Local shape approximation from shading

Exact shape cannot be inferred from a local analysis of shading, but for shape interpolation, a crude local approximation may be sufficient. The author explores the limits of such local approximations that are easy to compute. In particular, the shape of shading is used to approximate the surface in areas of monotonic change of intensity. This analysis is complemented by a method to compute the direction of a single point light source from the shading on occluding contours.