Copyright © 2010 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.
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This specification is an HTML specification. All the conformance requirements, conformance classes, definitions, dependencies, terminology, and typographical conventions described in the core HTML5 specification apply to this specification. [HTML5]
When the getContext()
method of a canvas
element is invoked with 2d
as the argument, a
CanvasRenderingContext2D
object must be returned.
There is only one CanvasRenderingContext2D
object
per canvas, so calling the getContext()
method with the
2d
argument a second time
must return the same object.
The 2D context represents a flat Cartesian surface whose origin (0,0) is at the top left corner, with the coordinate space having x values increasing when going right, and y values increasing when going down.
interface CanvasRenderingContext2D { // back-reference to the canvas readonly attribute HTMLCanvasElement canvas; // state void save(); // push state on state stack void restore(); // pop state stack and restore state // transformations (default transform is the identity matrix) void scale(in float x, in float y); void rotate(in float angle); void translate(in float x, in float y); void transform(in float m11, in float m12, in float m21, in float m22, in float dx, in float dy); void setTransform(in float m11, in float m12, in float m21, in float m22, in float dx, in float dy); // compositing attribute float globalAlpha; // (default 1.0) attribute DOMString globalCompositeOperation; // (default source-over) // colors and styles attribute any strokeStyle; // (default black) attribute any fillStyle; // (default black) CanvasGradient createLinearGradient(in float x0, in float y0, in float x1, in float y1); CanvasGradient createRadialGradient(in float x0, in float y0, in float r0, in float x1, in float y1, in float r1); CanvasPattern createPattern(in HTMLImageElement image, in DOMString repetition); CanvasPattern createPattern(in HTMLCanvasElement image, in DOMString repetition); CanvasPattern createPattern(in HTMLVideoElement image, in DOMString repetition); // line caps/joins attribute float lineWidth; // (default 1) attribute DOMString lineCap; // "butt", "round", "square" (default "butt") attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter") attribute float miterLimit; // (default 10) // shadows attribute float shadowOffsetX; // (default 0) attribute float shadowOffsetY; // (default 0) attribute float shadowBlur; // (default 0) attribute DOMString shadowColor; // (default transparent black) // rects void clearRect(in float x, in float y, in float w, in float h); void fillRect(in float x, in float y, in float w, in float h); void strokeRect(in float x, in float y, in float w, in float h); // path API void beginPath(); void closePath(); void moveTo(in float x, in float y); void lineTo(in float x, in float y); void quadraticCurveTo(in float cpx, in float cpy, in float x, in float y); void bezierCurveTo(in float cp1x, in float cp1y, in float cp2x, in float cp2y, in float x, in float y); void arcTo(in float x1, in float y1, in float x2, in float y2, in float radius); void rect(in float x, in float y, in float w, in float h); void arc(in float x, in float y, in float radius, in float startAngle, in float endAngle, in boolean anticlockwise); void fill(); void stroke(); void clip(); boolean isPointInPath(in float x, in float y); <ZZ> // accessible focus and caret selection management boolean drawFocusRing(in Element element, in float xPos, in float yPos, in float w, in float h, in optional boolean drawCustomRing(); int caretBlinkRate(); boolean setCaretSelectionRect(in Element element, in float xPos, in float yPos, in float w, in float h); </ZZ> // text attribute DOMString font; // (default 10px sans-serif) attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start") attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic") void fillText(in DOMString text, in float x, in float y, in optional float maxWidth); void strokeText(in DOMString text, in float x, in float y, in optional float maxWidth); TextMetrics measureText(in DOMString text); // drawing images void drawImage(in HTMLImageElement image, in float dx, in float dy, in optional float dw, in float dh); void drawImage(in HTMLImageElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh); void drawImage(in HTMLCanvasElement image, in float dx, in float dy, in optional float dw, in float dh); void drawImage(in HTMLCanvasElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh); void drawImage(in HTMLVideoElement image, in float dx, in float dy, in optional float dw, in float dh); void drawImage(in HTMLVideoElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh); // pixel manipulation ImageData createImageData(in float sw, in float sh); ImageData createImageData(in ImageData imagedata); ImageData getImageData(in float sx, in float sy, in float sw, in float sh); void putImageData(in ImageData imagedata, in float dx, in float dy, in optional float dirtyX, in float dirtyY, in float dirtyWidth, in float dirtyHeight); }; interface CanvasGradient { // opaque object void addColorStop(in float offset, in DOMString color); }; interface CanvasPattern { // opaque object }; interface TextMetrics { readonly attribute float width; }; interface ImageData { readonly attribute unsigned long width; readonly attribute unsigned long height; readonly attribute CanvasPixelArray data; }; interface CanvasPixelArray { readonly attribute unsigned long length; getter octet (in unsigned long index); setter void (in unsigned long index, in octet value); };
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]
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 path and the current bitmap are not part
of the drawing state. The current path is persistent, and can only
be reset using the beginPath()
method. The
current bitmap is a property of the canvas, not the context.
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.
The transformation matrix is applied to coordinates when creating shapes and paths.
When the context is created, the transformation matrix must initially be the identity transform. It may then be adjusted using the transformation methods.
The transformations must be performed in reverse order. For instance, if a scale transformation that doubles the width is applied, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.
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
(m11, m12, m21, m22, dx, dy)Changes the transformation matrix to apply the matrix given by the arguments as described below.
setTransform
(m11, m12, m21, m22, dx, dy)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(m11, m12, m21, m22, dx,
dy)
method must multiply the
current transformation matrix with the matrix described by:
m11 | m21 | dx |
m12 | m22 | dy |
0 | 0 | 1 |
The setTransform(m11, m12, m21, m22, dx,
dy)
method must reset the current
transform to the identity matrix, and then invoke the transform(m11, m12, m21, m22, dx,
dy)
method with the same arguments.
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
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.
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). [PORTERDUFF]
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
.
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.
fillStyle
[ = value ]Returns the current style used for filling shapes.
Can be set, to change the fill style.
The style can be either a string containing a CSS color, or a
CanvasGradient
or CanvasPattern
object. Invalid values are ignored.
The strokeStyle
attribute represents the color or style to use for the lines around
shapes, and the fillStyle
attribute represents the color or style to use inside the
shapes.
Both attributes can be either strings,
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
is not a valid color, 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, a U+002E FULL STOP (representing the decimal
point), one or more digits in the range 0-9 (U+0030 to U+0039)
representing the fractional part of the alpha value, and finally a
U+0029 RIGHT PARENTHESIS.
When the context is created, the strokeStyle
and fillStyle
attributes must
initially have the string value #000000
.
There are two types of gradients, linear gradients and radial
gradients, both represented by objects implementing the opaque
CanvasGradient
interface.
Once a gradient has been created (see below), stops are placed along it to define how the colors are distributed along the gradient. The color of the gradient at each stop is the color specified for that stop. Between each such stop, the colors and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the color to use at that offset. Before the first stop, the color must be the color of the first stop. After the last stop, the color must be the color of the last stop. When there are no stops, the gradient is transparent black.
addColorStop
(offset, color)Adds a color stop with the given color to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.
Throws an INDEX_SIZE_ERR
exception if the offset
it out of range. Throws a SYNTAX_ERR
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.
If any of the arguments are not finite numbers, throws a
NOT_SUPPORTED_ERR
exception.
createRadialGradient
(x0, y0, r0, x1, y1, r1)Returns a CanvasGradient
object that represents a
radial gradient that paints along the cone given by the circles
represented by the arguments.
If any of the arguments are not finite numbers, throws a
NOT_SUPPORTED_ERR
exception. If either of the radii
are negative throws an INDEX_SIZE_ERR
exception.
The addColorStop(offset, color)
method on the CanvasGradient
interface adds a new stop
to a gradient. If the offset is less than 0,
greater than 1, infinite, or NaN, then an
INDEX_SIZE_ERR
exception must be raised. If the color cannot be parsed as a CSS color, then a
SYNTAX_ERR
exception must be raised. Otherwise, the
gradient must have a new stop placed, at offset offset relative to the whole gradient, and with the
color obtained by parsing color as a CSS
<color> value. If multiple stops are added at the same offset
on a gradient, they must be placed in the order added, with the
first one closest to the start of the gradient, and each subsequent
one infinitesimally further along towards the end point (in effect
causing all but the first and last stop added at each point to be
ignored).
The createLinearGradient(x0, y0, x1,
y1)
method takes four arguments
that represent the start point (x0, y0) and end point (x1, y1) of the gradient. If any of the arguments to createLinearGradient()
are infinite or NaN, the method must raise a
NOT_SUPPORTED_ERR
exception. Otherwise, the method must
return a linear CanvasGradient
initialized with the
specified line.
Linear gradients must be rendered such that all points on a line perpendicular to the line that crosses the start and end points have the color at the point where those two lines cross (with the colors coming from the interpolation and extrapolation described above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.
If x0 = x1 and y0 = y1, then the linear gradient must paint nothing.
The createRadialGradient(x0, y0, r0,
x1, y1, r1)
method takes six arguments, the
first three representing the start circle with origin (x0, y0) and radius r0, and the last three representing the end circle
with origin (x1, y1) and
radius r1. The values are in coordinate space
units. If any of the arguments are infinite or NaN, a
NOT_SUPPORTED_ERR
exception must be raised. If either
of r0 or r1 are negative, an
INDEX_SIZE_ERR
exception must be raised. Otherwise,
the method must return a radial CanvasGradient
initialized with the two specified circles.
Radial gradients must be rendered by following these steps:
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).
Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.
The points in the radial gradient must be transformed as described by the current transformation matrix when rendering.
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 repeat
are repeat
(both directions), repeat-x
(horizontal only), repeat-y
(vertical only), and no-repeat
(neither). If the repetition argument is empty or null, the value
repeat
is used.
If the first argument isn't an img
,
canvas
, or video
element, throws a
TYPE_MISMATCH_ERR
exception. If the image has no
image data, throws an INVALID_STATE_ERR
exception. If
the second argument isn't one of the allowed values, throws a
SYNTAX_ERR
exception. If the image isn't yet fully
decoded, then the method returns null.
To create objects of this type, the createPattern(image, repetition)
method is used. The first argument gives the image to use as the
pattern (either an HTMLImageElement
,
HTMLCanvasElement
, or HTMLVideoElement
object). Modifying this image after calling the createPattern()
method
must not affect the pattern. The second argument must be a string
with one of the following values: repeat
,
repeat-x
, repeat-y
,
no-repeat
. If the empty string or null is
specified, repeat
must be assumed. If an
unrecognized value is given, then the user agent must raise a
SYNTAX_ERR
exception. User agents must recognize the
four values described above exactly (e.g. they must not do case
folding). The method must return a CanvasPattern
object
suitably initialized.
The image argument is an instance of either
HTMLImageElement
, HTMLCanvasElement
, or
HTMLVideoElement
. If the image is
of the wrong type or null, the implementation must raise a
TYPE_MISMATCH_ERR
exception.
If the image argument is an
HTMLImageElement
object whose complete
attribute is false, or
if the image argument is an
HTMLVideoElement
object whose readyState
attribute is either
HAVE_NOTHING
or HAVE_METADATA
, then the
implementation must return null.
If the image argument is an
HTMLCanvasElement
object with either a horizontal
dimension or a vertical dimension equal to zero, then the
implementation must raise an INVALID_STATE_ERR
exception.
Patterns must be painted so that the top left of the first image
is anchored at the origin of the coordinate space, and images are
then repeated horizontally to the left and right (if the
repeat-x
string was specified) or vertically up and
down (if the repeat-y
string was specified) or in all
four directions all over the canvas (if the repeat
string was specified). The images are not scaled by this process;
one CSS pixel of the image must be painted on one coordinate space
unit. Of course, patterns must actually be painted only where the
stroking or filling effect requires that they be drawn, and are
affected by the current transformation matrix.
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).
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 ]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 ]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 ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
The lineWidth
attribute gives the width of lines, in coordinate space units. On
getting, it must return the current value. On setting, zero,
negative, infinite, and NaN values must be ignored, leaving the
value unchanged; other values must change the current value to the
new value.
When the context is created, the lineWidth
attribute must
initially have the value 1.0
.
The lineCap
attribute
defines the type of endings that UAs will place on the end of
lines. The three valid values are butt
,
round
, and square
. The butt
value means that the end of each line has a flat edge perpendicular
to the direction of the line (and that no additional line cap is
added). The round
value means that a semi-circle with
the diameter equal to the width of the line must then be added on to
the end of the line. The square
value means that a
rectangle with the length of the line width and the width of half
the line width, placed flat against the edge perpendicular to the
direction of the line, must be added at the end of each line.
On getting, it must return the current value. On setting, if the
new value is one of the literal strings butt
,
round
, and square
, then the current value
must be changed to the new value; other values must ignored, leaving
the value unchanged.
When the context is created, the lineCap
attribute must
initially have the value butt
.
The lineJoin
attribute defines the type of corners that UAs will place where two
lines meet. The three valid values are bevel
,
round
, and miter
.
On getting, it must return the current value. On setting, if the
new value is one of the literal strings bevel
,
round
, and miter
, then the current value
must be changed to the new value; other values must be ignored,
leaving the value unchanged.
When the context is created, the lineJoin
attribute must
initially have the value miter
.
A join exists at any point in a subpath shared by two consecutive lines. When a subpath is closed, then a join also exists at its first point (equivalent to its last point) connecting the first and last lines in the subpath.
In addition to the point where the join occurs, two additional points are relevant to each join, one for each line: the two corners found half the line width away from the join point, one perpendicular to each line, each on the side furthest from the other line.
A filled triangle connecting these two opposite corners with a
straight line, with the third point of the triangle being the join
point, must be rendered at all joins. The lineJoin
attribute controls
whether anything else is rendered. The three aforementioned values
have the following meanings:
The bevel
value means that this is all that is
rendered at joins.
The round
value means that a filled arc connecting
the two aforementioned corners of the join, abutting (and not
overlapping) the aforementioned triangle, with the diameter equal to
the line width and the origin at the point of the join, must be
rendered at joins.
The miter
value means that a second filled triangle
must (if it can given the miter length) be rendered at the join,
with one line being the line between the two aforementioned corners,
abutting the first triangle, and the other two being continuations of
the outside edges of the two joining lines, as long as required to
intersect without going over the miter length.
The miter length is the distance from the point where the join occurs to the intersection of the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of the miter length to half the line width. If the miter length would cause the miter limit ratio to be exceeded, this second triangle must not be rendered.
The miter limit ratio can be explicitly set using the miterLimit
attribute. On getting, it must return the current value. On setting,
zero, negative, infinite, and NaN values must be ignored, leaving
the value unchanged; other values must change the current value to
the new value.
When the context is created, the miterLimit
attribute must
initially have the value 10.0
.
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 is not a valid color, 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 size of the blurring effect. (The units do
not map to coordinate space units, and are not affected by the
current transformation matrix.)
When the context is created, the shadowBlur
attribute must
initially have the value 0
.
On getting, the attribute must return its current value. On setting the attribute must be set to the new value, except if the value is negative, infinite or NaN, in which case the new value must be ignored.
Shadows are only drawn
if the opacity component of the alpha component of the color
of shadowColor
is
non-zero and either the shadowBlur
is non-zero, or
the shadowOffsetX
is non-zero, or the shadowOffsetY
is
non-zero.
When shadows are drawn, they must be rendered as follows:
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:
If shadowBlur
is less than
8, let σ be half the value of shadowBlur
; otherwise,
let σ be the square root of multiplying
the value of shadowBlur
by
2.
Perform a 2D Gaussian Blur on B, using σ as the standard deviation.
User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.
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).
There are three methods that immediately draw rectangles to the bitmap. They each take four arguments; the first two give the x and y coordinates of the top left of the rectangle, and the second two give the width w and height h of the rectangle, respectively.
The current transformation matrix must be applied to the following four coordinates, which form the path that must then be closed to get the specified rectangle: (x, y), (x+w, y), (x+w, y+h), (x, y+h).
Shapes are painted without affecting the current path, and are
subject to the clipping region,
and, with the exception of clearRect()
, also shadow effects, global alpha, and global composition
operators.
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 clear the pixels in the
specified rectangle that also intersect the current clipping region
to a fully transparent black, erasing any previous image. If either
height or width are zero, this method has no effect.
The fillRect(x, y, w, h)
method must paint the specified
rectangular area using the fillStyle
. If either height
or width are zero, this method has no effect.
The strokeRect(x, y, w, h)
method must stroke the specified
rectangle's path using the strokeStyle
, lineWidth
, lineJoin
, and (if
appropriate) miterLimit
attributes. If
both height and width are zero, this method has no effect, since
there is no path to stroke (it's a point). If only one of the two is
zero, then the method will draw a line instead (the path for the
outline is just a straight line along the non-zero dimension).
The context always has a current path. There is only one current path, it is not part of the drawing state.
A path has a list of zero or more subpaths. Each subpath consists of a list of one or more points, connected by straight or curved lines, and a flag indicating whether the subpath is closed or not. A closed subpath is one where the last point of the subpath is connected to the first point of the subpath by a straight line. Subpaths with fewer than two points are ignored when painting the path.
beginPath
()Resets the current path.
moveTo
(x, y)Creates a new subpath with the given point.
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)Adds the given point to the current subpath, connected to the previous one by a straight line.
quadraticCurveTo
(cpx, cpy, x, y)Adds the given point to the current path, connected to the previous one by a quadratic Bézier curve with the given control point.
bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current path, connected to the previous one by a cubic Bézier curve with the given control points.
arcTo
(x1, y1, x2, y2, radius)Adds a point to the current path, connected to the previous one by a straight line, then adds a second point to the current path, connected to the previous one by an arc whose properties are described by the arguments.
Throws an INDEX_SIZE_ERR
exception if the given
radius is negative.
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, is added to the path, connected to the previous point by a straight line.
Throws an INDEX_SIZE_ERR
exception if the given
radius is negative.
rect
(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
fill
()Fills the subpaths with the current fill style.
stroke
()Strokes the subpaths with the current stroke style.
clip
()Further constrains the clipping region to the given path.
isPointInPath
(x, y)Returns true if the given point is in the current path.
Initially, the context's path must have zero subpaths.
The points and lines added to the path by these methods must be transformed according to the current transformation matrix as they are added.
The beginPath()
method must empty the list of subpaths so that the context once
again has zero subpaths.
The moveTo(x, y)
method must
create a new subpath with the specified point as its first (and
only) point.
When the user agent is to ensure there is a subpath
for a coordinate (x, y), the
user agent must check to see if the context has any subpaths, and if
it does not, then the user agent must create a new subpath with the
point (x, y) as its first
(and only) point, as if the moveTo()
method had been
called.
The closePath()
method must do nothing if the context has no subpaths. Otherwise, it
must mark the last subpath as closed, create a new subpath whose
first point is the same as the previous subpath's first point, and
finally add this new subpath to the path.
If the last subpath had more than one point in its
list of points, then this is equivalent to adding a straight line
connecting the last point back to the first point, thus "closing"
the shape, and then repeating the last (possibly implied) moveTo()
call.
New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the context's paths.
The lineTo(x, y)
method must
ensure there is a subpath for (x, y) if the context has
no subpaths. Otherwise, it must connect the last point in the
subpath to the given point (x, y) using a straight line, and must then add the given
point (x, y) to the
subpath.
The quadraticCurveTo(cpx, cpy, x,
y)
method must ensure there
is a subpath for (cpx,
cpy), and then must connect the last
point in the subpath to the given point (x, y) using a quadratic Bézier curve with control
point (cpx, cpy), and must
then add the given point (x, y) to the subpath. [BEZIER]
The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)
method must
ensure there is a subpath for (cp1x, cp1y), and then must
connect the last point in the subpath to the given point (x, y) using a cubic Bézier
curve with control points (cp1x, cp1y) and (cp2x, cp2y). Then, it must add the point (x, y) to the subpath. [BEZIER]
The arcTo(x1, y1, x2,
y2, radius)
method must first ensure there is a subpath for (x1, y1). Then, the behavior depends on the
arguments and the last point in the subpath, as described below.
Negative values for radius must cause the
implementation to raise an INDEX_SIZE_ERR
exception.
Let the point (x0, y0) be the last point in the subpath.
If the point (x0, y0) is equal to the point (x1, y1), or if the point (x1, y1) is equal to the point (x2, y2), or if the radius radius is zero, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
Otherwise, let The Arc be the shortest arc given by circumference of the circle that has radius radius, and that has one point tangent to the half-infinite line that crosses the point (x0, y0) and ends at the point (x1, y1), and that has a different point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point (x2, y2). The points at which this circle touches these two lines are called the start and end tangent points respectively. The method must connect the point (x0, y0) to the start tangent point by a straight line, adding the start tangent point to the subpath, and then must connect the start tangent point to the end tangent point by The Arc, adding the end tangent point to the subpath.
The arc(x, y, radius,
startAngle, endAngle, anticlockwise)
method draws an arc. If
the context has any subpaths, then the method must add a straight
line from the last point in the subpath to the start point of the
arc. In any case, it must draw the arc between the start point of
the arc and the end point of the arc, and add the start and end
points of the arc to the subpath. The arc and its start and end
points are defined as follows:
Consider a circle that has its origin at (x, y) and that has radius radius. The points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the positive x-axis, are the start and end points respectively.
If the anticlockwise argument is 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 circle.
Otherwise, the arc is the path along the circumference of this circle from the start point to the end point, going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise. Since the points are on the circle, as opposed to being simply angles from zero, the arc can never cover an angle greater than 2π radians. If the two points are the same, or if the radius is zero, then the arc is defined as being of zero length in both directions.
Negative values for radius must cause the
implementation to raise an INDEX_SIZE_ERR
exception.
The rect(x, y, w, h)
method must create a new subpath
containing just the four points (x, y), (x+w,
y), (x+w, y+h),
(x, y+h), with those four points connected by straight
lines, and must then mark the subpath as closed. It must then create
a new subpath with the point (x, y) as the only point in the subpath.
The fill()
method must fill all the subpaths of the current path, using
fillStyle
, and using
the non-zero winding number rule. Open subpaths must be implicitly
closed when being filled (without affecting the actual
subpaths).
Thus, if two overlapping but otherwise independent subpaths have opposite windings, they cancel out and result in no fill. If they have the same winding, that area just gets painted once.
The stroke()
method
must calculate the strokes of all the subpaths of the current path,
using the lineWidth
,
lineCap
, lineJoin
, and (if
appropriate) miterLimit
attributes, and
then fill the combined stroke area using the strokeStyle
attribute.
Since the subpaths are all stroked as one, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.
Paths, when filled or stroked, must be painted without affecting the current path, and must be subject to shadow effects, global alpha, the clipping region, and global composition operators. (Transformations affect the path when the path is created, not when it is painted, though the stroke style is still affected by the transformation during painting.)
Zero-length line segments must be pruned before stroking a path. Empty subpaths must be ignored.
The clip()
method must create a new clipping region by calculating
the intersection of the current clipping region and the area
described by the current path, using the non-zero winding number
rule. Open subpaths must be implicitly closed when computing the
clipping region, without affecting the actual subpaths. The new
clipping region replaces the current clipping region.
When the context is initialized, the clipping region must be set to the rectangle with the top left corner at (0,0) and the width and height of the coordinate space.
The isPointInPath(x, y)
method must
return true if the point given by the x and y coordinates passed to the method, when treated as
coordinates in the canvas coordinate space unaffected by the current
transformation, is inside the current path as determined by the
non-zero winding number rule; and must return false
otherwise. Points on the path itself are considered to be inside the
path. If either of the arguments is infinite or NaN, then the method
must return false.
When a canvas is interactive, authors should include focusable elements in the element's fallback content corresponding to each focusable part of the canvas.
To indicate which focusable part of the canvas is currently
focused, authors should use the drawFocusRing()
method,
passing it the element for which a ring is being drawn. This method
only draws the focus ring if the element is focused or is a descendant of the element with focus, so that it can
simply be called whenever drawing the element, without checking
whether the element is focused or not first. The top-left position of the
the control must be given in the x and y arguments.
drawFocusRing
(element, x, y, w, h, [ drawCustomRing ])If the given element is focused, draws a focus ring around the current drawing path, following the platform conventions for focus rings. This draws users attention and allows a magnifier to magnify around the associated element based on the accessibility information provided, such as its role.
The given coordinates (x, y) are relative to the upper left corner of the canvas element. These coordinates are combined with supplied width and height (w, h) to form the bounding rectangle of the focus ring for that element. Authors who choose to draw a custom focus ring and use other shapes to form the ring (circle, ellipse, polygon, etc.) may do so as long as they provide a rectangle (x, y, w, h) that best fits the focus ring shape. These coordinates are converted to screen coordinates within the user agent. The user agent then combines the converted coordinates with (w, h) for the focused element to compute the bounding rectangle of the associated element in the accessibility API for that element. When focus moves to the associated element a screen magnifier can then use the bounding rectangle, along with other associated accessibility properties to align the magnifier with the visible focus.
If the drawCustomRing argument is true, the author wishes to draw a custom focus ring around the element. Here, the user agent will only draw the focus ring if the user has specifally configured the operating sytem to draw focus rings in a particular manner, in which case the author would not be approved to draw a custom focus ring. (For example, high contrast focus rings.)
Returns true if:
Otherwise, returns false.
When the method returns true, the author is expected to manually draw a focus ring.
The drawFocusRing(element, x, y, [drawCustomRing])
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.
Transform the given point (x, y) according to the current transformation matrix.
Combine the (w, h) with the converted point to a rectangle on which to draw a focus ring.
If the user has configured the operating system to draw focus rings in a certain way (e.g. high-contrast focus rings), or if the drawFocusRing argument is absent or false, then the user agent MUST draw a focus ring of the appropriate style along the path defined by the by the coverted bounding rectangle.
If platform conventions take into consideration the role, or widget type, of an element the user agent must draw the focus ring based on the final role computed from the native host language semantics and the supplied role attribute.
If the user agent draws the focus ring, the drawing should not be subject to the shadow effects, the global alpha, or the global composition operators, but should be subject to the clipping region.
If your user agent supports a platform accessibility API, the user agent MUST use the (transformed) rectangle in screen coordinates for the associated focused element and store it as the bounding rectangle in the accessibility API implementation for that element.
Return true.
When an interactive canvas implementation is used to render editable fallback content that contains a caret or selectable text, it is essential that all users be able to follow their interaction. For some users, such as those requiring screen magnification, it is essential to report to the assistive technology the last rendered screen position of either the end of the last visible selected character, or the caret position when the associated text has focus or a descendant of an element with focus. When this last rendered screen position is the selection position, the screen position must be equivalten to the the pixel location of the outer top left or right corner of the last character selected in the direction (left or right) of the selection.
To indicate the pixel location of the last rendered of either caret or selection position on the focused canvas, authors must use the setCaretSelectionRect()
method,
passing it the element associated with the last drawn of either the caret position or the selected text. Some systems, follow a convention whereby caret position does not track the selected text, in which case the last drawn of either must be tracked. To do this the top left pixel coordinate of the caret or selection position must be given in the x and y arguments. Additionally, the w and h are supplied to indicate the width of the caret and height of the caret or selection point. Authors should compute the height based on the font height of the associated character, in pixels, and the width of the caret or nearest associated selected character in pixels.
setCaretSelectionRect
(element, x, y, w, h)If the given element is focused or a descendent of the focused element, sets the last drawn of either the caret or selection position.
The given coordinates (x, y) are relative to the outer edge of the caret or selection position where the edge represents the left or right edge in the direction of the user's selection. For user agents that support accessibility API, these coordinates are converted to screen coordinates within the user agent and used to notify the assistive technology of the change in either the current caret or selection position. An assistive technology, such as a screen magnifier, may then track the text or selection position as the user interacts with the canvas
The w and h are supplied to indicate the respective width of the caret and height of the caret or selection point. Authors should compute the height based on the font height of the associated character, in pixels, and the width of the caret or nearest associated selected character.
Returns true if the given element is focused or a document descendant of an element with focus.
Otherwise, returns false.
The setCaretSelectionRect(element, x, y, w, h)
method, when invoked, must run the following steps:
If element is not focused or is not a document descendant of the element with whose context the method is associated, then return false and abort these steps.
Authors must ensure that the given point (x, y) represents the pixel location of the top-left or top-right corner of the last drawn of either the:
the outer edge of the elements last visible selected character in the direction (left or right) of the selection, or
the caret position within the element.
Transform the given point (x, y) according to the current transformationmatrix.
When element is a descendent of an element with focus there must be only one descendent of the element with focus that has either a caret or selection position and the last call to this function must be for the last caret or selection drawn.
Authors must ensure the w and h are supplied to indicate the respective width of the caret and height of the caret or selection point. Authors should compute the height based on the font height of the associated character, in pixels, and the width of the caret or nearest associated selected character in pixels.
If your user agent supports a platform accessibility API the user agent MUST use the (transformed) coordinate on the canvas and compute a caret or selection position (or rectangle) in screen coordinates and provide it to an assistive technology through the supported accessibility API implementation for that element.
If the user resizes or moves the user agent window the user agent report must reflect the revised (x, y, w, h) position (or rectangle) in the accessibility API mapping.
Return true.
When drawing a blinking caret the author must adhere to the blink rates in systems that support this feature. High blink rates may cause epileptic seizures in some users. User agents must provide the system caret blink rate to content authors. Default system caret blink rate settings are roughly once every 500 milliseconds.
To access the system caret blink rate in canvas use the caretBlinkRate()
method
caretBlinkRate
()Returns the blink rate of the system in milliseconds if supported.
Otherwise, returns -1.
The caretBlinkRate()
method, when invoked, must run the following steps:
If the operating system supports a caret blink rate setting the user agent must return an integer value in milliseconds.
If the operating system does not support a caret blink rate setting the user agent must return an integer value of minus one.
When the caret blink rate value returned:
is greater than zero the author, when drawing a blinking caret, must reflect the blink rate returned by this method.
is less than zero the author, when drawing a blinking caret, must determine the blink rate.
is zero the author SHOULD visibly render the caret.
This canvas
element has a couple of checkboxes:
<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) { context.save(); context.font = '10px sans-serif'; context.textAlign = 'left'; context.textBaseline = 'middle'; var metrics = context.measureText(element.labels[0].textContent); 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 (context.drawFocusRing(element, x, y, true)) { 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); drawCheckbox(context, document.getElementById('showB'), 20, 60); 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 - canvas.offsetLeft; var y = event.clientY - canvas.offsetTop; drawCheckbox(context, document.getElementById('showA'), 20, 40); if (context.isPointInPath(x, y)) document.getElementById('showA').checked = !(document.getElementById('showA').checked); drawCheckbox(context, document.getElementById('showB'), 20, 60); 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>
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 ]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 ]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
.
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
Returns the advance width of the text that was passed to the
measureText()
method.
The font
IDL
attribute, on setting, must be parsed the same way as the 'font'
property of CSS (but without supporting property-independent style
sheet syntax like 'inherit'), and the resulting font must be
assigned to the context, with the 'line-height' component forced to
'normal', with the 'font-size' component converted to CSS pixels,
and with system fonts being computed to explicit values. If the new
value is syntactically incorrect (including using
property-independent style sheet syntax like 'inherit' or
'initial'), then it must be ignored, without assigning a new font
value. [CSS]
Font names must be interpreted in the context of the
canvas
element's stylesheets; any fonts embedded using
@font-face
must therefore be available once
they are loaded. (If a font is referenced before it is fully loaded,
then it must be treated as if it was an unknown font, falling back
to another as described by the relevant CSS specifications.) [CSSFONTS]
Only vector fonts should be used by the user agent; if a user agent were to use bitmap fonts then transformations would likely make the font look very ugly.
On getting, the font
attribute must return the serialized form of the current font of the context
(with no 'line-height' component). [CSSOM]
For example, after the following statement:
context.font = 'italic 400 12px/2 Unknown Font, sans-serif';
...the expression context.font
would
evaluate to the string "italic 12px "Unknown Font", sans-serif
". The
"400" font-weight doesn't appear because that is the default
value. The line-height doesn't appear because it is forced to
"normal", the default value.
When the context is created, the font of the context must be set
to 10px sans-serif. When the 'font-size' component is set to lengths
using percentages, 'em' or 'ex' units, or the 'larger' or 'smaller'
keywords, these must be interpreted relative to the computed value
of the 'font-size' property of the corresponding canvas
element at the time that the attribute is set. When the
'font-weight' component is set to the relative values 'bolder' and
'lighter', these must be interpreted relative to the computed value
of the 'font-weight' property of the corresponding
canvas
element at the time that the attribute is
set. If the computed values are undefined for a particular case
(e.g. because the canvas
element is not in a
Document
), then the relative keywords must be
interpreted relative to the normal-weight 10px sans-serif
default.
The textAlign
IDL
attribute, on getting, must return the current value. On setting, if
the value is one of start
, end
, left
, right
, or center
, then the
value must be changed to the new value. Otherwise, the new value
must be ignored. When the context is created, the textAlign
attribute must
initially have the value start
.
The textBaseline
IDL attribute, on getting, must return the current value. On
setting, if the value is one of top
, hanging
, middle
, alphabetic
,
ideographic
,
or bottom
,
then the value must be changed to the new value. Otherwise, the new
value must be ignored. When the context is created, the textBaseline
attribute
must initially have the value alphabetic
.
The textBaseline
attribute's allowed keywords correspond to alignment points in the
font:
The keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
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:
Let font be the current font of the
context, as given by the font
attribute.
Replace all the space characters in text with U+0020 SPACE characters.
Form a hypothetical infinitely wide CSS line box containing
a single inline box containing the text text,
with all the properties at their initial values except the 'font'
property of the inline box set to font and the
'direction' property of the inline box set to the
directionality of the canvas
element. [CSS]
If the maxWidth argument was specified and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
Let the anchor point be a point on the
inline box, determined by the textAlign
and textBaseline
values, as
follows:
Horizontal position:
textAlign
is left
textAlign
is start
and the directionality of the
canvas
element is 'ltr'textAlign
is end
and the directionality of the
canvas
element is 'rtl'textAlign
is right
textAlign
is end
and the directionality of the
canvas
element is 'ltr'textAlign
is start
and the directionality of the
canvas
element 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
Paint the hypothetical inline box as the shape given by the text's glyphs, as transformed by the current transformation matrix, and anchored and sized so that before applying the current transformation matrix, the anchor point is at (x, y) and each CSS pixel is mapped to one coordinate space unit.
For fillText()
fillStyle
must be
applied to the glyphs and strokeStyle
must be
ignored. For strokeText()
the reverse
holds and strokeStyle
must be
applied to the glyph outlines and fillStyle
must be
ignored.
Text is painted without affecting the current path, and is subject to shadow effects, global alpha, the clipping region, and global composition operators.
The measureText()
method takes one argument, text. When the method
is invoked, the user agent must replace all the space characters in text with
U+0020 SPACE characters, and then must form a hypothetical
infinitely wide CSS line box containing a single inline box
containing the text text, with all the
properties at their initial values except the 'font' property of the
inline element set to the current font of the context, as given by
the font
attribute, and
must then return a new TextMetrics
object with its
width
attribute set to
the width of that inline box, in CSS pixels. [CSS]
The TextMetrics
interface is used for the objects
returned from measureText()
. It has one
attribute, width
, which is set
by the measureText()
method.
Glyphs rendered using fillText()
and strokeText()
can spill out
of the box given by the font size (the em square size) and the width
returned by measureText()
(the text
width). This version of the specification does not provide a way to
obtain the bounding box dimensions of the text. If the text is to be
rendered and removed, care needs to be taken to replace the entire
area of the canvas that the clipping region covers, not just the box
given by the em square height and measured text width.
A future version of the 2D context API may provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.
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
TYPE_MISMATCH_ERR
exception. If the image has no
image data, throws an INVALID_STATE_ERR
exception. If
the second argument isn't one of the allowed values, throws a
SYNTAX_ERR
exception. If the image isn't yet fully
decoded, then 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.
The image argument is an instance of either
HTMLImageElement
, HTMLCanvasElement
, or
HTMLVideoElement
. If the image is
of the wrong type or null, the implementation must raise a
TYPE_MISMATCH_ERR
exception.
If the image argument is an
HTMLImageElement
object whose complete
attribute is false, or
if the image argument is an
HTMLVideoElement
object whose readyState
attribute is either
HAVE_NOTHING
or HAVE_METADATA
, then the
implementation must return without drawing anything.
If the image argument is an
HTMLCanvasElement
object with either a horizontal
dimension or a vertical dimension equal to zero, then the
implementation must raise an INVALID_STATE_ERR
exception.
The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).
If the source rectangle is not entirely within the source image,
or if one of the sw or sh
arguments is zero, the implementation must raise an
INDEX_SIZE_ERR
exception.
The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).
When drawImage()
is
invoked, the region of the image specified by the source rectangle
must be painted on the region of the canvas specified by the
destination rectangle, after applying the current transformation
matrix to the points of the destination rectangle.
The original image data of the source image must be used, not the
image as it is rendered (e.g. width
and height
attributes on the source
element have no effect). The image data must be processed in the
original direction, even if the dimensions given are negative.
This specification does not define the algorithm to use when scaling the image, if necessary.
When a canvas is drawn onto itself, the drawing model requires the source to be copied before the image is drawn back onto the canvas, so it is possible to copy parts of a canvas onto overlapping parts of itself.
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.
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.
Throws a NOT_SUPPORTED_ERR
exception if the
argument is null.
getImageData
(sx, sy, sw, sh)Returns an ImageData
object containing the image
data for the given rectangle of the canvas.
Throws a NOT_SUPPORTED_ERR
exception if any of the
arguments are not finite. Throws an INDEX_SIZE_ERR
exception if the either of the width or height arguments are
zero.
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.
putImageData
(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ])Paints the data from the given ImageData
object
onto the canvas. If a dirty rectangle is provided, only the pixels
from that rectangle are painted.
The globalAlpha
and globalCompositeOperation
attributes, as well as the shadow attributes, are ignored for the
purposes of this method call; pixels in the canvas are replaced
wholesale, with no composition, alpha blending, no shadows,
etc.
If the first argument isn't an ImageData
object,
throws a TYPE_MISMATCH_ERR
exception. Throws a
NOT_SUPPORTED_ERR
exception if any of the other
arguments are not finite.
The createImageData()
method is used to instantiate new blank ImageData
objects. When the method is invoked with two arguments sw and sh, it must return an
ImageData
object representing a rectangle with a width
in CSS pixels equal to the absolute magnitude of sw and a height in CSS pixels equal to the absolute
magnitude of sh. When invoked with a single imagedata argument, it must return an
ImageData
object representing a rectangle with the same
dimensions as the ImageData
object passed as the
argument. The ImageData
object return must be filled
with transparent black.
The getImageData(sx, sy, sw,
sh)
method must return an
ImageData
object representing the underlying pixel data
for the area of the canvas denoted by the rectangle whose corners are
the four points (sx, sy),
(sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh), in canvas
coordinate space units. Pixels outside the canvas must be returned
as transparent black. Pixels must be returned as non-premultiplied
alpha values.
If any of the arguments to createImageData()
or
getImageData()
are
infinite or NaN, or if the createImageData()
method is invoked with only one argument but that argument is null,
the method must instead raise a NOT_SUPPORTED_ERR
exception. If either the sw or sh arguments are zero, the method must instead raise
an INDEX_SIZE_ERR
exception.
ImageData
objects must be initialized so that their
width
attribute
is set to w, the number of physical device
pixels per row in the image data, their height
attribute is
set to h, the number of rows in the image data,
and their data
attribute is initialized to a CanvasPixelArray
object
holding the image data. At least one pixel's worth of image data
must be returned.
The CanvasPixelArray
object provides ordered,
indexed access to the color components of each pixel of the image
data. The data must be represented in left-to-right order, row by
row top to bottom, starting with the top left, with each pixel's
red, green, blue, and alpha components being given in that order for
each pixel. Each component of each device pixel represented in this
array must be in the range 0..255, representing the 8 bit value for
that component. The components must be assigned consecutive indices
starting with 0 for the top left pixel's red component.
The CanvasPixelArray
object thus represents h×w×4 integers. The
length
attribute of a CanvasPixelArray
object must return this
number.
The object's indices of the supported indexed properties are the numbers in the range 0 .. h×w×4-1.
When a CanvasPixelArray
object is indexed to retrieve an indexed
property index, the value returned must be
the value of the indexth component in the
array.
When a CanvasPixelArray
object is indexed to modify an indexed
property index with value value, the value of the indexth
component in the array must be set to value.
The width and height (w and h) might be different from the sw and sh arguments to the above methods, e.g. if the canvas is backed by a high-resolution bitmap, or if the sw and sh arguments are negative.
The putImageData(imagedata, dx, dy, dirtyX, dirtyY, dirtyWidth, dirtyHeight)
method writes data from
ImageData
structures back to the canvas.
If any of the arguments to the method are infinite or NaN, the
method must raise a NOT_SUPPORTED_ERR
exception.
If the first argument to the method is null or not an
ImageData
object then the putImageData()
method
must raise a TYPE_MISMATCH_ERR
exception.
When the last four arguments are omitted, they must be assumed to
have the values 0, 0, the width
member of the imagedata structure, and the height
member of the imagedata structure, respectively.
When invoked with arguments that do not, per the last few
paragraphs, cause an exception to be raised, the putImageData()
method
must act as follows:
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.
Due to the lossy nature of converting to and from
premultiplied alpha color values, pixels that have just been set
using putImageData()
might be
returned to an equivalent getImageData()
as
different values.
The current path, transformation matrix,
shadow attributes, global alpha, the clipping region, and global composition
operator must not affect the getImageData()
and putImageData()
methods.
The data returned by getImageData()
is at the
resolution of the canvas backing store, which is likely to not be
one device pixel to each CSS pixel if the display used is a high
resolution display.
In the following example, the script generates an
ImageData
object so that it can draw onto it.
// canvas is a reference to a <canvas> element var context = canvas.getContext('2d'); // create a blank slate var data = context.createImageData(canvas.width, canvas.height); // create some plasma FillPlasma(data, 'green'); // green plasma // add a cloud to the plasma AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle // paint the plasma+cloud on the canvas context.putImageData(data, 0, 0); // support methods function FillPlasma(data, color) { ... } function AddCloud(data, x, y) { ... }
Here is an example of using getImageData()
and putImageData()
to
implement an edge detection filter.
<!DOCTYPE HTML> <html> <head> <title>Edge detection demo</title> <script> var image = new Image(); function init() { image.onload = demo; image.src = "image.jpeg"; } function demo() { var canvas = document.getElementsByTagName('canvas')[0]; var context = canvas.getContext('2d'); // draw the image onto the canvas context.drawImage(image, 0, 0); // get the image data to manipulate var input = context.getImageData(0, 0, canvas.width, canvas.height); // get an empty slate to put the data into var output = context.createImageData(canvas.width, canvas.height); // alias some variables for convenience // notice that we are using input.width and input.height here // as they might not be the same as canvas.width and canvas.height // (in particular, they might be different on high-res displays) var w = input.width, h = input.height; var inputData = input.data; var outputData = output.data; // edge detection for (var y = 1; y < h-1; y += 1) { for (var x = 1; x < w-1; x += 1) { for (var c = 0; c < 3; c += 1) { var i = (y*w + x)*4 + c; outputData[i] = 127 + -inputData[i - w*4 - 4] - inputData[i - w*4] - inputData[i - w*4 + 4] + -inputData[i - 4] + 8*inputData[i] - inputData[i + 4] + -inputData[i + w*4 - 4] - inputData[i + w*4] - inputData[i + w*4 + 4]; } outputData[(y*w + x)*4 + 3] = 255; // alpha } } // put the image data back after manipulation context.putImageData(output, 0, 0); } </script> </head> <body onload="init()"> <canvas></canvas> </body> </html>
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.
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".
For a full list of acknowledgements, please see the HTML5 specification. [HTML5]