OffscreenCanvas: getContext() method
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 givenImageBitmap
.
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 flaggfx.canvas.willReadFrequently.enable
is set totrue
(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 , orImageBitmapRenderingContext
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
See also
- The interface defining this method:
OffscreenCanvas
HTMLCanvasElement.getContext()
- Available rendering contexts:
CanvasRenderingContext2D
,WebGLRenderingContext
,WebGL2RenderingContext
,ImageBitmapRenderingContext
, andOffscreenCanvasRenderingContext2D