Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!rutgers!ucsd!usc!samsung!aplcen!haven!purdue!decwrl!shelby!eos!jbm
From: jbm@eos.UUCP (Jeffrey Mulligan)
Newsgroups: comp.graphics
Subject: Re: Color perception (Was another @#*! VGA article)
Message-ID: <5583@eos.UUCP>
Date: 15 Nov 89 19:34:08 GMT
References: <824@uwm.edu> <391@ucsvc.ucs.unimelb.edu.au> <3667@celit.fps.com>
Organization: NASA Ames Research Center, California
Lines: 36

hutch@fps.com (Jim Hutchison) writes:

>In <391@ucsvc.ucs.unimelb.edu.au> U5569462@ucsvc.ucs.unimelb.edu (DAVID CLUNIE):
>[contemplation of 6 bit DAC input/output deleted]
>>Which brings me to the point that I have read somewhere recently (can't
>>remember where) that the human eye CAN'T distinguish any more than 64 different
>>shades of grey. Is this so ? Do people beleive it ?

>Nope, I don't believe it.  Smoothly shaded surfaces will develop visible
>banding, shadows will have little definition, highlights will be boring.
>These are all the nifty things you will get with a limited color space.
>64 is not all that bad though, much nicer than 32 or 16.

The question as posed is not quite what hutch@fps answered.  Imagine
doing the following experiment:  start with a black field; now increase
the luminance of an adjacent field until the difference can just be
seen.  (This is called a just-noticeable difference or JND).  Now,
keep doing this.  Eventually, the added fields will become so bright
that the eye will adapt and some of the dark gray fields will look black.
At this point count how many JND's span this range.  A general rule of thumb
is that the JND is a fixed fraction of the base level (Weber's law),
usually around 1%.

It is fallacious to infer the number of JND's which span the visual range
from observations using graphic's systems with so-many-colors because
the sampling of the luminance range by the device may not match the
differential sensitivity of the visual system.  On an 8 bit system,
6 may be discriminable from 7, while 216 may not be discriminable from
217.


-- 

	Jeff Mulligan (jbm@aurora.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-3745