Section: New Results
Keywords : Human Motion, Motion Understanding.
Virtual reality to analyze interaction between humans
Virtual reality was previously used in several domains to train people performing a costly and complex task. In such applications, metaphors are generally used to interact with virtual objects and the subjects consequently do not react exactly as in the real world. In these applications, the feeling of being there (called presence) that ensures realism can thus only be analyzed through questionnaires. In sports, realism and presence can also be evaluated through the gestures performed by the subjects. Let us consider the thrower and goal-keeper hand-ball duel. Previous results demonstrated that real goalkeepers react realistically to virtual opponents. We also verified that a small modification in the opponents' gestures engendered modifications in the goalkeepers' parry whereas no modification was found for a same throw repeated two times. In neuroscience and sports science, anticipation is a control skill involved in duels between two players. According to elements considered in an opponent's gestures, people are able to predict events that will occur in a near future. In  , we demonstrated that this phenomenon is also recovered in a virtual environment. In this environment, the opponents' gestures are animated through a kinematic model that could engender unrealistic trajectories. Nevertheless, the animation module, even if it is based on simplifications, seems to reproduce the visual elements considered by goalkeepers.
A ViconMX motion capture system was bought in January 2005. This system is able to capture the motion of reflective markers in real-time (with an acceptable time shift). Thanks to this system, we are able to capture the motion of handball goalkeepers and react to their gestures in real-time. First, we have developed a real-time collision checker that states if the virtual ball is intercepted by the real goalkeeper's avatar (see figure 24 ).
In addition, the point of view is recalculated at each time step according to the subjects' head position. We are currently designing a protocol in collaboration with LPBEM of University Rennes 2 and UMR 6152 "Mouvement et Perception" in Marseille in order to make our system be applicable in neuroscience. The main applications are sports (providing performance indicators to trainers), neuroscience (understanding perception and decision-making) and virtual reality (understanding how making interactions in virtual worlds be felt realistic). Future works will tend to develop more complex interactions, involving several other players. We also wish to investigate how coupling this approach to more classical eye-tracking techniques.