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

Kevin Shaw kevs
Fri Apr 23 03:34:24 BST 2004


Dear All - 
I too have seen this before, possibly on another site, but it had an
explanation then. I kept it and am posting it at the end of this message.

The effect apparently has been known for some time as this quote from John
Ruskin 1879 suggests

"Every light is a shade, compared to the higher lights, till you come to the
sun; and every shade is a light, compared to the deeper shades, till you
come to the night."

An excellent description of lightness perception can be found at

http://persci.mit.edu/people/adelson/publications/gazzan.dir/gazzan.htm

I only wish I had written this paper myself!
Here is the original explanation of the checkerboard illusion
>>>
"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."

Happy Coloring

Kevin Shaw                 consultant colorist
kevs at finalcolor.com          www.finalcolor.com
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on 04/22/04 12:58 PM, jeffh at jeffh at nvbb.net wrote:

> thanks to Pat Howley of MTI for supporting the TIG.
> --
> 
> 
> http://www.ebaumsworld.com/checkershadow.html
> 
> Some of you may have seen this before, or other things

 
Kevin Shaw                 consultant colorist
kevs at finalcolor.com          www.finalcolor.com
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