Fig. 3

The visual system infers shape-from-shading of two-dimensional hexagonal [18] or circular [15] objects largely biased on the assumption that they are three dimensional objects illuminated by a single light source. There is no objective three-dimensional depth information in any of these two-dimensional patterns which are perceived to be elevated or depressed depending on whether the observer believes that the light source illuminating them is located superior or inferior (A and B), or to the left or right (C and D) of the image. The depth perceived in them is a visual illusion. Elevation and indentation can be reversed by rotating any of these objects 180 degrees in the horizontal plane or by an observer consciously changing the location of the light source believed to be responsible for the boundary shading