Very low bit rate coding is an important part of the MPEG-4 standard. To obtain maximum compression in videoconference sequences a technique called model-based coding can be used. With this technique, the face of the speaker is represented by a model, and its movement between frames is coded with a small set of parameters. As with
all MPEG standards, the encoder algorithm is not described and this leaves the door open to many possible implementations. In this paper, a summary of model-based coding is presented. Many fields have been studied, including 3D wire-frame models, face detection, facial feature extraction, motion parameter estimation, and face model
parameter coding. Several problems need to be solved before model-based video communication will become practical. Some of them are discussed in this paper, such as how to detect and locate a face in an image, how to extract parameters from an image to adapt a face model, and how to compress the animation parameters.