Perceiving light versus material

Humans rarely confuse variations in light intensity, such as shadows, shading, light sources and specular reflections, from variations in material properties, such as albedo or pigment. This review explores the cues, or regularities in the visual world that evidence suggests vision exploits to discriminate light from material. These cues include luminance relations, figural relations, 3D-shape, depth, colour, texture, and motion. On the basis of an examination of the cues together with the behavioural evidence that they are used by vision, I propose a set of heuristics that may guide vision in the task of distinguishing between light and material. I argue that while there is evidence for the use of these heuristics, little is known about their relative importance and the manner in which they are combined in naturalistic situations where there are multiple cues as to what is light and what is material. Finally, I discuss two theoretical frameworks, the generic view principle and Bayesian estimation, that are beginning to help us understand the visual processes involved in distinguishing between light and material.