http://www-bcs.mit.edu/people/adelson/checkershadow_illusion.html
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 shad
