Question

Are web workers allowed to access a canvas object?

Answer 1

Small update, as the question is now more than half a year old:

In Chrome/Chromium 6 you can now send a canvas ImageData object to a web worker, let the web worker make changes to the object and then write it back to the canvas using putImageData(..).

Google s Chromabrush does it this way, the source-code can be found here:

Update:

The latest development snapshots of Opera (10.70) and Firefox (4.0b1) also support passing ImageData objects to a web worker.

Update 2017:

Actual links from Github (easier to find needed files from Chromabrush):

Answer 2

No.

The postMessage spec was updated a few months back to allow you to post ImageData objects but as yet no one has implemented that behaviour (we re all getting there). The problem with canvas itself is that it s a DOM element and so doesn t work in a worker (there s no DOM).

This was raised recently on either the whatwg or web-apps mailing lists so i suspect we ll start looking at whether it s possible to provide a CanvasRenderingContext2D-like api in workers.

Answer 3

Newer browsers support OffscreenCanvas (see the browser compatibility in that doc), which is a canvas 2d context that runs in web workers, but automatically paints to the main thread.

The following is the basic example from that MDN doc.

In the main thread, create an OffscreenCanvas, then send it to a worker:

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

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

In the worker thread, use the offscreen canvas reference to create a context like you would normally do on the main thread, and perform any drawing commands that you want:

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);
};

(The requestAnimationFrame API exists inside the worker.)

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