The squares marked A and B are the [i]same shade of gray[/i]![br][br]To verify this use the [b][color=#1551b5]blue sliders[/color][/b] to slide squares A and B on and off the checkerboard.

Based on the work of Dr. Edward H. Adelson of MIT. See [url]http://persci.mit.edu/gallery/checkershadow[/url]