[whatwg] Canvas in workers

[Kyle Huey]

I'm referring to Web Workers.

http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html
https://developer.mozilla.org/en-US/docs/Web/Guide/Performance/Using_web_workers

- Kyle


On Sun, Oct 13, 2013 at 12:20 PM, Michael Norton <norto at me.com> wrote:

> Hi Kyle,
>
> I have read the editor's draft of canvas 2d context and I have some
> familiarity with DOM.  But I do not understand what you are referencing to
> "worker", missing some lingo here.  Can you expand on that?
>
> Cheers,
>
> Mike
>
> Sent by the hope boat.
>
> On Oct 13, 2013, at 12:12 AM, Kyle Huey <me at kylehuey.com> wrote:
>
> > I talked at length with Robert O'Callahan about what the DOM API for
> > supporting <canvas> in web workers should look like and we came up with
> the
> > following modifications to the spec.
> >
> >   1. Rename CanvasProxy to WorkerCanvas and only allow it to be
> >   transferred to workers.  I don't think we're interested in supporting
> >   cross-origin <canvas> via CanvasProxy (I would be curious to hear more
> >   about what the use cases are).
> >   2. Add a worker-only WorkerCanvas constructor that takes the desired
> >   width/height of the drawing surface.
> >   3. Remove the rendering context constructors and the setContext method
> >   on WorkerCanvas (née CanvasProxy).
> >   4. Copy all of the sensible non-node related things from
> >   HTMLCanvasElement to WorkerCanvas.  This would include
> >   - width and height as readonly attributes
> >      - getContext (to replace what we removed in step 3).  roc prefers to
> >      have getContext2D and getContextWebGL, and dispense with the string
> >      argument version entirely, but I don't have strong feelings.
> >      - toBlob.  We do not intend to implement toDataURL here.
> >   5. Add a "commit" method to WorkerCanvas.  For a WorkerCanvas obtained
> >   from a main thread <canvas> element, this would cause the buffer
> displayed
> >   on screen to swap.  For a WorkerCanvas created *de novo* on a worker
> >   thread, it would do nothing.  This commit method would also commit a
> minor
> >   violation of run-to-completion semantics, described below.
> >   6. We would rely on extracting ImageBitmaps from the WorkerCanvas and
> >   shipping them to the main thread via postMessage to allow synchronizing
> >   canvas updates with DOM updates.  We explored a couple other options
> but we
> >   didn't come up with anything else that allows synchronizing updates to
> >   multiple canvases from a worker.  This isn't really sketched out here.
> >
> > So the IDL would look something like:
> >
> >> [Constructor(unsigned long width, unsigned long height)]
> >>
> >> interface WorkerCanvas {
> >>
> >>  readonly attribute unsigned long width;
> >>
> >>  readonly attribute unsigned long height;
> >>
> >>
> >>  CanvasRenderingContext2D? getContext2D(any... args);
> >>
> >>  WebGLRenderingContext? getContextWebGL(any... args);
> >>
> >>
> >>  void toBlob(FileCallback? _callback, optional DOMString type, any...
> >> arguments);
> >>
> >>
> >>  bool commit();
> >>
> >> };
> >>
> >> WorkerCanvas implements Transferable;
> > Everything would be behave pretty much as one would expect, except
> perhaps
> > for the commit method.  The width and height of the canvas can be
> modified
> > on the main thread while the worker is drawing.  This would fire an event
> > off to the worker to update the WorkerCanvas's dimensions that would be
> > scheduled as if the main thread had postMessage()d something to the
> > worker.  But it's possible that the worker would attempt to draw to the
> > <canvas> before that update runs.  It's also possible that the worker
> would
> > simply draw in a loop without yielding.  To solve this, if commit is
> called
> > and the current dimensions on the main thread don't match the dimensions
> of
> > the WorkerCanvas it would fail (return false) and update the dimensions
> of
> > the WorkerCanvas before returning.  This is technically a violation of
> > run-to-completion semantics, but is needed to support workers that do not
> > yield.
> >
> > Thoughts?
> >
> > - Kyle
>