This specification is like no other — it has been processed with you, the humble web developer, in mind.
The focus of this specification is readability and ease of access. Unlike the full HTML Standard, this "developer's edition" removes information that only browser vendors need know. It is automatically produced from the full specification by our build tooling, and thus always in sync with the latest developments in HTML.
To read about its conception, construction, and future, read the original press release, and the blog post about its relaunch.
Finally, feel free to contribute on GitHub to make this edition better for everyone!
canvas elementPath2D objectsImageBitmap rendering contextOffscreenCanvas interfacecanvas elementa elements, img elements with
usemap attributes, button elements,
input elements whose type attribute are in
the Checkbox or Radio Button states, input elements that are
buttons, select elements with a multiple attribute or a display size greater than 1, and elements that would not be
interactive content except for having the tabindex attribute specified.width — Horizontal dimensionheight — Vertical dimensionHTMLCanvasElement.The canvas element provides scripts with a resolution-dependent bitmap canvas,
which can be used for rendering graphs, game graphics, art, 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 Web Components.
When authors use the canvas element, they must also provide content that, when
presented to the user, conveys essentially the same function or purpose as the
canvas's bitmap. 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, if scripting is enabled for
the canvas element, and if support for canvas elements has been enabled,
then the canvas element represents embedded content
consisting of a dynamically created image, the element's bitmap.
In non-interactive, static, visual media, if the canvas element has been
previously associated with a rendering context (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 represents
embedded content with the element's current bitmap and size. Otherwise, the element
represents its fallback content instead.
In non-visual media, and in visual media if scripting is
disabled for the canvas element or if support for canvas elements
has been disabled, the canvas element represents its fallback
content instead.
When a canvas element represents embedded content, the
user can still focus descendants of the canvas element (in the fallback
content). When an element is focused, it is the target of keyboard interaction
events (even though the element itself is not visible). This allows authors to make an interactive
canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no
effect on mouse interaction events.) [UIEVENTS]
An element whose nearest canvas element ancestor is being rendered
and represents embedded content is an element that is being used as
relevant canvas fallback content.
The canvas element has two attributes to control the size of the element's bitmap:
width and height. These attributes, when specified, must have
values that are valid non-negative integers. The width
attribute defaults to 300, and the height attribute
defaults to 150.
The intrinsic dimensions of the canvas element when it
represents embedded content are equal to the dimensions of the
element's bitmap.
The user agent must use a square pixel density consisting of one pixel of image data per
coordinate space unit for the bitmaps of a canvas and its rendering contexts.
A canvas element can be sized arbitrarily by a style sheet, its
bitmap is then subject to the 'object-fit' CSS property.
getContext(contextId [, ... ] )Returns an object that exposes an API for drawing on the canvas. The first argument
specifies the desired API, either "2d", "bitmaprenderer" or "webgl". Subsequent arguments are handled by that API.
This specification defines the "2d" and "bitmaprenderer" contexts below. There is also a
specification that defines a "webgl" context.
[WEBGL]
Returns null if the given context ID is not supported, or if the canvas has already been
initialized with another context type (e.g. trying to get a "2d" context after getting a "webgl" context).
toDataURL( [ type [, quality ] ] )Returns a data: URL for the image in the
canvas.
The first argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is "image/png"; that type is also used if the given type isn't
supported. The second argument applies if the type is an image format that supports variable
quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive
indicating the desired quality level for the resulting image.
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 of 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:,".)
toBlob(callback [, type [, quality ] ] )Creates a Blob object representing a file containing the image in the canvas,
and invokes a callback with a handle to that object.
The second argument, if provided, controls the type of the image to be returned (e.g. PNG or
JPEG). The default is "image/png"; that type is also used if the given type isn't
supported. The third argument applies if the type is an image format that supports variable
quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive
indicating the desired quality level for the resulting image.
transferControlToOffscreen()Returns a newly created OffscreenCanvas object that uses the
canvas element as a placeholder. Once the canvas element has become a
placeholder for an OffscreenCanvas object, its intrinsic size can no longer be
changed, and it cannot have a rendering context. The content of the placeholder canvas is
updated by calling the commit() method of the
OffscreenCanvas object's rendering context.
getContext('2d' [, { [ alpha: false ] } ] )Returns a CanvasRenderingContext2D object that is permanently bound to a
particular canvas element.
If the alpha setting is
provided and set to false, then the canvas is forced to always be opaque.
canvasReturns the canvas element.
The CanvasFillRule enumeration is used to select the fill rule
algorithm by which to determine if a point is inside or outside a path.
The value "nonzero" value
indicates the non-zero winding rule, wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path going in one direction is equal to the
number of times it crosses the path going in the other direction.
The "evenodd" value indicates
the even-odd rule, wherein
a point is considered to be outside a shape if the number of times a half-infinite straight
line drawn from that point crosses the shape's path is even.
If a point is not outside a shape, it is inside the shape.
The ImageSmoothingQuality enumeration is used to express a preference for the
interpolation quality to use when smoothing images.
The "low" value
indicates a preference for a low level of image interpolation quality. Low-quality image
interpolation may be more computationally efficient than higher settings.
The "medium" value
indicates a preference for a medium level of image interpolation quality.
The "high" value
indicates a preference for a high level of image interpolation quality. High-quality image
interpolation may be more computationally expensive than lower settings.
Bilinear scaling is an example of a relatively fast, lower-quality image-smoothing algorithm. Bicubic or Lanczos scaling are examples of image-smoothing algorithms that produce higher-quality output. This specification does not mandate that specific interpolation algorithms be used.
The output bitmap, when it is not directly displayed by the user agent,
implementations can, instead of updating this bitmap, merely remember the sequence of drawing
operations that have been applied to it until such time as the bitmap's actual data is needed
(for example because of a call to drawImage(), or
the createImageBitmap() factory method). In many
cases, this will be more memory efficient.
The bitmap of a canvas element is the one bitmap that's pretty much always going
to be needed in practice. The output bitmap of a rendering context, when it has one,
is always just an alias to a canvas element's bitmap.
Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its intrinsic size, or to enable double buffering so that graphics updates, like page scrolling for example, can be processed concurrently while canvas draw commands are being executed.
Objects that implement the CanvasState interface maintain a stack of drawing
states. Drawing states consist of:
strokeStyle, fillStyle, globalAlpha, lineWidth, lineCap, lineJoin, miterLimit, lineDashOffset, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, filter, globalCompositeOperation, font, textAlign, textBaseline, direction, imageSmoothingEnabled, imageSmoothingQuality.The current default path and the rendering context's bitmaps are not
part of the drawing state. The current default path is persistent, and can only be
reset using the beginPath() method. The bitmaps
depend on whether and how the rendering context is bound to a canvas element.
save()Pushes the current state onto the stack.
restore()Pops the top state on the stack, restoring the context to that state.
lineWidth [ = value ]lineWidth [ = value ]Returns the current line width.
Can be set, to change the line width. Values that are not finite values greater than zero are ignored.
lineCap [ = value ]lineCap [ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
The possible line cap styles are "butt", "round", and "square". Other values are ignored.
lineJoin [ = value ]lineJoin [ = value ]Returns the current line join style.
Can be set, to change the line join style.
The possible line join styles are "bevel", "round", and "miter". Other values are ignored.
miterLimit [ = value ]miterLimit [ = value ]Returns the current miter limit ratio.
Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.
setLineDash(segments)setLineDash(segments)Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.
getLineDash()getLineDash()Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).
lineDashOffsetlineDashOffsetReturns the phase offset (in the same units as the line dash pattern).
Can be set, to change the phase offset. Values that are not finite values are ignored.
font [ = value ]font [ = value ]Returns the current font settings.
Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.
Relative keywords and lengths are computed relative to the font of the canvas
element.
textAlign [ = value ]textAlign [ = value ]Returns the current text alignment settings.
Can be set, to change the alignment. The possible values are and their meanings are given
below. Other values are ignored. The default is "start".
textBaseline [ = value ]textBaseline [ = value ]Returns the current baseline alignment settings.
Can be set, to change the baseline alignment. The possible values and their meanings are
given below. Other values are ignored. The default is "alphabetic".
direction [ = value ]direction [ = value ]Returns the current directionality.
Can be set, to change the directionality. The possible values and their meanings are given
below. Other values are ignored. The default is "inherit".
The textAlign attribute's allowed keywords are
as follows:
start
Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).
end
Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).
left
Align to the left.
right
Align to the right.
center
Align to the center.
The textBaseline
attribute's allowed keywords correspond to alignment points in the
font:
The keywords map to these alignment points as follows:
top
hanging
middle
alphabetic
ideographic
bottom
The direction attribute's allowed keywords are
as follows:
ltr
Treat input to the text preparation algorithm as left-to-right text.
rtl
Treat input to the text preparation algorithm as right-to-left text.
inherit
Default to the directionality of the canvas element or Document
as appropriate.
The text preparation algorithm is as follows. It takes as input a string text
, a CanvasTextDrawingStyles object target, and an optional length
maxWidth. It returns an array of glyph shapes, each positioned on a common coordinate
space, a physical alignment whose value is one of left, right, and
center, and an inline box. (Most callers of this algorithm ignore the
physical alignment and the inline box.)
If maxWidth was provided but is less than or equal to zero or equal to NaN, then return an empty array.
Replace all ASCII whitespace in text with U+0020 SPACE characters.
Let font be the current font of target, as given
by that object's font attribute.
Apply the appropriate step from the following list to determine the value of direction:
direction attribute has the value "ltr"direction attribute has the value "rtl"Document with a non-null document elementForm a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]
If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.
The anchor point is a point on the inline box, and the physical
alignment is one of the values left, right, and center. These
variables are determined by the textAlign and
textBaseline values as follows:
Horizontal position:
textAlign is lefttextAlign is start and direction is
'ltr'textAlign is end and direction is 'rtl'textAlign is righttextAlign is end and direction is 'ltr'textAlign is start and direction is
'rtl'textAlign is centerVertical position:
textBaseline is toptextBaseline is hangingtextBaseline is middletextBaseline is alphabetictextBaseline is ideographictextBaseline is bottomLet result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin is at the anchor point.
Return result, physical alignment, and the inline box.
Objects that implement the CanvasPath interface have a path. A path has a list of zero or
more subpaths. Each subpath consists of a list of one or more points, connected by straight or
curved line segments, 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 only one point are ignored when painting the path.
Paths have a need new subpath flag. When this flag is set, certain APIs create a new subpath rather than extending the previous one. When a path is created, its need new subpath flag must be set.
When an object implementing the CanvasPath interface is created, its path must be initialized to zero subpaths.
moveTo(x, y)moveTo(x, y)Creates a new subpath with the given point.
closePath()closePath()Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
lineTo(x, y)lineTo(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
quadraticCurveTo(cpx, cpy, x, y)quadraticCurveTo(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
arcTo(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )arcTo(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.
If two radii are provided, then the first controls the width of the arc's ellipse, and the second controls the height. If only one is provided, or if they are the same, then the arc is from a circle. In the case of an ellipse, the rotation argument controls the clockwise inclination of the ellipse relative to the x-axis.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
arc(x, y, radius, startAngle, endAngle [, anticlockwise ] )arc(x, y, radius, startAngle, endAngle [, anticlockwise ] )Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.
Throws an "IndexSizeError" DOMException if the given
radius is negative.
rect(x, y, w, h)rect(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
Path2D objectsPath2D objects can be used to declare paths that are then later used on
objects implementing the CanvasDrawPath interface. In addition to many of the APIs
described in earlier sections, Path2D objects have methods to combine paths, and to
add text to paths.
Path2D()Creates a new empty Path2D object.
Path2D(path)Creates a new Path2D object that is a copy of the argument.
Path2D(paths [, fillRule ] )Creates a new Path2D object that describes a path that outlines the given paths, using the given fill rule.
Path2D(d)Creates a new path with the path described by the argument, interpreted as SVG path data. [SVG]
addPath(path [, transform ] )Adds to the path the path given by the argument.
Objects that implement the CanvasTransform interface have a current
transformation matrix, as well as methods (described in this section) to manipulate it. When
an object implementing the CanvasTransform interface is created, its transformation
matrix must be initialized to the identity transform.
The current transformation matrix is applied to coordinates when creating the
current default path, and when painting text, shapes, and Path2D
objects, on objects implementing the CanvasTransform interface.
Most of the API uses DOMMatrix objects rather than this API. This API
remains mostly for historical reasons.
scale(x, y)Changes the current transformation matrix to apply a scaling transformation with the given characteristics.
rotate(angle)Changes the current transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.
translate(x, y)Changes the current transformation matrix to apply a translation transformation with the given characteristics.
transform(a, b, c, d, e, f)Changes the current transformation matrix to apply the matrix given by the arguments as described below.
getTransform()Returns a copy of the current transformation matrix, as a newly created
DOMMatrix object.
setTransform(a, b, c, d, e, f)Changes the current transformation matrix to the matrix given by the arguments as described below.
setTransform(transform)Changes the current transformation matrix to the matrix represented by
the passed DOMMatrix2DInit dictionary.
resetTransform()Changes the current transformation matrix to the identity transform.
| a | c | e |
| b | d | f |
| 0 | 0 | 1 |
The arguments a, b, c, d, e, and f are sometimes called m11, m12, m21, m22, dx, and dy or m11, m21, m12, m22, dx, and dy. Care ought to be taken in particular with the order of the second and third arguments (b and c) as their order varies from API to API and APIs sometimes use the notation m12/m21 and sometimes m21/m12 for those positions.
Some methods on the CanvasDrawImage and CanvasFillStrokeStyles
interfaces take the union type CanvasImageSource as an argument.
This union type allows objects implementing any of the following interfaces to be used as image sources:
HTMLOrSVGImageElement (img or SVG image
elements)HTMLVideoElement (video elements)HTMLCanvasElement (canvas elements)ImageBitmapAlthough not formally specified as such, SVG image
elements are expected to be implemented nearly identical to img elements. That is,
SVG image elements share the fundamental concepts and features of
img elements.
The ImageBitmap interface can be created from a number of other
image-representing types, including ImageData.
When a user agent is required to check the usability of the image
argument, where image is a CanvasImageSource object, the
user agent must run these steps, which return either good, bad, or
aborted:
If image is an HTMLOrSVGImageElement object that is in the broken state, then throw an
"InvalidStateError" DOMException, return aborted,
and abort these steps.
If image is an HTMLOrSVGImageElement object that is not fully decodable, or if image is an
HTMLVideoElement object whose readyState
attribute is either HAVE_NOTHING or HAVE_METADATA, then return bad and abort these
steps.
If image is an HTMLOrSVGImageElement object with an
intrinsic width or intrinsic height (or both) equal to zero, then
return bad and abort these steps.
If image is an HTMLCanvasElement object with either a horizontal
dimension or a vertical dimension equal to zero, then return bad and abort these
steps.
If image is an ImageBitmap object with its
[[Detached]] internal slot value set to true, then throw an
"InvalidStateError" DOMException, return aborted,
and abort these steps.
Return good.
When a CanvasImageSource object represents an HTMLOrSVGImageElement,
the element's image must be used as the source image.
Specifically, when a CanvasImageSource object represents an animated image in an
HTMLOrSVGImageElement, the user agent must use the default image of the animation
(the one that the format defines is to be used when animation is not supported or is disabled),
or, if there is no such image, the first frame of the animation, when rendering the image for
CanvasRenderingContext2D APIs.
When a CanvasImageSource object represents an HTMLVideoElement, then
the frame at the current playback position when the method with the argument is
invoked must be used as the source image when rendering the image for
CanvasRenderingContext2D APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource
(i.e. after any aspect-ratio correction has been applied).
When a CanvasImageSource object represents an HTMLCanvasElement, the
element's bitmap must be used as the source image.
When a CanvasImageSource object represents an element that is being
rendered and that element has been resized, the original image data of the source image
must be used, not the image as it is rendered (e.g. width and
height attributes on the source element have no effect on how
the object is interpreted when rendering the image for CanvasRenderingContext2D
APIs).
When a CanvasImageSource object represents an ImageBitmap, the
object's bitmap image data must be used as the source image.
The image argument is not origin-clean if it is an
HTMLOrSVGImageElement or HTMLVideoElement whose origin is
not the same as the origin specified by the entry settings
object, or if it is an HTMLCanvasElement whose bitmap's origin-clean flag is false.
fillStyle [ = value ]Returns the current style used for filling shapes.
Can be set, to change the fill style.
The style can be either a string containing a CSS color, or a CanvasGradient or
CanvasPattern object. Invalid values are ignored.
strokeStyle [ = value ]Returns the current style used for stroking shapes.
Can be set, to change the stroke style.
The style can be either a string containing a CSS color, or a CanvasGradient or
CanvasPattern object. Invalid values are ignored.
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.
addColorStop(offset, color)Adds a color stop with the given color to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.
Throws an "IndexSizeError" DOMException if the offset
is out of range. Throws a "SyntaxError" DOMException if
the color cannot be parsed.
createLinearGradient(x0, y0, x1, y1)Returns a CanvasGradient object that represents a
linear gradient that paints along the line given by the
coordinates represented by the arguments.
createRadialGradient(x0, y0, r0, x1, y1, r1)Returns a CanvasGradient object that represents a
radial gradient that paints along the cone given by the circles
represented by the arguments.
If either of the radii are negative, throws an
"IndexSizeError" DOMException exception.
Patterns are represented by objects implementing the opaque CanvasPattern
interface.
createPattern(image, repetition)Returns a CanvasPattern object that uses the given image and repeats in the
direction(s) given by the repetition argument.
The allowed values for repetition are repeat (both
directions), repeat-x (horizontal only), repeat-y
(vertical only), and no-repeat (neither). If the repetition argument is empty, the value repeat is used.
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an "InvalidStateError" DOMException.
setTransform(transform)Sets the transformation matrix that will be used when rendering the pattern during a fill or stroke painting operation.
Objects that implement the CanvasRect interface provide the following methods for
immediately drawing rectangles to the bitmap. The methods 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.
clearRect(x, y, w, h)Clears all pixels on the bitmap in the given rectangle to transparent black.
fillRect(x, y, w, h)Paints the given rectangle onto the bitmap, using the current fill style.
strokeRect(x, y, w, h)Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.
fillText(text, x, y [, maxWidth ] )strokeText(text, x, y [, maxWidth ] )Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.
measureText(text)Returns a TextMetrics object with the metrics of the given text in the current
font.
widthactualBoundingBoxLeftactualBoundingBoxRightfontBoundingBoxAscentfontBoundingBoxDescentactualBoundingBoxAscentactualBoundingBoxDescentemHeightAscentemHeightDescenthangingBaselinealphabeticBaselineideographicBaselineReturns the measurement described below.
width attributeThe width of that inline box, in CSS pixels. (The text's advance width.)
actualBoundingBoxLeft attributeThe distance parallel to the baseline from the alignment point given by the textAlign attribute to the left side of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going left from the given alignment point.
The sum of this value and the next (actualBoundingBoxRight) can be wider than
the width of the inline box (width), in
particular with slanted fonts where characters overhang their advance width.
actualBoundingBoxRight attributeThe distance parallel to the baseline from the alignment point given by the textAlign attribute to the right side of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going right from the given alignment point.
fontBoundingBoxAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the top of the highest
bounding rectangle of all the fonts used to render the text, in CSS
pixels; positive numbers indicating a distance going up from the given baseline.
This value and the next are useful when rendering a background that have to have
a consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent attribute (and
its corresponding attribute for the descent) are useful when drawing a bounding box around
specific text.
fontBoundingBoxDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the bottom of the lowest
bounding rectangle of all the fonts used to render the text, in CSS
pixels; positive numbers indicating a distance going down from the given
baseline.
actualBoundingBoxAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the top of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going up from the given baseline.
This number can vary greatly based on the input text, even if the first font
specified covers all the characters in the input. For example, the actualBoundingBoxAscent of a lowercase
"o" from an alphabetic baseline would be less than that of an uppercase "F". The value can
easily be negative; for example, the distance from the top of the em box (textBaseline value "top") to the top of the bounding rectangle when
the given text is just a single comma "," would likely (unless the font is
quite unusual) be negative.
actualBoundingBoxDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the bottom of the bounding
rectangle of the given text, in CSS pixels; positive numbers
indicating a distance going down from the given baseline.
emHeightAscent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the highest top of the em
squares in the line box, in CSS pixels; positive numbers
indicating that the given baseline is below the top of that em square (so this value will usually
be positive). Zero if the given baseline is the top of that em square; half the font size if the
given baseline is the middle of that em square.
emHeightDescent attributeThe distance from the horizontal line indicated by the textBaseline attribute to the lowest bottom of the em
squares in the line box, in CSS pixels; positive numbers
indicating that the given baseline is below the bottom of that em square (so this value will
usually be negative). (Zero if the given baseline is the bottom of that em square.)
hangingBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the hanging baseline of the
line box, in CSS pixels; positive numbers indicating that
the given baseline is below the hanging baseline. (Zero if the given baseline is the hanging
baseline.)
alphabeticBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the alphabetic baseline of
the line box, in CSS pixels; positive numbers indicating
that the given baseline is below the alphabetic baseline. (Zero if the given baseline is the
alphabetic baseline.)
ideographicBaseline attributeThe distance from the horizontal line indicated by the textBaseline attribute to the ideographic baseline of
the line box, in CSS pixels; positive numbers indicating
that the given baseline is below the ideographic baseline. (Zero if the given baseline is the
ideographic baseline.)
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). Authors are encouraged
to use the bounding box values described above if this is an issue.
A future version of the 2D context API might 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.
Objects that implement the CanvasDrawPath interface have a current default
path. There is only one current default path, it is not part of the
drawing state. The current default path is a path, as described above.
beginPath()Resets the current default path.
fill( [ fillRule ] )fill(path [, fillRule ] )Fills the subpaths of the current default path or the given path with the current fill style, obeying the given fill rule.
stroke()stroke(path)Strokes the subpaths of the current default path or the given path with the current stroke style.
clip( [ fillRule ] )clip(path [, fillRule ] )Further constrains the clipping region to the current default path or the given path, using the given fill rule to determine what points are in the path.
resetClip()Unconstrains the clipping region.
isPointInPath(x, y [, fillRule ] )isPointInPath(path, x, y [, fillRule ] )Returns true if the given point is in the current default path or the given path, using the given fill rule to determine what points are in the path.
isPointInStroke(x, y)isPointInStroke(path, x, y)Returns true if the given point would be in the region covered by the stroke of the current default path or the given path, given the current stroke style.
This canvas element has a couple of checkboxes. The path-related commands are
highlighted:
<canvas height=400 width=750>
<label><input type=checkbox id=showA> Show As</label>
<label><input type=checkbox id=showB> Show Bs</label>
<!-- ... -->
</canvas>
<script>
function drawCheckbox(context, element, x, y, paint) {
context.save();
context.font = '10px sans-serif';
context.textAlign = 'left';
context.textBaseline = 'middle';
var metrics = context.measureText(element.labels[0].textContent);
if (paint) {
context.beginPath();
context.strokeStyle = 'black';
context.rect(x-5, y-5, 10, 10);
context.stroke();
if (element.checked) {
context.fillStyle = 'black';
context.fill();
}
context.fillText(element.labels[0].textContent, x+5, y);
}
context.beginPath();
context.rect(x-7, y-7, 12 + metrics.width+2, 14);
context.drawFocusIfNeeded(element);
context.restore();
}
function drawBase() { /* ... */ }
function drawAs() { /* ... */ }
function drawBs() { /* ... */ }
function redraw() {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
drawCheckbox(context, document.getElementById('showA'), 20, 40, true);
drawCheckbox(context, document.getElementById('showB'), 20, 60, true);
drawBase();
if (document.getElementById('showA').checked)
drawAs();
if (document.getElementById('showB').checked)
drawBs();
}
function processClick(event) {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
var x = event.clientX;
var y = event.clientY;
var node = event.target;
while (node) {
x -= node.offsetLeft - node.scrollLeft;
y -= node.offsetTop - node.scrollTop;
node = node.offsetParent;
}
drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
if (context.isPointInPath(x, y))
document.getElementById('showA').checked = !(document.getElementById('showA').checked);
drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
if (context.isPointInPath(x, y))
document.getElementById('showB').checked = !(document.getElementById('showB').checked);
redraw();
}
document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true);
document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false);
redraw();
</script>
drawFocusIfNeeded(element)drawFocusIfNeeded(path, element)If the given element is focused, draws a focus ring around the current default path or the given path, following the platform conventions for focus rings.
scrollPathIntoView()scrollPathIntoView(path)Scrolls the current default path or the given path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.
Objects that implement the CanvasDrawImage interface have the drawImage method to draw images.
drawImage(image, dx, dy)drawImage(image, dx, dy, dw, dh)drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)Draws the given image onto the canvas. The arguments are interpreted as follows:
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
data, throws an "InvalidStateError" DOMException.
ImageData(sw, sh)createImageData(sw, sh)Returns an ImageData object with the given dimensions. All the pixels in the
returned object are transparent black.
Throws an "IndexSizeError" DOMException if either of
the width or height arguments are zero.
createImageData(imagedata)Returns an ImageData object with the same dimensions as the argument. All the
pixels in the returned object are transparent black.
ImageData(data, sw [, sh ] )Returns an ImageData object using the data provided in the Uint8ClampedArray argument, interpreted using the given
dimensions.
As each pixel in the data is represented by four numbers, the length of the data needs to be a multiple of four times the given width. If the height is provided as well, then the length needs to be exactly the width times the height times 4.
Throws an "IndexSizeError" DOMException if the given
data and dimensions can't be interpreted consistently, or if either dimension is zero.
getImageData(sx, sy, sw, sh)Returns an ImageData object containing the image data for the given rectangle of
the bitmap.
Throws an "IndexSizeError" DOMException if the either
of the width or height arguments are zero.
widthheightReturns the actual dimensions of the data in the ImageData object, in
pixels.
dataReturns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.
putImageData(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ] )Paints the data from the given ImageData object onto the bitmap. If a dirty
rectangle is provided, only the pixels from that rectangle are painted.
The globalAlpha and globalCompositeOperation attributes, as
well as the shadow attributes, are ignored for the purposes of this method call; pixels in the
canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.
Throws an "InvalidStateError" DOMException if the
imagedata object's data's [[Detached]] internal slot value is true.
In the following example, the script generates an ImageData object so that it can
draw onto it.
// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');
// create a blank slate
var data = context.createImageData(canvas.width, canvas.height);
// create some plasma
FillPlasma(data, 'green'); // green plasma
// add a cloud to the plasma
AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle
// paint the plasma+cloud on the canvas
context.putImageData(data, 0, 0);
// support methods
function FillPlasma(data, color) { ... }
function AddCloud(data, x, y) { ... }
Here is an example of using getImageData() and putImageData() to implement an edge detection
filter.
<!DOCTYPE HTML>
<html lang="en">
<head>
<title>Edge detection demo</title>
<script>
var image = new Image();
function init() {
image.onload = demo;
image.src = "image.jpeg";
}
function demo() {
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
// draw the image onto the canvas
context.drawImage(image, 0, 0);
// get the image data to manipulate
var input = context.getImageData(0, 0, canvas.width, canvas.height);
// get an empty slate to put the data into
var output = context.createImageData(canvas.width, canvas.height);
// alias some variables for convenience
// In this case input.width and input.height
// match canvas.width and canvas.height
// but we'll use the former to keep the code generic.
var w = input.width, h = input.height;
var inputData = input.data;
var outputData = output.data;
// edge detection
for (var y = 1; y < h-1; y += 1) {
for (var x = 1; x < w-1; x += 1) {
for (var c = 0; c < 3; c += 1) {
var i = (y*w + x)*4 + c;
outputData[i] = 127 + -inputData[i - w*4 - 4] - inputData[i - w*4] - inputData[i - w*4 + 4] +
-inputData[i - 4] + 8*inputData[i] - inputData[i + 4] +
-inputData[i + w*4 - 4] - inputData[i + w*4] - inputData[i + w*4 + 4];
}
outputData[(y*w + x)*4 + 3] = 255; // alpha
}
}
// put the image data back after manipulation
context.putImageData(output, 0, 0);
}
</script>
</head>
<body onload="init()">
<canvas></canvas>
</body>
</html>
globalAlpha [ = value ]Returns the current alpha value applied to rendering operations.
Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are ignored.
globalCompositeOperation [ = value ]Returns the current composition operation, from the values defined in the Compositing and Blending specification. [COMPOSITE].
Can be set, to change the composition operation. Unknown values are ignored.
imageSmoothingEnabled [ = value ]Returns whether pattern fills and the drawImage() method will attempt to smooth images if
their pixels don't line up exactly with the display, when scaling images up.
Can be set, to change whether images are smoothed (true) or not (false).
imageSmoothingQuality [ = value ]Returns the current image-smoothing-quality preference.
Can be set, to change the preferred quality of image smoothing. The possible values are
"low", "medium" and "high". Unknown values are ignored.
All drawing operations on an object which implements the CanvasShadowStyles
interface are affected by the four global shadow attributes.
shadowColor [ = value ]Returns the current shadow color.
Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.
shadowOffsetX [ = value ]shadowOffsetY [ = value ]Returns the current shadow offset.
Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
shadowBlur [ = value ]Returns the current level of blur applied to shadows.
Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.
If the current composition operation is copy, then shadows
effectively won't render (since the shape will overwrite the shadow).
All drawing operations on an object which implements the CanvasFilters
interface are affected by the global filter
attribute.
filter [ = value ]Returns the current filter.
Can be set, to change the filter. Values that cannot be parsed as a <filter-function-list> value are ignored.
Since drawing is performed using filter value 'none' until an externally-defined
filter has finished loading, authors might wish to determine whether such a filter
has finished loading before proceeding with a drawing operation. One way to accomplish
this is to load the externally-defined filter elsewhere within the same page in some
element that sends a load event (for example, an SVG
use element), and wait for the load event to be
dispatched.
When a canvas is interactive, authors should include focusable elements in the element's fallback content corresponding to each focusable part of the canvas, as in the example above.
When rendering focus rings, to ensure that focus rings have the appearance of native focus
rings, authors should use the drawFocusIfNeeded() method, passing it the
element for which a ring is being drawn. This method only draws the focus ring if the element is
focused, so that it can simply be called whenever drawing the element, without
checking whether the element is focused or not first.
In addition to drawing focus rings, authors should use the scrollPathIntoView() method when an element in
the canvas is focused, to make sure it is visible on the screen (if applicable).
Authors should avoid implementing text editing controls
using the canvas element. Doing so has a large number of disadvantages:
This is a huge amount of work, and authors are most strongly encouraged to avoid doing any of
it by instead using the input element, the textarea element, or the
contenteditable attribute.
Here is an example of a script that uses canvas to draw pretty glowing lines.
<canvas width="800" height="450"></canvas>
<script>
var context = document.getElementsByTagName('canvas')[0].getContext('2d');
var lastX = context.canvas.width * Math.random();
var lastY = context.canvas.height * Math.random();
var hue = 0;
function line() {
context.save();
context.translate(context.canvas.width/2, context.canvas.height/2);
context.scale(0.9, 0.9);
context.translate(-context.canvas.width/2, -context.canvas.height/2);
context.beginPath();
context.lineWidth = 5 + Math.random() * 10;
context.moveTo(lastX, lastY);
lastX = context.canvas.width * Math.random();
lastY = context.canvas.height * Math.random();
context.bezierCurveTo(context.canvas.width * Math.random(),
context.canvas.height * Math.random(),
context.canvas.width * Math.random(),
context.canvas.height * Math.random(),
lastX, lastY);
hue = hue + 10 * Math.random();
context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)';
context.shadowColor = 'white';
context.shadowBlur = 10;
context.stroke();
context.restore();
}
setInterval(line, 50);
function blank() {
context.fillStyle = 'rgba(0,0,0,0.1)';
context.fillRect(0, 0, context.canvas.width, context.canvas.height);
}
setInterval(blank, 40);
</script>
The 2D rendering context for canvas is often used for sprite-based games. The
following example demonstrates this:
Here is the source for this example:
<!DOCTYPE HTML>
<meta charset="utf-8">
<title>Blue Robot Demo</title>
<style>
html { overflow: hidden; min-height: 200px; min-width: 380px; }
body { height: 200px; position: relative; margin: 8px; }
.buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; }
</style>
<canvas width="380" height="200"></canvas>
<script>
var Landscape = function (context, width, height) {
this.offset = 0;
this.width = width;
this.advance = function (dx) {
this.offset += dx;
};
this.horizon = height * 0.7;
// This creates the sky gradient (from a darker blue to white at the bottom)
this.sky = context.createLinearGradient(0, 0, 0, this.horizon);
this.sky.addColorStop(0.0, 'rgb(55,121,179)');
this.sky.addColorStop(0.7, 'rgb(121,194,245)');
this.sky.addColorStop(1.0, 'rgb(164,200,214)');
// this creates the grass gradient (from a darker green to a lighter green)
this.earth = context.createLinearGradient(0, this.horizon, 0, height);
this.earth.addColorStop(0.0, 'rgb(81,140,20)');
this.earth.addColorStop(1.0, 'rgb(123,177,57)');
this.paintBackground = function (context, width, height) {
// first, paint the sky and grass rectangles
context.fillStyle = this.sky;
context.fillRect(0, 0, width, this.horizon);
context.fillStyle = this.earth;
context.fillRect(0, this.horizon, width, height-this.horizon);
// then, draw the cloudy banner
// we make it cloudy by having the draw text off the top of the
// canvas, and just having the blurred shadow shown on the canvas
context.save();
context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0);
context.shadowColor = 'white';
context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas
context.shadowBlur = '5';
context.fillStyle = 'white';
context.textAlign = 'left';
context.textBaseline = 'top';
context.font = '20px sans-serif';
context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas
context.restore();
// then, draw the background tree
context.save();
context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0);
context.beginPath();
context.fillStyle = 'rgb(143,89,2)';
context.lineStyle = 'rgb(10,10,10)';
context.lineWidth = 2;
context.rect(0, this.horizon+5, 10, -50); // trunk
context.fill();
context.stroke();
context.beginPath();
context.fillStyle = 'rgb(78,154,6)';
context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves
context.fill();
context.stroke();
context.restore();
};
this.paintForeground = function (context, width, height) {
// draw the box that goes in front
context.save();
context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0);
context.beginPath();
context.rect(0, this.horizon - 5, 25, 25);
context.fillStyle = 'rgb(220,154,94)';
context.lineStyle = 'rgb(10,10,10)';
context.lineWidth = 2;
context.fill();
context.stroke();
context.restore();
};
};
</script>
<script>
var BlueRobot = function () {
this.sprites = new Image();
this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells
this.targetMode = 'idle';
this.walk = function () {
this.targetMode = 'walk';
};
this.stop = function () {
this.targetMode = 'idle';
};
this.frameIndex = {
'idle': [0], // first cell is the idle frame
'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6
'stop': [7], // last cell is the stopping animation
};
this.mode = 'idle';
this.frame = 0; // index into frameIndex
this.tick = function () {
// this advances the frame and the robot
// the return value is how many pixels the robot has moved
this.frame += 1;
if (this.frame >= this.frameIndex[this.mode].length) {
// we've reached the end of this animation cycle
this.frame = 0;
if (this.mode != this.targetMode) {
// switch to next cycle
if (this.mode == 'walk') {
// we need to stop walking before we decide what to do next
this.mode = 'stop';
} else if (this.mode == 'stop') {
if (this.targetMode == 'walk')
this.mode = 'walk';
else
this.mode = 'idle';
} else if (this.mode == 'idle') {
if (this.targetMode == 'walk')
this.mode = 'walk';
}
}
}
if (this.mode == 'walk')
return 8;
return 0;
},
this.paint = function (context, x, y) {
if (!this.sprites.complete) return;
// draw the right frame out of the sprite sheet onto the canvas
// we assume each frame is as high as the sprite sheet
// the x,y coordinates give the position of the bottom center of the sprite
context.drawImage(this.sprites,
this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height,
x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height);
};
};
</script>
<script>
var canvas = document.getElementsByTagName('canvas')[0];
var context = canvas.getContext('2d');
var landscape = new Landscape(context, canvas.width, canvas.height);
var blueRobot = new BlueRobot();
// paint when the browser wants us to, using requestAnimationFrame()
function paint() {
context.clearRect(0, 0, canvas.width, canvas.height);
landscape.paintBackground(context, canvas.width, canvas.height);
blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1);
landscape.paintForeground(context, canvas.width, canvas.height);
requestAnimationFrame(paint);
}
paint();
// but tick every 150ms, so that we don't slow down when we don't paint
setInterval(function () {
var dx = blueRobot.tick();
landscape.advance(dx);
}, 100);
</script>
<p class="buttons">
<input type=button value="Walk" onclick="blueRobot.walk()">
<input type=button value="Stop" onclick="blueRobot.stop()">
<footer>
<small> Blue Robot Player Sprite by <a href="https://johncolburn.deviantart.com/">JohnColburn</a>.
Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small>
<small> This work is itself licensed under a <a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">Creative
Commons Attribution-ShareAlike 3.0 Unported License</a>.</small>
</footer>
ImageBitmap rendering contextImageBitmapRenderingContext is a performance-oriented interface that provides a
low overhead method for displaying the contents of ImageBitmap objects. It uses
transfer semantics to reduce overall memory consumption. It also streamlines performance by
avoiding intermediate compositing, unlike the drawImage() method of
CanvasRenderingContext2D.
Using an img element as an intermediate for getting an image resource into a
canvas, for example, would result in two copies of the decoded image existing in memory at the
same time: the img element's copy, and the one in the canvas's backing store. This
memory cost can be prohibitive when dealing with extremely large images. This can be avoided by
using ImageBitmapRenderingContext.
Using ImageBitmapRenderingContext, here is how to transcode an image to the JPEG
format in a memory- and CPU-efficient way:
createImageBitmap(inputImageBlob).then(image => {
const canvas = document.createElement('canvas');
const context = canvas.getContext('bitmaprenderer');
context.transferFromImageBitmap(image);
canvas.toBlob(outputJPEGBlob => {
// Do something with outputJPEGBlob.
}, 'image/jpeg');
});
ImageBitmapRenderingContext interfacegetContext('bitmaprenderer' [, { [ alpha: false ] } ] )Returns an ImageBitmapRenderingContext object that is permanently bound to a
particular canvas element.
If the alpha setting is
provided and set to false, then the canvas is forced to always be opaque.
canvasReturns the canvas element that the context is bound to.
transferFromImageBitmap(imageBitmap)Transfers the underlying bitmap data
from imageBitmap to context, and the bitmap becomes the contents of the
canvas element to which context is bound.
transferFromImageBitmap(
null)Replaces contents of the canvas element to which context
is bound with a transparent black bitmap whose size corresponds to the width and height
content attributes of the canvas element.
OffscreenCanvas interfaceOffscreenCanvas objects are used to create rendering contexts, much like an
HTMLCanvasElement, but with no connection to the DOM. This makes it possible to
use canvas rendering contexts in workers.
An OffscreenCanvas object may hold a weak reference to a placeholder canvas element, which is
typically in the DOM, whose embedded content is provided by the OffscreenCanvas
object. The bitmap of the OffscreenCanvas object is pushed to the placeholder canvas element by
calling the commit() method of the
OffscreenCanvas object's rendering context. All rendering context types that
can be created by an OffscreenCanvas object must implement a commit() method. The exact behavior of the commit
method (e.g. whether it copies or transfers bitmaps) may vary, as defined by the rendering
contexts' respective specifications. Only the 2D context for offscreen canvases is defined
in this specification.
OffscreenCanvas(width,
height)Returns a new OffscreenCanvas object that is not linked to a placeholder canvas element, and
whose bitmap's size is determined by the width and height arguments.
getContext(contextType [, ... ] )Returns an object that exposes an API for drawing on the OffscreenCanvas
object. The first argument specifies the desired API, either "2d", or "webgl". Subsequent arguments are handled by that
API.
This specification defines the "2d" context below,
which is similar but distinct from the "2d"
context that is created from a canvas element. There is also a specification that
defines a "webgl" context.
[WEBGL]
Returns null if the canvas has already been initialized with another context type (e.g.
trying to get a "2d" context after getting a
"webgl" context).
width [
= value ]height [
= value ]These attributes return the dimensions of the OffscreenCanvas object's bitmap.
They can be set, to replace the bitmap with a new, transparent black bitmap of the specified dimensions (effectively resizing it).
If an OffscreenCanvas object whose dimensions were changed has
a placeholder canvas element, then
the placeholder canvas element's
intrinsic size will only be updated via the commit() method of the OffscreenCanvas
object's rendering context.
convertToBlob( [options] )Returns a promise that will fulfill with a new Blob object representing a file
containing the image in the OffscreenCanvas object.
The argument, if provided, is a dictionary that controls the encoding options of the image
file to be created. The type
field specifies the file format and has a default value of "image/png"; that type
is also used if the requested type isn't supported. If the image format supports variable
quality (such as "image/jpeg"), then the quality field is a number in the range 0.0
to 1.0 inclusive indicating the desired quality level for the resulting image.
transferToImageBitmap()Returns a newly created ImageBitmap object with the image in the
OffscreenCanvas object. The image in the OffscreenCanvas object is
replaced with a new blank image.
The OffscreenCanvasRenderingContext2D is a rendering context
interface for drawing to the bitmap of an
OffscreenCanvas object. It is similar to CanvasRenderingContext2D,
with the following differences:
text rendering is not supported;
there is no support for user interface features;
its canvas attribute refers to an
OffscreenCanvas object rather than a canvas element;
it has a commit() method for pushing the
rendered image to the context's OffscreenCanvas object's placeholder canvas element.
An OffscreenCanvasRenderingContext2D object has a bitmap that is initialized when the object is
created.
The bitmap has an origin-clean flag, which can be set to true or false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.
An OffscreenCanvasRenderingContext2D object also has an alpha flag, which can be set to true or false. Initially,
when the context is created, its alpha flag must be set to true. When an
OffscreenCanvasRenderingContext2D object has its alpha flag set to false, then its alpha channel must be
fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel
must be silently ignored.
An OffscreenCanvasRenderingContext2D object has an associated
OffscreenCanvas object, which is the OffscreenCanvas object
from which the OffscreenCanvasRenderingContext2D object was created.
commit()Copies the rendering context's bitmap to
the bitmap of the placeholder canvas
element of the associated OffscreenCanvas object. The copy
operation is asynchronous.
canvasReturns the associated OffscreenCanvas object.