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Re: [Public WebGL] WebGL back buffer contents




On Jan 28, 2010, at 1:43 AM, Tim Johansson wrote:


In recent conversations with people at NVIDIA they were pretty adamant
that we not mess with the OpenGL state, in particular the viewport,
upon changing the canvas size. Gregg's render target example is a good
reason to not automatically call glViewport() behind the scenes.
Another point is that it isn't obvious exactly when the implicit call
to glViewport() would be made -- whether it would be synchronous with
the setting of the width / height properties or whether it would
somehow be done lazily, before the first draw call after the canvas is
resized.


I used to think that doing this automatically would be a good idea but
because of the semantic details I don't any more.




We recently said that resizing should clear the canvas, which means we have to mess with the gl sate anyway. We will at least need to change bound FBO to 0 and disable scissor testing. Does it not qualify as messing with the state if we mess with it and then try to restore it?

Why would we have to change the FBO or scissor state? If you have an FBO set, then changing the size of the drawing buffer doesn't affect it, does it?



I agree that in the case of rendering to an FBO you would not want to change the viewport automatically, but I would rather have apps handle that case themselves and set the viewport back than have apps handle the most common case themselves. If the spec says that changing width or height behaves as if it calls viewport as soon as the width/height attribute is changed it would be clear what will happen. I would not even call it messing with the state as it just does what the app tells it when the app tells it to, that is change the size of the buffer and the viewport.


We could also do something more magical and say that if you have not changed the viewport yourself it will automatically change. If you ever did call viewport automatic viewport adjustment is disabled. That way it would never change the viewport for apps actually modifying the viewport and it would just work for apps who does not care about the viewport. So initial value for viewport would be "automatic", but there is no way to set it back to "automatic" if you change it yourself.

I'm not sure of your concern. What is the problem with requiring that the author calls ctx.viewport() after changing the width and height? The author has to know about viewports and keeping track of them. So why is that an issue?


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