Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!uunet!suntc!jh34607
From: jh34607@suntc.UUCP (john howell)
Newsgroups: comp.graphics
Subject: Re: How to map 24-bit RGB to 256 color palette?
Summary: Probably best to use a quantization scheme with a mixture of evenly
	 distributed colors and high frequency colors.
Keywords: RGB EGA VGA color requantization
Message-ID: <117@suntc.UUCP>
Date: 28 Aug 89 16:14:14 GMT
References: <3129@cbnewsm.ATT.COM> <7743@cbmvax.UUCP> <2009@uceng.UC.EDU>
Distribution: na
Organization: Deere & Co. Technical Center, Moline,IL
Lines: 36

In article <2009@uceng.UC.EDU>, mfinegan@uceng.UC.EDU (michael k finegan) writes:
> Problem 1: the histogram will be large (up to 262144 'bins')

Sure, I have used an array of 2 million bins (2^(7*3)) since the
computer I used at the time (PR1ME) didn't like an array with 16 million
bins (2^(8*3)).

> Problem 2: is Euclidian distance (at least in RGB color space) a good
> 	measure of closeness of color ?

Good question!  Perhaps a gamma factor should be thrown in.

> Problem 3: are the highest frequency colors in the histogram necessarily
>         the best to use? (Maybe there should be a minimum distance between
> 	bins used - (dithering?) image dependent ...)

The way I did this was to make sure I had a "smattering" of evenly
distributed colors in the final look up table.  For example, I would
have a 256 color look up table with 27 evenly distributed colors (each
corner (8), midpoint of each edge (12), center of each fave (6), center
of cube (1), of the color cube) and 229 quantitized colors.  This allows
good dithering to take place for all cases (I used Floyd-Steinberg
techniques).

> 
> 
> 					- Mike Finegan
> 					  mfinegan@uceng.UC.EDU

========================================================================
John Howell			uucp:		uunet!suntc!jrh
Deere & Company			MCImail:	360-4047
Technical Center		CompuServe:	[76666,2505]
3300 River Drive		FAX:		(309)765-3807
Moline, IL  61265		Voice:		(309)765-3784
========================================================================