HTMLCanvasElement.getContext()
The HTMLCanvasElement.getContext()
method returns a drawing context on the canvas, or null
if the context identifier is not supported.
Syntax
<var>canvas.getContext(contextType, contextAttributes);</var>
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 aCanvasRenderingContext2D
object representing a two-dimensional rendering context."webgl"
(or"experimental-webgl"
) which will create aWebGLRenderingContext
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 aWebGL2RenderingContext
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 aImageBitmapRenderingContext
which only provides functionality to replace the content of the canvas with a givenImageBitmap
.
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:
JavaScriptCopy Codecanvas.getContext("webgl", { antialias: false, depth: false });
2d context attributes:alpha
: Boolean that indicates if the canvas contains an alpha channel. If set tofalse
, 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 callinggetImageData()
frequently. This option is only available, if the flaggfx.canvas.willReadFrequently.enable
is set totrue
(which, by default, is only the case for B2G/Firefox OS). - (Blink only)
storage
: String that indicates which storage is used ("persistent" by default).
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"
, orImageBitmapRenderingContext
for"bitmaprenderer"
.
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 |
No support | 25 (25)[4] | No support | No support | No support |
2d alpha context attribute |
32 | 30 (30) | No support | (Yes) | No support |
|
(Yes) | 41 (41) | (Yes) | (Yes) | ? |
bitmaprenderer context | No support | 46 (46) | No support | No support | No support |
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 |
No support | No support | No support | No support | No support | No support |
2d alpha context attribute |
No support | No support | 30.0 (30) | No support | No support | No support |
failIfMajorPerformanceCaveat attribute |
? | ? | 41.0 (41) | ? | ? | ? |
bitmaprenderer context | No support | No support | 46.0 (46) | No support | No support | No support |
[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
. Starting with Gecko 42, the string "webgl2" is used behind the same preference.
See also
- The interface defining it,
HTMLCanvasElement
. OffscreenCanvas.getContext()
- Available rendering contexts:
CanvasRenderingContext2D
,WebGLRenderingContext
andWebGL2RenderingContext
andImageBitmapRenderingContext
.
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/api/htmlcanvaselement/getcontext