OffscreenCanvas: getContext() method

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.

Syntax

getContext(contextType, contextAttributes)

Parameters

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 Optional

You can use several context attributes when creating your rendering context, for example:

const gl = canvas.getContext("webgl", {
  antialias: false,
  depth: false,
});

2d context attributes:

alpha

A boolean value 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.

colorSpace Optional

Specifies the color space of the rendering context. Possible values are:

• "srgb" selects the sRGB color space. This is the default value.
• "display-p3" selects the display-p3 color space.

colorType Optional

Specifies the color type of the rendering context. Possible values are:

• "unorm8" sets the color channels to 8 bit unsigned values. This is the default value.
• "float16" sets the color channels to 16-bit floating-point values.

desynchronized

A boolean value that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop.

willReadFrequently

A boolean value 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.

WebGL context attributes:

alpha

A boolean value that indicates if the canvas contains an alpha buffer.

depth

A boolean value that indicates that the drawing buffer is requested to have a depth buffer of at least 16 bits.

stencil

A boolean value that indicates that the drawing buffer is requested to have a stencil buffer of at least 8 bits.

desynchronized

A boolean value that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop.

antialias

A boolean value that indicates whether or not to perform anti-aliasing if possible.

failIfMajorPerformanceCaveat

A boolean value that indicates if a context will be created if the system performance is low or if no hardware GPU is available.

powerPreference

A hint to the user agent indicating what configuration of GPU is suitable for the WebGL context. Possible values are:

"default"

Let the user agent decide which GPU configuration is most suitable. This is the default value.

"high-performance"

Prioritizes rendering performance over power consumption.

"low-power"

Prioritizes power saving over rendering performance.

premultipliedAlpha

A boolean value 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.

xrCompatible

A boolean value that hints to the user agent to use a compatible graphics adapter for an immersive XR device. Setting this synchronous flag at context creation is discouraged; rather call the asynchronous WebGLRenderingContext.makeXRCompatible() method the moment you intend to start an XR session.

Note: The WebGPU specification does not define any specific context attributes for getContext(). Instead, it provides configuration options via the GPUCanvasContext.configure() method.

Return value

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.

Exceptions

InvalidStateError DOMException

Throws if the canvas has transferred to another context scope, for example, to worker.

Examples

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

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

Specifications

Browser compatibility

See also

• The interface defining this method: OffscreenCanvas
• HTMLCanvasElement.getContext()
• Available rendering contexts: CanvasRenderingContext2D, WebGLRenderingContext, WebGL2RenderingContext, ImageBitmapRenderingContext, and OffscreenCanvasRenderingContext2D