A simple accelerated GL compositor

Felix Bellaby felix at bellaby.plus.com
Fri Jul 7 19:43:46 PDT 2006

Brian Paul wrote:
> > I note from some specs on the web that the card is supposed to support
> > alpha blending under OpenGL. This might mean that you need different
> > drivers, or that the card only supports alpha blending _without_ alpha
> > buffers (i.e. GL_CONSTANT_ALPHA blending). I suspect that the drivers
> > are the problem. However, if this really is a limitation of the card,
> > then we could still get transset-style transparency support using
> > constant alpha blending. 
> Many common OpenGL blending modes do not require alpha planes in the 
> destination buffer.  man glBlendFunc

True. I currently make no use of destination alpha values, and I may
drop them from the code. However, I do use source alpha planes in
implementing transparency and unless the mga drivers provide alpha
planes then I will have to switch to constant alpha blending. 

I primarily use source alpha planes in the code to accomodate alpha
visuals, enabling X applications to define their own alpha values. It
would seem from glxinfo that the mga drivers do not provide these alpha
visuals, so I have no pressing need for alpha planes with these drivers.

I have tried using constant alpha blending when the client windows use
non-alpha visuals, but it turns out to be broken on the nvidia drivers.
I will reinstate its use if some drivers genuinely have no alpha planes.


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