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Joint-Torque Control of Character Motions: Active Animations

We want to go beyond “passive rag-doll like” simulated characters towards more “active” intelligent self-driven solutions. The “puppet on strings” approach lacks dynamic interactive properties for engaging realistic and immersive virtual environments. This paper focuses on physics-based “self-driven characters” (e.g., character’s own joint-torques to control and move the limbs to accomplish specific tasks, such as walking) that can react in a life-like manner using physical properties (e.g., limb-strength, ground contacts, and mass). We start by explaining the kinematic principles of motion-control of articulated figures. The two types of motion-control for virtual characters are ‘position-based’ and ‘torque-based’. The position-based approach focuses on the linear inverse kinematic model, while the force-based approach extends the principle to generalized joint-torques. Primarily, the relationship between end-effectors (e.g., fingers and feet) current and desired targets to joint orientations and torques. We exploit the highly complex and ambiguous nature of an articulated character to produce primary and secondary cases. For example, projecting joint-torques onto the null-space transformation. This discusses and explains the principles and advantages of active torque controlled character animation solutions, including their dynamics and limitations.