[quake3-bugzilla] [Bug 4245] New: SDL selects wrong GLX visual on SGI IRIX

[bugzilla-daemon at icculus.org]

http://bugzilla.icculus.org/show_bug.cgi?id=4245

           Summary: SDL selects wrong GLX visual on SGI IRIX
           Product: ioquake3
           Version: SVN HEAD
          Platform: SGI
        OS/Version: IRIX
            Status: NEW
          Severity: major
          Priority: P3
         Component: Platform
        AssignedTo: zakk at icculus.org
        ReportedBy: baggett.patrick at gmail.com
         QAContact: quake3-bugzilla at icculus.org


Found on SGI Octane with MXE graphics running IRIX 6.5.28f

When using GL_SDL_GetAttribute() to find out how many R/G/B bits were in use in
16-bit color mode in GLimp_SetMode(), I found 12/12/12 (i.e. true 36-bit
color)! This really is a legal color mode on this machine (as reported by
glxinfo). It was selected because the only RGB visual without alpha is the
36-bit color mode. This is true on all of the SGI systems that can run Quake3,
so this problem is generalized to SGI/IRIX, not just a particular graphics
board.

According to the underlying function used by SDL, glXChooseVisual(), when
GLX_RED_SIZE is N > 0, it chooses the **LARGEST** size that is >= N. When N ==
0, the smallest size is preferred. In the case of 16-bit color, this gets R =
4, G = 4, B = 4, and A = 0 (implicitly). The largest values that meet this
requirement are 12/12/12/0, which very obviously isn't 16-bit color. Similar
story with 24-bit color -- selects the exact same visual.

The fix isn't super clean, but works great. Right after this part in
GLimp_SetMode():

      sdlcolorbits = 4;
      if (tcolorbits == 24)
          sdlcolorbits = 8;

Add:


#ifdef __sgi //Fix for SGIs grabbing too many bits of color

       if(sdlcolorbits == 4)
           sdlcolorbits = 0; //Use minimum size for 16-bit color

       //Need alpha or else SGIs choose 36+ bit RGB mode
       SDL_GL_SetAttribute( SDL_GL_ALPHA_SIZE, 1);
#endif

With this small addition, the code correctly selects 16-bit and 32-bit color
visuals and the performance hit of running true 36-bit color mode is removed.
The colors also look, well, 16-bit again. Until SDL uses the more modern method
of choosing a visual, that is glXChooseFBConfig() followed by
component-by-component matching to find an exact match if possible, this would
be required.

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