OffscreenCanvas

The OffscreenCanvas interface provides a canvas that can be rendered off screen. It is available in both the window and worker contexts.

OffscreenCanvas()

OffscreenCanvas constructor. Creates a new OffscreenCanvas object.

OffscreenCanvas.height

The height of the offscreen canvas.

OffscreenCanvas.width

The width of the offscreen canvas.

OffscreenCanvas.getContext()

Returns a rendering context for the offscreen canvas.

OffscreenCanvas.convertToBlob()

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.

One way to use the OffscreenCanvas API, is to use a rendering context 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.transferFromImageBitmap() 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.transferFromImageBitmap(bitmapOne);

// ... some more drawing for the second canvas using the gl context ...

// Commit rendering to the second canvas
var bitmapTwo = offscreen.transferToImageBitmap();
two.transferFromImageBitmap(bitmapTwo);

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 rendering context 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);
};

• WebGL Off the Main Thread – Mozilla Hacks
• CanvasRenderingContext2D
• ImageBitmap
• ImageBitmapRenderingContext
• HTMLCanvasElement.transferControlToOffscreen()
• WebGLRenderingContext.commit()