Web/API/OffscreenCanvas/getContext

This is an experimental technologyCheck 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

CanvasRenderingContext2D for "2d",
WebGLRenderingContext for "webgl" and "experimental-webgl",
WebGL2RenderingContext for "webgl2" and "experimental-webgl2" ' , or
ImageBitmapRenderingContext for "bitmaprenderer".

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 StandardThe definition of 'OffscreenCanvas.getContext()' in that specification.	Living Standard

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`getContext` Experimental'	Chrome Full support 69	Edge Full support ≤79	Firefox 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 No	Opera Full support 56	Safari No support No	WebView Android No support No	Chrome Android Full support 69	Firefox 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 48	Safari iOS No support No	Samsung Internet Android Full support 10.0
bitmaprenderer context Experimental'	Chrome Full support 76	Edge Full support ≤79	Firefox 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 No	Opera Full support 63	Safari No support No	WebView Android No support No	Chrome Android Full support 76	Firefox 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 54	Safari iOS No support No	Samsung Internet Android Full support 12.0
WebGL context Experimental'	Chrome Full support 69	Edge Full support ≤79	Firefox 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 No	Opera Full support 56	Safari No support No	WebView Android No support No	Chrome Android Full support 69	Firefox 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 48	Safari iOS No support No	Samsung Internet Android Full support 10.0
webgl2 context Experimental'	Chrome Full support 69	Edge Full support ≤79	Firefox 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 No	Opera Full support 56	Safari No support No	WebView Android No support No	Chrome Android Full support 69	Firefox 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 48	Safari iOS No support No	Samsung Internet Android Full support 10.0
2d context Experimental'	Chrome Full support 69	Edge Full support ≤79	Firefox No support No	IE No support No	Opera Full support 56	Safari No support No	WebView Android No support No	Chrome Android Full support 69	Firefox Android No support No	Opera Android Full support 48	Safari iOS No support No	Samsung 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.