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From: jeff@osc.edu (Jeff Faust)
Newsgroups: comp.windows.x
Subject: Special Colormap Allocation
Keywords: colormap dither XAllocCOlor
Message-ID: <1393@oscsunb.osc.edu>
Date: 15 Feb 91 17:15:13 GMT
Organization: Ohio Supercomputer Center, Columbus, OH, USA
Lines: 23

I have a graphics application which needs to allocate many colors in
the colormap in order to do quality shading. Specifically, 245 colors are
used for a special dithering algoritithm, 8 colors are used for user
interface colors, and the other 3 are leftover (assuming 256 colors).
I use a pretty good scheme now, but users still complain about the
colormap shifting when moving between applications. I could greatly
reduce the colormap shifting if I could examine carefully the default
colormap and try to match colors already allocated there to similiar
colors in my dithering colormap. In otherwords if there is a red in
the default colormap with a pixel value of 4, I would place a red in
my colormap at the pixel value of 4 as well. Unfortunately, X11 seems
to overlook this desire and it seems impossible (in a portable
fashion) to determine which colors are already allocated in the
default colormap. XAllocColor() and XAllocColors() come close but
there is now way to detect unallocated colors from alloced colors. I
don't want my code to bother to match what are garbage values in the
colormap. Does anyone have any ideas about how to improve this
situation?

Thanks:
Jeff Faust

jeff@totem.osgp.osc.edu