Grayscales with XDRIVER

Michael Shapiro shapiro at
Sat Jan 4 20:41:39 EST 1992

The GRASS graphics drivers use color lookup tables (reserving about 15
colors for vector "overlays"). This means that images with more than
240 colors for a 256 color display can't fit all its colors into the
hardware lookup table. In order to display the image a 6x6x6 color cube
is loaded into the hardware color registers (ie fixed mode) and the
nearest color is used instead of the "true" color. If you have an image
with a grey scale color map, and that image has more than 240 colors
(eg 1-255), then you get 6 levels of grey. The only solution is to
rescale the image data into the range 1-240 and set a grey scale for
the rescaled image.

In response to:
|When displaying grayscale images in the XDRIVER `monitor', I can only
|get about 6 levels of gray.  Changing the colormode with d.colormode
|has no effect.  I also recompiled other versions of XDRIVER with
|alternate (provided) Clr_table.c routines and saw no difference.
|Is there no way get more than 6 grey levels in an X-window?
|(SunOS 4.1, openwindows 2.0)
|Duane Wessels  <duane at | dwessels at>
|Pacific Northwest Laboratory, Richland, Washington  +1 509 376-4461

Michael Shapiro
U.S. Army CERL                  email:   shapiro at
Environmental Division          phone:   (217) 352-6511  ext 526
P.O. Box 9005                   fax:     (217) 373-7222
Champaign, Ill. 61826-9005

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