The world, the brain, and the speed of sight

(Commentary on Chapter 11: " The perception of shading and reflectance, EH Adelson and AP Pentland) Adelson & Pentland use an engaging metaphor to illustrate their position on scene analysis: interpretations are produced by a workshop that employs a set of specialists, each concerned with a single aspect of the scene. The authors argue that it is too expensive to have a supervisor coordinate the specialists and that it is too expensive to let them operate independently. They then show that a careful sequencing of the specialists leads to solutions of minimum cost, at least for their world of Mondrian panels. The authors admit that their approach is based on a relatively simple domain, and will have to be developed further if it is to be applied to more realistic situations. But can it really provide the basis for a better understanding of more general-purpose vision? In what follows, it will be shown that this approach is based on several rather strong hypotheses, some of which will have to be seriously modified or even replaced if an extension is to be made to more general domains. Three sets of issues are of particular concern here: the kinds of scenes that can be accurately interpreted this way, the relevance of such models to human vision, and the trade-offs between interpretative power and processing speed.