GRASS color questions

spf at spf at
Thu Feb 18 14:12:00 EST 1993


We're doing some experiments with color management in GRASS, and could
use some enlightenment from anyone who's travelled this way before.

What we want to do is restrict GRASS' use of color planes so that we
can reserve one or two for animated objects. (These objects can be drawn
and erased without changing the GRASS display; no GRASS refresh required.)

Two approaches have come to mind:

1. Tell GRASS (e.g. via the NCOLORS variable) that it only has 64 colors
   rather than 256.  We tried this, and gambled that it would therefore
   ignore the upper 2 planes.  It seems to have done this, and to have
   "clipped" the colors in the image that would have used these planes.
   However, it also changed the behavior of the way the 4.0 monitor brought
   up the display window.  While previously we'd see the window come up
   and then watch the data being painted, this time the window appeared
   with the data already present.  Any idea why this would have happened?

2. A simpler way would be to simply ensure that all raster files leave
   the upper planes alone.  But what's the mapping between raster cell
   value and color bit plane?  If my raster file has values ranging from
   (say) 0 to 63, does this only map me into the GRASS color table, or will
   this in effect leave the upper 2 planes alone?

As you can see, I don't have a very clear picture of GRASS's color
management, especially as implemented on X.  Any clues or references
would be greatly appreciated.

Steve Frysinger

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