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From: zech@leadsv.UUCP (Bill Zech)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Super Duper VGA boards
Summary: color lookup tables are better than direct mapping
Message-ID: <8937@leadsv.UUCP>
Date: 1 Dec 89 03:02:32 GMT
References: <8911250811.AA26716@cie.uoregon.edu> <10401@attctc.Dallas.TX.US> <4033@peora.ccur.com>
Organization: LMSC-LEADS, Sunnyvale, Ca.
Lines: 48

In article <4033@peora.ccur.com>, joel@peora.ccur.com (Joel Upchurch) writes:
> 
> It seems to me that if a board supports 16-bits/pixel that it would be
> better to have the colors map directly rather than going through a
> mapping table. With 5 bits each for red, green and blue that would
> give you 32k colors to work with one bit left over. This would eliminate
> the overhead of setting up the color mapping table as well the
> hardware to do the mapping.

Nope.  Color lookup tables (aka color indirection, etc.) is much
superior to direct mapping.  All high performance systems I have
ever used/designed employ them.  Why?  Suppose you want to change the
colors slightly.  Would you rather modify a small number of bytes in
a table, or re-write your entire 1 million pixel display.

There are many linear enhancement processes (contrast, brightness,
pseudo color, etc.) which are very efficient using LUTs.


> 
> Does anyone know how many different colors a normal monitor can display
> that the human eye can actually tell apart?

I don't know the exact number, but it's a lot.  I have done some
experiments with a good quality color system, and have observed
the following:

	If two *slightly* different shades of the same color
	are adjacent to each other, the human eye can almost
	always pick them out.

	Take those same two colors and spread them apart an
	inch or so with some other intervening color, and
	you can't tell them apart.

Because the human eye is so good at integrating, most pictures
don't really need very many colors.  Just look at some of the
available 256 color VGA GIF files that float around.  With a
properly chosen color map, many images take on a near-photo
quality.  With some dithering, you can do even better sometimes.
Generally, the lower the resolution, the more colors you need.
With very fine pixels, fewer colors are necessary because at
normal viewing distances, your eye blends and averages high
frequency color transitions into a smooth appearing image.


- Bill Zech
  Image Processing Consultant