Lightweight procedural animation with believable physical interactions

In this paper, we describe Twig, a fast, AI-friendly procedural animation system that supports easy authoring of new behaviors. The system provides a simplified dynamic simulation that is specifically designed to be easy to control. Characters are controlled by applying external forces directly to body parts, rather than by simulating joint torques. This puppetry-style of control provides the simplicity of kinematic control within an otherwise dynamic simulation. Although less realistic than motion capture or full biomechanical simulation, Twig produces compelling, responsive character behavior. Moreover, it is fast, stable, supports believable physical interactions between characters such as hugging, punching, and dragging, and makes it easy to author new behaviors.