[Public WebGL] using the same context with multiple canvases

Mitchell Williams [email protected]
Mon Mar 23 11:03:55 PDT 2015

If I understand the issue, it sure would be nice if this worked.

I tried to render the scene from a camera to another window (such as when
3DS Max can view a scene in perspective view, and that scene contains a
camera which we can see the scene from the camera's view in another window).

As an alternative, I rendered the scene as a texture map, but all contained
within a single canvas.

This would be nice to render a scene to multiple canvases.


On Mon, Mar 23, 2015 at 12:06 AM, Florian Bösch <[email protected]> wrote:

> The topic of how to draw to multiple canvases from a single context (from
> either the JS main thread or a worker) has not yet been resolved (correct
> me if I'm wrong).
> A possible resolution seems to be the canvas.setContext method. However,
> this seems to to be under specified, as just having a WebGL context is not
> sufficient, you also need a drawing buffer which carries attributes such as
> stencil, depth, bit-depth, MSAA, etc., and the WebGL 2 specification
> doesn't seem to contain anything about how to create either a WebGL 2
> context or a drawing buffer on its own (It just specifies that a getContext
> call needs to do both).
> Regardless of specification status or respective completeness, no
> implementation of setContext is available, let alone anything to create
> WebGL contexts and drawing buffers on their own to be used.
> I'll posit that this isn't an acceptable situation. I'll reiterate that
> quite frequently applications that do provide any kind of UI (like
> sketchfab, verold, goo, fabric) need to put WebGL driven content into many
> places inside that UI that isn't a single viewport. That content might find
> itself CSS transformed, composited, scrolled in/out of view, inside an
> overflow: scroll container, sandwiched between html elements etc. And that
> content might need to be updated at each animation frame.
> I'm frankly mystified why a topic we've been talking about for well over 3
> years and have been promised specifications/solutions for, that is widely
> recognized as an important issue to solve, is nearly unspecified and
> completely unsolved.

Mitch Williams

Check out my book "*WebGL Hotshot*" available at:

310-809-4836 (outside line)
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