Nilesh.org Notes on technology and everything else

Kottke
is mad over it, Andy
is amazed by it. I am blown away by it. My eyes hurt everytime I
see it. It’s this amazing illusion that every blogger is linking
to, on his site. So am I. Check it out.
Edward
H. Adelson has created this Checker-Shadow
Illusion. The squares A and B both have the same shade of grey.
Can you believe that?!! To believe it, as everyone there did, I
opened up the image in Gimp and checked the colours on the squares.
They both are really the same!!

This is the explanation that Edward give for the 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.

To convince you, Andy has put up these static and animated proofs
for the illusion.