A simple accelerated GL compositor

Tomasz Torcz zdzichu at irc.pl
Sat Jul 8 10:56:01 PDT 2006

On Fri, Jul 07, 2006 at 11:24:59PM +0100, 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.

  Tried that. Your composer runs, but screen goes black, only mouse
cursor is displayed and I can move it. By pressinc ^C I can return to
normal drawing.
> 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).

  Tried that, too. It doesn't work in way similar to very first failure.
Today I haven't patience to sprinkle glcompmgr.c with printf(), but I
suspect the same failure. On the side note, slightly more debugging
output (or any output) from glcompmgr could help.
> 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 will try with Matrox closed drivers. But I fear that they are to
very old X release and may now work with 6.9.0.

Tomasz Torcz                 Morality must always be based on practicality.
zdzichu at irc.-nie.spam-.pl                -- Baron Vladimir Harkonnen

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