|
 This
is not the proper place to go into Land's complete
Retinex
Theory of Color Vision.
(Note:
Retina of eye, plus Cortex of the brain, gave it its name.)
And I'm certainly not
the best person to do it. Nevertheless, after viewing the
color vision pages here some folks have commented: "Yes, but
how
does it work?"
Arrgghh! It can't be covered in a short page, which is why I
was avoiding it in the first place.
But -- you
can
find a lot on the subject in books and magazines on the
subjects of optics, opthamology, psychological and
biological studies, visual physiology, medicine, and other
related fields, in the library, on the web, and from book
stores, both new and used. The key words to search on are
obvious. If you are mainly interested in a few general
observations, I'll try to provide an overview, to get you
started. But being no expert, apologies if some particular
important factor is either not included, or is not described
below with technical precision, after all, I'm a
composer...!
Land
learned that our eyes and brain together evolved to detect
only some of the stimulus which vision provides. We view
things more "qualitatively"
than "quantitatively."
Our eyes don't measure photons, their wavelength and mean
energy, like an expensive laboratory instrument. We have no
need to do that, much as our hearing doesn't care a fig
about Hertz, or acoustic energy in Watts per second, and so
on, aside from units of measurement developed to match what
we hear, like the decibel (dB). We need our senses in order
to survive. That's far more useful to us than a list of
numbers. We need to know if a possible food or poison lies
among those leaves or berries we're considering eating,
don't we? Well, long ago we did care, much more than at our
local supermarket...! ;^)
If we saw the berries
in the daytime they would not appear exactly the same as
when we looked near sunset. The sunset light would be much
more orange-red than a noon impression. You could measure
the amount, and a camera's film or CCD does actually
measure, or at least record the true wavelength stimuli. But
our eyes instead "correct" for the different lighting
conditions and color. They sort of subtract out the
surrounding bias, and return an impression that remains
quite constant under many conditions. So we ignore our
incandescent and fluorescent lighting differences at home at
night, versus the light out of doors during the afternoon --
who cares?
Take two lamps, side by
side, shine them both on a wall. Place a red bulb one,
normal white bulb -- rather like the Land red-white lighting
we got started with. Now hold your hand up in front of both.
You should see two shadows. One of them will look red. The
other will look not gray, but cyan (blue-green)! It's the
complementary shade of the red. If the bulb were green, that
shadow would be the complement, or magenta; a yellow bulb,
and the shadow would appear blue. The effect had been
noticed earlier, and the visibly colored shadows one sees
are generally called "Goethe Shadows," for the author who
first described them (yeay
-- wotta poet!). I'd
noticed them during my childhood "experiments," and another
friend of mine tell me he had, too, at a similar age.
(Thanx
to Pete Z. for pointing out both of these neat bits and many
other ideas for these new
pages.)
What's going on is that
we're witnessing the way we automatically correct our
perceptions for a surrounding shade. It alters our
perception of the smaller object so surrounded. A red apple
looks red against a white plate, but put it on a blue or
green place mat, and the color will really jump out at you!
Contrast effect, which enhances the differences, that's
what's going on. Same principle as before.
We can't help but make
these corrections, it's a part of our human Retinex way of
seeing. It's something that helps us disregard
irrelevancies, like conditions in the large, to focus on the
details within those surroundings. And it goes on at all
sizes, nesting one surround effect within another within yet
another, most of the time. That banana is yellow, but in a
green dish, sitting on a blue and white tablecloth, covering
a reddish brown table, all under an ordinary light bulb
(yellow-white) on the ceiling, a cool-white (bluish)
fluorescent lamp next to the sofa, which is of a greenish
gray cloth, with beige and red pillows... the possibilities
are enormous. And our brains and eyes have become amazingly
adept at "canceling out" all those things, modifying the
perception of that solitary banana based on the total
accumulative effects of everything else I just named!
So, yes, it gets
complicated, but the idea is not. Retinex both aids the
actual color impressions and lighting contrasts, and
heightens the differences based on what color stimulus is
next to what other stimulus, next to yet another, and so on
like above. When we watch a Technicolor film from the 50's
we probably experience more than the camera and film
actually saw. Certainly we do with our color video gear and
computer monitors, even the printed color page. But early
theorists thought their descriptions were complete. The
ideas made sense, could be tested: we do
after all see an RGB image, sort of, most of the time. And
that seemed to be that.
Land received quite a
bit of flack when he attempted to make sense of why these
severely limited color worlds gave such surprising
sensations of full color, or at least much more color than
the older theories could explain. So Einstein's relativistic
theories supplemented Newton's, they refined rather than
"proved it wrong." And Land's Retinex supplement our earlier
models. That's the usual way science works out, seldom is
any that's really well established later discovered to be
completely off the wall. But it may be incomplete, at least
in certain conditions. Then a curious person investigates,
tries to make sense of, and we're off and running once
again!
So don't sweat it on
the Retinex. Just realize we're making instantaneous
comparisons from bit to bit, and automatically adjusting the
output to the brain based on the whole environment. Change
the environment, and you change the details you perceive.
That's why those black borders help on the Retinex views
you've been looking at: they isolate us from our surrounding
world of light and color, lest these interfere, or even
cancel out, the impression we're trying to study.
Make sense to you?
Well, that's my best shot, can't spend any more time on it.
The creation of all these pages, images and texts has been a
rather enormous job, and I do hope you'll look all of it
through, then investigate it further on your own. Can't be
your library for you, the web seems to spoil us all that
way. I know how it feels: "type in a few words or phrases,
push the button, hey presto!, here's all the skinny." Nope,
just the highlights, just a part of the story, one point of
view, at best. Like my pages are here. Then there's work to
do, to research into what everyone else has done of
significance on your topic of inquiry. And finally, you must
take what steps you are able to make from that juncture. You
can't always "pay back" in life, so you "pay forward," for
the next curious soul, bless them all...!
So don't just sit there
staring at this! You might want to return to the vision
pages you were looking at some minutes ago, or even sit down
to think this all though until most of it makes some
sense.
-Wendy
Carlos
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