Re: exposing CANVAS or something like it to Web Workers

On Mon, May 14, 2012 at 4:42 PM, Jonas Sicking <jonas@sicking.cc> wrote:

> On Mon, May 14, 2012 at 3:28 PM, Glenn Maynard <glenn@zewt.org> wrote:
> > On Mon, May 14, 2012 at 3:01 PM, Gregg Tavares (勤) <gman@google.com
> > wrote:
> >
> >> I'd like to work on exposing something like CANVAS to web workers.
> >>
> >> Ideally how over it works I'd like to be able to
> >>
> >> *) get a 2d context in a web worker
> >
> >
> > I'd recommend not trying to tackle 2d and 3d contexts at once, and only
> > worrying about WebGL to start.
> >
> > Another issue: rendering in a worker thread onto a canvas which is
> displayed
> > in the main thread.  This needs to be solved in a way that doesn't cause
> the
> > asynchronous nature of what's happening to be visible to scripts.
>  toDataURL
> > and toBlob would probably need to be prohibited on the canvas element.
>  I'm
> > not sure what the actual API would look like.
>
> If/when we do this, I think it should be done in such a way that the
> main window can't access the canvas object at all. Similar to what
> happens when an ArrayBuffer is transferred to a Worker using
> structured cloning. Once a canvas is transferred to a Worker, any
> access to it should throw or return null/0/"". If you want to transfer
> pixel data to the main thread, it seems less racy to do that by
> getting the pixel data in the Worker which owns the canvas and then
> transfer that to the main thread using postMessage.
>

How about separating the canvasy parts of canvas from canvas and the imagy
parts of image from image.

In other words, Imagine canvas is implemented like this

class Canvas : public HTMLElement {
  private:
    CanvasSurface* m_surface;  // everything about canvas that is not
HTMLElement
};

And that Image is similarly implemented as

class Image : public HTMLElement {
  private:
    Picture* m_picture;  // everything about Image that is not HTMLElement
}

now imagine you can instantiate inner implementation of these things. The
parts that are not HTMLElement

var canvasSurface = new CanvasSurface();
var ctx = canvasSurface.getContext("2d");
var pic = new Picture;
pic.src = "http://someplace.com/someimage.jpg";
pic.onload = function() {
   ctx.drawImage(pic, 0, );
}

Let's assume you can instantiate these things in either the page or a
worker. Nether can be transfered.

Would that work? What problems would that have?





>
> > This would also require some
> > equivalent to requestAnimationFrame in the worker thread.
>
> Agreed!
>
> / Jonas
>