This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.
The OffscreenCanvas.getContext()
method returns a drawing context for an offscreen canvas, or null
if the context identifier is not supported.
Note: This API is currently implemented for WebGL1 and WebGL2 contexts only. See bug 801176 for Canvas 2D API support from workers.
Syntax
offscreen.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 aCanvasRenderingContext2D
object representing a two-dimensional rendering context."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
" 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 also used in 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:
offscreen.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
var offscreen = new OffscreenCanvas(256, 256); var gl = offscreen.getContext("webgl"); gl; // WebGLRenderingContext gl.canvas; // OffscreenCanvas
Specifications
Specification | Status | Comment |
---|---|---|
WHATWG HTML Living Standard The definition of 'OffscreenCanvas.getContext()' in that specification. |
Living Standard |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | No support | 44 (44) [1] | No support | No support | No support |
webgl context | No support | 44 (44) [1] | No support | No support | No support |
webgl2 context | No support | 44 (44) [1] | No support | No support | No support |
2d context | No support | No support | No support | No support | No support |
bitmaprenderer context | No support | 46 (46) [1] | No support | No support | No support |
Feature | Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|
Basic support | No support | 44.0 (44) [1] | No support | No support | No support |
webgl context | No support | 44 (44) [1] | No support | No support | No support |
webgl2 context | No support | 44 (44) [1] | No support | No support | No support |
2d context | No support | No support | No support | No support | No support |
bitmaprenderer context | No support | 46 (46) [1] | No support | No support | No support |
[1] This feature is behind a feature preference setting. In about:config, set gfx.offscreencanvas.enabled
to true
.
See also
- The interface defining this method,
OffscreenCanvas
HTMLCanvasElement.getContext()
- Available rendering contexts:
CanvasRenderingContext2D
,WebGLRenderingContext
,WebGL2RenderingContext
, andImageBitmapRenderingContext
.