Shape from shading
from Michael’s Visual Phenomena & Optical Illusions
A slowly rotating 3×3 square, which contains bumps (“like eggs popping out”) and dents (“like cavities of an egg carton.”). In the beginning there is a dent at middle top. But after it has rotated to the bottom, it has mutated to a bump, and all other bumps to dents!
Rotate the angle manually, and change the colours.
This is a typical case of von Helmholtz’ “unconscious inference”: In fact, the dents/bumps are just circles, filled with a linear luminance gradient. We spontaneously infer third dimension, because usually light comes from above (“light from above prior”). And then an indentation would verily have a shadow at the top end. So in our visual system some object detector shouts “indent”…
After rotation, with light still assumed to come from above, the shadow now of course would correspond to the opposite local shape. This effect is very salient and belongs to the class of “pop out” phenomena (Ramachandran 1988).
Ramachandran VS (1988) Perception of shape from shading. Nature 331:163–166.
Ramachandran VS (1988) Perceiving Shape from Shading. Scientific American 259:76–83