Timewarp Rigid Body Simulation

1. Timewarp Rigid Body Simulation Brian Mirtich

2. Simulation Discontinuities • “Events that change the dynamic states or the equations of motion of some subset of the bodies” • Ex. “collisions, new contacts, transitions between rolling and sliding, and control law changes”

3. Retroactive Detection • If an event occurs then backtrack to the moment of the event and then process • All bodies must be backed up to the time • The event might have affected their motion • There is no framework for maintaining bodies at different times.

4. Conservative Advancement • Don’t integrate over a discontinuity • Lower bounds are maintained by a priority queue • Creeps up to a discontinuity

5. The Timewarp Algorithm • Avoids unnecessary synchronization • Comes from discrete event simulation(DES) • Enables the simulation to be run in parallel

6. Timewarp Continued • Each processor maintains part of the model • Each processor keeps a “Local Virtual Time” • Each processor advances its local time until an event occurs • Events are processed through messages

7. Timewarp Continued • Rollback occurs to the latest time before the message time • This time becomes the LVT • Messages sent after the new LVT are unsent via anti-messages • GVT is the minimum of all LVT, it is provably valid

8. Rigid Body Simulation • Each body is a process • There is a global event queue • Each event contains the time and the bodies that receive it • Four Events • Collision check events • Group check events • Redraw events • Callback events

9. Collisions and Rollback Isolated Bodies • Each collision check has a “safe time” which is the time the group of bodies was last verified to be disjoint • GVT is the minimum of all LVT’s and all event safe times • If a rollback is earlier than an event’s safe time it is rescheduled to the rollback time

10. Multibodies • One state queue serves for the whole multibody • Events are still handled on a per link basis however the whole multibody must be integrated

12. Contact Groups • Groups of rigid and multibodies in persistent contact • The components again must be integrated as a unit • When bodies join or leave a new contact group is formed

13. Group Events • If bodies A-B are in the same group but not currently in contact then standard A-B collision event • Group check events are fired at a user specified rate • While group check is enabled regular collision detection between A-B is disabled • Fission is checked for by performing a connected component algorithm

15. Collision Checks • Events are enabled between active pairs • Axis aligned bounding boxes are used for each rigid body state to generate a “swept volume” • A collision detection event is scheduled once an integration step causes the swept volumes to intersect

17. Callback Functions • Cannot access global data • Data that must persist across calls can use added slots to the body’s state

18. Possible Expansion • Full parallel implementation • How to minimize inter-processor communication? • How best to load balance? • How to handle shared data?