Reflections on Shading

Mutual illumination forms a significant component of image radiance that is not modeled by the image irradiance equation. Its effects, which are easy to observe in real scenes, lead to complicated structures in radiance. Because mutual illumination is a global interaction, accounting for its effects in shape from shading schemes is impossible in the general case. We argue that as a result, dense depth maps produced by schemes that assume the image irradiance equation are intrinsically inaccurate. Discontinuities in radiance are well behaved (in a sense made precise in the paper) under mutual illumination and are a robust shape cue as a result. We discuss the few other such sources known at present. Throughout, these points are illustrated by images of real scenes.