Why X server exposes multiple visuals with same depth/colormap ?

[Yann Droneaud]

Le jeudi 31 décembre 2009 à 11:54 +0100, patrick ficheux a écrit :
> Yann Droneaud a écrit :

> > Try glxinfo and you will get OpenGL framebuffer attributes.
> >   
> ... I don't understand the output of glxinfo ... :-(

Try glxinfo -v / -t for more details.

BTW, here's an example of glxinfo:

72 GLX Visuals
   visual  x  bf lv rg d st colorbuffer ax dp st accumbuffer  ms  cav
 id dep cl sp sz l  ci b ro  r  g  b  a bf th cl  r  g  b  a ns b eat

0xb4 24 tc  0 24  0 r  .  .  8  8  8  0  0  0  0  0  0  0  0  0 0 None
0xb5 24 tc  0 24  0 r  .  .  8  8  8  0  0  0  0 16 16 16  0  0 0 Slow
0xb6 24 tc  0 24  0 r  y  .  8  8  8  0  0  0  0  0  0  0  0  0 0 None
0xb7 24 tc  0 24  0 r  y  .  8  8  8  0  0  0  0 16 16 16  0  0 0 Slow
0xba 24 tc  0 24  0 r  .  .  8  8  8  0  0 24  0  0  0  0  0  0 0 None
0xbb 24 tc  0 24  0 r  .  .  8  8  8  0  0 24  0 16 16 16  0  0 0 Slow
0xbc 24 tc  0 24  0 r  y  .  8  8  8  0  0 24  0  0  0  0  0  0 0 None
0xbd 24 tc  0 24  0 r  y  .  8  8  8  0  0 24  0 16 16 16  0  0 0 Slow
0xc6 24 tc  0 32  0 r  .  .  8  8  8  8  0  0  0  0  0  0  0  0 0 None
0xc7 24 tc  0 32  0 r  .  .  8  8  8  8  0  0  0 16 16 16 16  0 0 Slow
0xc8 24 tc  0 32  0 r  y  .  8  8  8  8  0  0  0  0  0  0  0  0 0 None
0xc9 24 tc  0 32  0 r  y  .  8  8  8  8  0  0  0 16 16 16 16  0 0 Slow
0xcc 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0xcd 24 tc  0 32  0 r  .  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow
0xce 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  0  0  0  0  0  0 0 None
0xcf 24 tc  0 32  0 r  y  .  8  8  8  8  0 24  0 16 16 16 16  0 0 Slow

And the matching xdpyinfo output for some of the visual id listed
before:

visual:
    visual id:    0xb4
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff0000, 0xff00, 0xff
    significant bits in color specification:    8 bits
  

visual:                                                                      
    visual id:    0xcf
    class:    TrueColor
    depth:    24 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff0000, 0xff00, 0xff
    significant bits in color specification:    8 bits


All of those X visuals have same parameters regarding X, but they are
all different from an OpenGL point of view.

So, if you stick to "basic" X11 features, all those visuals provide the
same feature. You don't have to worry about them until you need a
special feature (Composite, OpenGL, etc.).

Regards.

-- 
Yann Droneaud