Recent work on visual cognition by Hoffman (e.g. 1983) challenges the traditional notion that the basic units in visual analysis, the cognitive primitives, are geometric figures.² He argues, rather, that the cognitive primitives for vision are properties of boundaries. For example, a region of a planar image will be interpreted in one way if it is analyzed with a convex edge and in another way if it is analyzed with a concave edge. Consider Figure 1 (from Hoffman's paper), where the rings can be taken to trace the trough or the crest of the waves, and the lines defining the waves are interpreted accordingly.