Computer AnimationLesson 12 - Ragdoll physics

Remi Ronfard, Nov 2019

12 Appeal

Ragdoll physics =« lifelike death »

Ragdoll physics is a type of procedural animation that is often used as a replacement for traditional static death animation. The term ragdoll comes from the problem that the articulated systems, due to the limits of the solvers used, tend to have little or zero joint stiffness, leading to a character collapsing much like a toy into comically improbable or compromising positions.

Rag-doll physics

Ragdoll physicsDead bodies -> all physics, no muscles

Ragdolls have been implemented using Featherstone's algorithm and spring-damper contacts (John Nagle, US Patent, 2000, Method and system for generating realistic collisions in graphical simulation). An alternative approach uses constraint solvers and idealized contacts (Physically Based Modeling Principles and Practice. Proc. SIGGRAPH '97. )

Procedural animation of ragdollsPhysics for multi-layered physical models in non-playing characters (bones / muscle / nervous systems) and deformable scenic elements from "simulated materials" in vehicles, etc. Alternative to pre-made animation: each game is unique, whilst still deterministic. Rather than detract from gameplay through overstimulation, the "natural" qualities of movement provide for a more immersive experience, and extended replayability.

Blended rag-dolls!Blended ragdoll works by playing a pre-made animation, but constraining the output of that animation to what a physical system would allow.

!Used in many games: ! Halo 2, Call of Duty 4, Left 4 Dead, Medal of

Honor Airborne, Unchartered Drake’s Fortune, etc.

Blended Ragdoll Example : Running

Blended Ragdoll Example : Running

Source: Kwon, Hodgins: Control Systems for Human Running using an Inverted Pendulum Model and a Reference Motion Capture Sequence. SCA 2010.

Blended Ragdoll (continued)

Captured motion is divided into small segments

Blended Ragdoll (continued)

Compute inverted pendulum model per segment

Blended Ragdoll (continued)

Adapt segment to the chosen trajectory and context

Quasi-physics

• Source: Sergey Levine and Jovan Popović. SCA 2012. Physically plausible simulation for character animation. 

Quasi-physics

• Source: Sergey Levine and Jovan Popović. SCA 2012. Physically plausible simulation for character animation. 

Quasi-physics

• Torque term

• Pose term

• End effector term

• Source: Sergey Levine and Jovan Popović. SCA 2012. Physically plausible simulation for character animation. 

Quasi-physics

• Goal constraint

• Equations of motion

• Source: Sergey Levine and Jovan Popović. SCA 2012. Physically plausible simulation for character animation. 

Quasi physics

Biologically Inspired Control of Physically Simulated Bipeds

Source: Torsten Reil and Colm Massey, Theory Biosci. 2001.

Biologically Inspired Control of Physically Simulated Bipeds

Puppet Master

Puppet Master

Puppet Master

Paper 12 - Deep Mimic