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Re: [Public WebGL] using the same context with multiple canvases

I've now specced the proposal for this; for details please see this post:


It's not exactly as I was describing earlier, but it's pretty close. 
(Turns out what I was proposing has some issues.)

On Wed, 14 Nov 2012, Gregg Tavares (社~T) wrote:
> Why do you want to blow away state? This seems like a perfectly reasonable
> thing to want to do
>    ctx.beginPath();
>    ctx.arc(...);
>    c1.setContext(ctx);
>    ctx.stroke();
>    c2.setContext(ctx);
>    ctx.stroke();
>    c3.setContext(ctx);
>    ctx.stroke();

I don't really understand why you would ever do that. Can you elaborate on 
the use case? I'm happy to support this if it makes sense, but in practice 
I think it's saner not to do that. Complications include things like the 
canvas bitmaps being different dimensions from each other (which do you 
use? right now I resize the scratch bitmap to match the canvas bitmap), 
what happens in the cross-worker case (what bitmap do you draw on, the 
previous scratch bitmap, the canvas bitmap?), etc. By clearing the scratch 
bitmap and state between each setContext() call, you solve a lot of these 
problems cleanly, and it is consistent with what has always happened with 
2D canvas whenever the dimensions are changed (which is conceptually 
equivalent to binding to a new canvas, IMHO).

On Wed, 14 Nov 2012, Florian Bsch wrote:
> On Wed, Nov 14, 2012 at 1:03 AM, Ian Hickson <ian@hixie.ch> wrote:
> > 
> > This question is still important; IMHO the answer to this question is 
> > related to where settings that apply to rendering to a specific canvas 
> > element's bitmap should be.
> GL contexts map via an affine transform set by the gl.viewport command 
> in the form of left, top, width, height.

Ah, ok. That makes sense. I'd say that it also makes sense to put the 
settings that apply to rendering to a specific canvas on the context too, 

> > My recommendation for WebGL would be to have an explicit method that 
> > pushes the bits to the screen, and that would also clear the buffer. 
> > So then (c) and (d) couldn't happen.
> I find this a noteworthy idea. Two remarks:
> 1) classically there exists a call in OpenGL called "flip" (which webgl
> does not have) which does exactly that in double buffering situations.
> 2) A call like it on a DrawingBuffer could help to flip exactly that
> drawing buffer you've actually drawn, avoiding to update drawing buffers
> which haven't actually been updated, and avoiding the preserveDrawingBuffer
> mess somewhat.

The way I specced the proposal for canvas is designed so that it should be 
possible to implement this for WebGL. See the e-mail I cited above for the 
hooks that WebGL would need to use to get this working.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'