When using the <canvas> element or the Canvas API, rendering, animation, and user interaction usually happen on the main execution thread of a web application. The computation relating to canvas animations and rendering can have a significant impact on application performance.

The OffscreenCanvas interface provides a canvas that can be rendered off screen, decoupling the DOM and the Canvas API so that the <canvas> element is no longer entirely dependent on the DOM. Rendering operations can also be run inside a worker context, allowing you to run some tasks in a separate thread and avoid heavy work on the main thread.

OffscreenCanvas is a transferable object.

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. See the API description for important notes on managing this ImageBitmap.

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 an 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 OffscreenCanvas as described above.

const one = document.getElementById("one").getContext("bitmaprenderer");
const two = document.getElementById("two").getContext("bitmaprenderer");

const offscreen = new OffscreenCanvas(256, 256);
const gl = offscreen.getContext("webgl");

// Perform some drawing for the first canvas using the gl context
const bitmapOne = offscreen.transferToImageBitmap();
one.transferFromImageBitmap(bitmapOne);

// Perform some more drawing for the second canvas
const 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.

The main.js script (main thread) may look like this:

const htmlCanvas = document.getElementById("canvas");
const offscreen = htmlCanvas.transferControlToOffscreen();

const worker = new Worker("offscreencanvas.js");
worker.postMessage({ canvas: offscreen }, [offscreen]);

While the offscreencanvas.js script (worker thread) can look like this:

onmessage = (evt) => {
  const canvas = evt.data.canvas;
  const gl = canvas.getContext("webgl");
  // Perform some drawing using the gl context
};

It's also possible to use requestAnimationFrame() in workers:

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

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

For a full example, see the OffscreenCanvas example source on GitHub or run the OffscreenCanvas example live.

• CanvasRenderingContext2D
• OffscreenCanvasRenderingContext2D
• ImageBitmap
• ImageBitmapRenderingContext
• HTMLCanvasElement.transferControlToOffscreen()
• requestAnimationFrame()
• WebGL Off the Main Thread – Mozilla Hacks (2016)