OffscreenCanvas: getContext() method
====================================

Baseline 2023
-------------

Newly availableSince March 2023, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

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

Syntax
------


```js
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).


`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:


```js
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` 
Non-standard
 (Firefox only)

 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).


`storage` 
Non-standard
 (Blink only)

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 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.


### Return value

A rendering context which is either a

* `OffscreenCanvasRenderingContext2D` for `"2d"`,
* `WebGLRenderingContext` for `"webgl"` and `"experimental-webgl"`,
* `WebGL2RenderingContext` for `"webgl2"` and `"experimental-webgl2"` 
Experimental
, or
* `ImageBitmapRenderingContext` for `"bitmaprenderer"`.

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

Examples
--------


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

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

Specifications
--------------

| Specification |
| --- |
| HTML Standard # dom-offscreencanvas-getcontext-dev |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

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