Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!uw-beaver!ssc-vax!coy
From: coy@ssc-vax (Stephen B Coy)
Newsgroups: comp.graphics
Subject: Re: Color quantization ideas: To histogram or not to histogram, comments?
Keywords: graphics quantization
Message-ID: <3931@ssc-bee.ssc-vax.UUCP>
Date: 1 May 91 19:32:33 GMT
References: <606@lysator.liu.se>
Sender: news@ssc-vax.UUCP
Reply-To: coy@ssc-vax.UUCP (Stephen B Coy)
Distribution: comp
Organization: Boeing Aerospace & Electronics
Lines: 36

In article <606@lysator.liu.se> zap@lysator.liu.se (Zap Andersson) writes:
>* Is there ANY sane reason whatsoever, when trying to quantize a 24 bit
>  image to an 8 bit image, to make a histogram?

Yes, sometimes it looks better when the popularity of the colors is
taken into account.  Sometimes it also looks worse.  So allow the
user to experiment and decide for each image.

>However, the RESULT of my renderer is MUCH better 8 bit images than using
>any 'standard' algorithm.

Rendering and color quantization are two seperate processes.  Are
you saying that your renderer is better than others you've seen or
that your quantization is better?

>To create one, you need to pre-quantize to a
>manageable number of colors, most often 5 bits per channel, yielding
>15 bits (32768 colors). BUT 5 BITS PER COLOR IS NOT ENOUGH!

You're right.  For most images 5 bits is not enough.  I've found for
converting to VGA that 6 bits works best.  Since VGA cards have
6-bit DACs this should be no surprise.  Keeping track of 18bit color
running under MSDOS is challenging but not impossible.

BTW  I've seen some of the anims from RayTracker.  Looks like a nice
program.  If you'd like to see what I consider fairly good
quantization get a copy of my ray tracer, Vivid.  It's somewhere out
there...  The quantization program I include does do a histogram but
allows you to limit the max count for a color.

Stephen Coy
coy@ssc-vax.UUCP