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From: tarolli@westcoast.esd.sgi.com (Gary Tarolli)
Newsgroups: comp.sys.sgi
Subject: Re: Who needs 24 bit colors
Summary: color banding
Keywords: color
Message-ID: <95622@sgi.sgi.com>
Date: 5 Apr 91 15:00:51 GMT
References: <8009@eos.arc.nasa.gov>
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Distribution: usa
Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <8009@eos.arc.nasa.gov>, prevost@eos.arc.nasa.gov (Michael Prevost) writes:
> What about the case where someone is doing smooth shading? You can burn up a lot
> of colors that are very close to the same but the eye is very sensitive to 
> comparing two colors side by side. If you don't want color banding you had 
> better have many bit planes.
> 

If you only have 4 or 8 bits of RGB then color banding is noticeable in some
cases (or many cases, depending on what kind of images you have).  When you
get up to 12-16 bits of RGB (total not per color channel), then I believe
the dithering hides most of the color banding.  In fact, in some cases (I
wont go as far as to say all) dithering can actually hide the color bands.

I haven't done enough comparisons to see if dithered 16 bits looks better
than 24 bits non-dithered.  I sort of doubt it.  Certainly 8 or 12 bits
dithered will not look as good as 24 bits, but then again it costs less.
But for sure, if you hold the number of bits
constant, dithering greatly improves the image.  The improvement is obviously
most apparent when there are few bitplanes.  Remarkably, dithering hides
most of the color bands even when there's only 8 bits.

Interestingly enough,  dithering is still usefull when you have 24 bits.
If you have a large polygon that is shaded with a very small color slope,
you will see Mach bands where the color changes.  As you pointed out,
the eye is very sensitive to comparing two colors side by side.  But if
you dither the 24 bit image, I believe the banding will disappear.

--------------------
	Gary Tarolli