drawing operations are affected by the four global shadow attributes.
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.
shadowOffsetX
[ = value ]shadowOffsetY
[ = value ]Returns the current shadow offset.
Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
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:
Let A be an infinite transparent black bitmap on which the source image for which a shadow is being created has been rendered.
Let B be an infinite transparent black bitmap, with a coordinate space and an origin identical to A.
Copy the alpha channel of A to B, offset by shadowOffsetX
in the
positive x direction, and shadowOffsetY
in the
positive y direction.
If shadowBlur
is greater than
0:
Let σ be half the value of
shadowBlur
.
Perform a 2D Gaussian Blur on B, using σ as the standard deviation.
User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.
Set the red, green, and blue components of every pixel in
B to the red, green, and blue components
(respectively) of the color of shadowColor
.
Multiply the alpha component of every pixel in B by the alpha component of the color of shadowColor
.
The shadow is in the bitmap B, and is rendered as part of the drawing model described below.
If the current composition operation is copy
, shadows effectively won't render
(since the shape will overwrite the shadow).
When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):
Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honored, and the stroke must itself also be subjected to the current transformation matrix.
When shadows are drawn, render the shadow from image A, using the current shadow styles, creating image B.
When shadows are drawn, multiply the alpha
component of every pixel in B by globalAlpha
.
When shadows are drawn, composite B within the clipping region over the current canvas bitmap using the current composition operator.
Multiply the alpha component of every pixel in A by globalAlpha
.
Composite A within the clipping region over the current canvas bitmap using the current composition operator.
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.
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>
All references are normative unless marked "Non-normative".
XMLHttpRequest
,
A. van Kesteren. W3C.