Animation Control with Dynamics B. Arnaldi, G. Dumont, G. Hégron, N. Magnenat-Thalmann, D. Thalmann Abstract This paper discusses the advantages and disavantages of kinematics and dynamics in motion control for animating three-dimensional characters. It presents a motion control system based on dynamics. Applications of such a system are especially in the area of walking and grasping. It is shown that a simulation of writing a letter is a typical dynamic process. For the animation of the hand itself, a kinematic approach seems to be more convenient. The problem of surface deformations is also briefly mentioned Keywords: kinematics, dynamics, three-dimensional character, walking, writing, grasping Introduction In this paper, we address the important problem of limb positioning and animating in human animation, e.g.: what are the angle values for the shoulder, elbow and wrist if the hand has to reach a certain position and orientation in space? How to realistically animate the arm to write a letter for example? This is mainly a problem of motion control. To solve this problem, several approaches have been described and classified [Parke 1982; Steketee and Badler 1985; Hanrahan and Sturman 1985; Magnenat-Thalmann and Thalmann 1985]. In a first step, Tost and Pueyo [1988], classified models in kinematic models and dynamic models. Kinematic models produce motion from positions, velocities and accelerations. Dynamic models describe motion by a set of forces and torques where kinematic data are derived. Systems can be also classified according to the specification of movements they allow. For example, Zeltzer [1985] classifies animation systems as being either guiding, animator-level or task-level systems. - Guiding: in these systems, the animator defines in advance the details of motion. There is no mechanisms for user-defined abstraction or adaptive motion. Guiding systems include motion recording, shape interpolation, key-transformation systems and notation-based systems. - Animator-level: these systems have been defined to allow the animator to specify motion algorithmically. - Task-level: these systems must schedule the execution of motor programs to control characters. Kinematics The direct-kinematics problem consists of finding the position and orientation of a manipulator (the external coordinates) with respect to a fixed-reference coordinate system as a function of time without regard to the forces or the moments that cause the motion. A typical kinematics approach is the parametric keyframe animation, which consists of specifying, for some key positions, various angles for