canvas elementStrictly inline-level embedded content.
figure element.
width
height
interface HTMLCanvasElement : HTMLElement {
attribute unsigned long width;
attribute unsigned long height;
DOMString toDataURL();
DOMString toDataURL(in DOMString type);
DOMObject getContext(in DOMString contextId);
};
Shouldn't allow inline-level content to be the content model when the parent's content model is strictly inline only.
The canvas element represents a
resolution-dependent bitmap canvas, which can be used for rendering
graphs, game graphics, or other visual images on the fly.
Authors should not use the canvas
element in a document when a more suitable element is available. For
example, it is inappropriate to use a canvas element to render a page heading: if the
desired presentation of the heading is graphically intense, it should be
marked up using appropriate elements (typically h1) and then styled using CSS and supporting
technologies such as XBL.
When authors use the canvas element,
they should also provide content that, when presented to the user, conveys
essentially the same function or purpose as the bitmap canvas. This
content may be placed as content of the canvas element. The contents of the canvas element, if any, are the element's fallback content.
In interactive visual media with scripting enabled, the canvas element is an embedded element with a dynamically created image.
In non-interactive, static, visual media, if the canvas element has been previously painted on
(e.g. if the page was viewed in an interactive visual medium and is now
being printed, or if some script that ran during the page layout process
painted on the element), then the canvas element must be treated as embedded
content with the current image and size. Otherwise, the element's fallback
content must be used instead.
In non-visual media, and in visual media with scripting
disabled, the canvas element's
fallback content must be used instead.
The canvas element has two attributes
to control the size of the coordinate space: width and height. These attributes, when
specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to
obtain their numeric values. If an attribute is missing, or if parsing its
value returns an error, then the default value must be used instead. The
width attribute
defaults to 300, and the height attribute defaults to 150.
The intrinsic dimensions of the canvas element equal the size of the coordinate
space, with the numbers interpreted in CSS pixels. However, the element
can be sized arbitrarily by a style sheet. During rendering, the image is
scaled to fit this layout size.
The size of the coordinate space does not necessarily represent the size of the actual bitmap that the user agent will use internally or during rendering. On high-definition displays, for instance, the user agent may internally use a bitmap with two device pixels per unit in the coordinate space, so that the rendering remains at high quality throughout.
The canvas must initially be fully transparent black.
Whenever the width and height attributes are set (whether to a new
value or to the previous value), the bitmap and any associated contexts
must be cleared back to their initial state and reinitialised with the
newly specified coordinate space dimensions.
The width and
height DOM
attributes must reflect the content attributes of
the same name.
Only one square appears to be drawn in the following example:
// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');
context.fillRect(0,0,50,50);
canvas.setAttribute('width', '300'); // clears the canvas
context.fillRect(0,100,50,50);
canvas.width = canvas.width; // clears the canvas
context.fillRect(100,0,50,50); // only this square remains
To draw on the canvas, authors must first obtain a reference to a context using the getContext(contextId) method of the canvas element.
This specification only defines one context, with the name "2d". If getContext()
is called with that exact string for tis contextId
argument, then the UA must return a reference to an object implementing
CanvasRenderingContext2D.
Other specifications may define their own contexts, which would return
different objects.
Vendors may also define experimental contexts using the syntax
vendorname-context,
for example, moz-3d.
When the UA is passed an empty string or a string specifying a context that it does not support, then it must return null. String comparisons must be literal and case-sensitive.
A future version of this specification will probably define a
3d context (probably based on the OpenGL ES API).
The toDataURL() method must,
when called with no arguments, return a data: URI
containing a representation of the image as a PNG file. [PNG].
The toDataURL(type) method (when called with one or
more arguments) must return a data: URI containing a
representation of the image in the format given by type. The possible values are MIME types with no
parameters, for example image/png, image/jpeg,
or even maybe image/svg+xml if the implementation actually
keeps enough information to reliably render an SVG image from the canvas.
Only support for image/png is required. User agents may
support other types. If the user agent does not support the requested
type, it must return the image using the PNG format.
User agents must convert the provided type to lower case before
establishing if they support that type and before creating the
data: URI.
When trying to use types other than image/png,
authors can check if the image was really returned in the requested format
by checking to see if the returned string starts with one the exact
strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus
the requested type was not supported.
Arguments other than the type must be ignored, and
must not cause the user agent to raise an exception (as would normally
occur if a method was called with the wrong number of arguments). A future
version of this specification will probably allow extra parameters to be
passed to toDataURL() to allow authors to more
carefully control compression settings, image metadata, etc.
Security: To prevent information leakage, the
toDataURL() and getImageData() methods should raise a security exception if the canvas has ever had an
image painted on it whose origin is different from
that of the script calling the method.
When the getContext() method of a canvas element is invoked with 2d as the argument, a CanvasRenderingContext2D
object is returned.
There is only one CanvasRenderingContext2D
object per canvas, so calling the getContext() method with the 2d argument a second time
must return the same object.
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 DOMObject strokeStyle; // (default black)
attribute DOMObject fillStyle; // (default black)
CanvasGradient createLinearGradient(in float x0, in float y0, in float x1, in float y1);
CanvasGradient createRadialGradient(in float x0, in float y0, in float r0, in float x1, in float y1, in float r1);
CanvasPattern createPattern(in HTMLImageElement image, DOMString repetition);
CanvasPattern createPattern(in HTMLCanvasElement image, DOMString repetition);
// line caps/joins
attribute float lineWidth; // (default 1)
attribute DOMString lineCap; // "butt", "round", "square" (default "butt")
attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter")
attribute float miterLimit; // (default 10)
// shadows
attribute float shadowOffsetX; // (default 0)
attribute float shadowOffsetY; // (default 0)
attribute float shadowBlur; // (default 0)
attribute DOMString shadowColor; // (default 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);
// drawing images
void drawImage(in HTMLImageElement image, in float dx, in float dy);
void drawImage(in HTMLImageElement image, in float dx, in float dy, in float dw, in float dh);
void drawImage(in HTMLImageElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh);
void drawImage(in HTMLCanvasElement image, in float dx, in float dy);
void drawImage(in HTMLCanvasElement image, in float dx, in float dy, in float dw, in float dh);
void drawImage(in HTMLCanvasElement image, in float sx, in float sy, in float sw, in float sh, in float dx, in float dy, in float dw, in float dh);
// pixel manipulation
ImageData getImageData(in float sx, in float sy, in float sw, in float sh);
void putImageData(in ImageData image, in float dx, in float dy);
// drawing text is not supported in this version of the API
// (there is no way to predict what metrics the fonts will have,
// which makes fonts very hard to use for painting)
};
interface CanvasGradient {
// opaque object
void addColorStop(in float offset, in DOMString color);
};
interface CanvasPattern {
// opaque object
};
interface ImageData {
readonly attribute long int width;
readonly attribute long int height;
readonly attribute int[] data;
};
The canvas attribute must
return the canvas element that the
context paints on.
Each context maintains a stack of drawing states. Drawing states consist of:
strokeStyle, fillStyle,
globalAlpha, lineWidth,
lineCap,
lineJoin, miterLimit, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, globalCompositeOperation.
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.
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 all drawing operations prior to their being rendered. It is also applied when creating the clip region.
When the context is created, the transformation matrix must initially be the identity transform. It may then be adjusted using the transformation methods.
The transformation matrix can become infinite, at which point nothing is drawn anymore.
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.
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. If
either argument is Infinity the transformation matrix must be marked as
infinite instead of the method throwing an exception.
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. If either argument is Infinity the transformation
matrix must be marked as infinite instead of the method throwing an
exception.
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 |
If any of the arguments are Infinity the transformation matrix must be marked as infinite instead of the method throwing an exception.
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. If any of the arguments are Infinity the transformation matrix
must be marked as infinite instead of the method throwing an exception.
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, 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 only recognise values that exactly match 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 recognise 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.
The strokeStyle attribute
represents the color or style to use for the lines around shapes, and the
fillStyle attribute
represents the color or style to use inside the shapes.
Both attributes can be either strings, CanvasGradients, or CanvasPatterns. On setting, strings must
be parsed as CSS <color> values and the color assigned, and CanvasGradient and CanvasPattern objects must be assigned
themselves. [CSS3COLOR] 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.
On getting, if the value is a color, then: if it has alpha equal to 1.0,
then the color must be returned as 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). If the value has alpha less than 1.0, then the value
must instead be returned in the CSS rgba()
functional-notation format: the literal string rgba
(U+0072 U+0067 U+0062 U+0061) followed by a U+0028 LEFT PARENTHESIS, a
base-ten integer in the range 0-255 representing the red component (using
digits 0-9, U+0030 to U+0039, in the shortest form possible), a literal
U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma
and a space, an integer for the blue component, another comma and space, a
U+0030 DIGIT ZERO, a U+002E FULL STOP (representing the decimal point),
one or more digits in the range 0-9 (U+0030 to U+0039) representing the
fractional part of the alpha value, and finally a U+0029 RIGHT
PARENTHESIS.
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.)
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.
The addColorStop(offset, color) method on
the CanvasGradient interface
adds a new stop to a gradient. If the offset is less
than 0 or greater than 1 then an INDEX_SIZE_ERR exception
must be raised. 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, representing
the start point (x0, y0) and end
point (x1, y1) of the gradient, in
coordinate space units, and must return a linear CanvasGradient initialised with that
line.
Linear gradients must be rendered such that at and before the starting point on the canvas the color at offset 0 is used, that at and after the ending point the color at offset 1 is used, and 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 described above).
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. The method must return a radial CanvasGradient initialised with those
two circles. If either of r0 or r1
are negative, an INDEX_SIZE_ERR exception must be raised.
Radial gradients must be rendered by following 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 of the gradient at offset 0.0 for all values of ω less than 0.0, the color at offset 1.0 for all values of ω greater than 1.0, and the color at the given offset for values of ω in the range 0.0 ≤ ω ≤ 1.0
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.
If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing.
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 only be painted where the relevant stroking or filling effects requires that they be drawn.
Support for actually painting gradients is optional. Instead of painting
the gradients, user agents may instead just paint the first stop's color.
However, createLinearGradient() and createRadialGradient() must always
return objects when passed valid arguments.
Patterns are represented by objects implementing the opaque CanvasPattern interface.
To create objects of this type, the createPattern(image,
repetition) method is used. The first argument gives the
image to use as the pattern (either an HTMLImageElement or an HTMLCanvasElement). Modifying this
image after calling the createPattern() method must not
affect the pattern. The second argument must be a string with one of the
following values: repeat, repeat-x, repeat-y, no-repeat. If the empty string or null is specified, repeat must be assumed. If an unrecognised value is given,
then the user agent must raise a SYNTAX_ERR exception. User
agents must recognise the four values described above exactly (e.g. they
must not do case folding). The method must return a CanvasPattern object suitably
initialised.
The image argument must be an instance of an
HTMLImageElement or HTMLCanvasElement. If the image is of the wrong type, the implementation must raise a
TYPE_MISMATCH_ERR exception. If the image
argument is an HTMLImageElement object whose complete attribute is false, 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 be scaled by this process; one CSS pixel of the image must
be painted on one coordinate space unit. Of course, patterns must only
actually painted where the stroking or filling effect requires that they
be drawn, and are affected by the current transformation matrix.
Support for patterns is optional. If the user agent doesn't support
patterns, then createPattern() must return null.
The lineWidth attribute
gives the default width of lines, in coordinate space units. On setting,
zero and negative values must be ignored, leaving the value unchanged.
When the context is created, the lineWidth attribute must initially have the
value 1.0.
The lineCap attribute defines
the type of endings that UAs shall place on the end of lines. The three
valid values are butt, round, and
square. The butt value means that the end of
each line is a flat edge perpendicular to the direction of the line. The
round value means that a semi-circle with the diameter equal
to the width of the line is then added on to the end of the line. The
square value means that at the end of each line is a
rectangle with the length of the line width and the width of half the line
width, placed flat against the edge perpendicular to the direction of the
line. On setting, any other value than the literal strings
butt, round, and square must be
ignored, leaving the value unchanged.
When the context is created, the lineCap attribute must initially have the value
butt.
The lineJoin attribute
defines the type of corners that that UAs will place where two lines meet.
The three valid values are round, bevel, and
miter.
On setting, any other value than the literal strings round,
bevel and miter must be ignored, leaving the
value unchanged.
When the context is created, the lineJoin attribute must initially have the
value miter.
The round value means that a filled arc connecting the
corners on the outside of the join, with the diameter equal to the line
width, and the origin at the point where the inside edges of the lines
touch, must be rendered at joins. The bevel value means that
a filled triangle connecting those two corners with a straight line, the
third point of the triangle being the point where the lines touch on the
inside of the join, must be rendered at joins. The miter
value means that a filled four- or five-sided polygon must be placed at
the join, with two of the lines being the perpendicular edges of the
joining lines, and the other two being continuations of the outside edges
of the two joining lines, as long as required to intersect without going
over the miter limit.
The miter length is the distance from the point where the lines touch on the inside of the join to the intersection of the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of the miter length to the line width. If the miter limit would be exceeded, then a fifth line must be added to the polygon, connecting the two outside lines, such that the distance from the inside point of the join to the point in the middle of this fifth line is the maximum allowed value for the miter length.
The miter limit ratio can be explicitly set using the miterLimit attribute.
On setting, zero and negative values must be ignored, leaving the value
unchanged.
When the context is created, the miterLimit attribute must initially have the
value 10.0.
All drawing operations are affected by the four global shadow attributes. Shadows form part of the source image during composition.
The shadowColor attribute
sets the color of the shadow.
When the context is created, the shadowColor attribute initially must be
fully-transparent black.
The shadowOffsetX and
shadowOffsetY
attributes specify the distance that the shadow will be offset in the
positive horizontal and positive vertical distance respectively. Their
values are in coordinate space units.
When the context is created, the shadow offset attributes initially have
the value 0.
The shadowBlur attribute
specifies the number of coordinate space units that the blurring is to
cover. On setting, negative numbers must be ignored, leaving the attribute
unmodified.
When the context is created, the shadowBlur attribute must initially have the
value 0.
Support for shadows is optional. When they are supported, then, when shadows are drawn, they must be rendered using the specified color, offset, and blur radius.
There are three methods that immediately draw rectangles to the bitmap. They each take four arguments; the first two give the x and y coordinates of the top left of the rectangle, and the second two give the width and height of the rectangle, respectively.
Shapes are painted without affecting the current path, and are subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.
Negative values for width and height must cause the implementation to
raise an INDEX_SIZE_ERR exception.
The clearRect() method must
clear the pixels in the specified rectangle to a fully transparent black,
erasing any previous image. If either height or width are zero, this
method has no effect.
The fillRect() method must
paint the specified rectangular area using the fillStyle.
If either height or width are zero, this method has no effect.
The strokeRect() method
must draw stroke the path that would be created for the outline of a
rectangle of the specified size 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.
Initially, the context's path must have zero subpaths.
The beginPath() method must
empty the list of subpaths so that the context once again has zero
subpaths.
The moveTo(x, y) method must create a
new subpath with the specified point as its first (and only) point.
The closePath() method must
do nothing if the context has no subpaths. Otherwise, it must mark the
last subpath as closed, create a new subpath whose first point is the same
as the previous subpath's first point, and finally add this new subpath to
the path. (If the last subpath had more than one point in its list of
points, then this is equivalent to adding a straight line connecting the
last point back to the first point, thus "closing" the shape, and then
repeating the last moveTo() call.)
New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the context's paths.
The lineTo(x, y) method must do
nothing if the context has no subpaths. Otherwise, it must connect the
last point in the subpath to the given point (x, y) using a straight line, and must then add the given point
(x, y) to the subpath.
The quadraticCurveTo(cpx, cpy, x, y) method must do nothing if the context has
no subpaths. Otherwise it must connect the last point in the subpath to
the given point (x, y) by a
quadratic curve with control point (cpx, cpy), and must then add the given point (x, y) to the subpath.
The bezierCurveTo(cp1x, cp1y, cp2x,
cp2y, x, y) method must do nothing if the context has
no subpaths. Otherwise, it must connect the last point in the subpath to
the given point (x, y) using a
bezier curve with control points (cp1x, cp1y) and (cp2x, cp2y). Then, it must add the point (x,
y) to the subpath.
The arcTo(x1, y1, x2, y2, radius) method must do
nothing if the context has no subpaths. If the context does have
a subpath, then the behaviour depends on the arguments and the last point
in the subpath.
Let the point (x0, y0) be the last point in the subpath. Let The Arc be the shortest arc given by circumference of the circle that has one point tangent to the line defined by the points (x0, y0) and (x1, y1), another point tangent to the line defined by the points (x1, y1) and (x2, y2), and that has radius radius. The points at which this circle touches these two lines are called the start and end tangent points respectively.
If the point (x2, y2) is on the line defined by the points (x0, y0) and (x1, y1) then the method must do nothing, as no arc would satisfy the above constraints.
Otherwise, the method must connect the point (x0, y0) to the start tangent point by a straight line, then connect the start tangent point to the end tangent point by The Arc, and finally add the start and end tangent points to the subpath.
Negative or zero values for radius must cause the
implementation to raise an INDEX_SIZE_ERR exception.
The arc(x, y, radius, startAngle, endAngle, anticlockwise) method draws an arc. If the
context has any subpaths, then the method must add a straight line from
the last point in the subpath to the start point of the arc. In any case,
it must draw the arc between the start point of the arc and the end point
of the arc, and add the start and end points of the arc to the subpath.
The arc and its start and end points are defined as follows:
Consider a circle that has its origin at (x, y) and that has radius radius. The points at startAngle and endAngle along the circle's circumference, measured in radians clockwise from the positive x-axis, are the start and end points respectively. The arc is the path along the circumference of this circle from the start point to the end point, going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise.
Negative or zero values for radius must cause the
implementation to raise an INDEX_SIZE_ERR exception.
The rect(x, y, w, h) method must create a new subpath containing
just the four points (x, y), (x+w, y), (x+w, y+h), (x, y+h), with those four points connected by straight lines, and
must then mark the subpath as closed. It must then create a new subpath
with the point (x, y) as the only
point in the subpath.
Negative values for w and h must
cause the implementation to raise an INDEX_SIZE_ERR
exception.
The fill()
method must fill each subpath of the current path in turn, using fillStyle,
and using the non-zero winding number rule. Open subpaths must be
implicitly closed when being filled (without affecting the actual
subpaths).
The stroke() method must stroke
each subpath of the current path in turn, using the strokeStyle, lineWidth,
lineJoin, and (if appropriate) miterLimit attributes.
Paths, when filled or stroked, must be painted without affecting the current path, and must be subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.
The transformation is applied to the path when it is drawn, not when the path is constructed. Thus, a single path can be constructed and then drawn according to different transformations without recreating the path.
The clip()
method must create a new clipping path by
calculating the intersection of the current clipping path and the area
described by the current path (after applying the current
transformation), using the non-zero winding number rule. Open
subpaths must be implicitly closed when computing the clipping path,
without affecting the actual subpaths.
When the context is created, the initial clipping path is the rectangle with the top left corner at (0,0) and the width and height of the coordinate space.
The isPointInPath(x, y) method must return
true if the point given by the x and y coordinates passed to the method, when treated as
coordinates in the canvas' coordinate space unaffected by the current
transformation, is within the area of the canvas that would be filled if
the current path was to be filled; and must return false otherwise.
To draw images onto the canvas, the drawImage method can be
used.
This method is overloaded with three variants: drawImage(image, dx, dy), drawImage(image, dx, dy, dw, dh), and drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh). (Actually it is overloaded with six; each of
those three can take either an HTMLImageElement or an HTMLCanvasElement for the image argument.) If not specified, the dw and dh arguments default to the
values of sw and sh, interpreted
such that one CSS pixel in the image is treated as one unit in the canvas
coordinate space. If the sx, sy,
sw, and sh arguments are omitted,
they default to 0, 0, the image's intrinsic width in image pixels, and the
image's intrinsic height in image pixels, respectively.
The image argument must be an instance of an
HTMLImageElement or HTMLCanvasElement. If the image is of the wrong type, the implementation must raise a
TYPE_MISMATCH_ERR exception. If one of the sy, sw, sw, and
sh arguments is outside the size of the image, or if
one of the dw and dh arguments is
negative, the implementation must raise an INDEX_SIZE_ERR
exception. If the image argument is an HTMLImageElement object whose complete attribute is false, then the
implementation must raise an INVALID_STATE_ERR exception.
When drawImage() is invoked, the specified region
of the image specified by the source rectangle (sx,
sy, sw, sh)
must be painted on the region of the canvas specified by the destination
rectangle (dx, dy, dw, dh).
Images are painted without affecting the current path, and are subject to transformations, shadow effects, global alpha, clipping paths, and global composition operators.
The getImageData(sx, sy, sw, sh) method must return an ImageData object representing the underlying
pixel data for the area of the canvas denoted by the rectangle which has
one corner at the (sx, sy)
coordinate, and that has width sw and height sh. Pixels outside the canvas must be returned as
transparent black. Pixels must be returned as non-premultiplied alpha
values.
ImageData objects must be
initialised so that their height attribute is set to
h, the number of rows in the image data, their width attribute is
set to w, the number of physical device pixels per row
in the image data, and the data attribute is initialised
to an array of h×w×4
integers. The pixels must be represented in this array in left-to-right
order, row by row, starting at the top left, with each pixel's red, green,
blue, and alpha components being given in that order. Each component of
each device pixel represented in this array must be in the range 0..255,
representing the 8 bit value for that component. At least one pixel must
be returned.
The width and height (w and h) might be different than the sw and sh arguments to the function, e.g. if the canvas is backed by a high-resolution bitmap.
If the getImageData(sx, sy, sw, sh) method is called with either the sw or sh arguments set to zero or
negative values, the method must raise an INDEX_SIZE_ERR
exception.
The putImageData(image, dx, dy) method must take the given ImageData structure, and draw it at the
specified location dx,dy in the
canvas coordinate space, mapping each pixel represented by the ImageData structure into one device pixel.
If the first argument to the method is not an object whose [[Class]]
property is ImageData, but all of
the following conditions are true, then the method must treat the first
argument as if it was an ImageData
object (and thus not raise the TYPE_MISMATCH_ERR exception):
width and height
attributes with integer values and a data attribute whose value is an integer array.
ImageData object's width is greater
than zero.
ImageData object's height is
greater than zero.
ImageData object's width multiplied
by its height multiplied by 4 is equal to the number
of entries in the ImageData
object's data
array.
ImageData object's data array only
contains entries that are in the range 0 to 255 inclusive.
The handling of pixel rounding when the specified coordinates do not exactly map to the device coordinate space is not defined by this specification, except that the following must result in no visible changes to the rendering:
context.putImageData(context.getImageData(x, y, w, h), x, y);
...for any value of x and y. In
other words, while user agents may round the arguments of the two methods
so that they map to device pixel boundaries, any rounding performed must
be performed consistently for both the getImageData() and putImageData() operations.
The current transformation matrix must not affect the getImageData() and putImageData() methods.
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. Thus,
while one could create an ImageData
object, one would net necessarily know what resolution the canvas
expected (how many pixels the canvas wants to paint over one coordinate
space unit pixel).
In the following example, the script first obtains the size of the
canvas backing store, and then generates a few new ImageData objects which can be used.
// canvas is a reference to a <canvas> element
// (note: this example uses JavaScript 1.7 features)
var context = canvas.getContext('2d');
var backingStore = context.getImageData(0, 0, canvas.width, canvas.height);
var actualWidth = backingStore.width;
var actualHeight = backingStore.height;
function CreateImageData(w, h) {
return {
height: h,
width: w,
data: [i for (i in function (n) { for (let i = 0; i < n; i += 1) yield 0 }(w*h*4)) ]
};
}
// create some plasma
var plasma = CreateImageData(actualWidth, actualHeight);
FillPlasma(plasma, 'green'); // green plasma
// create a cloud
var could = CreateImageData(actualWidth, actualHeight);
FillCloud(cloud, actualWidth/2, actualHeight/2); // put a cloud in the middle
// paint them on top of each other
context.putImageData(plasma, 0, 0);
context.putImageData(cloud, 0, 0);
function FillPlasma(data) { ... }
function FillCload(data, x, y) { ... }
When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):
globalAlpha.
map elementid
attribute is required.
interface HTMLMapElement : HTMLElement {
readonly attribute HTMLCollection areas;
readonly attribute HTMLCollection images;
};
The map element, in conjuction with any
area element descendants, defines an image map.
There must always be an id
attribute present on map elements.
The areas attribute
must return an HTMLCollection
rooted at the map element, whose filter
matches only area elements.
The images
attribute must return an HTMLCollection rooted at the
Document node, whose filter matches only img and object
elements that are associated with this map
element according to the image map processing model.
area elementStrictly inline-level content.
map element.
alt
coords
shape
href
target
ping
rel
media
hreflang
type
interface HTMLAreaElement : HTMLElement {
attribute DOMString alt;
attribute DOMString coords;
attribute DOMString shape;
attribute DOMString href;
attribute DOMString target;
attribute DOMString ping;
attribute DOMString rel;
readonly attribute DOMTokenList relList;
attribute DOMString media;
attribute DOMString hreflang;
attribute DOMString type;
};
The area element represents either a
hyperlink with some text and a corresponding area on an image map, or a dead area on an image map.
If the area element has an href attribute, then
the area element represents a hyperlink; the alt attribute, which must then be
present, specifies the text.
However, if the area element has no
href
attribute, then the area represented by the element cannot be selected,
and the alt attribute
must be omitted.
In both cases, the shape and coords attributes specify the area.
The shape
attribute is an enumerated attribute. The
following table lists the keywords defined for this attribute. The states
given in the first cell of the the rows with keywords give the states to
which those keywords map. Some of the keywords are non-conforming, as
noted in the last column.
| State | Keywords | Notes |
|---|---|---|
| Circle state | circ
| Non-conforming |
circle
| ||
| Default state | default
| |
| Polygon state | poly
| |
polygon
| Non-conforming | |
| Rectangle state | rect
| |
rectangle
| Non-conforming |
The attribute may be ommited. The missing value default is the rectangle state.
The coords
attribute must, if specified, contain a valid list of
integers. This attribute gives the coordinates for the shape described
by the shape
attribute. The processing for this attribute is described as part of the
image map processing model.
In the circle state,
area elements must have a coords attribute
present, with three integers, the last of which must be non-negative. The
first integer must be the distance in CSS pixels from the left edge of the
image to the center of the circle, the second integer must be the distance
in CSS pixels from the top edge of the image to the center of the circle,
and the third integer must be the radius of the circle, again in CSS
pixels.
In the default
state state, area elements must not
have a coords
attribute.
In the polygon state,
area elements must have a coords attribute with
at least six integers, and the number of integers must be even. Each pair
of integers must represent a coordinate given as the distances from the
left and the top of the image in CSS pixels respectively, and all the
coordinates together must represent the points of the polygon, in order.
In the rectangle
state, area elements must have a
coords attribute
with exactly four integers, the first of which must be less than the
third, and the second of which must be less than the fourth. The four
points must represent, respectively, the distance from the left edge of
the image to the top left side of the rectangle, the distance from the top
edge to the top side, the distance from the left edge to the right side,
and the distance from the top edge to the bottom side, all in CSS pixels.
When user agents allow users to follow hyperlinks created using the area element, as described in the next section,
the href,
target and
ping attributes
decide how the link is followed. The rel, media, hreflang, and type attributes may
be used to indicate to the user the likely nature of the target resource
before the user follows the link.
The target, ping, rel, media, hreflang, and type attributes
must be omitted if the href attribute is not present.
The activation behavior of area elements is to run the following steps:
DOMActivate event in
question is not trusted (i.e. a
click() method call was
the reason for the event being dispatched), and the area element's target attribute is ... then raise an INVALID_ACCESS_ERR
exception.
area element, if any.
One way that a user agent can enable users to follow
hyperlinks is by allowing area elements
to be clicked, or focussed and activated by the keyboard. This will cause the
aforementioned activation behavior to be
invoked.
The DOM attributes alt, coords, shape, href, target, ping, rel, media, hreflang, and type, each must reflect the respective content attributes of the same
name.
The DOM attribute relList must reflect the rel content attribute.
An image map allows geometric areas on an image to be associated with hyperlinks.
An image, in the form of an img element
or an object element representing an
image, may be associated with an image map (in the form of a map element) by specifying a usemap attribute on the
img or object element. The usemap attribute, if specified, must be a valid hashed ID reference to a map element.
If an img element or an object element representing an image has a usemap attribute specified, user agents must
process it as follows:
First, rules for parsing a hashed ID reference
to a map element must be followed. This
will return either an element (the map) or null.
If that returned null, then abort these steps. The image is not associated with an image map after all.
Otherwise, the user agent must collect all the area elements that are descendants of the map. Let those be the areas.
Having obtained the list of area
elements that form the image map (the areas),
interactive user agents must process the list in one of two ways.
If the user agent intends to show the text that the img element represents, then it must use the
following steps.
In user agents that do not support images, or that have
images disabled, object elements cannot
represent images, and thus this section never applies (the fallback
content is shown instead). The following steps therefore only apply to
img elements.
Remove all the area elements in areas that have no href attribute.
Remove all the area elements in areas that have no alt attribute, or whose alt attribute's value is
the empty string, if there is another area element in areas with
the same value in the href attribute and with a non-empty alt attribute.
Each remaining area element in areas represents a hyperlink.
Those hyperlinks should all be made available to the user in a manner
associated with the text of the img or
input element.
In this context, user agents may represent area and img
elements with no specified alt attributes, or
whose alt attributes are the empty string or some
other non-visible text, in a user-agent-defined fashion intended to
indicate the lack of suitable author-provided text.
If the user agent intends to show the image and allow interaction with
the image to select hyperlinks, then the image must be associated with a
set of layered shapes, taken from the area elements in areas, in
reverse tree order (so the last specified area element in the map is the
bottom-most shape, and the first element in the map,
in tree order, is the top-most shape).
Each area element in areas must be processed as follows to obtain a shape to
layer onto the image:
Find the state that the element's shape attribute represents.
Use the rules for parsing a list of integers to
parse the element's coords attribute, if it is present, and let
the result be the coords list. If the attribute is
absent, let the coords list be the empty list.
If the number of items in the coords list is less
than the minimum number given for the area element's current state, as per the
following table, then the shape is empty; abort these steps.
| State | Minimum number of items |
|---|---|
| Circle state | 3 |
| Default state | 0 |
| Polygon state | 6 |
| Rectangle state | 4 |
Check for excess items in the coords list as per
the entry in the following list corresponding to the shape attribute's
state:
If the shape
attribute represents the rectangle state, and the first number in
the list is numerically less than the third number in the list, then
swap those two numbers around.
If the shape
attribute represents the rectangle state, and the second number in
the list is numerically less than the fourth number in the list, then
swap those two numbers around.
If the shape
attribute represents the circle state, and the third number in
the list is less than or equal to zero, then the shape is empty; abort
these steps.
Now, the shape represented by the element is the one described for the
entry in the list below corresponding to the state of the shape attribute:
Let x be the first number in coords, y be the second number, and r be the third number.
The shape is a circle whose center is x CSS pixels from the left edge of the image and x CSS pixels from the top edge of the image, and whose radius is r pixels.
The shape is a rectangle that exactly covers the entire image.
Let xi be the (2i)th entry in coords, and yi be the (2i+1)th entry in coords (the first entry in coords being the one with index 0).
Let the coordinates be (xi, yi), interpreted in CSS pixels measured from the top left of the image, for all integer values of i from 0 to (N/2)-1, where N is the number of items in coords.
The shape is a polygon whose vertices are given by the coordinates, and whose interior is established using the even-odd rule. [GRAPHICS]
Let x1 be the first number in coords, y1 be the second number, x2 be the third number, and y2 be the fourth number.
The shape is a rectangle whose top-left corner is given by the coordinate (x1, y1) and whose bottom right corner is given by the coordinate (x2, y2), those coordinates being interpreted as CSS pixels from the top left corner of the image.
For historical reasons, the coordinates must be interpreted relative
to the displayed image, even if it stretched using CSS or the
image element's width and height attributes.
Mouse clicks on an image associated with a set of layered shapes per the
above algorithm must be dispatched to the top-most shape covering the
point that the pointing device indicated (if any), and then, must be
dispatched again (with a new Event object) to the image
element itself. User agents may also allow individual area elements representing hyperlinks to be selected and activated (e.g. using a
keyboard); events from this are not also propagated to the image.
Because a map element (and
its area elements) can be associated with
multiple img and object elements, it is possible for an area element to correspond to multiple focusable
areas of the document.
Image maps are live; if the DOM is mutated, then the user agent must act as if it had rerun the algorithms for image maps.
The width and
height attributes
on img, embed, object,
and video elements may be specified to
give the dimensions of the visual content of the element (the width and
height respectively, relative to the nominal direction of the output
medium), in CSS pixels. The attributes, if specified, must have values
that are valid positive non-zero integers.
The specified dimensions given may differ from the dimensions specified in the resource itself, since the resource may have a resolution that differs from the CSS pixel resolution. (On screens, CSS pixels have a resolution of 96ppi, but in general the CSS pixel resolution depends on the reading distance.) If both attributes are specified, then the ratio of the specified width to the specified height must be the same as the ratio of the logical width to the logical height in the resource. The two attributes must be omitted if the resource in question does not have both a logical width and a logical height.
To parse the attributes, user agents must use the rules for parsing dimension values. This will return either an integer length, a percentage value, or nothing. The user agent requirements for processing the values obtained from parsing these attributes are described in the rendering section. If one of these attributes, when parsing, returns no value, it must be treated, for the purposes of those requirements, as if it was not specified.
The width and height DOM attributes
on the embed, object, and video elements must reflect the content
attributes of the same name.