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From: fleming@balboa (Dennis Paul Fleming)
Newsgroups: comp.graphics
Subject: Re: VGA Colors
Message-ID: <3698@orion.cf.uci.edu>
Date: 23 Nov 89 02:58:12 GMT
References: <824@uwm.edu> <17400014@hpfcdj.HP.COM>
Sender: news@orion.oac.uci.edu
Reply-To: fleming@balboa.UUCP (Dennis Paul Fleming)
Organization: EE Dept., University of California, Irvine
Lines: 51

In article <17400014@hpfcdj.HP.COM> brian@hpfcdj.HP.COM (Brian Rauchfuss) writes:
>
>Everyone around here can distinquish 256 shades of gray, but a dark room is
>required.  64 shades are very easy to distinquish.
>
>Brian Rauchfuss

Your ability to distinguish probably depends on two gray levels being
neighbors.  If they are seperated then one's ability is curtailed.
There are two experements which point this out:
	1) Grossberg in his book on Neural Nets looks at an image 
	with the folowing greylevel pattern:

                           |\
                           | \
                           |  \
------------------------\  |   \-----------------------
                         \ |
                          \|
                           \

	The resulting perception is that the 2 grey patches on either
	side of the dicontinuity appear to be different grey levels.

	2) McCann and Land in their paper on the Retinex theory
	Journal of Optical Scientist (?) had an experiment where
	light was cast onto a sheet of paper with two areas of
	different reflectance.  The energy comming off of the paper
	was as follows:

                                      _____________________________
         \       |\                   |             |             |
          \      | \                  |             |             |
           \     |  \              /  |             |             |
            \    |   \            )-  |             |             |
             \   |    \            \  |     40%     |    80%      |
              \  |     \              | reflectance | reflectance |
               \ |      \             |             |             |
                \|       \            |___________________________|
	

	The result of this experiment was that when a pencil or
	other obstruction was places at the dicontinuity both
	grey areas appeared the same value.  because of the position
	of the light and the relative values the total energy
	coming from each grea patch was the same, and so percieved
	as such.

Dennis Fleming
Dept of Electrical and Computer Engineering
University of California, Irvine