This research was supported by NSF grant DCR-8502009. Implementation of the SCERPO system relied upon the extensive facilities and software of the NYU vision laboratory, which are due to the efforts of Robert Hummel, Jack Schwartz, and many others. Robert Hummel, in particular, provided many important kinds of technical and practical assistance during the implementation process. Mike Overton provided help with the numerical aspects of the design. Much of the theoretical basis for this research was developed while the author was at the Stanford Artificial Intelligence Laboratory, with the help of Tom Binford, Rod Brooks, Chris Goad, David Marimont, Andy Witkin, and many others.