The OffscreenCanvas.getContext()
method returns a drawing context for an offscreen canvas, or null
if the context identifier is not supported.
The OffscreenCanvas.getContext()
method returns a drawing context for an offscreen canvas, or null
if the context identifier is not supported.
js
getContext(contextType, contextAttributes)
contextType
A string containing the context identifier defining the drawing context associated to the canvas. Possible values are:
2d
Creates a OffscreenCanvasRenderingContext2D
object representing a two-dimensional rendering context.
webgl
Creates a WebGLRenderingContext
object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 1 (OpenGL ES 2.0).
webgl2
Creates a WebGL2RenderingContext
object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 2 (OpenGL ES 3.0).
bitmaprenderer
Creates a ImageBitmapRenderingContext
which only provides functionality to replace the content of the canvas with a given ImageBitmap
.
Note: The identifiers "experimental-webgl"
or "experimental-webgl2"
are also used in implementations of WebGL. These implementations have not reached test suite conformance, or the graphic drivers situation on the platform is not yet stable. The Khronos Group certifies WebGL implementations under certain conformance rules.
contextAttributes
You can use several context attributes when creating your rendering context, for example:
js
offscreen.getContext("webgl", { antialias: false, depth: false });
2d context attributes:
alpha
Boolean that indicates if the canvas contains an alpha channel. If set to false
, the browser now knows that the backdrop is always opaque, which can speed up drawing of transparent content and images then.
willReadFrequently
Non-standard (Firefox only) Boolean that indicates whether or not a lot of read-back operations are planned. This will force the use of a software (instead of hardware accelerated) 2D canvas and can save memory when calling getImageData()
frequently. This option is only available, if the flag gfx.canvas.willReadFrequently.enable
is set to true
(which, by default, is only the case for B2G/Firefox OS).
storage
Non-standard (Blink only)String that indicates which storage is used ("persistent" by default).
WebGL context attributes:
alpha
Boolean that indicates if the canvas contains an alpha buffer.
depth
Boolean that indicates that the drawing buffer is requested to have a depth buffer of at least 16 bits.
stencil
Boolean that indicates that the drawing buffer is requested to have a stencil buffer of at least 8 bits.
antialias
Boolean that indicates whether or not to perform anti-aliasing if possible.
premultipliedAlpha
Boolean that indicates that the page compositor will assume the drawing buffer contains colors with pre-multiplied alpha.
preserveDrawingBuffer
If the value is true the buffers will not be cleared and will preserve their values until cleared or overwritten by the author.
failIfMajorPerformanceCaveat
Boolean that indicates if a context will be created if the system performance is low.
A rendering context which is either a
OffscreenCanvasRenderingContext2D
for "2d"
,WebGLRenderingContext
for "webgl"
and "experimental-webgl"
,WebGL2RenderingContext
for "webgl2"
and "experimental-webgl2"
Experimental , orImageBitmapRenderingContext
for "bitmaprenderer"
.If the contextType
doesn't match a possible drawing context, null
is returned.
js
const offscreen = new OffscreenCanvas(256, 256); const gl = offscreen.getContext("webgl"); gl; // WebGLRenderingContext gl.canvas; // OffscreenCanvas
Specification |
---|
HTML Standard # dom-offscreencanvas-getcontext-dev |
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
2d_context |
69 | 79 | 105 | No | 56 | 16.4 | No | 69 | 105 | 48 | 16.4 | 10.0 |
getContext |
69 | 79 | 105 | No | 56 | 16.4 | 69 | 69 | 105 | 48 | 16.4 | 10.0 |
bitmaprenderer_context |
76 | 79 | 105 | No | 63 | 16.4 | No | 76 | 105 | 54 | 16.4 | 12.0 |
webgl2_context |
69 | 79 | 105 | No | 56 | No | No | 69 | 105 | 48 | No | 10.0 |
webgl_context |
69 | 79 | 105 | No | 56 | No | No | 69 | 105 | 48 | No | 10.0 |
OffscreenCanvas
HTMLCanvasElement.getContext()
CanvasRenderingContext2D
, WebGLRenderingContext
, WebGL2RenderingContext
, ImageBitmapRenderingContext
, and OffscreenCanvasRenderingContext2D
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https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas/getContext