Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!nrl-cmf!cmcl2!husc6!cca!mirror!rayssd!raybed2!applicon!bambi!peterson!peterson
From: peterson@peterson.applicon.UUCP
Newsgroups: comp.sys.mac.programmer
Subject: Re: Utilities for RGB to 8-Bit
Message-ID: <77900002@peterson>
Date: 26 May 88 14:45:00 GMT
References: <92@ai.etl.army.mil>
Lines: 25
Nf-ID: #R:ai.etl.army.mil:-9200:peterson:77900002:000:1289
Nf-From: peterson.applicon.UUCP!peterson    May 26 10:45:00 1988


	The problem here is that you have a true 24 bit image and you
want to display it as an 8 bit image.  This has been an ongoing problem
and there is no perfect solution.  The previous two responses would
work fine if you had only 256 pixels in your image, but that would be 
VERY small!  One way to do this would be to do a scan through the image
sampling it in 256 places (evenly).  Then assign these colors to the
Mac CLUT.  Now display the image using the Mac's ability to find the closest
color available for each pixel.  This gives you a unique color map for
each picture.
	Another way, as suggested, is dithering.  The idea is to try
to cover the entire 16 million colors with 256 by making a CLUT that
has a very large range of colors.  Doing this will make very large
steps between shades and will make your picture look "banded."
To reduce this banding, you have to alternate between the two nearest
colors depending on how close the real color is to the available ones.
There are many articles on dithering (also in "Fundamentals of Interactive
Computer Graphics" by Foley & Van Dam).
	I hope this helps.  I wish I could provide an algorithm, but
that would take up too much space here!

Joe Peterson
Schlumberger Technologies
Billerica, Ma

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