Checker shadow illusion

The checker shadow illusion is an optical illusion published by Edward H. Adelson, professor of vision science at MIT, in 1995.

[1] The image depicts a checkerboard with light and dark squares, partly shadowed by another object.

The optical illusion is that the area labeled A appears to be a darker color than the area labeled B.

However, within the context of the two-dimensional image, they are of identical brightness, i.e., they would be printed with identical mixtures of ink, or displayed on a screen with pixels of identical color.

[1] While being one of the most well-known contrast illusions, there are similar effects which cause two regions of identical color to appear differently depending on context:

The regions marked A and B are the same shade of gray.
A region of the same shade has been drawn connecting A and B.