three-dimensional surface identifications. Once the shape of the three- dimensional surface is hypothesized as cylindrical or spherical, information such as the direction of the light source and the cylinder/sphere radius may also be estimated by examining the topographic labels.
The facet model for image processing estimates the underlying surface of a gray-tone intensity image and uses this estimate in processing the image. The facet model has been used in edge detection, several kinds of segmentation algorithms, and to construct the topographic primal sketch of the image. The topographic primal sketch, a pixel classification scheme, has been used to aid in the estimation of three-dimensional shape from two-dimensional views of shaded objects. In all of these different algorithms, the facet model has been shown to be a robust model for understanding and processing images.