OffscreenCanvas.getContext()

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

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", leading to the creation of a CanvasRenderingContext2D object representing a two-dimensional rendering context.
• "webgl" which will create 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" which will create 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" which will create 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 implementation 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

Currently drafted as a proposal: OffscreenCanvas.

Browser compatibility

[1] This feature is behind a feature preference setting. In about:config, set gfx.offscreencanvas.enabled to true.

See also

• The interface defining this method, OffscreenCanvas
• HTMLCanvasElement.getContext()
• Available rendering contexts: CanvasRenderingContext2D, WebGLRenderingContext, WebGL2RenderingContext, and ImageBitmapRenderingContext.