HTMLCanvasElement.getContext()

The HTMLCanvasElement.getContext() method returns a drawing context on the canvas, or null if the context identifier is not supported.

Syntax

canvas.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" (or "experimental-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" (or "experimental-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).

Note: The identifiers "experimental-webgl" or "experimental-webgl2" are used in new 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:

canvas.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.

Return value

A RenderingContext which is either a

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

Examples

Given this <canvas> element:

<canvas id="canvas" width="300" height="300"></canvas>

You can get a 2d context of the canvas with the following code:

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
console.log(ctx); // CanvasRenderingContext2D { ... }

Now you have the 2D rendering context for a canvas and you can draw within it.

Specifications

Specification Status Comment
WHATWG HTML Living Standard
The definition of 'HTMLCanvasElement.getContext' in that specification.
Living Standard No change since the latest snapshot, HTML5
HTML5.1
The definition of 'HTMLCanvasElement.getContext' in that specification.
Working Draft  
HTML5
The definition of 'HTMLCanvasElement.getContext' in that specification.
Recommendation Snapshot of the WHATWG HTML Living Standard containing the initial definition.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (2d context) 4 3.6 (1.9.2) 9 9 3.1
webgl context 9[1]
33
3.6 (1.9.2)[1]
24 (24)
11.0[2] 9.0[3] 5.1[2]
webgl2 context Not supported 25 (25)[4] Not supported Not supported Not supported
2d alpha context attribute 32 30 (30) Not supported (Yes) Not supported
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (2d context) (Yes) (Yes) 1.0 (1.9.2) (Yes) (Yes) (Yes)
webgl context ? ? (Yes)[2] ? ? ?
webgl2 context Not supported Not supported Not supported Not supported Not supported Not supported
2d alpha context attribute Not supported Not supported 30.0 (30) Not supported Not supported Not supported

[1] Chrome 9 and Gecko 1.9.2 initially implemented this as experimental-webgl. Since Chrome 33 and Gecko 24 it is implemented as the standard webgl.

[2] Internet Explorer 11, WebKit 5.1 and Firefox Mobile implemented this as experimental-webgl.

[3] Opera 9 implemented this as experimental-webgl, behind a user preference, in version 15.0 the user preference got removed.

[4] Gecko 25 implements this as experimental-webgl2 behind the user preference webgl.enable-prototype-webgl2.

See also

Document Tags and Contributors

Contributors to this page: markg, Sebastianz, fscholz, Nexii, teoli
Last updated by: markg,