Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!apollo!oj
From: oj@apollo.uucp (Ellis Oliver Jones)
Newsgroups: comp.graphics
Subject: Re: Color table database
Message-ID: <3b892ab5.d5b2@apollo.uucp>
Date: 18 Apr 88 13:41:00 GMT
References: <1059@cod.NOSC.MIL> <4418@batcomputer.tn.cornell.edu>
Reply-To: oj@apollo.UUCP (Ellis Oliver Jones)
Followup-To: comp.graphics
Distribution: na
Organization: Apollo Computer, Chelmsford, MA
Lines: 33
Keywords: X11, color ramps
Summary: use ramps...

In article <1059@cod.NOSC.MIL> thrapp@cod.NOSC.MIL (Gary R. Thrapp) writes:
>I am very interested in obtaining a set of RGB (or convertible to RGB)
>values to construct a 64, 128, or 256 element color table with a wide
>variety of colors including things like shades of brown?

I've had pretty good success reducing 24-plane TrueColor images for display
on 8 planes by constructing a color map with coarse, but uniform, ramps for 
each of the primary colors.

One such color map (originally dreamed up by Allen Akin at DEC, and
suggested as a standard shared color map for X11) provides six levels each
of red, green, and blue, using a total of 216 (6x6x6) color cells.  This is
handy in an 8-plane workstation context because it leaves forty cells free).

Another scheme I've used has seven levels of red and green, and five of blue
(245 cells).  This gives slightly better image quality for some color images, but
is bad for gray ramps -- they change color slightly.

Of course, there's always 8 shades of red, 8 of green, and 4 of blue (in other
words 3 bits each of red and green, and 2 bits of blue).

To fit your color map to your image adaptively (rather than just coarsening
the colors of your image) see Color Image Quantization for Frame Buffer Display
by Paul Heckbert on p 297ff of the 1982 SIGgraph proceedings (from Heckbert's
Pre-Jello Period  :-)

If you are going to use as simple a color-space reduction as this, you probably
should consider dithering your image into the coarser color space.  
If you have less than eight planes (256 color
cells) to work with, you definitely should dither.
(see Rogers, Procedural Elements for Computer Graphics, p107).

/Ollie Jones (speaking for myself, not necessarily for Apollo Computer, Inc.)