From: The World In Your Head by Steven Lehar

Fig. 4.1 (A) A line drawing stimulates a volumetric spatial percept with an explicit depth value at every point on every visible surface, and an amodal percept of hidden rear surfaces. (B) Perception viewed as a transformation from a two-dimensional stimulus to a three-dimensional percept. (C) The central Y vertex from (A), which tends to be perceived as a corner in depth. (D) A dynamic rod-and-rail model of the emergence of the depth percept in (C) by relaxation of local constraints.