Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!iuvax!cica!tut.cis.ohio-state.edu!hydroplane.cis.ohio-state.edu!ray
From: ray@hydroplane.cis.ohio-state.edu (william c ray)
Newsgroups: comp.graphics
Subject: Re: Gray levels and color
Keywords: color gray
Message-ID: <54868@tut.cis.ohio-state.edu>
Date: 14 Jul 89 04:36:30 GMT
References: <20722@hodge.UUCP>
Sender: news@tut.cis.ohio-state.edu
Reply-To: william c ray <ray@cis.ohio-state.edu>
Organization: Ohio State University Computer and Information Science
Lines: 37

in a previous article someone says...  'I once knew a fella who could %
reconstruct color data from a B/W photo, he could read resistor codes on
b/w photos w/out the color there.  somehow he could actualy see the color
in the grayscale, because he never guessed and was always right... how did
he do it?"

answer... he was a better guesser than you thought...  in any color-to-BW
scheme, there are multiple hues which will record (generic term here, not necc
meaning screen graphics) as the same value (shade of gray).  ie, if the
pigments were right in the bands, several bands could have recorded as the
exact same value on a print.  there would be no way of knowing what the
origional colors were (none)  to make matters worse (and your friends guessing
ability even better...) there is not one BW film available (that I know of)
which records colors as we see them.  as a matter of fact, depending on the
film, the same color in two different prints would record as two very different
values.  for example, a photo of, say a 200 ohm resistor 
               (thats red          black  brown) would record on 
TRI X          (       black       black  dark-grey)   and on say
Panatomic X    (       light-grey  black  dark-grey)

This is due to the fact that TRI X is not particularly sensitive to red
light (like as in not hardly at all) while Panatomic X is more sensitive to
Red light than it is to Blue (under tungsten lighting conditions, when 
unaffected by ultra-violet)  What this means is that given an apple, a
green pepper, and (can anyone think of a largish blue food substance?) oh,
why not a VERY large blueberry, and using the correct films, I can make
any one of them appear black and/or any one of them appear white in the
final image  (actualy this is a bit tough to do to the green without filters,
but we wont go into that)

therefore, as you can see, determining which color is which from the grayscale
that it records at is an utterly impossible proposition.  (this means that
the fella who could read resistors in B/W was GOOD...)

later
Will Ray
ray@cis.ohio-state.edu