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 shadow trees.
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 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 representsembedded content with the element's current bitmap and size. Otherwise, the element
represents its fallback content instead.
When a canvas element representsembedded 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]
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 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 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.
The bitmaps of canvas elements, the bitmaps of ImageBitmap objects,
as well as some of the bitmaps of rendering contexts, such as those described in the sections on
the CanvasRenderingContext2D and ImageBitmapRenderingContext objects
below, have an origin-clean flag, which can be
set to true or false. Initially, when the canvas element or ImageBitmap
object is created, its bitmap's origin-clean
flag must be set to true.
A canvas element can have a rendering context bound to it. Initially, it does not
have a bound rendering context. To keep track of whether it has a rendering context or not, and
what kind of rendering context it is, a canvas also has a canvas context mode, which is initially none but can be changed to either placeholder, 2d, bitmaprenderer, webgl, webgl2, or webgpu by algorithms defined in this specification.
Whenever the width and height content attributes are set, removed, changed, or
redundantly set to the value they already have, then the user agent must perform the action
from the row of the following table that corresponds to the canvas element's context mode.
Returns an object that exposes an API for drawing on the canvas. contextId
specifies the desired API: "2d", "bitmaprenderer", "webgl", "webgl2", or "webgpu". options is handled by that API.
This specification defines the "2d" and "bitmaprenderer" contexts below. The WebGL
specifications define the "webgl" and "webgl2" contexts. WebGPU defines the "webgpu" context. [WEBGL][WEBGPU]
Returns null if contextId 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).
The getContext(contextId, options)
method of the canvas element, when invoked, must run these steps:
If options is not an object, then set
options to null.
Set options to the result of convertingoptions to a JavaScript value.
Run the steps in the cell of the following table whose column header matches this
canvas element's canvas context
mode and whose row header matches contextId:
* For example, the "webgl" or "webgl2" value in the case of a user agent having exhausted
the graphics hardware's abilities and having no software fallback implementation.
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:,".)
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.
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 natural 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.
The toDataURL(type, quality) method,
when invoked, must run these steps:
If this canvas element's bitmap has no pixels (i.e. either its horizontal
dimension or its vertical dimension is zero) then return the string "data:,". (This is the shortest data: URL; it represents the empty string in a text/plain resource.)
If this canvas element's bitmap has pixels (i.e., neither its horizontal
dimension nor its vertical dimension is zero), then set result to a copy of this
canvas element's bitmap.
A CanvasRenderingContext2D object has an output bitmap that
is initialized when the object is created.
The output 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.
The CanvasRenderingContext2D object also has an alpha boolean. When a
CanvasRenderingContext2D object's alpha is 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.
Thus, the bitmap of such a context starts off as opaque black instead
of transparent black; clearRect()
always results in opaque black pixels, every fourth byte from getImageData() is always 255, the putImageData() method effectively ignores every
fourth byte in its input, and so on. However, the alpha component of styles and images drawn
onto the canvas are still honoured up to the point where they would impact the output
bitmap's alpha channel; for instance, drawing a 50% transparent white square on a freshly
created output bitmap with its alpha set
to false will result in a fully-opaque gray square.
The CanvasRenderingContext2D object also has a desynchronized boolean. When a
CanvasRenderingContext2D object's desynchronized is true, then the user agent may
optimize the rendering of the canvas to reduce the latency, as measured from input events to
rasterization, by desynchronizing the canvas paint cycle from the event loop, bypassing the
ordinary user agent rendering algorithm, or both. Insofar as this mode involves bypassing the
usual paint mechanisms, rasterization, or both, it might introduce visible tearing artifacts.
The user agent usually renders on a buffer which is not being displayed, quickly
swapping it and the one being scanned out for presentation; the former buffer is called
back buffer and the latter front buffer. A popular technique for reducing latency is called
front buffer rendering, also known as single buffer rendering, where rendering happens in
parallel and racily with the scanning out process. This technique reduces the latency at the price
of potentially introducing tearing artifacts and can be used to implement in total or part of the
desynchronized boolean.
[MULTIPLEBUFFERING]
The desynchronized boolean
can be useful when implementing certain kinds of applications, such as drawing applications,
where the latency between input and rasterization is critical.
On most devices the user agent needs to decide whether to store the canvas's
output bitmap on the GPU (this is also called "hardware accelerated"), or on the CPU
(also called "software"). Most rendering operations are more performant for accelerated canvases,
with the major exception being readback with getImageData(), toDataURL(), or toBlob(). CanvasRenderingContext2D objects with
will read frequently equal to true tell
the user agent that the webpage is likely to perform many readback operations and that it is
advantageous to use a software canvas.
The CanvasRenderingContext2D 2D rendering context represents a flat linear
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. The x-coordinate of the right-most edge is equal to the width of the rendering
context's output bitmap in CSS pixels; similarly, the
y-coordinate of the bottom-most edge is equal to the height of the rendering context's
output bitmap in CSS pixels.
The size of the coordinate space does not necessarily represent the size of the actual bitmaps
that the user agent will use internally or during rendering. On high-definition displays, for
instance, the user agent may internally use bitmaps with four device pixels per unit in the
coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can
similarly be implemented using oversampling with bitmaps of a higher resolution than the final
image on the display.
Using CSS pixels to describe the size of a rendering context's
output bitmap does not mean that when rendered the canvas will cover an equivalent
area in CSS pixels. CSS pixels are reused
for ease of integration with CSS features, such as text layout.
In other words, the canvas element below's rendering context has a 200x200
output bitmap (which internally uses CSS pixels as a
unit for ease of integration with CSS) and is rendered as 100x100 CSS
pixels:
Only one square appears to be drawn in the following example:
// canvas is a reference to a <canvas> elementvar 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
The canvas attribute must return the value it was
initialized to when the object was created.
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 nonzero 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.
4.12.5.1.1 Implementation notes
This section is non-normative.
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 natural 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.
4.12.5.1.2 The canvas state
Objects that implement the CanvasState interface maintain a stack of drawing
states. Drawing states consist of:
The rendering context's bitmaps are not part of the drawing state, as they
depend on whether and how the rendering context is bound to a canvas element.
Objects that implement the CanvasState mixin have a context lost boolean, that is initialized to false
when the object is created. The context lost
value is updated in the context lost steps.
Returns true if the rendering context was lost. Context loss can occur due to driver
crashes, running out of memory, etc. In these cases, the canvas loses its backing storage and
takes steps to reset the rendering context to its default state.
The save() method
steps are to push a copy of the current drawing state onto the drawing state stack.
The restore()
method steps are to pop the top entry in the drawing state stack, and reset the drawing state it
describes. If there is no saved state, then the method must do nothing.
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.
Returns the phase offset (in the same units as the line dash pattern).
Can be set, to change the phase offset. Values that are not finite values are ignored.
Objects that implement the CanvasPathDrawingStyles interface have attributes and
methods (defined in this section) that control how lines are treated by the object.
The lineWidth attribute gives the width of lines, in
coordinate space units. On getting, it must return the current value. On setting, zero, negative,
infinite, and NaN values must be ignored, leaving the value unchanged; other values must change
the current value to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineWidth attribute must initially have the value
1.0.
The lineCap attribute defines the type of endings that
UAs will place on the end of lines. The three valid values are "butt",
"round", and "square".
On getting, it must return the current value. On setting, the current value must be changed
to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineCap attribute must initially have the value
"butt".
The lineJoin attribute defines the type of corners that
UAs will place where two lines meet. The three valid values are "bevel",
"round", and "miter".
On getting, it must return the current value. On setting, the current value must be changed
to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
lineJoin attribute must initially have the value
"miter".
When the lineJoin attribute has the value "miter", strokes use the miter limit ratio to decide how to render joins. The
miter limit ratio can be explicitly set using the miterLimit
attribute. On getting, it must return the current value. On setting, zero, negative, infinite, and
NaN values must be ignored, leaving the value unchanged; other values must change the current
value to the new value.
When the object implementing the CanvasPathDrawingStyles interface is created, the
miterLimit attribute must initially have the value
10.0.
Each CanvasPathDrawingStyles object has a dash list, which is either
empty or consists of an even number of non-negative numbers. Initially, the dash list
must be empty.
The setLineDash(segments) method, when invoked, must run
these steps:
If any value in segments is not finite (e.g. an Infinity or a NaN value), or
if any value is negative (less than zero), then return (without throwing an exception;
user agents could show a message on a developer console, though, as that would be helpful for
debugging).
If the number of elements in segments is odd, then let segments
be the concatenation of two copies of segments.
When the getLineDash() method is invoked, it must return a
sequence whose values are the values of the object's dash list, in the same
order.
It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching
ants" effect. The phase can be set using the lineDashOffset attribute. On getting, it must
return the current value. On setting, infinite and NaN values must be ignored, leaving the value
unchanged; other values must change the current value to the new value.
When a user agent is to trace a path, given an object style
that implements the CanvasPathDrawingStyles interface, it must run the following
algorithm. This algorithm returns a new path.
Remove from path any subpaths containing no lines (i.e. subpaths with
just one point).
Replace each point in each subpath of path other than the first point
and the last point of each subpath by a join that joins the line leading to that point to
the line leading out of that point, such that the subpaths all consist of two points (a starting
point with a line leading out of it, and an ending point with a line leading into it), one or
more lines (connecting the points and the joins), and zero or more joins (each connecting one
line to another), connected together such that each subpath is a series of one or more lines with
a join between each one and a point on each end.
Add a straight closing line to each closed subpath in path connecting
the last point and the first point of that subpath; change the last point to a join (from the
previously last line to the newly added closing line), and change the first point to a join (from
the newly added closing line to the first line).
If style's dash list is empty, then jump to the step
labeled convert.
Let pattern width be the concatenation of all the entries of
style's dash list, in coordinate space units.
For each subpath subpath in path, run the
following substeps. These substeps mutate the subpaths in pathin
vivo.
Let subpath width be the length of all the lines of subpath, in coordinate space units.
Let offset be the value of style's lineDashOffset, in coordinate space
units.
While offset is greater than pattern width,
decrement it by pattern width.
While offset is less than zero, increment it by pattern
width.
Define L to be a linear coordinate line defined along all lines in
subpath, such that the start of the first line in the subpath is defined
as coordinate 0, and the end of the last line in the subpath is defined as coordinate subpath width.
Let position be zero minus offset.
Let index be 0.
Let current state be off (the other states being on
and zero-on).
Dash on: Let segment length be
the value of style's dash
list's indexth entry.
Increment position by segment length.
If position is greater than subpath width,
then end these substeps for this subpath and start them again for the next subpath; if there
are no more subpaths, then jump to the step labeled convert instead.
If segment length is nonzero, then let current state be
on.
Increment index by one.
Dash off: Let segment
length be the value of style's dash list's indexth entry.
Let start be the offset position on L.
Increment position by segment length.
If position is less than zero, then jump to the step labeled
post-cut.
If start is less than zero, then let start be
zero.
If position is greater than subpath width,
then let end be the offset subpath width on L. Otherwise, let end be the offset position on L.
Jump to the first appropriate step:
If segment length is zero and current state is
off
Do nothing, just continue to the next step.
If current state is off
Cut the line on which end finds itself short at end and place a point there, cutting in two the subpath that it was in;
remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.
The point has a directionality for the purposes of drawing line caps (see below).
The directionality is the direction that the original line had at that point (i.e. when L was defined above).
Otherwise
Cut the line on which start finds itself into two at start and place a point there, cutting in two the subpath that it was in, and
similarly cut the line on which end finds itself short at end and place a point there, cutting in two the subpath that it was in,
and then remove all line segments, joins, points, and subpaths that are between start and end.
If start and end are the same point, then this
results in just the line being cut in two and two points being inserted there, with nothing
being removed, unless a join also happens to be at that point, in which case the join must
be removed.
Post-cut: If position is greater than subpath width, then jump to the step labeled convert.
If segment length is greater than zero, then let
positioned-at-on-dash be false.
Increment index by one. If it is equal to the number of entries in
style's dash list, then let index be
0.
Return to the step labeled dash on.
Convert: This is the step that converts the path to a new path that represents its
stroke.
Create a new path that describes the edge of the areas
that would be covered if a straight line of length equal to style's
lineWidth was swept along each subpath in path while being kept at an angle such that the line is orthogonal to the path
being swept, replacing each point with the end cap necessary to satisfy style's lineCap attribute as
described previously and elaborated below, and replacing each join with the join necessary to
satisfy style's lineJoin
type, as defined below.
Caps: Each point has a flat edge perpendicular to the direction of the line
coming out of it. This is then augmented according to the value of style's lineCap. The "butt" value means that no additional line cap is added. The "round" value means that a semi-circle with the diameter equal to
style's lineWidth width must
additionally be placed on to the line coming out of each point. The "square" value means that a rectangle with the length of style's lineWidth width and the
width of half style's lineWidth width, placed flat against the edge
perpendicular to the direction of the line coming out of the point, must be added at each
point.
Points with no lines coming out of them must have two caps placed back-to-back as if it was
really two points connected to each other by an infinitesimally short straight line in the
direction of the point's directionality (as defined above).
Joins: In addition to the point where a join occurs, two additional points
are relevant to each join, one for each line: the two corners found half the line width away
from the join point, one perpendicular to each line, each on the side furthest from the other
line.
A triangle connecting these two opposite corners with a straight line, with the third point
of the triangle being the join point, must be added at all joins. The lineJoin attribute controls whether anything else is
rendered. The three aforementioned values have the following meanings:
The "bevel" value means that this is all that is rendered at
joins.
The "round" value means that an arc connecting the two aforementioned
corners of the join, abutting (and not overlapping) the aforementioned triangle, with the
diameter equal to the line width and the origin at the point of the join, must be added at
joins.
The "miter" value means that a second triangle must (if it can given
the miter length) be added at the join, with one line being the line between the two
aforementioned corners, abutting the first triangle, and the other two being continuations of
the outside edges of the two joining lines, as long as required to intersect without going over
the miter length.
The miter length is the distance from the point where the join occurs to the intersection of
the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of
the miter length to half the line width. If the miter length would cause the miter limit ratio
(as set by style's miterLimit attribute) to be exceeded, then this second
triangle must not be added.
The subpaths in the newly created path must be oriented such that for any point, the number
of times a half-infinite straight line drawn from that point crosses a subpath is even if and
only if the number of times a half-infinite straight line drawn from that same point crosses a
subpath going in one direction is equal to the number of times it crosses a subpath going in the
other direction.
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".
Can be set, to change the font variant caps. The possible values and their meanings are given
below. Other values are ignored. The default is "normal".
In this example, the canvas will display text using mycanvasfont.ttf as
its font.
This is an example of how font resolution can happen using OffscreenCanvas.
Assuming a canvas element with ID c2 which is transferred to
a worker like so:
In this example, the canvas will display a text using myfont.ttf.
Notice that the font is only loaded inside the worker, and not in the document context.
The font IDL attribute, on setting, must be parsed as a CSS <'font'> value (but
without supporting property-independent style sheet syntax like 'inherit'), and the resulting font
must be assigned to the context, with the 'line-height' component forced to 'normal',
with the 'font-size' component converted to CSS pixels,
and with system fonts being computed to explicit values. If the new value is syntactically
incorrect (including using property-independent style sheet syntax like 'inherit' or 'initial'),
then it must be ignored, without assigning a new font value. [CSS]
Font family names must be interpreted in the context of the font style source
object when the font is to be used; any fonts embedded using @font-face or loaded using FontFace objects that are visible to the
font style source object must therefore be available once they are loaded. (Each font style source
object has a font source, which determines what fonts are available.) If a font
is used before it is fully loaded, or if the font style source object does not have
that font in scope at the time the font is to be used, then it must be treated as if it was an
unknown font, falling back to another as described by the relevant CSS specifications.
[CSSFONTS][CSSFONTLOAD]
...the expression context.font would evaluate to the string "italic 12px "Unknown Font", sans-serif". The "400"
font-weight doesn't appear because that is the default value. The line-height doesn't appear
because it is forced to "normal", the default value.
When the object implementing the CanvasTextDrawingStyles interface is created, the
font of the context must be set to 10px sans-serif. When the 'font-size' component is
set to lengths using percentages, 'em' or 'ex' units, or the 'larger' or
'smaller' keywords, these must be interpreted relative to the computed value of the
'font-size' property of the font style source object at the time that
the attribute is set, if it is an element. When the 'font-weight' component is set to
the relative values 'bolder' and 'lighter', these must be interpreted relative to the
computed value of the 'font-weight' property of the font style
source object at the time that the attribute is set, if it is an element. If the computed values are undefined for a particular case (e.g. because
the font style source object is not an element or is not being
rendered), then the relative keywords must be interpreted relative to the normal-weight
10px sans-serif default.
The textAlign IDL attribute, on getting, must return
the current value. On setting, the current value must be changed to the new value. When the object
implementing the CanvasTextDrawingStyles interface is created, the textAlign attribute must initially have the value start.
The textBaseline IDL attribute, on getting, must
return the current value. On setting, the current value must be changed to the new value. When the
object implementing the CanvasTextDrawingStyles interface is created, the textBaseline attribute must initially have the value
alphabetic.
The direction IDL attribute, on getting, must return
the current value. On setting, the current value must be changed to the new value. When the object
implementing the CanvasTextDrawingStyles interface is created, the direction attribute must initially have the value "inherit".
Objects that implement the CanvasTextDrawingStyles interface have attributes that
control the spacing between letters and words. Such objects have associated letter spacing and word spacing values, which are CSS
<length> values. Initially, both must be the result of parsing "0px" as a CSS
<length>.
The fontKerning IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontKerning attribute must
initially have the value "auto".
The fontStretch IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontStretch attribute must
initially have the value "normal".
The fontVariantCaps IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the fontVariantCaps attribute must
initially have the value "normal".
The textRendering IDL attribute, on
getting, must return the current value. On setting, the current value must be changed to the new
value. When the object implementing the CanvasTextDrawingStyles interface is created,
the textRendering attribute must
initially have the value "auto".
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:
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:
If the target object's direction attribute has the value "ltr"
and with all other properties set to their initial values.
If maxWidth was provided and the hypothetical width of the
inline box in the hypothetical line box is greater than
maxWidthCSS 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:
Let result be an array constructed by iterating over each glyph in the
inline box from left to right (if any), adding to the array, for each glyph, the
shape of the glyph as it is in the inline box, positioned on a coordinate space
using CSS pixels with its origin is at the anchor
point.
Return result, physical alignment, and the inline
box.
4.12.5.1.5 Building paths
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.
path.arc(x, y, radius, startAngle, endAngle [, counterclockwise ])
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.
path.ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise])
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.
Adds a new closed subpath to the path representing the given rounded rectangle.
radii is either a list of radii or a single radius representing the corners of the
rectangle in pixels. If a list is provided, the number and order of these radii function in the
same way as the CSS 'border-radius' property. A single radius behaves the same way
as a list with a single element.
If w and h are both greater than or equal to 0, or if both are smaller
than 0, then the path is drawn clockwise. Otherwise, it is drawn counterclockwise.
When w is negative, the rounded rectangle is flipped horizontally, which means
that the radius values that normally apply to the left corners are used on the right and vice
versa. Similarly, when h is negative, the rounded rect is flipped vertically.
When a value r in radii is a number, the corresponding corner(s) are
drawn as circular arcs of radius r.
When a value r in radii is an object with { x, y
} properties, the corresponding corner(s) are drawn as elliptical arcs whose x
and y radii are equal to r.x and r.y, respectively.
When the sum of the radii of two corners of the same edge is greater than the
length of the edge, all the radii of the rounded rectangle are scaled by a factor of
length / (r1 + r2). If multiple edges have this property, the scale factor
of the edge with the smallest scale factor is used. This is consistent with CSS behavior.
Throws a RangeError if radii is a list whose
size is not one, two, three, or four.
Throws a RangeError if a value in radii is a
negative number, or is an { x, y } object whose x
or y properties are negative numbers.
The following methods allow authors to manipulate the paths
of objects implementing the CanvasPath interface.
The moveTo(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Create a new subpath with the specified point as its first (and only) point.
When the user agent is to ensure there is a subpath for a coordinate (x,
y) on a path, the user agent must check to see if
the path has its need new subpath flag set. If it
does, then the user agent must create a new subpath with the point (x, y) as
its first (and only) point, as if the moveTo() method
had been called, and must then unset the path's need new
subpath flag.
The closePath() method, when invoked, must do nothing
if the object's path has no subpaths. Otherwise, it must mark the last subpath as closed, create a
new subpath whose first point is the same as the previous subpath's first point, and finally add
this new subpath to the path.
If the last subpath had more than one point in its list of points, then this is
equivalent to adding a straight line connecting the last point back to the first point of the last
subpath, thus "closing" the subpath.
New points and the lines connecting them are added to subpaths using the methods described
below. In all cases, the methods only modify the last subpath in the object's path.
The lineTo(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
If the point (x0, y0) is equal to the point (x1,
y1), or if the point (x1, y1) is equal to the point
(x2, y2), or if radius is zero, then add the point
(x1, y1) to the subpath, and connect that point to the previous point
(x0, y0) by a straight line.
Otherwise, if the points (x0, y0), (x1, y1),
and (x2, y2) all lie on a single straight line, then add the point
(x1, y1) to the subpath, and connect that point to the previous point
(x0, y0) by a straight line.
Otherwise, let The Arc be the shortest arc given by circumference of the circle
that has radius radius, and that has one point tangent to the half-infinite line that
crosses the point (x0, y0) and ends at the point (x1,
y1), and that has a different point tangent to the half-infinite line that ends at the
point (x1, y1) and crosses the point (x2, y2). The
points at which this circle touches these two lines are called the start and end tangent points
respectively. Connect the point (x0, y0) to the start tangent point by a
straight line, adding the start tangent point to the subpath, and then connect the start tangent
point to the end tangent point by The Arc, adding the end tangent point to the
subpath.
The arc(x,
y, radius, startAngle, endAngle,
counterclockwise) method, when invoked, must run the ellipse method
steps with this, x, y, radius, radius, 0,
startAngle, endAngle, and counterclockwise.
This makes it equivalent to ellipse()
except that both radii are equal and rotation is 0.
The ellipse(x,
y, radiusX, radiusY, rotation, startAngle,
endAngle, counterclockwise) method, when invoked, must run the
ellipse method steps with this, x, y, radiusX,
radiusY, rotation, startAngle, endAngle, and
counterclockwise.
The determine the point on an ellipse steps, given ellipse, and
angle, are:
Let eccentricCircle be the circle that shares its origin with
ellipse, with a radius equal to the semi-major axis of ellipse.
Let outerPoint be the point on eccentricCircle's circumference at
angle measured in radians clockwise from ellipse's semi-major
axis.
Let chord be the line perpendicular to ellipse's major axis between
this axis and outerPoint.
Return the point on chord that crosses ellipse's
circumference.
The ellipse method steps, given canvasPath, x, y,
radiusX, radiusY, rotation, startAngle,
endAngle, and counterclockwise, are:
If any of the arguments are infinite or NaN, then return.
Otherwise, the start point is the result of running the determine the point on an
ellipse steps given this ellipse and startAngle, the end point is the result
of running the determine the point on an ellipse steps given this ellipse and
endAngle, and the arc is the path along the circumference of this ellipse from the
start point to the end point, going counterclockwise if counterclockwise is true,
and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles
from zero, the arc can never cover an angle greater than 2π
radians.
Even if the arc covers the entire circumference of the ellipse and there are no
other points in the subpath, the path is not closed unless the closePath() method is appropriately invoked.
The rect(x,
y, w, h) method, when invoked, must run these
steps:
If any of the arguments are infinite or NaN, then return.
Create a new subpath containing just the four points (x, y),
(x+w, y), (x+w,
y+h), (x, y+h), in that order, with those
four points connected by straight lines.
Mark the subpath as closed.
Create a new subpath with the point (x, y) as the only point in the
subpath.
Otherwise append «[ "x" → radius,
"y" → radius ]» to
normalizedRadii.
Let upperLeft, upperRight, lowerRight, and
lowerLeft be null.
If normalizedRadii's size is 4, then set upperLeft to
normalizedRadii[0], set upperRight to normalizedRadii[1], set
lowerRight to normalizedRadii[2], and set lowerLeft to
normalizedRadii[3].
If normalizedRadii's size is 3, then set upperLeft to
normalizedRadii[0], set upperRight and lowerLeft to
normalizedRadii[1], and set lowerRight to
normalizedRadii[2].
If normalizedRadii's size is 2, then set upperLeft and
lowerRight to normalizedRadii[0] and set upperRight and
lowerLeft to normalizedRadii[1].
If normalizedRadii's size is 1, then set upperLeft,
upperRight, lowerRight, and lowerLeft to
normalizedRadii[0].
Corner curves must not overlap. Scale all radii to prevent this:
Path2D 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.
Create a copy of all the subpaths in path. Let this copy be known as
c.
Transform all the coordinates and lines in c by the transform matrix
matrix.
Let (x, y) be the last point in the last subpath
of c.
Add all the subpaths in c to a.
Create a new subpath in a with (x, y) as the only point
in the subpath.
4.12.5.1.7 Transformations
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 matrix.
The transformations must be performed in reverse order.
For instance, if a scale transformation that doubles the width is applied to the
canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn,
and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be
a square.
The scale(x,
y) method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Add the scaling transformation described by the arguments to the current
transformation matrix. The x argument represents the scale factor in the
horizontal direction and the y argument represents the scale factor in the vertical
direction. The factors are multiples.
The rotate(angle) method, when invoked, must
run these steps:
If angle is infinite or NaN, then return.
Add the rotation transformation described by the argument to the current
transformation matrix. The angle argument represents a clockwise rotation angle
expressed in radians.
The translate(x, y) method, when
invoked, must run these steps:
If either of the arguments are infinite or NaN, then return.
Add the translation transformation described by the arguments to the current
transformation matrix. The x argument represents the translation distance in
the horizontal direction and the y argument represents the translation distance in the
vertical direction. The arguments are in coordinate space units.
The transform(a, b, c,
d, e, f) method, when invoked, must run these
steps:
If any of the arguments are infinite or NaN, then return.
Replace the current transformation matrix with the result of multiplying the
current transformation matrix with the matrix described by:
a
c
e
b
d
f
0
0
1
The arguments a, b, c, d,
e, and f are sometimes called m11, m12,
m21, m22, dx, and dy or m11,
m21, m12, m22, dx, and dy. Care 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.
The getTransform() method, when invoked, must return
a newly created DOMMatrix representing a copy of the current transformation
matrix matrix of the context.
Although 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.
To check the usability of the image argument, where image
is a CanvasImageSource object, run these steps:
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 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 style can be either a string containing a CSS color, or a CanvasGradient or
CanvasPattern object. Invalid values are ignored.
Objects that implement the CanvasFillStrokeStyles interface have attributes and
methods (defined in this section) that control how shapes are treated by the object.
Such objects have associated fill
style and stroke style
values, which are either CSS colors, CanvasPatterns, or CanvasGradients.
Initially, both must be the result of parsing
the string "#000000".
When the value is a CSS color, it must not be affected by the transformation matrix when used
to draw on bitmaps.
When set to a CanvasPattern or CanvasGradient object,
changes made to the object after the assignment do affect subsequent stroking or filling of
shapes.
The serialization of a color for a color
value is a string, computed as follows: if it has alpha equal to 1.0, then the string is a
lowercase six-digit hex value, prefixed with a "#" character (U+0023 NUMBER SIGN), with the first
two digits representing the red component, the next two digits representing the green component,
and the last two digits representing the blue component, the digits being ASCII lower hex digits. 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 ASCII digits in the shortest form possible), a literal
U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer
for the blue component, another comma and space, a U+0030 DIGIT ZERO, if the alpha value is
greater than zero then a U+002E FULL STOP (representing the decimal point), if the alpha value is
greater than zero then one or more ASCII digits representing the fractional part of
the alpha, and finally a U+0029
RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with
the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same
alpha value.
There are three types of gradients, linear gradients, radial gradients, and conic gradients,
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.
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.
Returns a CanvasGradient object that represents a conic gradient that paints
clockwise along the rotation around the center represented by the arguments.
The addColorStop(offset,
color) method on the CanvasGradient, when invoked, must run
these steps:
No element is passed to the parser because CanvasGradient objects
are canvas-neutral — a CanvasGradient object created by one
canvas can be used by another, and there is therefore no way to know which is the
"element in question" at the time that the color is specified.
If multiple stops are added at the same offset on a gradient, then they must be placed in the
order added, with the first one closest to the start of the gradient, and each subsequent one
infinitesimally further along towards the end point (in effect causing all but the first and
last stop added at each point to be ignored).
The createLinearGradient(x0, y0,
x1, y1) method takes four arguments that represent the start
point (x0, y0) and end point (x1, y1) of the gradient.
The method, when invoked, must return a linear CanvasGradient initialized with the
specified line.
Linear gradients must be rendered such that all points on a line perpendicular to the line that
crosses the start and end points have the color at the point where those two lines cross (with the
colors coming from the interpolation and extrapolation described
above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.
If x0 = x1 and y0 = y1, then the linear
gradient must paint nothing.
The createRadialGradient(x0, y0,
r0, x1, y1, r1) method takes six
arguments, the first three representing the start circle with origin (x0,
y0) and radius r0, and the last three representing the end circle with
origin (x1, y1) and radius r1. The values are in coordinate space
units. If either of r0 or r1 are negative, then an
"IndexSizeError"CanvasGradient initialized with the
two specified circles.
Radial gradients must be rendered by following these steps:
If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must
paint nothing. Return.
Let x(ω) = (x1-x0)ω + x0
Let y(ω) = (y1-y0)ω + y0
Let r(ω) = (r1-r0)ω + r0
Let the color at ω be the color at that position on the gradient
(with the colors coming from the interpolation and extrapolation
described above).
For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with
radius r(ω) at position (x(ω), y(ω)), with the
color at ω, but only painting on the parts of the bitmap that have not
yet been painted on by earlier circles in this step for this rendering of the gradient.
This effectively creates a cone, touched by the two circles defined in the
creation of the gradient, with the part of the cone before the start circle (0.0) using the color
of the first offset, the part of the cone after the end circle (1.0) using the color of the last
offset, and areas outside the cone untouched by the gradient (transparent black).
The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.
The createConicGradient(startAngle,
x, y) method takes three arguments, the first argument,
startAngle, represents the angle in radians at which the gradient begins, and the last
two arguments, (x, y), represent the center of the gradient in CSS pixels. The method, when invoked, must return a conic
CanvasGradient initialized with the specified center and angle.
It follows the same rendering rule as CSS 'conic-gradient' and it is
equivalent to CSS 'conic-gradient(from adjustedStartAnglerad at
xpx ypx, angularColorStopList)'. Here:
adjustedStartAngle is given by startAngle + π/2;
angularColorStopList is given by the color stops that have been added to the
CanvasGradient using addColorStop(), with the color stop offsets
interpreted as percentages.
Gradients must be painted only where the relevant stroking or filling effects requires that
they be drawn.
Patterns are represented by objects implementing the opaque CanvasPattern
interface.
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"pattern.setTransform(transform)
If repetition is the empty string, then set it to "repeat".
If repetition is not identical to one of "repeat", "repeat-x", "repeat-y",
or "no-repeat", then throw a
"SyntaxError"pattern be a new CanvasPattern object with the image
image and the repetition behavior given by repetition.
Modifying the image used when creating a CanvasPattern object
after calling the createPattern() method must
not affect the pattern(s) rendered by the CanvasPattern object.
Patterns have a transformation matrix, which controls how the pattern is used when it is
painted. Initially, a pattern's transformation matrix must be the identity matrix.
The setTransform(transform) method,
when invoked, must run these steps:
Place a copy of the image on the bitmap, anchored such that its top left corner is at the
origin of the coordinate space, with one coordinate space unit per CSS
pixel of the image, then place repeated copies of this image horizontally to the left and
right, if the repetition behavior is "repeat-x", or vertically up and
down, if the repetition behavior is "repeat-y", or in all four
directions all over the bitmap, if the repetition behavior is "repeat".
If the original image data is a bitmap image, then the value painted at a point in the area
of the repetitions is computed by filtering the original image data. When scaling up, if the
imageSmoothingEnabled attribute is
set to false, then the image must be rendered using nearest-neighbor interpolation. Otherwise,
the user agent may use any filtering algorithm (for example bilinear interpolation or
nearest-neighbor). User agents which support multiple filtering algorithms may use the value of
the imageSmoothingQuality attribute
to guide the choice of filtering algorithm. When such a filtering algorithm requires a pixel
value from outside the original image data, it must instead use the value from wrapping the
pixel's coordinates to the original image's dimensions. (That is, the filter uses 'repeat'
behavior, regardless of the value of the pattern's repetition behavior.)
Transform the resulting bitmap according to the pattern's transformation matrix.
Replace any part of the image outside the area in which the pattern is to be rendered with
transparent black.
The resulting bitmap is what is to be rendered, with the same origin and same
scale.
If a radial gradient or repeated pattern is used when the transformation matrix is singular,
then the resulting style must be transparent black (otherwise the gradient or pattern
would be collapsed to a point or line, leaving the other pixels undefined). Linear gradients and
solid colors always define all points even with singular transformation matrices.
4.12.5.1.10 Drawing rectangles to the bitmap
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.
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).
If both w and h are zero, the path has a single subpath
with just one point (x, y), and no lines, and this method
thus has no effect (the trace a path algorithm returns an empty path in that
case).
If just one of either w or h is zero, then the path has
a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.
Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w,
y+h), and (x, y+h),
connected to each other in that order by straight lines.
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.
Objects that implement the CanvasText interface provide the following methods for
rendering text.
The fillText(text, x, y,
maxWidth) and strokeText(text, x, y,
maxWidth) methods 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 invoked, the user agent must run these steps:
If any of the arguments are infinite or NaN, then return.
Run the text preparation algorithm, passing it text, the object
implementing the CanvasText interface, and, if the maxWidth argument was
provided, that argument. Let glyphs be the result.
Move all the shapes in glyphs to the right by xCSS pixels and down by yCSS
pixels.
Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one
coordinate space unit.
The measureText(text) method steps are to
run the text preparation algorithm, passing it text and the object
implementing the CanvasText interface, and then using the returned inline
box must return a new TextMetrics object with members behaving as described in
the following list: [CSS]
The 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 attribute
The 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.
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.
The 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 attribute
The 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 attribute
The distance from the horizontal line indicated by the textBaseline attribute to the highest top of the em
squares in the inline 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 attribute
The distance from the horizontal line indicated by the textBaseline attribute to the lowest bottom of the em
squares in the inline box, in CSS pixels; positive numbers
indicating that the given baseline is above the bottom of that em square. (Zero if the given baseline
is the bottom of that em square.)
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.
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.
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.
The beginPath() method steps are to empty the list of
subpaths in this's current default path so that it once again has zero
subpaths.
Where the following method definitions use the term intended path for a
Path2D-or-null path, it means path itself if it is a
Path2D object, or the current default path otherwise.
When the intended path is a Path2D object, the coordinates and lines
of its subpaths must be transformed according to the current transformation matrix on the object
implementing the CanvasTransform interface when used by these methods (without
affecting the Path2D object itself). When the intended path is the current
default path, it is not affected by the transform. (This is because transformations
already affect the current default path when it is constructed, so applying it when
it is painted as well would result in a double transformation.)
The fill(fillRule) method steps are to run the
fill steps given this, null, and fillRule.
The fill(path, fillRule) method
steps are to run the fill steps given this, path, and
fillRule.
The fill steps, given a CanvasDrawPathcontext, a
Path2D-or-null path, and a fill rulefillRule,
are to fill all the subpaths of the intended path for path, using
context's fill style,
and using the fill rule indicated by fillRule. Open subpaths must be
implicitly closed when being filled (without affecting the actual subpaths).
The stroke() method steps are to run the stroke
steps given this and null.
The stroke(path) method steps are to run
the stroke steps given this and path.
As a result of how the algorithm to trace a path is defined,
overlapping parts of the paths in one stroke operation are treated as if their union was what was
painted.
The stroke style is affected by the transformation during painting, even
if the current default path is used.
The clip(fillRule) method steps are to run the
clip steps given this, null, and fillRule.
The clip(path, fillRule) method
steps are to run the clip steps given this, path, and
fillRule.
The clip steps, given a CanvasDrawPathcontext, a
Path2D-or-null path, and a fill rulefillRule,
are to create a new clipping region by calculating the intersection of
context's current clipping region and the area described by the intended
path for path, using the fill rule indicated by
fillRule. Open subpaths must be implicitly closed when computing the clipping region,
without affecting the actual subpaths. The new clipping region replaces the current clipping
region.
When the context is initialized, its current clipping region must be set to the largest
infinite surface (i.e. by default, no clipping occurs).
The isPointInPath(x, y,
fillRule) method steps are to return the result of the is point in
path steps given this, null, x, y, and
fillRule.
The isPointInPath(path, x,
y, fillRule) method steps are to return the result of the
is point in path steps given this, null, x, y,
and fillRule.
The is point in path steps, given a CanvasDrawPathcontext,
a Path2D-or-null path, two numbers x and y, and a
fill rulefillRule, are:
If x or y are infinite or NaN, then return false.
If the point given by the x and y coordinates, when treated as
coordinates in the canvas coordinate space unaffected by the current transformation, is inside
the intended path for path as determined by the fill rule
indicated by fillRule, then return true. Open subpaths must be implicitly closed when
computing the area inside the path, without affecting the actual subpaths. Points on the path
itself must be considered to be inside the path.
Return false.
The isPointInStroke(x, y)
method steps are to return the result of the is point in stroke steps given
this, null, x, and y.
The isPointInStroke(path, x,
y) method steps are to return the result of the is point in stroke
steps given this, path, x, and y.
The is point in stroke steps, given a CanvasDrawPathcontext, a Path2D-or-null path, and two numbers x
and y, are:
If x or y are infinite or NaN, then return false.
If the point given by the x and y coordinates, when treated as
coordinates in the canvas coordinate space unaffected by the current transformation, is inside
the path that results from tracing the intended
path for path, using the nonzero winding rule, and using
context's line styles as set by its CanvasPathDrawingStyles mixin, then
return true. Points on the resulting path must be considered to be inside the path.
Return false.
This canvas element has a couple of checkboxes. The path-related commands are
highlighted:
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.
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 CanvasUserInterface interface provide the following
methods to control drawing focus rings and scrolling paths into view.
The drawFocusIfNeeded(element)
method, when invoked, must run these steps:
If element is not focused or is not a descendant of the element with
whose context the method is associated, then return.
Draw a focus ring of the appropriate style along the intended path, following platform
conventions.
Some platforms only draw focus rings around elements that have been focused from
the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus
rings around some elements at all unless relevant accessibility features are enabled. This API
is intended to follow these conventions. User agents that implement distinctions based on the
manner in which the element was focused are encouraged to classify focus driven by the focus() method based on the kind of user interaction event from which
the call was triggered (if any).
Inform the user that the focus is at the location given by the
intended path. User agents may wait until the next time the event loop reaches its
update the rendering step to optionally inform the user.
User agents should not implicitly close open subpaths in the intended path when drawing the
focus ring.
This might be a moot point, however. For example, if the focus ring is drawn as an
axis-aligned bounding rectangle around the points in the intended path, then whether the subpaths
are closed or not has no effect. This specification intentionally does not specify precisely how
focus rings are to be drawn: user agents are expected to honor their platform's native
conventions.
The scrollPathIntoView() method, when invoked,
must run these steps:
Let specifiedRectangle be the rectangle of the bounding box of the intended
path.
Let notionalChild be a hypothetical element that is a rendered child of the
canvas element whose dimensions are those of specifiedRectangle.
Optionally, inform the user that the caret or selection (or both)
cover specifiedRectangle of the canvas. The user agent may wait until the next time
the event loop reaches its update the rendering step to optionally
inform the user.
"Inform the user", as used in this section, does not imply any persistent state
change. It could mean, for instance, calling a system accessibility API to notify assistive
technologies such as magnification tools so that the user's magnifier moves to the given area of
the canvas. However, it does not associate the path with the element, or provide a region for
tactile feedback, etc.
4.12.5.1.14 Drawing images
Objects that implement the CanvasDrawImage interface have the drawImage() method to
draw images.
This method can be invoked with three different sets of arguments:
If usability is bad, then return (without drawing anything).
Establish the source and destination rectangles as follows:
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 output bitmap's coordinate space. If the
sx, sy, sw, and sh arguments are omitted, then they
must default to 0, 0, the image's natural width in image pixels, and the image's
natural height in image pixels, respectively. If the image has no natural
dimensions, then the concrete object size must be used instead, as determined
using the CSS "Concrete Object
Size Resolution" algorithm, with the specified size having neither a definite width
nor height, nor any additional constraints, the object's natural properties being those of the
image argument, and the default object size being the size of the
output bitmap. [CSSIMAGES]
The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh),
(sx, sy+sh).
The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh),
(dx, dy+dh).
When the source rectangle is outside the source image, the source rectangle must be clipped
to the source image and the destination rectangle must be clipped in the same proportion.
When the destination rectangle is outside the destination image (the
output bitmap), the pixels that land outside the output bitmap are
discarded, as if the destination was an infinite canvas whose rendering was clipped to the
dimensions of the output bitmap.
If one of the sw or sh arguments is zero, then return. Nothing is
painted.
Paint the region of the image argument specified by the source rectangle
on the region of the rendering context's output bitmap specified by the
destination rectangle, after applying the current
transformation matrix to the destination rectangle.
The image data must be processed in the original direction, even if the dimensions given are
negative.
When scaling up, if the imageSmoothingEnabled attribute is set to
true, the user agent should attempt to apply a smoothing algorithm to the image data when it is
scaled. User agents which support multiple filtering algorithms may use the value of the imageSmoothingQuality attribute to guide
the choice of filtering algorithm when the imageSmoothingEnabled attribute is set to
true. Otherwise, the image must be rendered using nearest-neighbor interpolation.
This specification does not define the precise algorithm to use when scaling an
image down, or when scaling an image up when the imageSmoothingEnabled attribute is set to
true.
When a canvas element is drawn onto itself, the drawing
model requires the source to be copied before the image is drawn, so it is possible to
copy parts of a canvas element onto overlapping parts of itself.
If the original image data is a bitmap image, then the value painted at a point in the
destination rectangle is computed by filtering the original image data. The user agent may use
any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the
filtering algorithm requires a pixel value from outside the original image data, it must instead
use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behavior.)
When the filtering algorithm requires a pixel value from outside the source rectangle but inside
the original image data, then the value from the original image data must be used.
Thus, scaling an image in parts or in whole will have the same effect. This does
mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in
the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is
surrounded by a border of transparent black, or by copying sprites to be scaled
into temporary canvas elements and drawing the scaled sprites from there.
Returns an ImageData object with the given dimensions and the color space
indicated by settings. All the pixels in the returned object are transparent
black.
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.
Returns an ImageData object with the given dimensions. The color space of the
returned object is the color space of
context unless overridden by settings. All the pixels in the returned
object are transparent black.
Returns an ImageData object containing the image data for the given rectangle of
the bitmap. The color space of the returned object is the color space of context unless overridden
by settings.
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 properties, 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.
Let the source rectangle be the rectangle whose corners are the four points
(sx, sy), (sx+sw,
sy), (sx+sw, sy+sh), (sx,
sy+sh).
Set the pixel values of imageData to be the pixels of this's
output bitmap in the area specified by the source rectangle in the bitmap's
coordinate space units, converted from this's color space to imageData's colorSpace using 'relative-colorimetric'
rendering intent.
Set the pixels values of imageData for areas of the source rectangle that are
outside of the output bitmap to transparent black.
Return imageData.
To initialize an ImageData object imageData, given a
positive integer number of rows rows, a positive integer number of pixels per row
pixelsPerRow, an optional ImageDataSettingssettings, an optional source, and an optional
PredefinedColorSpacedefaultColorSpace:
If source was given, then initialize the data attribute of imageData to
source.
Otherwise (source was not given), initialize the data attribute of imageData to a new Canvas
Pixel ArrayBuffer for its storage, and must have a
zero start offset and a length equal to the length of its storage, in bytes. The Canvas
Pixel ArrayBuffer must have the correct size to
store rows × pixelsPerRow pixels.
When the last four arguments to this method are omitted, they must be assumed to have the
values 0, 0, the width member of the imagedata structure, and the height
member of the imagedata structure, respectively.
The method, when invoked, must act as follows:
Let buffer be imagedata's data attribute value's [[ViewedArrayBuffer]] internal
slot.
If dirtyHeight is negative, then let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be
equal to the absolute magnitude of dirtyHeight.
If dirtyX is negative, then let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.
If dirtyY is negative, then let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.
If dirtyX+dirtyWidth is greater than the width attribute of the imagedata argument, then
let dirtyWidth be the value of that width
attribute, minus the value of dirtyX.
If dirtyY+dirtyHeight is greater than the height attribute of the imagedata argument, then
let dirtyHeight be the value of that height attribute, minus the value of dirtyY.
If, after those changes, either dirtyWidth or dirtyHeight are negative
or zero, then return without affecting any bitmaps.
For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the
four channels of the pixel with coordinate (x, y) in
the imagedata data structure's Canvas Pixel
ArrayBuffer to the pixel with coordinate (dx+x, dy+y)
in the rendering context's output bitmap.
Due to the lossy nature of converting between color spaces and converting to and
from premultiplied alpha color values, pixels
that have just been set using putImageData(),
and are not completely opaque, might be returned to an equivalent getImageData() as different values.
In the following example, the script generates an ImageData object so that it can
draw onto it.
// canvas is a reference to a <canvas> elementvar context = canvas.getContext('2d');// create a blank slatevar 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 methodsfunction FillPlasma(data, color){...}function AddCloud(data, x, y){...}
<!DOCTYPE HTML><htmllang="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 manipulatevar input = context.getImageData(0,0, canvas.width, canvas.height);// get an empty slate to put the data intovar 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 detectionfor(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><bodyonload="init()"><canvas></canvas></body></html>
Here is an example of color space conversion applied when drawing a solid color and
reading the result back using and getImageData().
<!DOCTYPE HTML><htmllang="en"><title>Color space image data demo</title><canvas></canvas><script>const canvas = document.querySelector('canvas');const context = canvas.getContext('2d',{colorSpace:'display-p3'});// Draw a red rectangle. Note that the hex color notation// specifies sRGB colors.
context.fillStyle ="#FF0000";
context.fillRect(0,0,64,64);// Get the image data.const pixels = context.getImageData(0,0,1,1);// This will print 'display-p3', reflecting the default behavior// of returning image data in the canvas's color space.
console.log(pixels.colorSpace);// This will print the values 234, 51, and 35, reflecting the// red fill color, converted to 'display-p3'.
console.log(pixels.data[0]);
console.log(pixels.data[1]);
console.log(pixels.data[2]);</script>
The global alpha value gives an alpha value
that is applied to shapes and images before they are composited onto the output bitmap.
The value ranges from 0.0 (fully transparent) to 1.0 (no additional transparency). It must
initially have the value 1.0.
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).
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.
Objects that implement the CanvasImageSmoothing interface have attributes that
control how image smoothing is performed.
The imageSmoothingEnabled attribute, on
getting, must return the last value it was set to. On setting, it must be set to the new value.
When the object implementing the CanvasImageSmoothing interface is created, the
attribute must be set to true.
The imageSmoothingQuality attribute, on
getting, must return the last value it was set to. On setting, it must be set to the new value.
When the object implementing the CanvasImageSmoothing interface is created, the
attribute must be set to "low".
4.12.5.1.18 Shadows
All drawing operations on an object which implements the CanvasShadowStyles
interface are affected by the four global shadow attributes.
On setting, the new value must be parsed
with this canvas element and the color assigned. If parsing the value results in
failure then it must be ignored, and the attribute must retain its previous value.
[CSSCOLOR]
The shadowOffsetX and shadowOffsetY
attributes specify the distance that the shadow will be offset in the positive horizontal and
positive vertical distance respectively. Their values are in coordinate space units. They are not
affected by the current transformation matrix.
When the context is created, the shadow offset attributes must initially have the value
0.
On getting, they must return their current value. On setting, the attribute being set must be
set to the new value, except if the value is infinite or NaN, in which case the new value must be
ignored.
The shadowBlur attribute specifies the level of the
blurring effect. (The units do not map to coordinate space units, and are not affected by the
current transformation matrix.)
When the context is created, the shadowBlur
attribute must initially have the value 0.
On getting, the attribute must return its current value. On setting the attribute must be set
to the new value, except if the value is negative, infinite or NaN, in which case the new value
must be ignored.
Shadows are only drawn if the opacity component of
the alpha component of the color of shadowColor is
nonzero and either the shadowBlur is nonzero, or
the shadowOffsetX is nonzero, or the shadowOffsetY is nonzero.
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.
Can be set, to change the filter. Values can either be the string "none" or a string parseable as a <filter-value-list>. Other
values are ignored.
Such objects have an associated current
filter, which is a string. Initially the current filter is set to the string "none". Whenever the value of the current filter is the string "none" filters will be disabled for the context.
If the given value is "none", then set this's current filter to "none"
and return.
Let parsedValue be the result of parsing the given values as a <filter-value-list>. If any
property-independent style sheet syntax like 'inherit' or 'initial' is present, then this
parsing must return failure.
Though context.filter = "none" will disable
filters for the context, context.filter = "", context.filter = null,
and context.filter =
undefined are all treated as unparseable inputs and the value of the current filter is left unchanged.
Coordinates used in the value of the current
filter are interpreted such that one pixel is equivalent to one SVG user space unit and to
one canvas coordinate space unit. Filter coordinates are not affected by the current transformation matrix. The current
transformation matrix affects only the input to the filter. Filters are applied in the
output bitmap's coordinate space.
When the value of the current filter is a
string parsable as a <filter-value-list> which defines lengths using
percentages or using 'em' or 'ex' units, these must be interpreted
relative to the computed value of the 'font-size' property of the
font style source object at the time that the attribute is set. If the computed values are undefined for a particular case (e.g. because
the font style source object is not an element or is not being
rendered), then the relative keywords must be interpreted relative to the default value of
the font attribute. The 'larger' and 'smaller' keywords
are not supported.
If the value of the current filter is a
string parseable as a <filter-value-list> with a reference to an SVG filter in
the same document, and this SVG filter changes, then the changed filter is used for the next draw
operation.
If the value of the current filter is a
string parseable as a <filter-value-list> with a reference to an SVG filter in
an external resource document and that document is not loaded when a drawing operation is
invoked, then the drawing operation must proceed with no filtering.
4.12.5.1.20 Working with externally-defined SVG filters
This section is non-normative.
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.
4.12.5.1.21 Drawing model
When a shape or image is painted, user agents must follow these steps, in the order given (or
act as if they do):
Render the shape or image onto an infinite transparent black bitmap, creating
image A, as described in the previous sections. For shapes, the current fill, stroke,
and line styles must be honored, and the stroke must itself also be subjected to the current
transformation matrix.
When the current filter is set to a
value other than "none" and all the externally-defined filters it
references, if any, are in documents that are currently loaded, then use image A as
the input to the current filter, creating
image B. If the current filter
is a string parseable as a <filter-value-list>, then draw using the current filter in the same manner as SVG.
Otherwise, let B be an alias for A.
When shadows are drawn, render the shadow from image B,
using the current shadow styles, creating image C.
When compositing onto the output bitmap, pixels that would fall outside of the
output bitmap must be discarded.
4.12.5.1.22 Best practices
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:
Mouse placement of the caret has to be reimplemented.
Keyboard movement of the caret has to be reimplemented (possibly across lines, for multiline
text input).
Scrolling of the text control has to be implemented (horizontally for long lines, vertically
for multiline input).
Native features such as copy-and-paste have to be reimplemented.
Native features such as spell-checking have to be reimplemented.
Native features such as drag-and-drop have to be reimplemented.
Native features such as page-wide text search have to be reimplemented.
Native features specific to the user, for example custom text services, have to be
reimplemented. This is close to impossible since each user might have different services
installed, and there is an unbounded set of possible such services.
Bidirectional text editing has to be reimplemented.
For multiline text editing, line wrapping has to be implemented for all relevant
languages.
Text selection has to be reimplemented.
Dragging of bidirectional text selections has to be reimplemented.
Platform-native keyboard shortcuts have to be reimplemented.
Platform-native input method editors (IMEs) have to be reimplemented.
Undo and redo functionality has to be reimplemented.
Accessibility features such as magnification following the caret or selection have to be
reimplemented.
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.
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><htmllang="en"><metacharset="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><canvaswidth="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 cellsthis.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 frameIndexthis.tick =function(){// this advances the frame and the robot// the return value is how many pixels the robot has movedthis.frame +=1;if(this.frame >=this.frameIndex[this.mode].length){// we've reached the end of this animation cyclethis.frame =0;if(this.mode !=this.targetMode){// switch to next cycleif(this.mode =='walk'){// we need to stop walking before we decide what to do nextthis.mode ='stop';}elseif(this.mode =='stop'){if(this.targetMode =='walk')this.mode ='walk';elsethis.mode ='idle';}elseif(this.mode =='idle'){if(this.targetMode =='walk')this.mode ='walk';}}}if(this.mode =='walk')return8;return0;},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 100ms, so that we don't slow down when we don't paint
setInterval(function(){var dx = blueRobot.tick();
landscape.advance(dx);},100);</script><pclass="buttons"><inputtype=buttonvalue="Walk"onclick="blueRobot.walk()"><inputtype=buttonvalue="Stop"onclick="blueRobot.stop()"><footer><small> Blue Robot Player Sprite by <ahref="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 <arel="license"href="https://creativecommons.org/licenses/by-sa/3.0/">Creative
Commons Attribution-ShareAlike 3.0 Unported License</a>.</small></footer>
ImageBitmapRenderingContext 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');});
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.
The canvas attribute must return the
value it was initialized to when the object was created.
The step of compositing over an opaque black bitmap ought to be
elided whenever equivalent results can be obtained more efficiently by other means.
When a user agent is required to set an ImageBitmapRenderingContext's output
bitmap, with a context argument that is an
ImageBitmapRenderingContext object and an optional argument bitmap that
refers to bitmap data, it must run these
steps:
OffscreenCanvas 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.
Returns an object that exposes an API for drawing on the OffscreenCanvas object.
contextId specifies the desired API: "2d", "bitmaprenderer", "webgl", "webgl2", or "webgpu". options is 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. The WebGL specifications define the
"webgl" and "webgl2" contexts. WebGPU defines the
"webgpu" context. [WEBGL][WEBGPU]
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).
An OffscreenCanvas object has an internal bitmap that is initialized when the object
is created. The width and height of the bitmap are
equal to the values of the width and height attributes of the OffscreenCanvas
object. Initially, all the bitmap's pixels are transparent black.
An OffscreenCanvas object can have a rendering context bound to it. Initially,
it does not have a bound rendering context. To keep track of whether it has a rendering context
or not, and what kind of rendering context it is, an OffscreenCanvas object also
has a context mode, which is initially none but can be changed to either 2d, bitmaprenderer, webgl, webgl2, webgpu, or detached by algorithms defined in this
specification.
The constructor OffscreenCanvas(width,
height), when invoked, must create a new OffscreenCanvas
object with its bitmap initialized to a rectangular
array of transparent black pixels of the dimensions specified by width and
height; and its width and height attributes initialized to width and
height respectively.
Initialize value's bitmap to a
rectangular array of transparent black pixels with width given by
dataHolder.[[Width]] and height given by dataHolder.[[Height]].
If dataHolder.[[PlaceholderCanvas]] is not null, set value's placeholder canvas element to
dataHolder.[[PlaceholderCanvas]] (while maintaining the weak reference
semantics).
The getContext(contextId,
options) method of an OffscreenCanvas object, when invoked,
must run these steps:
If options is not an object, then set
options to null.
Set options to the result of convertingoptions to a JavaScript value.
Run the steps in the cell of the following table whose column header matches this
OffscreenCanvas object's context
mode and whose row header matches contextId:
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.
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 convertToBlob(options) method,
when invoked, must run the following steps:
Set this OffscreenCanvas object's bitmap to reference a newly created bitmap of the same
dimensions and color space as the previous bitmap, and with its pixels initialized to
transparent black, or opaque black if the rendering context's alpha flag is set to false.
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.
The offscreen 2D context creation algorithm, which is passed a
target (an OffscreenCanvas object) and optionally some arguments,
consists of running the following steps:
If the algorithm was passed some arguments, let arg be the first such
argument. Otherwise, let arg be undefined.
Set context's bitmap to a newly
created bitmap with the dimensions specified by the width and height attributes of target, and set
target's bitmap to the same bitmap (so that they are shared).
Implementations are encouraged to short-circuit the graphics update steps of
the window event loop for the purposes of updating the contents of a placeholder canvas element to the
display. This could mean, for example, that the commit() method can copy the bitmap contents directly
to a graphics buffer that is mapped to the physical display location of the placeholder canvas element. This or
similar short-circuiting approaches can significantly reduce display latency, especially in cases
where the commit() method is invoked from a
worker event loop and the window event loop of the placeholder canvas element is busy.
However, such shortcuts cannot have any script-observable side-effects. This means that the
committed bitmap still needs to be sent to the placeholder canvas element, in case the
element is used as a CanvasImageSource, as an ImageBitmapSource, or in
case toDataURL() or toBlob() are called on it.
The canvas APIs provide mechanisms for specifying the color space of the canvas's
backing store. The default backing store color space for all canvas APIs is
'srgb'.
Color space conversion must be applied to the canvas's backing store when rendering the canvas
to the output device. This color space conversion must be identical to the color space conversion
that would be applied to an img element with a color profile that specifies the same
color space as the canvas's backing store.
When drawing content to a 2D context, all inputs must be converted to the context's color space before drawing. Interpolation of
gradient color stops must be performed on color values after conversion to the context's color space. Alpha blending must be performed
on values after conversion to the context's color
space.
There do not exist any inputs to a 2D context for which the color space is
undefined. The color space for CSS colors is defined in CSS Color. The color space
for images that specify no color profile information is assumed to be 'srgb', as
specified in the Color Spaces of Untagged
Colors section of CSS Color. [CSSCOLOR]
4.12.5.5 Serializing bitmaps to a file
When a user agent is to create a
serialization of the bitmap as a file, given a type and an optional
quality, it must create an image file in the format given by type. If an
error occurs during the creation of the image file (e.g. an internal encoder error), then the
result of the serialization is null. [PNG]
The image file's pixel data must be the bitmap's pixel data scaled to one image pixel per
coordinate space unit, and if the file format used supports encoding resolution metadata, the
resolution must be given as 96dpi (one image pixel per CSS pixel).
If type is supplied, then it must be interpreted as a MIME
type giving the format to use. If the type has any parameters, then it must be treated as
not supported.
For example, the value "image/png" would mean to generate a PNG
image, the value "image/jpeg" would mean to generate a JPEG image, and the value
"image/svg+xml" would mean to generate an SVG image (which would require that the
user agent track how the bitmap was generated, an unlikely, though potentially awesome,
feature).
User agents must support PNG ("image/png"). User agents may support other types.
If the user agent does not support the requested type, then it must create the file using the PNG
format. [PNG]
For image types that do not support an alpha channel, the serialized image must be the bitmap
image composited onto an opaque black background using the source-over compositing operator.
For image types that support color profiles, the serialized image must include a color profile
indicating the color space of the underlying bitmap. For image types that do not support color
profiles, the serialized image must be converted to the 'srgb' color space using
'relative-colorimetric' rendering intent.
Thus, in the 2D context, calling the drawImage() method to render the output of the toDataURL() or toBlob() method to the canvas, given the appropriate dimensions,
has no visible effect beyond, at most, clipping colors of the canvas to a more narrow gamut.
If type is an image format that supports variable quality (such as
"image/jpeg"), quality is given, and type is not
"image/png", then, if Type(quality) is
Number, and quality is in the range 0.0 to 1.0 inclusive, the user agent must treat
quality as the desired quality level. Otherwise, the user agent must use its default
quality value, as if the quality argument had not been given.
The use of type-testing here, instead of simply declaring quality as
a Web IDL double, is a historical artifact.
Different implementations can have slightly different interpretations of
"quality". When the quality is not specified, an implementation-specific default is used that
represents a reasonable compromise between compression ratio, image quality, and encoding
time.
Information leakage can occur if scripts from one origin can
access information (e.g. read pixels) from images from another origin (one that isn't the same).
To mitigate this, bitmaps used with canvas elements and ImageBitmap
objects are defined to have a flag indicating whether they are origin-clean. All bitmaps start with their origin-clean set to true. The flag is set to false
when cross-origin images are used.
The flag can be reset in certain situations; for example, when changing the value of the
width or the height content attribute of the canvas element
to which a CanvasRenderingContext2D is bound, the bitmap is
cleared and its origin-clean flag is reset.
4.12.5.7 Premultiplied alpha and the 2D rendering context
Premultiplied alpha refers to one way of
representing transparency in an image, the other being non-premultiplied alpha.
Under non-premultiplied alpha, the red, green, and blue channels of a pixel represent that
pixel's color, and its alpha channel represents that pixel's opacity.
Under premultiplied alpha, however, the red, green, and blue channels of a pixel represent the
amounts of color that the pixel adds to the image, and its alpha channel represents the amount
that the pixel obscures whatever is behind it.
For instance, assuming the color channels range from 0 (off) to 255 (full intensity), these
example colors are represented in the following ways:
CSS color representation
Premultiplied representation
Non-premultiplied representation
Description of color
Image of color blended above other content
rgba(255, 127, 0, 1)
255, 127, 0, 255
255, 127, 0, 255
Completely-opaque orange
rgba(255, 255, 0, 0.5)
127, 127, 0, 127
255, 255, 0, 127
Halfway-opaque yellow
Unrepresentable
255, 127, 0, 127
Unrepresentable
Additive halfway-opaque orange
Unrepresentable
255, 127, 0, 0
Unrepresentable
Additive fully-transparent orange
rgba(255, 127, 0, 0)
0, 0, 0, 0
255, 127, 0, 0
Fully-transparent ("invisible") orange
rgba(0, 127, 255, 0)
0, 0, 0, 0
255, 127, 0, 0
Fully-transparent ("invisible") turquoise
Converting a color value from a non-premultiplied
representation to a premultiplied one involves multiplying the color's red, green, and
blue channels by its alpha channel (remapping the range of the alpha channel such that "fully
transparent" is 0, and "fully opaque" is 1).
Converting a color value from a premultiplied
representation to a non-premultiplied one involves the inverse: dividing the color's red,
green, and blue channels by its alpha channel.
As certain colors can only be represented under premultiplied alpha (for instance, additive
colors), and others can only be represented under non-premultiplied alpha (for instance,
"invisible" colors which hold certain red, green, and blue values even with no opacity); and
division and multiplication on 8-bit integers (which is how canvas's colors are currently stored)
entails a loss of precision, converting between premultiplied and non-premultiplied alpha is a
lossy operation on colors that are not fully opaque.
It is important for canvas bitmaps to represent colors using premultiplied alpha
because it affects the range of representable colors. While additive colors cannot currently be
drawn onto canvases directly because CSS colors are non-premultiplied and cannot represent them,
it is still possible to, for instance, draw additive colors onto a WebGL canvas and then draw that
WebGL canvas onto a 2D canvas via drawImage().
