Shape representations from shading primitives

Diffuse interreflections mean that surface shading and shape are related in ways that are difficult to untangle; in particular, distant and invisible surfaces may affect the shading field that one sees. The effects of distant surfaces are confined to relatively low spatial frequencies in the shading field, meaning that we can expect signatures, called shading primitives, corresponding to shape properties. We demonstrate how these primitives can be used to support the construction of useful shape representations. Approaches to this include testing hypotheses of geometric primitives for consistency with the shading field, and looking for shading events that are distinctive of some shape event. We show that these approaches can be composed, leading to an attractive process of representation that is intrinsically bottom up. This representation can be extracted from images of real scenes, and that the representation is diagnostic.