How to see a simple world: an exegesis of some computer programs for scene analysis.

The Platonic assumption that the world is made up entirely of objects with flat surfaces obviously does not hold; and yet, as with so many other simplifications of reality for the sake of tractability, it has been immensely productive in establishing a paradigm for scene analysis. There is a coherent evolving body of research based on the notion that a polyhedral world is the simplest we can consider without eliminating any of the essential aspects of scene analysis, namely, the picture-taking process, models, lighting, support, occlusion, and so on. The thesis is that once we achieve ways of dealing intelligently with those aspects for a simple, but nonetheless real, world we could then consider the fuzzy world of teddy bears (Michie, 1974) and the like. This should not be taken as suggesting that each of those aspects presents simply a separate, independent subproblem to be solved. The most important question to be faced was how to write programs that coordinate the use of these separate, but interrelated, knowledge systems to achieve sensible picture interpretations. Roberts (Roberts, 1965) was the first to give an answer to this question. We shall examine his answer in some detail, because he exposed in it the issues that became themes of the first decade of scene analysis.