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

[patrick ficheux]

Hello,

It seems that X server exposes several visuals with same caracteristics 
(depth / color).

For example, on my Debian, xdpyinfo displays 4 identical Visuals: 
TrueColor/24 planes (see output below) and only one visual TrueColor/32 
planes.
With window/cygwin, XWin (the X server) exposes 15 identical visuals 
TrueColor / 24 planes

Is there a reason to expose several identical visuals ?
If yes, could you give me an example ?

Regards



output of xdpyinfo on debian

name of display:    :0.0
version number:    11.0
[...]
number of supported pixmap formats:    7
supported pixmap formats:
    depth 1, bits_per_pixel 1, scanline_pad 32
    depth 4, bits_per_pixel 8, scanline_pad 32
    depth 8, bits_per_pixel 8, scanline_pad 32
    depth 15, bits_per_pixel 16, scanline_pad 32
    depth 16, bits_per_pixel 16, scanline_pad 32
    depth 24, bits_per_pixel 32, scanline_pad 32
    depth 32, bits_per_pixel 32, scanline_pad 32
[...]
default screen number:    0
number of screens:    1
screen #0:
[...]
  depth of root window:    24 planes
[...]
  default colormap:    0x20
  default number of colormap cells:    256
[...]
  number of visuals:    5
  default visual id:  0x22
  visual:
    visual id:    0x22
    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:    0x23
    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:    0x24
    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:    0x25
    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:    0x3f
    class:    TrueColor
    depth:    32 planes
    available colormap entries:    256 per subfield
    red, green, blue masks:    0xff0000, 0xff00, 0xff
    significant bits in color specification:    8 bits