canvas elementwidthheightinterface HTMLCanvasElement : HTMLElement {
attribute unsigned long ;
attribute unsigned long ;
attribute unsigned long width;
attribute unsigned long height;
DOMString toDataURL();
DOMString toDataURL(in DOMString type);
DOMObject getContext(in DOMString contextId);
};
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 , media, if the
canvas element is with script
,the canvas element represents 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 represents embedded content with the current image and size.
Otherwise, the element's element represents its fallback content must be
used instead.
In non-visual media, and in visual media with scripting disabled , if the canvas
element's element
is without
script ,the canvas element represents its 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 reinitialized 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
When the canvas is initialized it must be set to fully transparent black.
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 its 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.
Arguments other than the contextId 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 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] .
If the canvas has no pixels (i.e. either
its horizontal dimension or its vertical dimension is zero) then
the method must return the string " data:, ". (This is the shortest data: URI; it represents the empty string in a
text/plain
resource.)
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. (The
one exception to this is if the canvas has either no height or no
width, in which case the result might simply be "
data:,
".)
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 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);
image, DOMString repetition);
image, DOMString repetition);
CanvasPattern createPattern(in HTMLImageElement image, in DOMString repetition);
CanvasPattern createPattern(in HTMLCanvasElement 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);
// 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);
void fillText(in DOMString text, in float x, in float y, in float maxWidth);
void strokeText(in DOMString text, in float x, in float y);
void strokeText(in DOMString text, in float x, in float y, in float maxWidth);
TextMetrics measureText(in DOMString text);
// 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 createImageData(in float sw, in float sh);
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);
// 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)
void putImageData(in ImageData imagedata, in float dx, in float dy, in 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 long int ;
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.
Unless otherwise stated, for the 2D context interface, any method call with a numeric argument whose value is infinite or a NaN value must be ignored.
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.
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 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.
If the angle argument is
infinite, the method call must be ignored.
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-atopsource-insource-outsource-over (default)destination-atopsource-atop but using the destination
image instead of the source image and vice versa.destination-insource-in but using the destination image
instead of the source image and vice versa.destination-outsource-out but using the destination image
instead of the source image and vice versa.destination-oversource-over but using the destination
image instead of the source image and vice versa.lightercopyxorvendorName - operationNameThese values are all case-sensitive — they must be used exactly
as shown. User agents must only
recognise not recognize values
that do not 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 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
.
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. [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 the serialisation 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 serialisation 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.
The addColorStop( offset , color ) method
on the CanvasGradient
interface adds a new stop to a gradient. If the offset is less than 0 or
0, greater than 1 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, representing
arguments that, after being subjected to
the current
transformation matrix ,represent the start point ( x0
, y0 ) and end point ( x1 ,
y1 ) of the gradient, in
coordinate space units, and gradient.
If any of the arguments to createLinearGradient() are infinite or NaN, the method must raise an
INDEX_SIZE_ERR exception. Otherwise, the method must return a
linear CanvasGradient
initialised initialized with that
the specified 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 x 0 = x 1 and y 0 = y 1 , 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, or if any of the
arguments are infinite or NaN, an INDEX_SIZE_ERR
exception must be raised. Otherwise, the
method must return a radial CanvasGradient initialized with the two specified circles, after
transforming them according to the current
transformation matrix .
Radial gradients must be rendered by following these steps:
If x 0 = x 1 and y 0 = y 1 and r 0 = r 1 ,then the radial gradient must paint nothing. Abort these steps.
Let x( ω ) = ( x 1 - x 0 ) ω + x 0
Let y( ω ) = ( y 1 - y 0 ) ω + y 0
Let r( ω ) = ( r 1 - r 0 ) ω + r 0
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.
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
only 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) createPattern( image ,repetition )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 unrecognized value is given, then the user agent
must raise a SYNTAX_ERR exception. User agents must
recognise recognize the four values described above exactly
(e.g. they must not do case folding). The method must return a
CanvasPattern object
suitably initialised. initialized.
The image argument must be an instance of an
HTMLImageElement or
HTMLCanvasElement .
If the image is of the wrong type, 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, then the
implementation must raise an INVALID_STATE_ERR
exception.
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 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 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, as its image argument, an
animated image, the poster frame of the animation, or the first
frame of the animation if there is no poster frame, must be
used.
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 zero, negative,
infinite, and negative
NaN 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 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 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
must then be 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, must be added at the end of each 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 bevelbevel roundmiter .
On setting, any other value than the literal strings
round bevelbevel roundmiter 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 join exists at any point in a
filled arc 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 corners on first and last
lines in the outside of
subpath.
In addition to the join, with point where
the diameter equal join occurs, two additional points are relevant to
each join, one for each line: the two corners
found half the line width, and
width away from the origin at join point, one
perpendicular to each line, each on the point where side furthest
from the inside edges other line.
A filled triangle connecting these two
opposite corners with a straight line, with the third point
of the lines touch, 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 triangle
arc connecting those the two
aforementioned corners with a straight line, the third point of the
triangle being join, abutting (and not overlapping) the
point where aforementioned triangle, with the lines touch on diameter
equal to the inside line width and the origin at the point of the
join, must be rendered at joins.
The miter value means that a second filled four- or
five-sided polygon triangle must
(if it can given the miter length) be
placed rendered at the join, with two of the lines one
line being the perpendicular edges
of line between the joining lines, 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 limit. length.
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 of the two continuation lines to
the line width. If the miter limit
length would be exceeded, then a fifth line this
second triangle must not 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. rendered.
The miter limit ratio can be explicitly set using the miterLimit
attribute. On setting, zero zero, negative, infinite, and negative NaN 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.
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 serialisation
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. [CSS3COLOR]
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
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. 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, if the value is greater than or
equal to zero, then setting the
attribute must be set to the new value;
otherwise, value, except if the
new value is negative, infinite or NaN, in which case the new value
must be ignored.
Support for shadows is optional. When they are supported, then, when shadows are drawn, they must be rendered as follows:
Let A be the source image for which a shadow is being created.
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.
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 paths
region , and, with the exception of
clearRect() , also shadow effects , global alpha , and global composition
operators .
Negative values for width and height must
cause the implementation to raise an INDEX_SIZE_ERR
exception. The clearRect() clearRect(
x ,y ,w ,h )path
region to a fully transparent black,
erasing any previous image. If either height or width are zero,
this method has no effect.
The fillRect() fillRect(
x ,y ,w ,h )fillStyle . If either height or width are
zero, this method has no effect.
The strokeRect() strokeRect( x ,y ,w ,h )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 coordinates given in the
arguments points and lines added
to the path by these methods must be
transformed according to the current transformation matrix
before applying the calculations described
below and before adding any points to the path. 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.
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 ) 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 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 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 do nothing if the context has no
subpaths. If the context does have a subpath, then the
behaviour behavior depends on the arguments and the last
point in the subpath.
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. Let
If the point ( The Arc x0 ,y0 be
) is equal to the shortest arc given by circumference of point ( x1 ,y1 ), or if the
circle point
( x1
,y1 ) is equal to the point ( x2 ,y2 ), then the method
must add the point ( x1 ,y1 ) to the subpath,
and connect that has one point
tangent to the line defined 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: if the direction from (
x0 ,y0 ) to
( x1 , y1 ) is the same as the direction from ( x1 ,y1 ) to (
x2 ,y2 ), another then method must add
the point tangent ( x1 ,y1 ) to the
line defined subpath, and connect that point to the previous point
( x0
,y0 ) by a straight line;
otherwise, the points direction from ( x0 ,y0 ) to (
x1 , y1 ) and is the opposite of the
direction from ( x1 ,y1 ) to (
x2 , y2 ), and that has radius the method
must add a point ( radius
. The points at which this circle touches these two lines are
called x ∞ ,y ∞
) to the start subpath, and
end tangent points respectively. If
connect that point to the previous point ( x0 ,y0 ) by a straight
line, where ( x
∞ ,y ∞ ) is the point
that is infinitely far away from ( x1 ,y1 ), that lies on the
same line as ( x0 ,y0 ), (
x1 ,y1 ),
and ( x2 , y2
) ), and
that is on the same side of (
x1 ,y1 ) on
that line defined as ( x2 ,y2 ).
Otherwise, let The Arc be the shortest arc given by circumference of the points 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 )
then and
crosses the method must do nothing, as
no arc would satisfy point (
x2 ,y2 ).
The points at which this circle touches these two lines are
called the above constraints.
start and end tangent points
respectively.
Otherwise, the 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 , and finally add adding
the start and end tangent points point 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 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
angles are equal, 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.
Negative values for w and h must cause the
implementation to raise an INDEX_SIZE_ERR exception. The
fill() method must fill
each subpath all
the subpaths of the current path in
turn, 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
stroke each subpath calculate the strokes of all the subpaths of the
current path in turn, path, using the strokeStyle
lineWidthlineWidth
lineCaplineJoin , and (if appropriate) miterLimit attributes. 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 transformations , shadow effects , global alpha , the clipping paths
region , and global composition
operators . The transformation is applied
to (Transformations affect the
path when it the
path is drawn, created, not when it is
painted, though the path
stroke style is constructed. Thus, a single path can be constructed and
then drawn according to different transformations without
recreating 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 path
region by calculating the
intersection of the current clipping path 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 path, region, without affecting the actual subpaths.
The new clipping region replaces the current
clipping region.
When the context is created,
initialized, the initial clipping path
is 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'
canvas coordinate space unaffected by
the current transformation, is within
inside the area current path; 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.
The font DOM
attribute, on setting, must be parsed the same way as the 'font'
property of CSS (but without supporting property-independent
stylesheet syntax like 'inherit'), and the resulting font must be
assigned to the context, with the 'line-height' component forced to
'normal'. [CSS]
Font names must be interpreted in the
context of the canvas element's
stylesheets; any fonts embedded using that@font-face must therefore be available. [CSSWEBFONTS]
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. [CSSOM]
When the context is created, the font of
the context must be filled
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.
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.
The textAlign DOM
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 DOM attribute, on getting, must return the current
path 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:
tophangingmiddlealphabeticideographicbottomThe 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 browsing 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 element set to font and
the 'direction' property of the inline element set to the
'direction' property 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 filled; 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 lefttextAlign is start and the
'direction' property on the canvas element has a computed value of 'ltr'textAlign is end and the
'direction' property on the canvas element has a computed value of 'rtl'textAlign is righttextAlign is end and the
'direction' property on the canvas element has a computed value of 'ltr'textAlign is start and the
'direction' property on the canvas element has a computed value of 'rtl'textAlign is centerVertical position:
textBaseline is toptextBaseline is hangingtextBaseline is middletextBaseline is alphabetictextBaseline is ideographictextBaseline is bottomPaint 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 browsing context, as given by the font attribute,
and must then return false
otherwise. 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 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
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 must be an instance of an
HTMLImageElement or
HTMLCanvasElement .
If the image is of the wrong type, type or null,
the implementation must raise a TYPE_MISMATCH_ERR
exception.
If one of the sy image argument is
an HTMLImageElement object whose complete attribute is false, 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 , and sy + sh arguments is outside
).
If the size
of source rectangle is not entirely
within the source image, or if
one of the dw sw and
or dh sh arguments
is negative, zero, the implementation must raise an
INDEX_SIZE_ERR exception. If
The destination rectangle is the
rectangle whose corners are the four points
( image dx ,dy argument is
an ), ( HTMLImageElement dx + dw object whose complete attribute is false, then the
implementation must raise an INVALID_STATE_ERR
exception. , dy ), (
dx
+ dw ,dy + dh ), ( dx ,dy + dh ).
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
), rectangle, after applying the
current transformation matrix
. to the
points of the destination rectangle.
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, as its image argument, an
animated image, the poster frame of the animation, or the first
frame of the animation if there is no poster frame, must be
used.
Images are painted without affecting the current path, and are
subject to shadow effects ,
global
alpha , the clipping paths
region , and global composition
operators .
The createImageData( sw ,sh ) method 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 ,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 which has its top left corner
at whose corners are the
four points ( sx ,
sy ) coordinate, and that
has width ), ( sx + sw and
height , 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 either the sw or sh arguments are zero,
the method must instead raise an INDEX_SIZE_ERR exception.
ImageData objects must be
initialised 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 the data attribute is
initialised initialized 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 values of the data array may be changed (the length of the
array, and the other attributes in ImageData objects, are all read-only). On
setting, JS undefined values must be converted to
zero. Other values must first be converted to numbers using
JavaScript's ToNumber algorithm, and if the result is not a number,
NaN value, a
TYPE_MISMATCH_ERR exception must be raised. If the
result is less than 0, it must be clamped to zero. If the result is
more than 255, it must be clamped to 255. If the number is not an
integer, it must be rounded to the nearest integer using the IEEE
754r roundTiesToEven rounding mode. [ECMA262] [IEEE754R]
The width and height ( w and
h ) might be different from the sw and sh arguments to the function, above
methods, e.g. if the canvas is backed by a high-resolution
bitmap. If the getImageData( sx , sy , sw ,
sh ) method is called with either bitmap, or if the sw
or and
sh arguments set to zero
or negative values, the method must raise an INDEX_SIZE_ERR
exception. are negative.
The putImageData( imagedata , dx , dy , dirtyX ,dirtyY ,dirtyWidth ,dirtyHeight ) method
must take the given writes data from ImageData structure,
and place it at the specified location ( dx , dy ) in the canvas
coordinate space, mapping each pixel represented by
structures back to the ImageData structure into one device pixel.
canvas.
If the first argument to the method is null or not an object whose
[[Class]] property is ImageData object that was returned by , but all of the
following conditions are true,createImageData() or getImageData() then the method must treat the first argument as if it was
an ImageData putImageData()object (and thus not method
must raise the a TYPE_MISMATCH_ERR exception): The method's first argument is
exception.
If any of the arguments to the method are
infinite or NaN, the method must raise an object with INDEX_SIZE_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 attributes
member of the imagedata structure,
respectively.
When invoked with integer values and a arguments that do not, per the last few paragraphs,
cause an exception to be raised, the data
putImageData()attribute whose value method
must act as follows:
Let dx device 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 dy device 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 an
enumerable list negative, let
dirtyX be dirtyX +
dirtyWidth
,and let dirtyWidth be equal to
the absolute magnitude of values that
are either JS Numbers or dirtyWidth .
If dirtyHeight is
negative, let dirtyY be
dirtyY
+ dirtyHeight ,and let dirtyHeight be equal
to the JS value undefined .
absolute magnitude of dirtyHeight .
If dirtyX is negative,
let dirtyWidth be
ImageData 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 object's 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 zero. The the ImageData heightobject's 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 greater
than zero. negative or zero, stop these
steps without affecting the canvas.
Otherwise, for all values of
x and y where
ImageData dirtyX ≤ x < object's dirtyX +
dirtyWidth
width multiplied by its and height
dirtyY ≤ y < multiplied by 4 is equal to dirtyY + dirtyHeight ,copy the number
four channels of entries the pixel with
coordinate ( x ,y ) in the
imagedata
data structure to the pixel with coordinate
( ImageData x device object's +
x data , property. In the y device
data + y property, undefined
values must be treated as zero, any numbers below zero must be
clamped to zero, any numbers above 255 must be clamped to 255, and
any numbers that are not integers must be rounded to
) in the nearest
integer using underlying pixel data
of the IEEE 754r roundTiesToEven
rounding mode. [IEEE754R] 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), 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 path, transformation matrix ,
shadow attributes ,
global
alpha , the clipping path
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. Thus, while one could create an ImageData object, one would not 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 a blank slatevar data = CreateImageData(actualWidth, actualHeight);var data = context.createImageData(canvas.width, canvas.height); // create some plasma FillPlasma(data, 'green'); // green plasma // add a cloud to the plasmaAddCloud(data, actualWidth/2, actualHeight/2); // put a cloud in the middleAddCloud(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 AddCload(data, x, y) { ... }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);
// edge detection
// get an empty slate to put the data into
var output = context.crateImageData(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 = new Array(w*h*4);
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
var output = {
width: w,
height: h,
data: outputData
};
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, creating image A
, as described in the previous sections. For shapes, the current
fill, stroke, and line styles must be honoured, honored, and
the stroke must itself also be subjected to the current
transformation matrix.
If shadows are supported:
Render the shadow from image A , using the current shadow styles, creating image B .
Multiply the alpha component of every pixel in B by globalAlpha .
Within the clipping path,
region, composite B
over the current canvas bitmap using the current composition
operator.
Multiply the alpha component of every pixel in A by globalAlpha .
Within the clipping path,
region, composite A
over the current canvas bitmap using the current composition
operator.
The canvas APIs must perform
colour color correction at only two points: when
rendering images with their own gamma correction and color space information onto the canvas, to
convert the image to the color space used by the canvas (e.g. using
the drawImage() method with an HTMLImageElement object), and when
rendering the actual canvas bitmap to the output device.
Thus, in the 2D context, colors used to draw shapes
onto the canvas will exactly match colors obtained through the
getImageData() method.
The toDataURL() method must not include color
space information in the resource returned. Where the output format allows it, the color of pixels
in resources created by toDataURL() must
match those returned by the getImageData() method.
In user agents that support CSS, the color space used by a
canvas element must match the
color space used for processing any colors for that element in
CSS.
The gamma correction and color space
information of images must be handled in such a way that an image
rendered directly using an img element would
use the same colors as one painted on a canvas element that is then itself rendered. Furthermore, the
rendering of images that have no color correction information (such
as those returned by the toDataURL() method) must be rendered with no color
correction.
Thus, in the 2D context,
calling the drawImage()
method to render the output of the
toDataURL() method to the canvas, given the appropriate dimensions,
has no visible effect.
canvas elementsInformation leakage can occur if scripts from one origin are exposed to images from another origin (one that isn't the same ).
To mitigate this, canvas elements are defined to have a flag indicating whether
they are origin-clean
.All canvas elements must start with their origin-clean set to
true. The flag must be set to false if any of the following actions
occur:
The element's 2D context's
drawImage()
method is called with an HTMLImageElement whose origin is not the
same as that of
the Document
object that owns the canvas element.
The element's 2D context's
drawImage()
method is called with an HTMLCanvasElement whose origin-clean flag is
false.
The element's 2D context's
fillStyle
attribute is set to a CanvasPattern object that was created from an HTMLImageElement whose origin is not the
same as that of
the Document
object that owns the canvas element.
The element's 2D context's
fillStyle
attribute is set to a CanvasPattern object that was created from an HTMLCanvasElement whose origin-clean flag is
false.
The element's 2D context's
strokeStyle
attribute is set to a CanvasPattern object that was created from an HTMLImageElement whose origin is not the
same as that of
the Document
object that owns the canvas element.
The element's 2D context's
strokeStyle
attribute is set to a CanvasPattern object that was created from an HTMLCanvasElement whose origin-clean flag is
false.
Whenever the toDataURL() method of a canvas element
whose origin-clean
flag is set to false is called, the method
must immediately raise a security exception.
Whenever the getImageData() method of the 2D context of a canvas element whose origin-clean flag is set
to false is called, the method must immediately raise a security
exception.
map elementname interface HTMLMapElement : HTMLElement {
attribute DOMString name;
readonly attribute HTMLCollection areas;
readonly attribute HTMLCollection images;
};
The map element, in conjuction conjunction
with any area element descendants,
defines an image map .
There The name
attribute gives the map a name so that it can
be referenced. The attribute must always be an
present and must have a non-empty value.
Whitespace is significant in this attribute's value. If the
id attribute
present on map elements. is also specified, both attributes must have the same
value.
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.
The DOM attribute name must
reflect
the content attribute of the same
name.
area elementmap element
ancestor.altcoordsshapehreftargetpingrelmediahreflangtypeinterface 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.
omitted. 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 shape must reflect the
shape content
attribute, limited
to only known values .
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
hash-name 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 hash-name
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 x i be the (2 i ) th entry in coords , and y i be the (2 i +1) th entry in coords (the first entry in coords being the one with index 0).
Let the coordinates be ( x i , y i ), 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 math element from
the MathML
namespace falls into the
embedded
content category for the purposes
of the content models in this specification.
User agents must handle text other
than inter-element whitespace found in MathML elements whose content models do not
allow raw text by pretending for the purposes of MathML content
models, layout, and rendering that that text is actually wrapped in
an mtext
element in the MathML namespace
.(Such text is not, however,
conforming.)
User agents must act as if any MathML
element whose contents does not match the element's content model
was replaced, for the purposes of MathML layout and rendering, by
an merror
element in the MathML namespace
containing some appropriate error
message.
To enable authors to use MathML tools that only accept MathML in its XML form, interactive HTML user agents are encouraged to provide a way to export any MathML fragment as a namespace-well-formed XML fragment.
The svg element from
the SVG
namespace falls into the
embedded
content category for the purposes
of the content models in this specification.
To enable authors to use SVG tools that only accept SVG in its XML form, interactive HTML user agents are encouraged to provide a way to export any SVG fragment as a namespace-well-formed XML fragment.
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.