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Re: a link to some interesting illusions





Stating that the eye-brain system perceives color independently of
illumination is in general false (although it is true in various
situations). This can be easily proved by anyone who has bought an article
of clothing in a store and then noticed that the color appears very
different when taken outside.

However the idea that the 3 color additive model predicts the color seen, is
also false. The 3 color additive model works well for color television and
for photos, but has great difficulty with real life situations. If you go
to http://www.hhmi.org/senses/b140.html you will see the following text
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"Color constancy is the most important property of the color system,"
declares neurobiologist Semir Zeki of University College, London. Color
would be a poor way of labeling objects if the perceived colors kept
shifting under different conditions, he points out. But the eye is not a
camera. Instead, the eye-brain pathway constitutes a kind of computer—vastly
more complex and powerful than any that human engineers have built—designed
to construct a stable visual representation of reality.

The key to color constancy is that we do not determine the color of an
object in isolation; rather, the object's color derives from a comparison of
the wavelengths reflected from the object and its surround. In the rosy
light of dawn, for instance, a yellow lemon will reflect more long-wave
light and therefore might appear orange; but its surrounding leaves also
reflect more long-wave light. The brain compares the two and cancels out the
increases.
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I would submit that the real life situation is more general than the color
TV or photo. situation. In the real life situation you never see red as
blue, but you may see a different shade of red because of the illumination.
In other words color constancy is the first order effect with some color
shift due to illumination a second order effect. This was vividly
demonstrated by Land many years ago. Any photographer knows that you
perceive colors in one way and your camera can come out with a completely
different picture.

The CIE system is actually an engineering system designed to produce good
quality reproduction of man made images. In that task it works well. But
because of the color constancy of perception it is often difficult to set up
good demonstrations of the additive color scheme without going to great
lengths to control stray illumination. The CIE system might work even
better if it used some of the insights from the retinex theory.

The main difference of opinion is that I think the general situation is
color constancy while LW thinks it is the exception. I doubt that any
amount of argument can settle this. I suppose that I would agree that it is
the exception for couch potatoes who watch a lot of TV. In most normal
situations both models give the same results, but when there is
disagreement, I would submit that the eye's processing becomes the
determining factor.

John M. Clement
Houston, TX