Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!elroy!usc!orion.cf.uci.edu!uci-ics!venera.isi.edu!raveling
From: raveling@venera.isi.edu (Paul Raveling)
Newsgroups: comp.graphics
Subject: Re: 24-bit RGB color to 8-bit color ?
Message-ID: <8071@venera.isi.edu>
Date: 25 Apr 89 15:41:04 GMT
References: <538@rna.UUCP>
Reply-To: raveling@venera.isi.edu (Paul Raveling)
Organization: Information Sciences Institute, Univ. of So. California
Lines: 28

In article <538@rna.UUCP> dan@rna.UUCP (Dan Ts'o) writes:
>
>	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) ?

	There are lots around these days.  One is embedded in the
	imglib library that I just put out for anonymous ftp.

	If you're looking for good image quality, though, least
	error doesn't always correlate with best image quality
	according to human judgement.  Another measure with some
	merit is one I call chromatic retention; it's essentially
	the ratio of how many colors the output image retains
	to the number of colors the input image had at some given
	resolution in RGB space.  It appears to correlate best
	with image quality at about 3-bit resolution per RGB component
	(i.e., partitioning RGB space into 256 equal cubes).

	Even though mean squared error and chromatic retention
	have some relation to perceived quality of quantized images,
	neither is entirely appropriate.  Does anyone know of other
	promising measures?


----------------
Paul Raveling
Raveling@isi.edu