Computer Graphics Lab (LIG), Swiss Federal Institute of Technology (EPFL) CH-1015 Lausanne, Switzerland This paper presents a model of nonverbal communication and interpersonal relationship between virtual actors. Nonverbal communication improves their believability. They react not only to the presence of the other actors but also to their postures. Furthermore, their interpersonal relationships are affected by the issue of social interactions. To avoid homogenous group behaviors, each actor is set with a different character profile. We present an application of this model to create actors involved in social interactions in a virtual public garden. The animation of virtual actors is based on the library AGENTlib which is dedicated to the management of agent entities able to coordinate perception and action.
A public garden is a place where social interactions occur. People come there for a walk and to meet each other. Therefore, it is an ideal place to study social interactions and changes in social relationships. The way social interactions take place affects relationships. The friendliness or interest of a person for another one evolves according to the attitude adopted by this other person during a communication. Its attitude might have been for instance very disappointing or surprisingly nice. As people are usually not talking loudly, for other people not involved in the conversation, it is hard to hear a word people are saying and only gestures and postures can be watched. Nevertheless, by watching this body language, it is possible to infer the type of relationship between two or three persons and how the conversation is taking place. For instance, how well they know each other or how interested they are in the conversation. Thus, the use of a model of nonverbal communication for virtual actors can increase their believability although they do not use an audible spoken language. They adopt and respond to postures. We use this model to create a virtual public garden where synthetic actors come for a walk and try to engage in communication with other actors. Virtual actors reacting only to the presence of the others lack of believability and we believe that this can be improved by the use of elements of nonverbal communication. As for real people, virtual actors should have a specific behavior and the issue of a communication should affect them. Therefore, virtual actors are provided with a character and a description of their interpersonal relationship with other actors. Their character influences how they evaluate a posture and an emotional state describes their happiness. Actors interact by adopting postures in response to the postures of the others. The response posture corresponds to an attitude resulting from the attitude of the other person. But, in the same context, the reaction of an actor will be different from that of another because its character influences its style of nonverbal communication and its perception of postures and the postures it adopts. The interpersonal relationship between two actors determines if they begin a communication and the issue of this communication affects their relationship. The remainder of the paper is organized as follows. In the next section, we review some papers related to behavioral animation and social behaviors. In section 3, we give a definition of nonverbal communication and we describe the elements of nonverbal communication we focus on. In section 4, we present the behavioral model we implemented to control the social behavior of actors using nonverbal communication. We explain how they evaluate and select postures according to their character and how their relationships evolve. In section 5, we describe how the public garden and the actors are modeled with the libraries SCENElib and BODYlib  dedicated to the creation of...