A Taxonomy of Complex Models for Visualizing Humans Nadia Magnenat Thalmann University of Geneva Geneva, Switzerland and HEC, University of Montreal Daniel Thalmann Computer Graphics Lab Swiss Federal Institute of Technology Lausanne, Switzerland Abstract This paper proposes a new classification of models for animating synthetic actors both according to the method of controlling motion and according to the kinds of interactions the actors have. The paper discusses the use of geometrical, physical and behavioral models for animating human characters. Various situations of a virtual scene are described, including the simulation of the motion of a single virtual human, the interaction between two virtual humans and the interaction between a virtual human and a real animator. Specific problems are emphasized such as cloth animation, facial emotions, individualized walking, vision-based obstacle avoidance and physically-based interactive motion control using 3D input devices. 1. Introduction The classification of approaches to computer animation can help us impose conceptual order in an area characterized by rapid and piece-meal innovation in many directions simultaneously, to systematically analyze the differences and similarities among these approaches, and to better understand the way in which the field has been evolving. In this paper, we show the evolution over time of research into animation models. The first computerized models to be defined in Computer Animation were mainly geometric. Since computer animation derives from traditional animation, the first trend was to imitate how traditional animators produce traditional films. The accent was put more on the graphic result per se rather than models, for example for the creation of a synthetic actor ( synthetic actors will be named actors in this paper). To make the movement more realistic, physics-based models have been introduced. The problem with these models is that all actors behave the same way. Because humans do not act solely according to physical laws, behavioral models have been introduced more recently to take into account the individuality of a character. Besides physical laws, another kind of control is necessary for simulating human motions. Concurrently with the evolution of motion control models, there have been major developments in the relationship between an actor and his environment. The emergence of techniques like A.I., object-oriented programming, new increases of computer speed and new interactive devices have made it possible to take into account the interactions of an actor with his environment, an actor with another actor and an actor with the animator. These kinds of interaction and our 3 categories of models (geometric, physical and behavioral) give rise to a classificatory array with 4 rows and 3 columns. Each case in the table represents existing and potential approaches to animation control, with an increasing complexity from top to bottom and from left to right.