Again, this goes beyond the scope of the topic addressed by this model, which concerns the perception of three- dimensional form. I also omit discussion of texture, color, motion, symmetry, binocular disparity, perception of familiar forms, and a host of other topics relevent to the perception of three-dimensional form. Marr and Biederman also omitted discussion of the special case of 2-D perception, indeed I cannot think of a single model of three-dimensional spatial perception that addresses the special case of line drawings perceived as 2-D rather than 3-D forms. Perhaps I should state explicitly that this model is not presented as a solution to the entire problem of biological vision, but merely as an approach to addressing some issues that have remained unaddressed for too long.