Path: utzoo!attcan!uunet!cs.utexas.edu!usc!apple!well!gors
From: gors@well.UUCP (Gordon Stewart)
Newsgroups: comp.graphics
Subject: Re: How to map 24-bit RGB to best EGA/VGA palette?
Keywords: RGB EGA VGA color
Message-ID: <13319@well.UUCP>
Date: 24 Aug 89 15:36:36 GMT
References: <3129@cbnewsm.ATT.COM> <7743@cbmvax.UUCP>
Reply-To: gors@well.UUCP (Gordon Stewart)
Distribution: na
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
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The classic paper on this subject is in SIGGRAPH notes (1982?) by
Paul Heckbert, (paraphrased) "Color Image Quantization for Frame
Buffer Display."

Several improvements are possible:  some of these are mentioned in the
article, such as using different criteria for selecting a sub-range
to partition.

I myself have a color-selection algorithm which obviates truncating
the pixels to 15 bits.  It doesn't use a histogram, but a multi-way
partition method, and if applied using Heckbert's criterion (longest
box),it is equivalent to the Heckbert method using a 24-bit histogram.

The process, as described in Heckbert, is two-fold (possibly three-fold)"

	1) Select the best N colors;
	2) Render the image via mapping function;

opt.	3) While rendering, apply a dithering scheme to trade spatial
	   for color resolution (effective if the target N colors are
	   few - say, 16 or less - though this varies from image to image)


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