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Physics artifacts
Game physics simulations can exhibit various artifacts related to collision detection, dynamics and constraint solving. These include issues with bounding volumes, single contact points, feature clipping, contact caching, mass ratios, constraint solving order and more. The document discusses the causes and potential solutions for each type of artifact.
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Physics artifacts
- 2. Game Physics Artifacts ErwinCoumans, AMD
- 3. Intro • Collision shapeand contact artifacts • Dynamics and constraint solving artifacts
- 4. SCE Physics Effectsand Bullet • http://bullet.googlecode.com
- 5. Forward Dynamics CollisionDetection Detect pairs Forward Dynamics Setup constraints Solve constraints Integrate position Apply gravity Collision Data Dynamics Data Compute AABBs Predict transforms Collision shapes Object AABBs Overlapping pairs World transforms velocities Mass Inertia Constraints (contacts, joints) Compute contact points Contact points Time StepStart End
- 6. bounding volume artifacts •axis aligned bounding boxes (AABB) • if bounds are too small, you might get shootouts
- 7. Single contact pointartifacts
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- 11. Multiple contact points •Feature clipping • Persistent contact cache • Perturbation to fill the contact cache
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- 15. Persistent Contact Cache •Add a single point at a time to a cache • Refresh cache, update or remove points
- 16. Updating local contactpoints Shape A Shape B a n b d a n b d‘
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- 18. Contact reduction • Keepup to 4 contact points • keep point with deepest penetration • maximize area of the points
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- 20. contact caching artifacts Sliding clears cache
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- 23. Hybrid method • Singleshot manifold to build a full cache • Only add a single point to a full cache • Google for btPersistentManifold
- 24. Perturbation artifact • Curvedobjects start rolling • Use damping to prevent it
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- 26. triangle A triangleB internal edge 3D view 2D view
- 27. triangle A triangle B internaledge 2D view
- 28. triangle A trianglenormal B internal edge 2D view corrected contact normal
- 29. triangle A triangle normalB internal edge 2D view corrected contact normal
- 30. triangle A triangle normalB internal edge 2D view triangle normal A edge voronoi region triangle B
- 31. triangle A triangle normalB internal edge 2D view triangle normal A edge voronoi region triangle B
- 32. Internal edges • Needto solve some more corner cases
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- 34. collision shape artifacts •Avoid concave triangle meshes • Use visual debugging tools
- 35. shape scale artifacts •large scale appears as slow motion • avoid scaling, skew, shear in rigid transforms • Use local scaling instead of global scaling
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- 39. Even smaller timesteps
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- 41. Predictive contact constraints •Add contact constraints using closest distance/points computation
- 42. Predictive contact constraints •The constraint solver will allow the object to move up to the given distance
- 43. Fix your timestep • fixed time step versus variable timestep • clamping causes slow-motion
- 44. Well of despair Slower framerate More sub stepsSlower simulation
- 45. mass ratio artifacts •pgs constraint solver doesn't handle it well • heavy of top of light
- 46. mass ratio artifacts •Keep masses all very similar • Promising research in NNCG method (quadratic convergence) See http://iphys.wordpress.com/ and http://jinngine.googlecode.com
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- 48. Jinngine NNCG javaapplet
- 49. warm starting artifacts •applied impulses are similar each frame, cache them • full warm starting can add energy so scale it down • assume that the configuration hardly change • this assumption doesn’t always hold, joint limits etc.
- 50. constraint solving orderartifacts • complex interaction between various constraints • randomizing can help convergence
- 51. friction artifacts • coupledversus decoupled friction • friction pyramid approximation • clamping of friction directions • friction and warm starting
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- 53. Friction clamping artifact •Use velocity direction as clamping axis
- 54. center of massartifacts
- 55. Animated (kinematic) objects •also known as kinematic objects • use infinite mass and inertia • update the linear and angular velocity
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- 57. inertia tensor artifacts •objects don't rotate
- 58. inertia tensor artifacts •angular motion causing instability and jitter • Shaking ragdoll limbs • Increase inertia tensor • Or aggressive deactivation
- 59. References • Game PhysicsPearls, book by AK Peters (2010) • http://bulletphysics.org • http://iphys.wordpress.com/