A simple accelerated GL compositor

Brian Paul brian.paul at tungstengraphics.com
Fri Jul 7 15:27:31 PDT 2006

Felix Bellaby wrote:
> Felix Bellaby wrote:
>>The problem is probably in the colorbuffer/a column, which gives the
>>alpha depth of your visuals. GL uses the alpha channel to provide
>>support for blending and transparency. It would appear that the mga
>>driver does not provide access to the Matrox G550 alpha buffers.  Can
>>anyone confirm that this in fact the case ?
>>You might still find that my code will work without transparency on your
>>platform. Try setting GLX_ALPHA_SIZE to 0 where it appears.
>>A quick web search suggests that there are OpenGL drivers for your card
>>that do provide alpha blending. You might need to use them to get
>>meaningful accelerated GL compositing. Unfortunately, my non-existant
>>knowledge of your hardware makes me a poor person to ask. 
> Thinking this over, glxinfo is only really telling us that the mga
> Matrox drivers do not support X visuals with alpha buffers. It might
> still be possible to create a GLXPixmap with an alpha buffer.
> Could you try changing GLX_DRAWABLE_TYPE (line 1154) to GLX_PIXMAP_BIT
> (i.e. drop the redundant GLX_WINDOW_BIT) ? This _might_ get you past the
> ChooseFBConfig even with GLX_ALPHA_SIZE set to 1 (line 1152).
> If you find that setting GLX_ALPHA_SIZE to 0 (line 1152) remains the
> only way to get a valid FBConfig then either the mga drivers do not
> provide access to the alpha buffers or there aren't any.
> 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


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