Using Web Workers for drawing using native canvas functions
It's possible to send a CanvasPixelArray obtained via getImageData to a worker script, and let the worker script manipulate the pixels in its background thread, and eventually post the manipulated pixel array back.
However, I'm using native canvas drawing functions, like drawImage. The drawImage calls are currently blocking the UI thread. This causes slow redraw of buttons, and a noticeable delay when clicking on a button, just to name a few drawbacks. (Edit: A small improvement can now be accomplished with ctx.imageSmoothingEnabled = false, at least on WebKit with the webkit prefix.)
I'd like to move the drawing from the main thread into a background thread using Web Workers. However, I don't seem to be able to send the canvas nor the context to the worker.
I did find this notice on MDN:
Note: As usual, background threads—including workers—cannot manipulate the DOM. If actions taken by the background thread need to result in changes to the DOM, they should post messages back to their creators to do that work.
But I would like to leave the DOM as-is; I simply want to draw things on a canvas element. Is this possible, or are Web Workers really only allowed to calculate and not to draw?
(Or is it perhaps possible to use functions like drawImage to manipulate a CanvasPixelArray instead of drawing on a canvas?)
Answer
[Edit ~5 years later: some of this is beginning to change, and there are new web platform features that actually allow rendering to a canvas from a Worker! See this blog for more info: https://hacks.mozilla.org/2016/01/webgl-off-the-main-thread/ - the rest of the answer is provided for 2011-era information ;)]
Web workers can only calculate, not modify the DOM or make any calls to draw to a canvas. However like you say you can post an array of pixels to a web worker to process it there and post it back. Since that's asynchronous I don't see why that would cause any slowdown in the UI thread, unless you deliberately block until the web worker responds (which you shouldn't).
So it seems odd that your drawImage calls are taking so long it's affecting the UI. Most canvases are hardware-accelerated these days so they should skip along nicely. My guess is that you're drawing via a web worker to a canvas pixel array every frame, which effectively means you're software rendering the canvas in javascript. Javascript is still far too slow to do this - even C++ software renderers are kind of slow, which is why hardware acceleration is important. So you can render something to a canvas pixel array in a web worker once, and then when you get your result cache it in to an Image once, and then draw that Image to the canvas as much as you like. That should still be really fast.
Edit: you might want to look in to WebGL, where you can write fragment shaders which are effectively little programs to process pixel effects. They run entirely on the graphics card so they're stupidly fast.