Combining Incremental and Parallel Methods for Large- scale Physics Simulation OpenCL Physics 1 Sheldon Brown, Site Director Daniel Tracy, Programmer Analyst Erik Hill, Programmer Analyst

Brief Overview OpenCL Physics 2

A Tale of Two Engines Original Scalable City physics engine built for rich virtual environments Took advantage of “coherence” of scene Could handle much larger, richer environments than previous physics engines available. Massively multi-user operation motivated high-throughput, GPU-based engine Represented objects as “sets of constraints” Implemented in OpenCL for targetability/longevity 3

Synthesis We are combining both methods to achieve the highest scalability for large virtual environments The highest levels of physical interaction in the richest environmental context The result is one of the technical achievements that make Scalable City possible 4

Recently Finished Work OpenCL Physics 5

ScalableEngine-CLEngine Integration Integrated “incremental” & parallel methods CLPhysics performs the hard work on a subset of the universe (active objects) Physically active objects and objects near them All other objects removed from the physics subsystem Incremental physics & state management system now performs maintenance on this set Broad phase efficiently triggers object activations Changes sent via single compressed buffer to GPU On-board OpenCL kernel used to place data correctly 6

ScalableEngine-CLEngine Integration cont’d Transforms can now be set directly from CPU Data transmitted w/ same method as object activation Used for AI-controlled animations, inverse kinematics Transforms are sent back to CPU en masse Active objects’ transforms change constantly Only active objects are part of this set Moved body-to-constraint translation to GPU Higher performance due to uniform processing Accelerates communication due to smaller data size 7

Proposed Work 8

Proposed Work: What’s left undone? CLEngine should trigger object deactivation based on prolonged contact with heightmap Broad phase updates are still en masse Should transmit delta only for comm. speed Requires more sophisticated tracking on GPU Collision event feedback for audio There are still behavioral anomalies present System behavior should be similar to CPU engine 9