Web Workers and Canvas

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:

• Main thread
• Web worker

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):

• Sending imageData to worker
• Receiving data

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.