From: The World In Your Head by Steven Lehar

Fig. 4.4 Boundary completion in the Gestalt bubble model. (A) A single fading line creates a crease in the perceptual surface. (B) Two fading lines generate a crease joining them, especially when (C) the spatial context is consistent with this interpretation as seen in this figure. (D) Alternatively, a fading line can promote an occlusion percept, which (E) tends to join up with other edges in a collinear configuration, especially when (F) the spatial context is consistent with this interpretation.