This illusion is based on a billboard showing a bathtub shot at an angle. As we walk from one end of the picture to the other, the bathtub seems to stretch and shrink. Why? Each change in location results in a different retinal image. When processed in the usual way, each of these images results in a different 3D percept. Walking past the real bathtub will also produce a series of retinal images, different from those produced by the picture. All of these images will elicit a single, common 3D interpretation, and thus shape constancy.
The three pink- and blue-colored parallelograms are the same. All blue lines are equal in length; all pink lines are also equal. Box B is simply Box C rotated counterclockwise.
But the three parallelograms look different, and boxes B and C look different.
Our visual system assumes that the diagonals in A and C are foreshortened and “stretches” them perceptually. The pink lines in B should be foreshortened and stretched, just as they are in C. But our visual system doesn’t stretch a horizontal quite as much as it stretches a diagonal. Why not?