Figure 1.

Gradient shading creates an illusion of depth to human viewers. (a) Tokens look concave or convex depending on the shading. We generally assume light comes from above, but can switch this percept. However, we cannot see all tokens in the same form simultaneously, which Ramachandran [3] terms the common light source rule. (b) This sense of depth is lost in tokens where light and dark areas are distinct. (c) A cuttlefish showing asymmetrical shading of the white square (WS) component. (Online version in colour.)