1. Haptic Cloth Rendering 6th Dutch-Belgian Haptics Meeting TUDelft, 21 st June 2006 Lode Vanacken Expertise centre for Digital Media (EDM) Hasselt University (UHasselt)

2. 21-06-20066th Dutch-Belgian Haptics Meeting 2 Introduction Comparing techniques in both domains: –Integration Techniques –Collision Detection –Haptic Rendering Haptic Cloth Rendering Cloth SimulationHaptics

3. 21-06-20066th Dutch-Belgian Haptics Meeting 3 Overview Cloth Simulation Integration Techniques Collision Detection Haptic Rendering Haptic Cloth Rendering

4. 21-06-20066th Dutch-Belgian Haptics Meeting 4 Cloth Simulation Using Mass Spring Systems –Combination of Particles and Springs –Several techniques to connect particles

5. 21-06-20066th Dutch-Belgian Haptics Meeting 5 Overview Cloth Simulation Integration Techniques Collision Detection Haptic Rendering Haptic Cloth Rendering

6. 21-06-20066th Dutch-Belgian Haptics Meeting 6 Integration Techniques Moving the Cloth? –Movement through Newton’s dynamics –Position/Velocity of the particles need to be updated –Represented as Initial Value Problem –IVP can be solved using a first order Ordinary Differential Equation (ODE) –Several numerical methods exist Explicit, Implicit, IMEX, Verlet Integration,…

7. 21-06-20066th Dutch-Belgian Haptics Meeting 7 Integration Techniques Verlet Integration –Popular in Molecular Dynamics –Basic Verlet Scheme –High accuracy: O(h 4 ) with only one force evaluation –No explicit Velocity computation –Two other schemes Velocity Leapfrog

8. 21-06-20066th Dutch-Belgian Haptics Meeting 8 Integration Techniques After IT step: Inverse Dynamics –Changing the positions/velocities –Stability increases

9. 21-06-20066th Dutch-Belgian Haptics Meeting 9 Overview Cloth Simulation Integration Techniques Collision Detection Haptic Rendering Haptic Cloth Rendering

10. 21-06-20066th Dutch-Belgian Haptics Meeting 10 Collision Detection Achieving an interactive cloth (e.g. draping a table) Two phases –Broad phase: efficiently decreases the number of possibly colliding objects –Narrow phase: detects the areas of an object that possibly collides and checks these areas for collision

11. 21-06-20066th Dutch-Belgian Haptics Meeting 11 Collision Detection Only Object-Object collisions Main problem –Deformable Object  Pre-computed structure becomes invalid Two popular techniques –Bounding Volume Hierarchies (BVH) –Spatial Subdivision

12. 21-06-20066th Dutch-Belgian Haptics Meeting 12 Bounding Volume Hierachies BVH is a tree structure Primitives are recursively subdivided Pre-computed but here also rebuild/repair Bounding Volumes –Sphere, AABB, OBB,… –Good Enclosing + Fast Intersection test

13. 21-06-20066th Dutch-Belgian Haptics Meeting 13 Arity is important! –# nodes to update –# recursion depth Collision testing is performed top- down Repairing the BVH –Top-down, Bottom-up or Hybrid –Brown et al. combine Bottom-up with a priority queue sorted on depth Bounding Volume Hierachies

14. 21-06-20066th Dutch-Belgian Haptics Meeting 14 Spatial Subdivision Divide the objects in space –Subdivision of the Complete Space –Each object has its own subdivision Optimized Spatial Hashing (OSH) –Divide the space implicitly into small grid cells –Hash function: 3D grid cells  1D hash table –No 3D Structure! –Infinite Regular Spatial Grids –Two phases

15. 21-06-20066th Dutch-Belgian Haptics Meeting 15 Overview Cloth Simulation Integration Techniques Collision Detection Haptic Rendering Haptic Cloth Rendering

16. 21-06-20066th Dutch-Belgian Haptics Meeting 16 Haptic Rendering Two popular Rigid Body techniques –Penalty Based Methods Uses penetration depth Pop-Through/Multiple Objects –Constraint Based Methods Solution to above problems Representative object attached to the user using a spring

17. 21-06-20066th Dutch-Belgian Haptics Meeting 17 Haptic Rendering High cost of dynamic simulation  Adapt HR algorithm –Intermediate model [Mark et al.] Plane updated at simulation speed –Local model [Mendoza et al.] Treat part of object as rigid body in the haptic loop –Forcegrid [Mazella et al.] Uniform grid that functions as a buffer structure

18. 21-06-20066th Dutch-Belgian Haptics Meeting 18 Overview Cloth Simulation Integration Techniques Collision Detection Haptic Rendering Haptic Cloth Rendering

19. 21-06-20066th Dutch-Belgian Haptics Meeting 19 Haptic Cloth Rendering Choose appropriate techniques Cloth Simulation –Some forces: Gravity, Viscous drag –Rectangular surface –Cloth’s mass –Smooth Shading –Texturing

20. 21-06-20066th Dutch-Belgian Haptics Meeting 20 Haptic Cloth Rendering Integration: –Empirically Taxonomy M. Hauth Comparison of Implemented ITs –Mathematically Second most stable/accurate Leapfrog Verlet

21. 21-06-20066th Dutch-Belgian Haptics Meeting 21 Haptic Cloth Rendering Haptic Rendering –Realized using HAL –Constraint Based Methods [Ruspini et al.] –Force Shading [Morgenbesser et al.] –Cloth simulator integrated as a separate library Implemented and compared the other Collision Detection Techniques

22. 21-06-20066th Dutch-Belgian Haptics Meeting 22 Haptic Cloth Rendering Sphere and AABB Bounding Volume Hierarchies with possible arity of 2 or 4 –Modification of the repair algorithm of Brown et al. Priority queue is sorted on IDs Last added parent is saved Use array (see Table) Recursion is completely removed!

23. 21-06-20066th Dutch-Belgian Haptics Meeting 23 Haptic Cloth Rendering Optimized Spatial Hashing –Removed the first phase Evaluation of the CD algorithms

24. 21-06-20066th Dutch-Belgian Haptics Meeting 24 Haptic Cloth Rendering Comparison in update time (#updates <> Ms)

25. 21-06-20066th Dutch-Belgian Haptics Meeting 25 Haptic Cloth Rendering Comparison in Haptic Loop

26. 21-06-20066th Dutch-Belgian Haptics Meeting 26 Conclusion Who Won? –In our situation a sphere BVH with arity 4 is the best CD algorithm. Still a few problems –Force discontinuities arise –Solutions: Interpolate between two cloth representations Integrate locally at haptic update rate

27. 21-06-20066th Dutch-Belgian Haptics Meeting 27 Questions?