OffscreenCanvas.getContext()

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The OffscreenCanvas.getContext() method returns a drawing context for an offscreen canvas, or null if the context identifier is not supported.

Note: This API is currently implemented for WebGL1 and WebGL2 contexts only. See bug 801176 for Canvas 2D API support from workers.

Syntax

offscreen.getContext(contextType, contextAttributes);

Parameters

contextType
Is a DOMString containing the context identifier defining the drawing context associated to the canvas. Possible values are:
  • "2d" creates a CanvasRenderingContext2D 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:

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.
  • (Gecko only) willReadFrequently: 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).
  • (Blink only) storage: 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 has a depth buffer of at least 16 bits.
  • stencil: Boolean that indicates that the drawing buffer has a stencil buffer of at least 8 bits.
  • antialias: Boolean that indicates whether or not to perform anti-aliasing.
  • 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.

Return value

A RenderingContext which is either a

If the contextType doesn't match a possible drawing context, null is returned.

Examples

var offscreen = new OffscreenCanvas(256, 256);
var gl = offscreen.getContext("webgl");

gl; // WebGLRenderingContext
gl.canvas; // OffscreenCanvas

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'OffscreenCanvas.getContext()' in that specification.
Living Standard

Browser compatibility

DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
getContext
Experimental
Chrome Full support 69Edge Full support ≤79Firefox Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
IE No support NoOpera Full support 56Safari No support NoWebView Android No support NoChrome Android Full support 69Firefox Android Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
Opera Android Full support 48Safari iOS No support NoSamsung Internet Android Full support 10.0
bitmaprenderer context
Experimental
Chrome Full support 76Edge Full support ≤79Firefox Partial support 46
Notes Disabled
Partial support 46
Notes Disabled
Notes See bug 1390089.
Disabled From version 46: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
IE No support NoOpera Full support 63Safari No support NoWebView Android No support NoChrome Android Full support 76Firefox Android Partial support 46
Notes Disabled
Partial support 46
Notes Disabled
Notes See bug 1390089.
Disabled From version 46: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
Opera Android Full support 54Safari iOS No support NoSamsung Internet Android Full support 12.0
WebGL context
Experimental
Chrome Full support 69Edge Full support ≤79Firefox Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
IE No support NoOpera Full support 56Safari No support NoWebView Android No support NoChrome Android Full support 69Firefox Android Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
Opera Android Full support 48Safari iOS No support NoSamsung Internet Android Full support 10.0
webgl2 context
Experimental
Chrome Full support 69Edge Full support ≤79Firefox Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
IE No support NoOpera Full support 56Safari No support NoWebView Android No support NoChrome Android Full support 69Firefox Android Partial support 44
Notes Disabled
Partial support 44
Notes Disabled
Notes See bug 1390089.
Disabled From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.
Opera Android Full support 48Safari iOS No support NoSamsung Internet Android Full support 10.0
2d context
Experimental
Chrome Full support 69Edge Full support ≤79Firefox No support NoIE No support NoOpera Full support 56Safari No support NoWebView Android No support NoChrome Android Full support 69Firefox Android No support NoOpera Android Full support 48Safari iOS No support NoSamsung Internet Android Full support 10.0

Legend

Full support
Full support
Partial support
Partial support
No support
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.

See also