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From: rice@swatsun (Dan Rice)
Newsgroups: sci.misc
Subject: Re: Color
Message-ID: <1463@pompeii.swatsun.UUCP>
Date: Tue, 8-Dec-87 02:04:25 EST
Article-I.D.: pompeii.1463
Posted: Tue Dec  8 02:04:25 1987
Date-Received: Sun, 13-Dec-87 13:11:19 EST
References: <162300002@uiucdcsb> <3290002@hpindda.HP.COM>
Organization: Swarthmore College, Swarthmore PA
Lines: 68
Summary: Color mixing

In article <3290002@hpindda.HP.COM>, dfc@hpindda.HP.COM (Don Coolidge) writes:
> Did I actually write this? That should teach me to not post a response 
> late on a Friday afternoon...:-)
> 
> > Your two questions are actually one...red, blue, and yellow are the primary
> >PIGMENTS, but red, blue, and green are the primary LIGHT COLORS. Red pigment
> >absorbs blue and green light; yellow absorbs blue and red; blue absorbs red
> >and green. Mix blue and yellow pigment and everything is absorbed except
> >green wavelengths, which are reflected. 
> 
> Ok, if the green is absorbed, it's clearly not reflected. I can't remember
> exactly how to get there from here, though...help, someone!

	O.K.  The conventional ``primaries'' for light are red, green, and blue.
A pigment that absorbs red will reflect green and blue (what actually happens is
more complex, involving integrals of energy per wavelength and visual pigment
sensitivities, but it winds up like this), the combination of which is usually
known as cyan.  A green-absorber reflects red and blue, so appears magenta, and
a blue-absorber reflects red and green, and thus is yellow.  Cyan is a greenish
blue and magenta a bluish red, so the red, yellow, and blue pigments you used in
kindergarten are not far off, and in fact can be mixed to form a decent set of
colors.  Notice that they add together into murky brown, not true black, since
they don't absorb all of the light incident on them; in color printing, even
with cyan, magenta, and yellow, it is customary to use black ink for the darkest
areas.
	In fact, the concept of a primary color is meaningless; any three colors
(excepting some degenerate cases) will serve to create a more-or-less wide range
of colors.  One standard color chart (the C.I.E diagram) looks like this, in a
crude rendition:

    |---- Green
    v

   /-\
  |   |
 |     | 
 |      |
|    .  |     . = white
|        |
|        |
----------  <---- Blue

^
| Red

All the colors of a given brightness fall within a horseshoe-shaped boundary,
where the monochromatic colors (colors formed by a single wavelength of light)
make up the boundary.  This diagram has the nice property that a mix of X%
of color A and (100-X)% of color B falls X% along the line between A and B.
Thus, arbitrary mixes of two colors span a line, and mixes of three non-
collinear colors (the degenerate case mentioned above) span a triangle.
You can see from the diagram that it's advantageous to span the triangle with
vertices red, green, blue, since that triangle has the greatest area, and also
is the only triangle that includes the purples (the bottom line between blue
and red).  In reality, though, there are no reproduction processes that even
come close to this ideal triangle; instead they span smaller triangles in the
middle.  The moral is that everyone pretends that their color system is based
completely on logic and necessity, whereas in fact all are compromises, and none
truly create real (spectral) colors.  A photograph can never create the
sensation of a rainbow, for instance.

	Well, enough lecture.  I hope this clears up a few things.

-- 
- Dan Rice, Swarthmore College, Swarthmore PA 19081
``I hear you're mad about Brubeck... I like your eyes, I like him too...''
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