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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

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 new OffscreenCanvas 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 the OffscreenCanvas.

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.

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 ...
};

You can also use requestAnimationFrame in workers

onmessage = function(evt) {
  const canvas = evt.data.canvas.
  const gl = canvas.getContext("webgl");

  function render(time) {
    // ... some drawing using the gl context ...
    requestAnimationFrame(render);
  }
  requestAnimationFrame(render);
};

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'OffscreenCanvas' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support602 ?443 No Yes1 No
OffscreenCanvas() constructor No ?464 No No No
height No ?443 No No No
width No ?443 No No No
getContext No ?443 No No No
toBlob No ?464 No No No
transferToImageBitmap No ?464 No No No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes1602 ?443 Yes1 No No
OffscreenCanvas() constructor No42 ?464 No No4.0
height No No ?443 No No No
width No No ?443 No No No
getContext No No ?443 No No No
toBlob No No ?464 No No No
transferToImageBitmap No No ?464 No No No

1. This feature is behind the Experimental canvas features preference.

2. From version 60: this feature is behind the Experimental canvas features preference. To change preferences in Chrome, visit chrome://flags.

3. From version 44: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.

4. From version 46: this feature is behind the gfx.offscreencanvas.enabled preference. To change preferences in Firefox, visit about:config.

See also

Schlagwörter des Dokuments und Mitwirkende

Mitwirkende an dieser Seite: gmanpersona, fscholz, PawelAP, adria, dtipson
Zuletzt aktualisiert von: gmanpersona,