Newsgroups: sci.image.processing
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From: ogata@netcom.com (Jefferson Ogata)
Subject: Re: Help with reducing the number of colors
Message-ID: <ogataD47xMn.A1K@netcom.com>
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References: <c88_9502082202@saluton.iaf.nl> <3hcr0t$nbr@cosmos.imag.fr> <kgriffin-1702951711370001@dialup-7.triode.apana.org.au>
Date: Sat, 18 Feb 1995 23:08:47 GMT
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In article <kgriffin-1702951711370001@dialup-7.triode.apana.org.au> kgriffin@triode.apana.org.au (Keith Griffin) writes:
[snip]
>depending on the results you want of course, I would use an application
>such as Adobe Photoshop to reduce the image from 24/32-bit to 8-bit. It
>does a very good job if you must reduce the file size or the pixel
>information.
>
>---------------------------------------------------------------
>if I grow up, I want to be  | Keith Griffin                   |
>the chef at the restaurant  | I'm not institutionalised (yet) |
>at the end of the internet  | kgrifffin@triode.apana.org.au   |
>---------------------------------------------------------------

Actually, in spite of its many useful features, the palette
selection algorithm in Photoshop is extremely poor (at least it
was in 2.5.1). For a simple example try creating a 256x256
image and drop a horizontal gradient from red (255,0,0) to
green (0,255,0) onto it. This image will have exactly 256
colors. Now quantize the image with no dithering to 8 bits.
You will notice terrible banding and if you look at the
colormap you will see that Photoshop only found about 35 colors
to represent the image, apparently spaced at intervals of
about 8 in rgb value. Perhaps they used a low-resolution
histogram.

A better Mac program for color quantization is DeBabelizer.
Don't get me wrong--Photoshop is a great piece of software. I
just wouldn't recommend it for quantizing images to lower
color resolution. Please note that I haven't tested 3.0.1 out
with respect to this; Adobe might have made some improvements.

-- 
Jefferson Ogata                                            ogata@netcom.com
"Animals without backbones hid from each other or fell down. Clamasaurs and
 oysterettes appeared as appetizers. Then came the sponges, which sucked up
 about ten percent of all life."                         - Firesign Theatre
