Baseline 2023 *
Newly available
Note: This feature is available in Web Workers.
The OffscreenCanvas.getContext() method returns a drawing context for an offscreen canvas, or null if the context identifier is not supported, or the offscreen canvas has already been set to a different context mode.
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).
"webgpu"
Creates a GPUCanvasContext object representing a three-dimensional rendering context for WebGPU render pipelines. This context is only available on browsers that implement WebGPU API.
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.
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. In Firefox 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).
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",GPUCanvasContext for "webgpu",ImageBitmapRenderingContext for "bitmaprenderer".If the context identifier is not supported, or the canvas has already been set to a different context mode, null is returned.
InvalidStateError DOMException
Throws if the canvas has transferred to another context scope, for example, to worker.
const offscreen = new OffscreenCanvas(256, 256);
const gl = offscreen.getContext("webgl");
gl; // WebGLRenderingContext
gl.canvas; // OffscreenCanvas
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