[tig] This is very grey to me...

bobineinc at yahoo.com bobineinc
Fri Apr 23 00:14:52 BST 2004


Hi
This is one of several official explanations 
  
""Why does the illusion work?The visual system needs
to determine the color of objects in the world. In
this case the problem is to determine the gray shade
of the checks on the floor. Just measuring the light
coming from a surface (the luminance) is not enough: a
cast shadow will dim a surface, so that a white
surface in shadow may be reflecting less light than a
black surface in full light. The visual system uses
several tricks to determine where the shadows are and
how to compensate for them, in order to determine the
shade of gray "paint" that belongs to the surface. 
The first trick is based on local contrast. In shadow
or not, a check that is lighter than its neighboring
checks is probably lighter than average, and vice
versa. In the figure, the light check in shadow is
surrounded by darker checks. Thus, even though the
check is physically dark, it is light when compared to
its neighbors. The dark checks outside the shadow,
conversely, are surrounded by lighter checks, so they
look dark by comparison.A second trick is based on the
fact that shadows often have soft edges, while paint
boundaries (like the checks) often have sharp edges.
The visual system tends to ignore gradual changes in
light level, so that it can determine the color of the
surfaces without being misled by shadows. In this
figure, the shadow looks like a shadow, both because
it is fuzzy and because the shadow casting object is
visible.The "paintness" of the checks is aided by the
form of the "X-junctions" formed by 4 abutting checks.
This type of junction is usually a signal that all the
edges should be interpreted as changes in surface
color rather than in terms of shadows or lighting. 
As with many so-called illusions, this effect really
demonstrates the success rather than the failure of
the visual system. The visual system is not very good
at being a physical light meter, but that is not its
purpose. The important task is to break the image
information down into meaningful components, and
thereby perceive the nature of the objects in view.
Adelson""

I personally think that is not an illusion but reality
or both. These kind of illusion happen all the time in
telecine and everybody seem to see the same thing. We
all see colors/contrast of multiples objects as part
of the environment but today color devices can only
see color/contrast of one object/pixel at the time.
There is a lot of research out there for smarter and
faster color software/hardware to see color/contrast
like the human eyes but we will need better/faster
CPU.

Jais @ bobine ( pronounce Bo Been)




=====
Disclaimer
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