Rod Constraints for Simplified Ragdolls

Abstract

Physics-based animation has become a standard feature in modern games. Typically, the bones in a character's animation rig are each associated with a simulated rigid body, leading to a jointed assembly commonly called a ragdoll. The high density of animation bones in the spine area can cause instability and performance issues, so we are motivated to find a simplified physical representation for this region. We approximate the spine region of a ragdoll as an inextensible elastic curve, building a circular arc constraint based on the Kirchhoff rod model. Our simplified spine shows improved performance and stability over the standard group of socket joints, and proves to be more controllable. To model general elastic rods we use soft position constraints in place of forces, leading to a stable maximal coordinate formulation of inextensible Kirchhoff rods.