Ka.

chvnx:

First time viewers of this display invariably do not see the 16 circles segmented from the background. Rather, they see a series of rectangles that they frequently describe as “door panels”. The illusion pits segmentation cues against what appears to be a very strong prior to interpret the image as a series of 3-D structures “coffers” with closed boundaries. (A coffer is a decorative sunken panel.) It appears that the prior involves both closure and shape-from shading assumptions. The Coffer Illusion is a variation on Gianni Sarcone’s “Op Art illusion”.

via illusioncontest.neuralcorrelate.com