GPU Particles. Jason Lubken

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1 GPU Particles Jason Lubken 1

2 Particles Nvidia CUDA SDK 2

3 Particles Nvidia CUDA SDK 3

4 SPH Smoothed Particle Hydrodynamics Nvidia PhysX SDK 4

5 SPH Smoothed Particle Hydrodynamics Nvidia PhysX SDK 5

6 Advection, Wind Fields & Weather Effects GPU Rainfall Pierre Rousseau, Vincent Jolivet, Djamchild Ghazanfarpour

7 Problem Problem: CPU - GPU communication is too slow for particles Move everything to the GPU Problem: Manage computation & storage for potentially O(n 2 ) particle interactions Optimize & tune 7

8 Papers UberFlow: A GPU-Based Particle Engine Peter Kipfer, Mark Segal, Rüdiger Westermann Fast N-Body Simulation with CUDA Lars Nyland, Mark Harris, Jan Prins 8

9 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 9

10 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 10

11 Particle Creation Encode particle properties in one or more textures Position (RGB) Creation time (A) Velocity (RGB) Type (A) Cycle particle regeneration using modulo 11

12 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 12

13 Particle Movement: Euler Integration Iterative Storage: Velocity 2 Position 2 Update: v = v + a t p = p + v t 13

14 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 14

15 Sorting for Depth Assign an id to each particle Compute distance to viewer Bitonic sort on distance --retain id-- Move position & velocity storage 15

16 Assigning Ids Problem: After sorting, find a particle s original location in a large texture using only a single float id Distance between consecutive float values is not constant Millions of particles > 65,536 particles One bit of exponent is not enough Precompute the texture of unique ids on the CPU 16

17 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 17

18 Adding Sorting Keys Problem: Resolve collisions without O(n 2 ) comparisons Construct spacial keys Like: x/g 2 + y/g + z First use aligned cells Then use staggered cells Resolve collisions 18

19 UberFlow Particle creation Particle movement Particle sorting for depth or particle interaction (Optional) Particle interaction Boundary enforcement Rendering 19

20 UberFlow Rotation & rotational velocity Particle types Surface normal Color Shape Wind field 20

21 UberFlow 21

22 UberFlow 22

23 Papers UberFlow: A GPU-Based Particle Engine Peter Kipfer, Mark Segal, Rüdiger Westermann Fast N-Body Simulation with CUDA Lars Nyland, Mark Harris, Jan Prins 23

24 N-Body Simulation All particles effect one another directly Brute force without spacial partitioning: O(n 2 ) Simplification using far-field effects possible No collision detection Integration problems with close proximity & high force Eplison check & different integrator used 24

25 Force Computation fij = G ( (mi mj) / rij 2 ) ( rij / rij ) mi & mj are mass r is distance between particle i and particle j G is gravitational constant First term is force Second term is direction 25

26 Force Optimization CPU does half the computation Leapfrog-Vertlet integration Smoother than Euler integration Particle positions & acceleration needed --no velocity texture lookup-- Remove mi from computation Work to overcome integration problems at close proximity also removes fii 26

27 Summation Optimization Tile computation of gravitational force Loop unrolling 32 body-body interaction Split rows for small N 27

28 N-Body Simulation 28

29 N-Body Simulation 29

30 Ideas UberFlow Clever use of ids seems computationally intensive How do you tune this? N-Body Simulation Force splatting to alpha buffer Particle control from CPU using small texture 30

31 References UberFlow: A GPU-Based Particle Engine 2004, Peter Kipfer, Mark Segal, Rüdiger Westermann Fast N-Body SImulation with CUDA, Lars Nyland, Mark Harris, Jan Prins (GPU Gems 3) Everything About Particle Effects, Lutz Lata Broad Phase Collision Detection with CUDA, Scott Le Grand http.developer.nvidia.com/gpugems3/gpugems3_ch32.html (GPU Gems 3) 31

32 Particle Movement Closed or Parametric Form Vertlet Integration Euler Integration Others 32

33 Particle Movement: Vertlet Integration Iterative Storage: Position 3 Update: p = 2p - p + a t 2 33

34 Particle Movement: Closed or Parametric Form Stateless Storage: Position x 2 Velocity Update: p = p o + v o t + ½a t 2 34