Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!rochester!rocksanne!bozo!kirby
From: kirby@bozo.wbst128.xerox.com (Mike Kirby (co-op))
Newsgroups: comp.graphics
Subject: Re: 24-bit RGB color to 8-bit color ?
Message-ID: <735@rocksanne.UUCP>
Date: 25 Apr 89 13:40:57 GMT
References: <538@rna.UUCP>
Sender: news@rocksanne.UUCP
Reply-To: kirby@bozo.UUCP (Mike Kirby (co-op))
Organization: WRC, XEROX
Lines: 33


>   Does anyone have a program to take an image with 24-bit color info
>and build an 8-bit color map that is "good" (find the 256 color map entries
>that produce the least error) ?

>        Note that I am not asking for a 24->8 bit dithering program. It would
>seem that the program I want must internally build a histogram of the
>occurences of each of the 16M possibilities and then doles each of the
>256 available entries according to overall minimum color error (ideally,
>including psychophysical facts).

A program that was supposed to do something like this floated around
on the net last year (don't remember who it was from, or what the name
of the method was though).  Either way, we didn't get very good 
results from it.  The 24->8 conversion tended to leave an enourmous ammount
of banding as a result of the loss of the colors.  Something like a 24-8
floyd-steinberg error diffusion algorithm, or some other method of dithering
or error diffusion would probably have much better results then a histogram
derived colormap.  

If nobody else beats me to it, I'll mail you the program tomorrow night
that does what you want.  

Has anyone else come up with a method of converting 24-8 that provided
a steady colormap.  This is desired because when doing animation,
changing the colormap produces a fair ammount of flicker.  At school
we experimented with using the colormap generator and that resulted in
the problem of banding & flicker. (needless to say we settled on black and 
white animation only).   We never got around to implimenting f/s.

Michael Kirby
mpk9172@ritvax.bitnet 		Works...
mpk9172%ritcv@cs.rit.ed		usually works with a strong tail wind...