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From: mab@pixar.UUCP (Malcolm Blanchard)
Newsgroups: comp.graphics,comp.windows.x
Subject: Re: Luminance from RGB
Summary: What is white? - a war story
Message-ID: <2663@pixar.UUCP>
Date: 11 Nov 88 17:42:10 GMT
References: <8811042303.AA21505@dawn.steinmetz.GE.COM> <76649@sun.uucp>
Organization: Pixar -- Marin County, California
Lines: 30

The discussion of luminance computations and the subsequent discussion
of the meaning of white reminds me of an experience I had a few years ago
when Pixar was a division of Lucasfilm and we were working on an effect
for "Young Sherlock Holmes".  Aesthetic decisions were being made by
people sitting in front of color monitors.  The digital images were
transferred to film using three color lasers.  The film was printed and
then projected in a screening room.  I decided that this was an great
place to implement a what-you-see-is-what-you-get color system.  And
so I delved into the murky depths of colorimetry in the hope of
developing a color-correction program that would produce
the same color in the screening room that was measured on the color
monitors.  This a difficult problem (in fact, in its strictest sense, an
impossible one, since the color gamuts of the two systems have mutually
exclusive regions).  I took into account the CIE coordinates of the
monitor's phosphors, its color balance, the sensitivity of the color film
to each of the lasers, cross-talk between the film layers, effects of
film processing, the spectral characteristics of the print film's dye
layers, and the spectral characteristics of a standard projector
bulb.  Several steps in this process are extremely non-linear, but I
was able to achieve some good results by using some piece-wise linear
approximations.  I felt a great sense of success when I used a colormeter
to confirm that the CIE coordinates on the silver screen did, indeed,
closely match those on the tiny screen.  We color corrected a few shots
and showed them to the effects director.  He's response was, "Why does
this look so blue"?  It turns out that when we look at a TV we're accustomed
to a blue balance and when we're sitting in a theater we expect a yellow
balance.  The digital color correction was abandoned and the production
relied on the film lab to produce an aesthetic balance.  Thus proving
to me that science may work, but computer graphics and film making are
still largely a matter of art.