Shape Distortion in Computer-Assisted Keyframe Animation

Distortion of one object shape into another is a tool that has been used effectively in two dimensions by animators in entertainment, education and communication for many years. This paper describes a method for computer support of a three-dimensional extension of this technique. If two object descriptions are topologically equivalent, then one object can be continuously deformed into the other by interpolation between the vertex positions of the two descriptions. Using a formal definition of polygon-based object descriptions, an algorithm for deciding the topological equivalence of two object descriptions (and for constructing a vertex correspondence) is developed. If the two descriptions are not topologically equivalent, some modifications to the vertices and edges of the descriptions are required before distortion of one into the other can be accomplished. This algorithm also computes an intermediate object description by the addition of duplicate vertices and degenerate edges and faces to one or both initial object descriptions. The intermediate object description can be distorted from one object to the other by vertex position interpolation.