[whatwg] Counterproposal for canvas in workers

Robert O'Callahan robert at ocallahan.org
Thu Oct 17 16:01:51 PDT 2013

On Fri, Oct 18, 2013 at 10:56 AM, Justin Novosad <junov at google.com> wrote:

> On Thu, Oct 17, 2013 at 5:50 PM, Rik Cabanier <cabanier at gmail.com> wrote:
>> Creating temporary canvases is still possible. I'm unsure how it would be
>> different from a worker.
>> An advantage would be that you can draw to the temporary canvases in
>> parallel to using them. Only PIXEL access is disallowed, you can still call
>> drawImage using a canvas that has outstanding tasks.
> Right. The write-only restriction would only apply to canvas contexts that
> commit (push their tasks) directly down to the compositor. You could still
> create a canvas that is local to the worker, rasterize it in the worker and
> do readbacks in the worker, create ImageBitmaps from it, etc.

I'm not sure that you and Rik are talking about the same thing, since he's
still talking about "outstanding tasks". If you are talking about the same
thing, I don't know what it is. I'd like to see some concrete details for
what you'd change in the current WorkerCanvas proposal. For the sake of
clarity I've put (my understand of) it here:

With WorkerCanvas and transferToImageBitmap, you can draw multiple layers
in parallel (and actually draw, not just queue drawing commands) by
creating multiple workers, having them each produce an ImageBitmap, and
compositing those ImageBitmaps together by stacking <img> elements or
drawing them all to a single canvas. It uses more memory but you get more

In fact, an implementation could choose to take the deferred-drawing
approach instead. You would queue up drawing commands in the WorkerCanvas
(or the drawing context), and then transferToImageBitmap would not
immediately render but produce an ImageBitmap implementation encapsulating
the list of drawing commands to be drawn later, wherever/whenever that
ImageBitmap ended up being used. I think for commit() the implementation
would always want to force rasterization on the worker (or possibly some
dedicated canvas-rendering thread); you could forward a list of drawing
commands to the compositor thread for rasterization but I don't think
there's any reason to do that (and some good reasons not to).

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