It looks to me that the GRASS XDRIVER source code (in
src/display/devices/XDRIVER/XDRIVER) sets the Visual to whatever the
default is for the hardware. On a machine with an 8-bit display
that should be the X PseudoColor Visual. On a 24-bit display that
I would guess would be the X DirectColor Visual. PseudoColor gives
a possible 256 colors, while DirectColor gives about 16 million.
The default Visual is requested by the XDRIVER in Graph_Set.c routine
GetRealNumberOfColors().
I'm no X-programmer, but it seems to me that since the XDRIVER does
not automatically set the X Visual to Pseudocolor, the XDRIVER should
take advantage of 24-bit color if the machine has it...
Bill Baker
bakerwl@corral.uwyo.edu