A General Version of Crow's Shadow Volumes

In 1977 Frank Crow introduced a new class of algorithm for the generation of shadows. His technique, based on the concept of shadow volumes, assumes a polygonal database and a constrained environment. For example, polyhedrons must be closed, and polygons must be planar. This article presents a new version of Crow's algorithm, developed at the Universite de Montreal, which attempts a less constrained environment. The method has allowed the handling of both open and closed models and nonplanar polygons with the viewpoint anywhere, including any shadow volume. It does not, however, sacrifice the essential features of Crow's original version: penetration between polygons is allowed and any number of light sources can be defined anywhere in 3D space, including the view volume and any shadow volume. The method has been used successfully in the film Tony de Peltrie and is easily incorporated into an existing scan-line, hidden-surface algorithm.