Colortable mystery: black isn't black

Greg Koerper greg at
Thu May 6 19:11:37 EDT 1993

Strange, but true!

I've been capturing raster images of the GRASS display window and sending
them through the pnm utilities for conversion to PostScript for some time now.
But just recently I stumbled on a problem.

I printed an raster map with no data set to RGB 0 0 0 (black), but it came out
dithered on the PS printer.  I have since begun storing the original raster
dumps and exploring with the color editor of the xv utility.  Interestingly
enough, black (as in d.erase black) was being stored as 42 42 42.  I tried 
setting d.colormode fixed, but to no avail.

I asked two other GRASS users to do the same, i.e. d.erase black, raster
capture, and probe with the color editor.  One user did get RGB 0 0 0,
the other a 42 42 42 mix as I had.  In neither case were there many unique
colors in the capture (nothing approaching 256).

Can anyone throw me a rope on this one?


Greg Koerper                            Internet: greg at
ManTech Environmental Technology, Inc.  UUCP:     hplabs!hp-pcd!orstcs!koerper
US EPA Environmental Research Lab
200 SW 35th St., JSB                    
Corvallis, OR   97333           "The 90s will make the 60s look like the 50s. 
(503) 754-4490                          Just ask your kids."
 fax:  (503) 754-4338

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