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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
interface provides a canvas that can be rendered off screen. It is available in both the window and worker contexts.
Note: This API is currently implemented for WebGL1 and WebGL2 contexts only. See bug 801176 for Canvas 2D API support from workers.
Constructors
OffscreenCanvas()
OffscreenCanvas
constructor. Creates a newOffscreenCanvas
object.
Properties
OffscreenCanvas.height
- The height of the offscreen canvas.
OffscreenCanvas.width
- The width of the offscreen canvas.
Methods
OffscreenCanvas.getContext()
- Returns a rendering context for the offscreen canvas.
OffscreenCanvas.toBlob()
- Creates a
Blob
object representing the image contained in the canvas.
OffscreenCanvas.transferToImageBitmap()
- Creates an
ImageBitmap
object from the most recently rendered image of theOffscreenCanvas
.
Examples
Synchronous display of frames produced by an OffscreenCanvas
One way to use the OffscreenCanvas
API, is to use a RenderingContext
that has been obtained from an OffscreenCanvas
object to generate new frames. Once a new frame has finished rendering in this context, the transferToImageBitmap()
method can be called to save the most recent rendered image. This method returns an ImageBitmap
object, which can be used in a variety of Web APIs and also in a second canvas without creating a transfer copy.
To display the ImageBitmap
, you can use a ImageBitmapRenderingContext
context, which can be created by calling canvas.getContext("bitmaprenderer")
on a (visible) canvas element. This context only provides functionality to replace the canvas's contents with the given ImageBitmap
. A call to ImageBitmapRenderingContext.transferImageBitmap()
with the previously rendered and saved ImageBitmap
from the OffscreenCanvas, will display the ImageBitmap
on the canvas and transfer its ownership to the canvas. A single OffscreenCanvas
may transfer frames into an arbitrary number of other ImageBitmapRenderingContext
objects.
Given these two <canvas>
elements
<canvas id="one"></canvas> <canvas id="two"></canvas>
the following code will provide the rendering using an OffscreenCanvas
as described above.
var one = document.getElementById("one").getContext("bitmaprenderer"); var two = document.getElementById("two").getContext("bitmaprenderer"); var offscreen = new OffscreenCanvas(256, 256); var gl = offscreen.getContext('webgl'); // ... some drawing for the first canvas using the gl context ... // Commit rendering to the first canvas var bitmapOne = offscreen.transferToImageBitmap(); one.transferImageBitmap(bitmapOne); // ... some more drawing for the second canvas using the gl context ... // Commit rendering to the second canvas var bitmapTwo = offscreen.transferToImageBitmap(); two.transferImageBitmap(bitmapTwo);
Asynchronous display of frames produced by an OffscreenCanvas
Another way to use the OffscreenCanvas
API, is to call transferControlToOffscreen()
on a <canvas>
element, either on a worker or the main thread, which will return an OffscreenCanvas
object from an HTMLCanvasElement
object from the main thread. Calling getContext()
will then obtain a RenderingContext
from that OffscreenCanvas
.
In order to make frames visible, you can call commit()
on that RenderingContext
, so that frames will be pushed back to the original <canvas>
element.
Note that, in Firefox, this API is currently only implemented for the WebGL context (WebGLRenderingContext.commit()
). For Canvas 2D API support from workers, see bug 801176.
main.js (main thread code):
var htmlCanvas = document.getElementById("canvas"); var offscreen = htmlCanvas.transferControlToOffscreen(); var worker = new Worker("offscreencanvas.js"); worker.postMessage({canvas: offscreen}, [offscreen]);
offscreencanvas.js (worker code):
onmessage = function(evt) { var canvas = evt.data.canvas. var gl = canvas.getContext("webgl"); // ... some drawing using the gl context ... // Push frames back to the original HTMLCanvasElement gl.commit(); };
Specifications
Currently drafted as a proposal: OffscreenCanvas.
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | No support | 44 (44) [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 |
[1] This feature is behind a feature preference setting. In about:config, set gfx.offscreencanvas.enabled
to true
.