HTML: The Living Standard — Edition for Web Developers

Review Draft: search element with form functionality — Last Updated 1 December 2021

1. 1. 1. 4.12.5 The canvas element
         1. 4.12.5.1 The 2D rendering context
            1. 4.12.5.1.1 Implementation notes
            2. 4.12.5.1.2 The canvas state
            3. 4.12.5.1.3 Line styles
            4. 4.12.5.1.4 Text styles
            5. 4.12.5.1.5 Building paths
            6. 4.12.5.1.6 Path2D objects
            7. 4.12.5.1.7 Transformations
            8. 4.12.5.1.8 Image sources for 2D rendering contexts
            9. 4.12.5.1.9 Fill and stroke styles
            10. 4.12.5.1.10 Drawing rectangles to the bitmap
            11. 4.12.5.1.11 Drawing text to the bitmap
            12. 4.12.5.1.12 Drawing paths to the canvas
            13. 4.12.5.1.13 Drawing focus rings and scrolling paths into view
            14. 4.12.5.1.14 Drawing images
            15. 4.12.5.1.15 Pixel manipulation
            16. 4.12.5.1.16 Compositing
            17. 4.12.5.1.17 Image smoothing
            18. 4.12.5.1.18 Shadows
            19. 4.12.5.1.19 Filters
            20. 4.12.5.1.20 Working with externally-defined SVG filters
            21. 4.12.5.1.21 Best practices
            22. 4.12.5.1.22 Examples
         2. 4.12.5.2 The ImageBitmap rendering context
            1. 4.12.5.2.1 Introduction
            2. 4.12.5.2.2 The ImageBitmapRenderingContext interface
         3. 4.12.5.3 The OffscreenCanvas interface
            1. 4.12.5.3.1 The offscreen 2D rendering context
         4. 4.12.5.4 Serializing bitmaps to a file
         5. 4.12.5.5 Premultiplied alpha and the 2D rendering context

4.12.5 The canvas element

Categories:

Flow content.

Phrasing content.

Embedded content.

Palpable content.

Contexts in which this element can be used:

Where embedded content is expected.

Content model:

Transparent, but with no interactive content descendants except for a elements, img elements with usemap attributes, button elements, input elements whose type attribute are in the Checkbox or Radio Button states, input elements that are buttons, and select elements with a multiple attribute or a display size greater than 1.

Tag omission in text/html:

Neither tag is omissible.

Content attributes:

Global attributes

width — Horizontal dimension

height — Vertical dimension

Accessibility considerations:

For authors.

For implementers.

DOM interface:

Uses HTMLCanvasElement.

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 interactive visual media, if scripting is enabled for the canvas element, and if support for canvas elements has been enabled, then the canvas element represents embedded content consisting of a dynamically created image, the element's bitmap.

In non-interactive, static, visual media, if the canvas element has been previously associated with a rendering context (e.g. if the page was viewed in an interactive visual medium and is now being printed, or if some script that ran during the page layout process painted on the element), then the canvas element represents embedded content with the element's current bitmap and size. Otherwise, the element represents its fallback content instead.

In non-visual media, and in visual media if scripting is disabled for the canvas element or if support for canvas elements has been disabled, the canvas element represents its fallback content instead.

When a canvas element represents embedded content, the user can still focus descendants of the canvas element (in the fallback content). When an element is focused, it is the target of keyboard interaction events (even though the element itself is not visible). This allows authors to make an interactive canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no effect on mouse interaction events.) [UIEVENTS]

An element whose nearest canvas element ancestor is being rendered and represents embedded content is an element that is being used as relevant canvas fallback content.

The canvas element has two attributes to control the size of the element's bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The width attribute defaults to 300, and the height attribute defaults to 150.

When setting the value of the width or height attribute, if the context mode of the canvas element is set to placeholder, the user agent must throw an "InvalidStateError" DOMException and leave the attribute's value unchanged.

The intrinsic dimensions of the canvas element when it represents embedded content are equal to the dimensions of the element's bitmap.

The user agent must use a square pixel density consisting of one pixel of image data per coordinate space unit for the bitmaps of a canvas and its rendering contexts.

A canvas element can be sized arbitrarily by a style sheet, its bitmap is then subject to the 'object-fit' CSS property.

context = canvas.getContext(contextId [, options ])

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).

url = canvas.toDataURL([ type [, quality ] ])

Returns a data: URL for the image in the canvas.

The first argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is "image/png"; that type is also used if the given type isn't supported. The second argument applies if the type is an image format that supports variable quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive indicating the desired quality level for the resulting image.

When trying to use types other than "image/png", authors can check if the image was really returned in the requested format by checking to see if the returned string starts with one of the exact strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus the requested type was not supported. (The one exception to this is if the canvas has either no height or no width, in which case the result might simply be "data:,".)

canvas.toBlob(callback [, type [, quality ] ])

Creates a Blob object representing a file containing the image in the canvas, and invokes a callback with a handle to that object.

The second argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is "image/png"; that type is also used if the given type isn't supported. The third argument applies if the type is an image format that supports variable quality (such as "image/jpeg"), and is a number in the range 0.0 to 1.0 inclusive indicating the desired quality level for the resulting image.

canvas.transferControlToOffscreen()

Returns a newly created OffscreenCanvas object that uses the canvas element as a placeholder. Once the canvas element has become a placeholder for an OffscreenCanvas object, its intrinsic size can no longer be changed, and it cannot have a rendering context. The content of the placeholder canvas is updated by calling the commit() method of the OffscreenCanvas object's rendering context.

4.12.5.1 The 2D rendering context

context = canvas.getContext('2d' [, { [ alpha: true ] [, desynchronized: false ] [, colorSpace: 'srgb'] [, willReadFrequently: false ]} ])

Returns a CanvasRenderingContext2D object that is permanently bound to a particular canvas element.

If the alpha member is false, then the context is forced to always be opaque.

If the desynchronized member is true, then the context might be desynchronized.

The colorSpace member specifies the color space of the rendering context.

If the willReadFrequently member is true, then the context is marked for readback optimization.

context.canvas

Returns the canvas element.

attributes = context.getContextAttributes()

Returns an object whose:

• alpha member is true if the context has an alpha channel, or false if it was forced to be opaque.
• desynchronized member is true if the context can be desynchronized.
• colorSpace member is a string indicating the context's color space.
• willReadFrequently member is true if the context is marked for readback optimization.

The PredefinedColorSpace enumeration is used to specify the color space of the canvas's backing store.

The "srgb" value indicates the 'srgb' color space.

The "display-p3" value indicates the 'display-p3' color space.

Algorithms for converting between color spaces are found in the Predefined color spaces section of CSS Color. [CSSCOLOR]

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

The output bitmap, when it is not directly displayed by the user agent, implementations can, instead of updating this bitmap, merely remember the sequence of drawing operations that have been applied to it until such time as the bitmap's actual data is needed (for example because of a call to drawImage(), or the createImageBitmap() factory method). In many cases, this will be more memory efficient.

The bitmap of a canvas element is the one bitmap that's pretty much always going to be needed in practice. The output bitmap of a rendering context, when it has one, is always just an alias to a canvas element's bitmap.

Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its intrinsic size, or to enable double buffering so that graphics updates, like page scrolling for example, can be processed concurrently while canvas draw commands are being executed.

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 current transformation matrix.
• The current clipping region.
• The current values of the following attributes: strokeStyle, fillStyle, globalAlpha, lineWidth, lineCap, lineJoin, miterLimit, lineDashOffset, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, filter, globalCompositeOperation, font, textAlign, textBaseline, direction, letterSpacing, fontKerning, fontStretch, fontVariantCaps, textRendering, wordSpacing, imageSmoothingEnabled, imageSmoothingQuality.
• The current dash list.

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.

context.save()

Pushes the current state onto the stack.

context.restore()

Pops the top state on the stack, restoring the context to that state.

context.reset()

Resets the rendering context, which includes the backing buffer, the drawing state stack, path, and styles.

context.isContextLost()

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.

4.12.5.1.3 Line styles

context.lineWidth [ = value ]

styles.lineWidth [ = value ]

Returns the current line width.

Can be set, to change the line width. Values that are not finite values greater than zero are ignored.

context.lineCap [ = value ]

styles.lineCap [ = value ]

Returns the current line cap style.

Can be set, to change the line cap style.

The possible line cap styles are "butt", "round", and "square". Other values are ignored.

context.lineJoin [ = value ]

styles.lineJoin [ = value ]

Returns the current line join style.

Can be set, to change the line join style.

The possible line join styles are "bevel", "round", and "miter". Other values are ignored.

context.miterLimit [ = value ]

styles.miterLimit [ = value ]

Returns the current miter limit ratio.

Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.

context.setLineDash(segments)

styles.setLineDash(segments)

Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.

segments = context.getLineDash()

segments = styles.getLineDash()

Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).

context.lineDashOffset

styles.lineDashOffset

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.

4.12.5.1.4 Text styles

context.font [ = value ]

styles.font [ = value ]

Returns the current font settings.

Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.

Relative keywords and lengths are computed relative to the font of the canvas element.

context.textAlign [ = value ]

styles.textAlign [ = value ]

Returns the current text alignment settings.

Can be set, to change the alignment. The possible values are and their meanings are given below. Other values are ignored. The default is "start".

context.textBaseline [ = value ]

styles.textBaseline [ = value ]

Returns the current baseline alignment settings.

Can be set, to change the baseline alignment. The possible values and their meanings are given below. Other values are ignored. The default is "alphabetic".

context.direction [ = value ]

styles.direction [ = value ]

Returns the current directionality.

Can be set, to change the directionality. The possible values and their meanings are given below. Other values are ignored. The default is "inherit".

context.letterSpacing [ = value ]

styles.letterSpacing [ = value ]

Returns the current spacing between characters in the text.

Can be set, to change spacing between characters. Postive values spreads characters further apart, while negative values brings them closer together. The default is 0.

context.fontKerning [ = value ]

styles.fontKerning [ = value ]

Returns the current font kerning settings.

Can be set, to change the font kerning. The possible values and their meanings are given below. Other values are ignored. The default is "auto".

context.fontStretch [ = value ]

styles.fontStretch [ = value ]

Returns the current font stretch settings.

Can be set, to change the font stretch. The possible values and their meanings are given below. Other values are ignored. The default is "normal".

context.fontVariantCaps [ = value ]

styles.fontVariantCaps [ = value ]

Returns the current font variant caps settings.

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".

context.textRendering [ = value ]

styles.textRendering [ = value ]

Returns the current text rendering settings.

Can be set, to change the text rendering. The possible values and their meanings are given below. Other values are ignored. The default is "auto".

context.wordSpacing [ = value ]

styles.wordSpacing [ = value ]

Returns the current spacing between words in the text.

Can be set, to change spacing between words. Postive values spreads words further apart, while negative values brings them closer together. The default is 0.

The textAlign attribute's allowed keywords are as follows:

start

Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).

end

Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).

left

Align to the left.

right

Align to the right.

center

Align to the center.

The textBaseline attribute's allowed keywords correspond to alignment points in the font:

The keywords map to these alignment points as follows:

top

The top of the em square

hanging

The hanging baseline

middle

The middle of the em square

alphabetic

The alphabetic baseline

ideographic

The ideographic-under baseline

bottom

The bottom of the em square

The direction attribute's allowed keywords are as follows:

ltr

Treat input to the text preparation algorithm as left-to-right text.

rtl

Treat input to the text preparation algorithm as right-to-left text.

inherit

Default to the directionality of the canvas element or Document as appropriate.

The fontKerning attribute's allowed keywords are as follows:

auto

Kerning is applied at the discretion of the user agent.

normal

Kerning is applied.

none

Kerning is not applied.

The fontStretch attribute's allowed keywords are as follows:

ultra-condensed

Same as CSS 'font-stretch' 'ultra-condensed' setting.

extra-condensed

Same as CSS 'font-stretch' 'extra-condensed' setting.

condensed

Same as CSS 'font-stretch' 'condensed' setting.

semi-condensed

Same as CSS 'font-stretch' 'semi-condensed' setting.

normal

The default setting, where width of the glyphs is at 100%.

semi-expanded

Same as CSS 'font-stretch' 'semi-expanded' setting.

expanded

Same as CSS 'font-stretch' 'expanded' setting.

extra-expanded

Same as CSS 'font-stretch' 'extra-expanded' setting.

ultra-expanded

Same as CSS 'font-stretch' 'ultra-expanded' setting.

The fontVariantCaps attribute's allowed keywords are as follows:

normal

None of the features listed below are enabled.

small-caps

Same as CSS 'font-variant-caps' 'small-caps' setting.

all-small-caps

Same as CSS 'font-variant-caps' 'all-small-caps' setting.

petite-caps

Same as CSS 'font-variant-caps' 'petite-caps' setting.

all-petite-caps

Same as CSS 'font-variant-caps' 'all-petite-caps' setting.

unicase

Same as CSS 'font-variant-caps' 'unicase' setting.

titling-caps

Same as CSS 'font-variant-caps' 'titling-caps' setting.

The textRendering attribute's allowed keywords are as follows:

auto

Same as 'auto' in SVG text-rendering property.

optimizeSpeed

Same as 'optimizeSpeed' in SVG text-rendering property.

optimizeLegibility

Same as 'optimizeLegibility' in SVG text-rendering propeerty.

geometricPrecision

Same as 'geometricPrecision' in SVG text-rendering property.

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.)

1. If maxWidth was provided but is less than or equal to zero or equal to NaN, then return an empty array.

2. Replace all ASCII whitespace in text with U+0020 SPACE characters.

3. Let font be the current font of target, as given by that object's font attribute.

4. 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"

   Let direction be 'ltr'.

   If the target object's direction attribute has the value "rtl"

   Let direction be 'rtl'.

   If the target object's font style source object is an element

   Let direction be the directionality of the target object's font style source object.

   If the target object's font style source object is a Document with a non-null document element

   Let direction be the directionality of the target object's font style source object's document element.

   Otherwise

   Let direction be 'ltr'.

5. Form a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with its CSS properties set as follows:

   Property	Source
   'direction'	direction
   'font'	font
   'font-kerning'	target's fontKerning
   'font-stretch'	target's fontStretch
   'font-variant-caps'	target's fontVariantCaps
   'letter-spacing'	target's letterSpacing
   SVG text-rendering	target's textRendering
   'white-space'	'pre'
   'word-spacing'	target's wordSpacing

   and with all other properties set to their initial values.

6. If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.

7. The anchor point is a point on the inline box, and the physical alignment is one of the values left, right, and center. These variables are determined by the textAlign and textBaseline values as follows:

   Horizontal position:

   If textAlign is left

   If textAlign is start and direction is 'ltr'

   If textAlign is end and direction is 'rtl'

   Let the anchor point's horizontal position be the left edge of the inline box, and let physical alignment be left.

   If textAlign is right

   If textAlign is end and direction is 'ltr'

   If textAlign is start and direction is 'rtl'

   Let the anchor point's horizontal position be the right edge of the inline box, and let physical alignment be right.

   If textAlign is center

   Let the anchor point's horizontal position be half way between the left and right edges of the inline box, and let physical alignment be center.

   Vertical position:

   If textBaseline is top

   Let the anchor point's vertical position be the top of the em box of the first available font of the inline box.

   If textBaseline is hanging

   Let the anchor point's vertical position be the hanging baseline of the first available font of the inline box.

   If textBaseline is middle

   Let the anchor point's vertical position be half way between the bottom and the top of the em box of the first available font of the inline box.

   If textBaseline is alphabetic

   Let the anchor point's vertical position be the alphabetic baseline of the first available font of the inline box.

   If textBaseline is ideographic

   Let the anchor point's vertical position be the ideographic-under baseline of the first available font of the inline box.

   If textBaseline is bottom

   Let the anchor point's vertical position be the bottom of the em box of the first available font of the inline box.

8. 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.

9. 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.

context.moveTo(x, y)

path.moveTo(x, y)

Creates a new subpath with the given point.

context.closePath()

path.closePath()

Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.

context.lineTo(x, y)

path.lineTo(x, y)

Adds the given point to the current subpath, connected to the previous one by a straight line.

context.quadraticCurveTo(cpx, cpy, x, y)

path.quadraticCurveTo(cpx, cpy, x, y)

Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.

context.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

path.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.

context.arcTo(x1, y1, x2, y2, radius)

path.arcTo(x1, y1, x2, y2, radius)

Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context.arc(x, y, radius, startAngle, endAngle [, counterclockwise ])

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.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context.ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, counterclockwise])

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.

Throws an "IndexSizeError" DOMException if the given radius is negative.

context.rect(x, y, w, h)

path.rect(x, y, w, h)

Adds a new closed subpath to the path, representing the given rectangle.

context.roundRect(x, y, w, h, radii)

path.roundRect(x, y, w, h, radii)

Adds a new closed subpath to the path representing the given rounded rectangle. radii represents a list of radii for the corners of the rectangle, in pixels. The number and order of these radii function in the same way as the CSS 'border-radius' property.

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 not a list of size 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.

4.12.5.1.6 Path2D objects

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.

path = new Path2D()

Creates a new empty Path2D object.

path = new Path2D(path)

When path is a Path2D object, returns a copy.

When path is a string, creates the path described by the argument, interpreted as SVG path data. [SVG]

path.addPath(path [, transform ])

Adds to the path the path given by the argument.

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 current transformation matrix is applied to coordinates when creating the current default path, and when painting text, shapes, and Path2D objects, on objects implementing the CanvasTransform interface.

context.scale(x, y)

Changes the current transformation matrix to apply a scaling transformation with the given characteristics.

context.rotate(angle)

Changes the current transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.

context.translate(x, y)

Changes the current transformation matrix to apply a translation transformation with the given characteristics.

context.transform(a, b, c, d, e, f)

Changes the current transformation matrix to apply the matrix given by the arguments as described below.

matrix = context.getTransform()

Returns a copy of the current transformation matrix, as a newly created DOMMatrix object.

context.setTransform(a, b, c, d, e, f)

Changes the current transformation matrix to the matrix given by the arguments as described below.

context.setTransform(transform)

Changes the current transformation matrix to the matrix represented by the passed DOMMatrix2DInit dictionary.

context.resetTransform()

Changes the current transformation matrix to the identity matrix.

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.

4.12.5.1.8 Image sources for 2D rendering contexts

Some methods on the CanvasDrawImage and CanvasFillStrokeStyles interfaces take the union type CanvasImageSource as an argument.

This union type allows objects implementing any of the following interfaces to be used as image sources:

• HTMLOrSVGImageElement (img or SVG image elements)
• HTMLVideoElement (video elements)
• HTMLCanvasElement (canvas elements)
• ImageBitmap
• VideoFrame

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:

1. Switch on image:

   HTMLOrSVGImageElement

   If image's current request's state is broken, then throw an "InvalidStateError" DOMException.

   If image is not fully decodable, then return bad.

   If image has an intrinsic width or intrinsic height (or both) equal to zero, then return bad.

   HTMLVideoElement

   If image's readyState attribute is either HAVE_NOTHING or HAVE_METADATA, then return bad.

   HTMLCanvasElement

   OffscreenCanvas

   If image has either a horizontal dimension or a vertical dimension equal to zero, then throw an "InvalidStateError" DOMException.

   ImageBitmap

   VideoFrame

   If image's [[Detached]] internal slot value is set to true, then throw an "InvalidStateError" DOMException.

2. Return good.

When a CanvasImageSource object represents an HTMLOrSVGImageElement, the element's image must be used as the source image.

Specifically, when a CanvasImageSource object represents an animated image in an HTMLOrSVGImageElement, the user agent must use the default image of the animation (the one that the format defines is to be used when animation is not supported or is disabled), or, if there is no such image, the first frame of the animation, when rendering the image for CanvasRenderingContext2D APIs.

When a CanvasImageSource object represents an HTMLVideoElement, then the frame at the current playback position when the method with the argument is invoked must be used as the source image when rendering the image for CanvasRenderingContext2D APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource (i.e., after any aspect-ratio correction has been applied).

When a CanvasImageSource object represents an HTMLCanvasElement, the element's bitmap must be used as the source image.

When a CanvasImageSource object represents an element that is being rendered and that element has been resized, the original image data of the source image must be used, not the image as it is rendered (e.g. width and height attributes on the source element have no effect on how the object is interpreted when rendering the image for CanvasRenderingContext2D APIs).

When a CanvasImageSource object represents an ImageBitmap, the object's bitmap image data must be used as the source image.

When a CanvasImageSource object represents a VideoFrame, the object's pixel data must be used as the source image, and the source image's dimensions must be the object's [[display width]] and [[display height]].

An object image is not origin-clean if, switching on image's type:

HTMLOrSVGImageElement

image's current request's image data is CORS-cross-origin.

HTMLVideoElement

image's media data is CORS-cross-origin.

HTMLCanvasElement

ImageBitmap

image's bitmap's origin-clean flag is false.

4.12.5.1.9 Fill and stroke styles

context.fillStyle [ = value ]

Returns the current style used for filling shapes.

Can be set, to change the fill style.

The style can be either a string containing a CSS color, or a CanvasGradient or CanvasPattern object. Invalid values are ignored.

context.strokeStyle [ = value ]

Returns the current style used for stroking shapes.

Can be set, to change the stroke style.

The style can be either a string containing a CSS color, or a CanvasGradient or CanvasPattern object. Invalid values are ignored.

There are 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.

gradient.addColorStop(offset, color)

Adds a color stop with the given color to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.

Throws an "IndexSizeError" DOMException if the offset is out of range. Throws a "SyntaxError" DOMException if the color cannot be parsed.

gradient = context.createLinearGradient(x0, y0, x1, y1)

Returns a CanvasGradient object that represents a linear gradient that paints along the line given by the coordinates represented by the arguments.

gradient = context.createRadialGradient(x0, y0, r0, x1, y1, r1)

Returns a CanvasGradient object that represents a radial gradient that paints along the cone given by the circles represented by the arguments.

If either of the radii are negative, throws an "IndexSizeError" DOMException exception.

gradient = context.createConicGradient(startAngle, x, y)

Returns a CanvasGradient object that represents a conic gradient that paints clockwise along the rotation around the center represented by the arguments.

Patterns are represented by objects implementing the opaque CanvasPattern interface.

pattern = context.createPattern(image, repetition)

Returns a CanvasPattern object that uses the given image and repeats in the direction(s) given by the repetition argument.

The allowed values for repetition are repeat (both directions), repeat-x (horizontal only), repeat-y (vertical only), and no-repeat (neither). If the repetition argument is empty, the value repeat is used.

If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no data, throws an "InvalidStateError" DOMException.

pattern.setTransform(transform)

Sets the transformation matrix that will be used when rendering the pattern during a fill or stroke painting operation.

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.

context.clearRect(x, y, w, h)

Clears all pixels on the bitmap in the given rectangle to transparent black.

context.fillRect(x, y, w, h)

Paints the given rectangle onto the bitmap, using the current fill style.

context.strokeRect(x, y, w, h)

Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.

4.12.5.1.11 Drawing text to the bitmap

context.fillText(text, x, y [, maxWidth ])

context.strokeText(text, x, y [, maxWidth ])

Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.

metrics = context.measureText(text)

Returns a TextMetrics object with the metrics of the given text in the current font.

metrics.width

metrics.actualBoundingBoxLeft

metrics.actualBoundingBoxRight

metrics.fontBoundingBoxAscent

metrics.fontBoundingBoxDescent

metrics.actualBoundingBoxAscent

metrics.actualBoundingBoxDescent

metrics.emHeightAscent

metrics.emHeightDescent

metrics.hangingBaseline

metrics.alphabeticBaseline

metrics.ideographicBaseline

Returns the measurement described below.

width attribute

The width of that inline box, in CSS pixels. (The text's advance width.)

actualBoundingBoxLeft attribute

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.

fontBoundingBoxAscent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the ascent metric of the first available font, in CSS pixels; positive numbers indicating a distance going up from the given baseline.

This value and the next are useful when rendering a background that have to have a consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent attribute (and its corresponding attribute for the descent) are useful when drawing a bounding box around specific text.

fontBoundingBoxDescent attribute

The distance from the horizontal line indicated by the textBaseline attribute to the descent metric of the first available font, in CSS pixels; positive numbers indicating a distance going down from the given baseline.

actualBoundingBoxAscent attribute

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.)

hangingBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the hanging baseline of the inline box, in CSS pixels; positive numbers indicating that the given baseline is below the hanging baseline. (Zero if the given baseline is the hanging baseline.)

alphabeticBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the alphabetic baseline of the inline box, in CSS pixels; positive numbers indicating that the given baseline is below the alphabetic baseline. (Zero if the given baseline is the alphabetic baseline.)

ideographicBaseline attribute

The distance from the horizontal line indicated by the textBaseline attribute to the ideographic-under baseline of the inline box, in CSS pixels; positive numbers indicating that the given baseline is below the ideographic-under baseline. (Zero if the given baseline is the ideographic-under baseline.)

Glyphs rendered using fillText() and strokeText() can spill out of the box given by the font size (the em square size) and the width returned by measureText() (the text width). Authors are encouraged to use the bounding box values described above if this is an issue.

A future version of the 2D context API might provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.

4.12.5.1.12 Drawing paths to the canvas

Objects that implement the CanvasDrawPath interface have a current default path. There is only one current default path, it is not part of the drawing state. The current default path is a path, as described above.

context.beginPath()

Resets the current default path.

context.fill([ fillRule ])

context.fill(path [, fillRule ])

Fills the subpaths of the current default path or the given path with the current fill style, obeying the given fill rule.

context.stroke()

context.stroke(path)

Strokes the subpaths of the current default path or the given path with the current stroke style.

context.clip([ fillRule ])

context.clip(path [, fillRule ])

Further constrains the clipping region to the current default path or the given path, using the given fill rule to determine what points are in the path.

context.isPointInPath(x, y [, fillRule ])

context.isPointInPath(path, x, y [, fillRule ])

Returns true if the given point is in the current default path or the given path, using the given fill rule to determine what points are in the path.

context.isPointInStroke(x, y)

context.isPointInStroke(path, x, y)

Returns true if the given point would be in the region covered by the stroke of the current default path or the given path, given the current stroke style.

<canvas height=400 width=750>
 <label><input type=checkbox id=showA> Show As</label>
 <label><input type=checkbox id=showB> Show Bs</label>
 <!-- ... -->
</canvas>
<script>
 function drawCheckbox(context, element, x, y, paint) {
   context.save();
   context.font = '10px sans-serif';
   context.textAlign = 'left';
   context.textBaseline = 'middle';
   var metrics = context.measureText(element.labels[0].textContent);
   if (paint) {
     context.beginPath();
     context.strokeStyle = 'black';
     context.rect(x-5, y-5, 10, 10);
     context.stroke();
     if (element.checked) {
       context.fillStyle = 'black';
       context.fill();
     }
     context.fillText(element.labels[0].textContent, x+5, y);
   }
   context.beginPath();
   context.rect(x-7, y-7, 12 + metrics.width+2, 14);

   context.drawFocusIfNeeded(element);
   context.restore();
 }
 function drawBase() { /* ... */ }
 function drawAs() { /* ... */ }
 function drawBs() { /* ... */ }
 function redraw() {
   var canvas = document.getElementsByTagName('canvas')[0];
   var context = canvas.getContext('2d');
   context.clearRect(0, 0, canvas.width, canvas.height);
   drawCheckbox(context, document.getElementById('showA'), 20, 40, true);
   drawCheckbox(context, document.getElementById('showB'), 20, 60, true);
   drawBase();
   if (document.getElementById('showA').checked)
     drawAs();
   if (document.getElementById('showB').checked)
     drawBs();
 }
 function processClick(event) {
   var canvas = document.getElementsByTagName('canvas')[0];
   var context = canvas.getContext('2d');
   var x = event.clientX;
   var y = event.clientY;
   var node = event.target;
   while (node) {
     x -= node.offsetLeft - node.scrollLeft;
     y -= node.offsetTop - node.scrollTop;
     node = node.offsetParent;
   }
   drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
   if (context.isPointInPath(x, y))
     document.getElementById('showA').checked = !(document.getElementById('showA').checked);
   drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
   if (context.isPointInPath(x, y))
     document.getElementById('showB').checked = !(document.getElementById('showB').checked);
   redraw();
 }
 document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true);
 document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false);
 redraw();
</script>

4.12.5.1.13 Drawing focus rings and scrolling paths into view

context.drawFocusIfNeeded(element)

context.drawFocusIfNeeded(path, element)

If the given element is focused, draws a focus ring around the current default path or the given path, following the platform conventions for focus rings.

context.scrollPathIntoView()

context.scrollPathIntoView(path)

Scrolls the current default path or the given path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.

4.12.5.1.14 Drawing images

Objects that implement the CanvasDrawImage interface have the drawImage method to draw images.

context.drawImage(image, dx, dy)

context.drawImage(image, dx, dy, dw, dh)

context.drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)

Draws the given image onto the canvas. The arguments are interpreted as follows:

If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no data, throws an "InvalidStateError" DOMException.

4.12.5.1.15 Pixel manipulation

imagedata = new ImageData(sw, sh [, settings])

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.

Throws an "IndexSizeError" DOMException if either of the width or height arguments are zero.

imagedata = new ImageData(data, sw [, sh [, settings ] ])

Returns an ImageData object using the data provided in the Uint8ClampedArray argument, interpreted using the given dimensions and the color space indicated by settings.

As each pixel in the data is represented by four numbers, the length of the data needs to be a multiple of four times the given width. If the height is provided as well, then the length needs to be exactly the width times the height times 4.

Throws an "IndexSizeError" DOMException if the given data and dimensions can't be interpreted consistently, or if either dimension is zero.

imagedata = context.createImageData(imagedata)

Returns an ImageData object with the same dimensions and color space as the argument. All the pixels in the returned object are transparent black.

imagedata = context.createImageData(sw, sh [, settings])

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.

Throws an "IndexSizeError" DOMException if either of the width or height arguments are zero.

imagedata = context.getImageData(sx, sy, sw, sh [, settings])

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.

Throws an "IndexSizeError" DOMException if the either of the width or height arguments are zero.

imagedata.width

imagedata.height

Returns the actual dimensions of the data in the ImageData object, in pixels.

imagedata.data

Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.

imagedata.colorSpace

Returns the color space of the pixels.

context.putImageData(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ])

Paints the data from the given ImageData object onto the bitmap. If a dirty rectangle is provided, only the pixels from that rectangle are painted.

The globalAlpha and globalCompositeOperation attributes, as well as the shadow attributes, are ignored for the purposes of this method call; pixels in the canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.

Throws an "InvalidStateError" DOMException if the imagedata object's data attribute value's [[ViewedArrayBuffer]] internal slot is detached.

// canvas is a reference to a <canvas> element
var context = canvas.getContext('2d');

// create a blank slate
var data = context.createImageData(canvas.width, canvas.height);

// create some plasma
FillPlasma(data, 'green'); // green plasma

// add a cloud to the plasma
AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle

// paint the plasma+cloud on the canvas
context.putImageData(data, 0, 0);

// support methods
function FillPlasma(data, color) { ... }
function AddCloud(data, x, y) { ... }

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>Edge detection demo</title>
  <script>
   var image = new Image();
   function init() {
     image.onload = demo;
     image.src = "image.jpeg";
   }
   function demo() {
     var canvas = document.getElementsByTagName('canvas')[0];
     var context = canvas.getContext('2d');

     // draw the image onto the canvas
     context.drawImage(image, 0, 0);

     // get the image data to manipulate
     var input = context.getImageData(0, 0, canvas.width, canvas.height);

     // get an empty slate to put the data into
     var output = context.createImageData(canvas.width, canvas.height);

     // alias some variables for convenience
     // In this case input.width and input.height
     // match canvas.width and canvas.height
     // but we'll use the former to keep the code generic.
     var w = input.width, h = input.height;
     var inputData = input.data;
     var outputData = output.data;

     // edge detection
     for (var y = 1; y < h-1; y += 1) {
       for (var x = 1; x < w-1; x += 1) {
         for (var c = 0; c < 3; c += 1) {
           var i = (y*w + x)*4 + c;
           outputData[i] = 127 + -inputData[i - w*4 - 4] -   inputData[i - w*4] - inputData[i - w*4 + 4] +
                                 -inputData[i - 4]       + 8*inputData[i]       - inputData[i + 4] +
                                 -inputData[i + w*4 - 4] -   inputData[i + w*4] - inputData[i + w*4 + 4];
         }
         outputData[(y*w + x)*4 + 3] = 255; // alpha
       }
     }

     // put the image data back after manipulation
     context.putImageData(output, 0, 0);
   }
  </script>
 </head>
 <body onload="init()">
  <canvas></canvas>
 </body>
</html>

<!DOCTYPE HTML>
<html lang="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>

4.12.5.1.16 Compositing

context.globalAlpha [ = value ]

Returns the current alpha value applied to rendering operations.

Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are ignored.

context.globalCompositeOperation [ = value ]

Returns the current composition operation, from the values defined in Compositing and Blending. [COMPOSITE].

Can be set, to change the composition operation. Unknown values are ignored.

4.12.5.1.17 Image smoothing

context.imageSmoothingEnabled [ = value ]

Returns whether pattern fills and the drawImage() method will attempt to smooth images if their pixels don't line up exactly with the display, when scaling images up.

Can be set, to change whether images are smoothed (true) or not (false).

context.imageSmoothingQuality [ = value ]

Returns the current image-smoothing-quality preference.

Can be set, to change the preferred quality of image smoothing. The possible values are "low", "medium" and "high". Unknown values are ignored.

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.

context.shadowColor [ = value ]

Returns the current shadow color.

Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.

context.shadowOffsetX [ = value ]

context.shadowOffsetY [ = value ]

Returns the current shadow offset.

Can be set, to change the shadow offset. Values that are not finite numbers are ignored.

context.shadowBlur [ = value ]

Returns the current level of blur applied to shadows.

Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.

If the current composition operation is copy, then shadows effectively won't render (since the shape will overwrite the shadow).

4.12.5.1.19 Filters

All drawing operations on an object which implements the CanvasFilters interface are affected by the global filter attribute.

context.filter [ = value ]

Returns the current filter.

Can be set, to change the filter. Values that cannot be parsed as a <filter-value-list> value are ignored.

4.12.5.1.20 Working with externally-defined SVG filters

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 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.

4.12.5.1.22 Examples

<canvas width="800" height="450"></canvas>
<script>

 var context = document.getElementsByTagName('canvas')[0].getContext('2d');

 var lastX = context.canvas.width * Math.random();
 var lastY = context.canvas.height * Math.random();
 var hue = 0;
 function line() {
   context.save();
   context.translate(context.canvas.width/2, context.canvas.height/2);
   context.scale(0.9, 0.9);
   context.translate(-context.canvas.width/2, -context.canvas.height/2);
   context.beginPath();
   context.lineWidth = 5 + Math.random() * 10;
   context.moveTo(lastX, lastY);
   lastX = context.canvas.width * Math.random();
   lastY = context.canvas.height * Math.random();
   context.bezierCurveTo(context.canvas.width * Math.random(),
                         context.canvas.height * Math.random(),
                         context.canvas.width * Math.random(),
                         context.canvas.height * Math.random(),
                         lastX, lastY);

   hue = hue + 10 * Math.random();
   context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)';
   context.shadowColor = 'white';
   context.shadowBlur = 10;
   context.stroke();
   context.restore();
 }
 setInterval(line, 50);

 function blank() {
   context.fillStyle = 'rgba(0,0,0,0.1)';
   context.fillRect(0, 0, context.canvas.width, context.canvas.height);
 }
 setInterval(blank, 40);

</script>

<!DOCTYPE HTML>
<html lang="en">
<meta charset="utf-8">
<title>Blue Robot Demo</title>
<style>
  html { overflow: hidden; min-height: 200px; min-width: 380px; }
  body { height: 200px; position: relative; margin: 8px; }
  .buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; }
</style>
<canvas width="380" height="200"></canvas>
<script>
 var Landscape = function (context, width, height) {
   this.offset = 0;
   this.width = width;
   this.advance = function (dx) {
     this.offset += dx;
   };
   this.horizon = height * 0.7;
   // This creates the sky gradient (from a darker blue to white at the bottom)
   this.sky = context.createLinearGradient(0, 0, 0, this.horizon);
   this.sky.addColorStop(0.0, 'rgb(55,121,179)');
   this.sky.addColorStop(0.7, 'rgb(121,194,245)');
   this.sky.addColorStop(1.0, 'rgb(164,200,214)');
   // this creates the grass gradient (from a darker green to a lighter green)
   this.earth = context.createLinearGradient(0, this.horizon, 0, height);
   this.earth.addColorStop(0.0, 'rgb(81,140,20)');
   this.earth.addColorStop(1.0, 'rgb(123,177,57)');
   this.paintBackground = function (context, width, height) {
     // first, paint the sky and grass rectangles
     context.fillStyle = this.sky;
     context.fillRect(0, 0, width, this.horizon);
     context.fillStyle = this.earth;
     context.fillRect(0, this.horizon, width, height-this.horizon);
     // then, draw the cloudy banner
     // we make it cloudy by having the draw text off the top of the
     // canvas, and just having the blurred shadow shown on the canvas
     context.save();
     context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0);
     context.shadowColor = 'white';
     context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas
     context.shadowBlur = '5';
     context.fillStyle = 'white';
     context.textAlign = 'left';
     context.textBaseline = 'top';
     context.font = '20px sans-serif';
     context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas
     context.restore();
     // then, draw the background tree
     context.save();
     context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0);
     context.beginPath();
     context.fillStyle = 'rgb(143,89,2)';
     context.lineStyle = 'rgb(10,10,10)';
     context.lineWidth = 2;
     context.rect(0, this.horizon+5, 10, -50); // trunk
     context.fill();
     context.stroke();
     context.beginPath();
     context.fillStyle = 'rgb(78,154,6)';
     context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves
     context.fill();
     context.stroke();
     context.restore();
   };
   this.paintForeground = function (context, width, height) {
     // draw the box that goes in front
     context.save();
     context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0);
     context.beginPath();
     context.rect(0, this.horizon - 5, 25, 25);
     context.fillStyle = 'rgb(220,154,94)';
     context.lineStyle = 'rgb(10,10,10)';
     context.lineWidth = 2;
     context.fill();
     context.stroke();
     context.restore();
   };
 };
</script>
<script>
 var BlueRobot = function () {
   this.sprites = new Image();
   this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells
   this.targetMode = 'idle';
   this.walk = function () {
     this.targetMode = 'walk';
   };
   this.stop = function () {
     this.targetMode = 'idle';
   };
   this.frameIndex = {
     'idle': [0], // first cell is the idle frame
     'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6
     'stop': [7], // last cell is the stopping animation
   };
   this.mode = 'idle';
   this.frame = 0; // index into frameIndex
   this.tick = function () {
     // this advances the frame and the robot
     // the return value is how many pixels the robot has moved
     this.frame += 1;
     if (this.frame >= this.frameIndex[this.mode].length) {
       // we've reached the end of this animation cycle
       this.frame = 0;
       if (this.mode != this.targetMode) {
         // switch to next cycle
         if (this.mode == 'walk') {
           // we need to stop walking before we decide what to do next
           this.mode = 'stop';
         } else if (this.mode == 'stop') {
           if (this.targetMode == 'walk')
             this.mode = 'walk';
           else
             this.mode = 'idle';
         } else if (this.mode == 'idle') {
           if (this.targetMode == 'walk')
             this.mode = 'walk';
         }
       }
     }
     if (this.mode == 'walk')
       return 8;
     return 0;
   },
   this.paint = function (context, x, y) {
     if (!this.sprites.complete) return;
     // draw the right frame out of the sprite sheet onto the canvas
     // we assume each frame is as high as the sprite sheet
     // the x,y coordinates give the position of the bottom center of the sprite
     context.drawImage(this.sprites,
                       this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height,
                       x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height);
   };
 };
</script>
<script>
 var canvas = document.getElementsByTagName('canvas')[0];
 var context = canvas.getContext('2d');
 var landscape = new Landscape(context, canvas.width, canvas.height);
 var blueRobot = new BlueRobot();
 // paint when the browser wants us to, using requestAnimationFrame()
 function paint() {
   context.clearRect(0, 0, canvas.width, canvas.height);
   landscape.paintBackground(context, canvas.width, canvas.height);
   blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1);
   landscape.paintForeground(context, canvas.width, canvas.height);
   requestAnimationFrame(paint);
 }
 paint();
 // but tick every 100ms, so that we don't slow down when we don't paint
 setInterval(function () {
   var dx = blueRobot.tick();
   landscape.advance(dx);
 }, 100);
</script>
<p class="buttons">
 <input type=button value="Walk" onclick="blueRobot.walk()">
 <input type=button value="Stop" onclick="blueRobot.stop()">
<footer>
 <small> Blue Robot Player Sprite by <a href="https://johncolburn.deviantart.com/">JohnColburn</a>.
 Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small>
 <small> This work is itself licensed under a <a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">Creative
 Commons Attribution-ShareAlike 3.0 Unported License</a>.</small>
</footer>

4.12.5.2 The ImageBitmap rendering context

4.12.5.2.1 Introduction

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.

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');
});

4.12.5.2.2 The ImageBitmapRenderingContext interface

context = canvas.getContext('bitmaprenderer' [, { [ alpha: false ] } ])

Returns an ImageBitmapRenderingContext object that is permanently bound to a particular canvas element.

If the alpha setting is provided and set to false, then the canvas is forced to always be opaque.

context.canvas

Returns the canvas element that the context is bound to.

context.transferFromImageBitmap(imageBitmap)

Transfers the underlying bitmap data from imageBitmap to context, and the bitmap becomes the contents of the canvas element to which context is bound.

context.transferFromImageBitmap(null)

Replaces contents of the canvas element to which context is bound with a transparent black bitmap whose size corresponds to the width and height content attributes of the canvas element.

4.12.5.3 The OffscreenCanvas interface

OffscreenCanvas is an EventTarget, so both OffscreenCanvasRenderingContext2D and WebGL can fire events at it. OffscreenCanvasRenderingContext2D can fire contextlost and contextrestored, and WebGL can fire webglcontextlost and webglcontextrestored. [WEBGL]

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.

offscreenCanvas = new OffscreenCanvas(width, height)

Returns a new OffscreenCanvas object that is not linked to a placeholder canvas element, and whose bitmap's size is determined by the width and height arguments.

context = offscreenCanvas.getContext(contextId [, options ])

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).

offscreenCanvas.width [ = value ]

offscreenCanvas.height [ = value ]

These attributes return the dimensions of the OffscreenCanvas object's bitmap.

They can be set, to replace the bitmap with a new, transparent black bitmap of the specified dimensions (effectively resizing it).

If an OffscreenCanvas object whose dimensions were changed has a placeholder canvas element, then the placeholder canvas element's intrinsic size will only be updated via the commit() method of the OffscreenCanvas object's rendering context.

promise = offscreenCanvas.convertToBlob([options])

Returns a promise that will fulfill with a new Blob object representing a file containing the image in the OffscreenCanvas object.

The argument, if provided, is a dictionary that controls the encoding options of the image file to be created. The type field specifies the file format and has a default value of "image/png"; that type is also used if the requested type isn't supported. If the image format supports variable quality (such as "image/jpeg"), then the quality field is a number in the range 0.0 to 1.0 inclusive indicating the desired quality level for the resulting image.

canvas.transferToImageBitmap()

Returns a newly created ImageBitmap object with the image in the OffscreenCanvas object. The image in the OffscreenCanvas object is replaced with a new blank image.

The following are the event handlers (and their corresponding event handler event types) supported, as event handler IDL attributes, by all objects implementing the OffscreenCanvas interface:

4.12.5.3.1 The offscreen 2D rendering context

The OffscreenCanvasRenderingContext2D object is a rendering context for drawing to the bitmap of an OffscreenCanvas object. It is similar to the CanvasRenderingContext2D object, with the following differences:

• there is no support for user interface features;

• its canvas attribute refers to an OffscreenCanvas object rather than a canvas element;

• it has a commit() method for pushing the rendered image to the context's OffscreenCanvas object's placeholder canvas element.

An OffscreenCanvasRenderingContext2D object has a bitmap that is initialized when the object is created.

The bitmap has an origin-clean flag, which can be set to true or false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.

An OffscreenCanvasRenderingContext2D object also has an alpha flag, which can be set to true or false. Initially, when the context is created, its alpha flag must be set to true. When an OffscreenCanvasRenderingContext2D object has its alpha flag set to false, then its alpha channel must be fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel must be silently ignored.

An OffscreenCanvasRenderingContext2D object also has a color space setting of type PredefinedColorSpace. The color space for the context's bitmap is set to the context's color space.

An OffscreenCanvasRenderingContext2D object has an associated OffscreenCanvas object, which is the OffscreenCanvas object from which the OffscreenCanvasRenderingContext2D object was created.

offscreenCanvasRenderingContext2D.commit()

Copies the rendering context's bitmap to the bitmap of the placeholder canvas element of the associated OffscreenCanvas object. The copy operation is synchronous. Calling this method is not needed for the transfer, since it happens automatically during the event loop execution.

offscreenCanvas = offscreenCanvasRenderingContext2D.canvas

Returns the associated OffscreenCanvas object.

4.12.5.4 Serializing bitmaps to a file

4.12.5.5 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.

A CanvasRenderingContext2D's output bitmap and an OffscreenCanvasRenderingContext2D's bitmap must use premultiplied alpha to represent transparent colors.

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().