Ball Catching: An Example of Psychologically-based Behavioural Animation

Behavioural animation aims to simulate the behaviour of a character so as to automatically produce sections of animation. An important aspect is simulation of perceptuo-motor behaviour: the ways in which a virtual actor senses and reacts to its virtual environment. The fact that all of the information about the virtual environment is available to the computer makes the problem easier than the equivalent problem in robotics but, to balance this, there is a greater requirement to produce realistic human-like behaviour. These two features combine to make this a novel and interesting area of study. This paper presents a new approach: in which results from experimental psychology are used as a basis for these algorithms. This provides us with guiding principles for the design of perceptuomotor routines at a higher level of abstraction than the computer vision methods while maintaining a strong basis in real vision. an animated character. These actors can contain representations of various non-visual aspects of a character. In particular, behavioural animation gives autonomous, computer controlled behaviour patterns to an actor. This means that, given sufficiently sophisticated behaviours an animation can be produced purely by interaction between actors and the environment and also between individual actors. This has achieved excellent results with simple animal behaviour, for example models of flocking birds [lo] and full fish behaviour [ 151. Most behaviour required of actors involves reacting to their environment, or at least taking the environment into account. For this to happen there needs to be some means by which the actor learns about the environment. This means that simulating perception is an integral part of simulating behaviour. All work on virtual actors has addressed this problem to some degree but it has been of particular interest to Renault, Noser, Magnenat-Thalmann and Thalmann [9, 81, Blumberg and Galyean [l] and Terzopoulos and Rabie [ll]. As a demonstration of this approach: we describe a simulation of how a cricket or baseball fielder runs to catch a ball. This is a real world problem that requires very specific perceptuo-motor skills and a tight coupling between vision and action. There is extensive work on this area in the experimental psychology literature. Using psychological theories produces a simple, efficient computer algorithm which produces a realistic running path. This example supports the thesis that results from experimental psychology provide a sound basis for behavioural animation. The next section summarises previous work in this area and presents a methodology based on using results from empirical, visual psychology to produce algorithms for behavioural animation. Sections 3 and 4 describe an example of this methodology, simulating how an outfielder runs to catch a ball. If a batsman in cricket hits a ball upwards it will move in a roughly parabolic path and the fielder has to run so as to end up at the point at which the ball can be caught. This is a problem which has not been significantly addressed in the computer animation literature but has been studied extensively studied by experimental psychologists. Section 3 presents the current range of theories expressed in the psychology literature. Section 4 discusses how these can be used to produce an algorithm for behavioural animation. 2 Simulated Vision