Behavior Planning for Character Animation

1. Behavior Planning for Character Animation Manfred Lau and James Kuffner Carnegie Mellon University

2. Problem

3. Key ideas Motions abstracted as high-level behaviors and organized into a finite state machine (FSM). (in contrast to connections of individual poses) Build search tree of behavior states and perform global planning in both space and time. (in contrast to local policies)

4. Overview Environment FSM

5. Overview Behavior Planner Environment FSM

6. Overview Behavior Planner Solution Path(Sequence of Behaviors) Environment FSM Animation

7. Overview Dynamic Environments Terrain with small slopes

8. Overview Different behavior preferences Variety of characters

9. Related Work Motion Planning Kuffner 98Shiller et al. 01Bayazit et al. 02Choi et al. 03Pettre et al. 03Koga et al. 94Kalisiak and van de Panne 01Yamane et al. 04 GlobalNavigation Choi et al. 03 Manipulation andwhole-body motions

10. Related Work Re-playing original motion capture data Arikan and Forsyth 02Kovar et al. 02Lee et al. 02Pullen and Bregler 02Gleicher et al. 03 Move Trees / Steering Approaches Brogan and Hodgins 97Menache 99Reynolds 99Mizuguchi et al. 01 Kovar et al. 02 Reynolds 99

11. Our Approach Manually-Constructed Behavior FSM + Scalability + Search Efficiency + Memory Usage + Intuitive Structure – Requires segmented motion data – Requires FSM with appropriate transitions

12. Behavior Finite-State Machine

13. Environment Representation 2D Heightfield Obstacle Growth in Robot Path PlanningUdupa 77Lozano-Pérez and Wesley 83

14. Behavior Planner – A* search initialize Tree and Queue while Queue is not empty remove sbest if Goal reached return sbest if appropriate expand sbest end return no possible solution position orientation time cost

15. State-dependent Transitions if appropriate expand sbest Retrieve (from FSM) the states that sbest can transition to Updates position, orientation, cost, time Collision checking

16. Visualization of search tree

17. Motion Generation / Blending Sequence of behaviors  converted to actual motion Blending at frames near transition points Linearly interpolate root positions Smooth-in, smooth-out slerp interpolation for joint rotations

18. Generated Animation fromComputed Behavior Sequence

19. Environment-dependent Transitions Transition regions near obstacles (computed automatically fromenvironment geometry) Action must completely traverse corresponding obstacle (pass underneath, cross over, etc.)

20. Generated Animation fromComputed Behavior Sequence

21. Dynamic Obstacles State and Time-dependent Transitions Movement of dynamic obstacles needs to be predictable envt E(time)

22. Time-dependent obstacle

23. Time-dependent obstacle

24. Planning for Multiple Characters Centralized PlanningPlan jointly for all characters using all possible combinations of behaviors + returns globally optimal solutions – search time exponential in # of characters and behaviors Prioritized PlanningPlan for each character in turn according to priority – not as general – returns globally non-optimal solutions + linear time in the # of characters

25. Multiple Characters – Prioritized Planning

26. Multiple Characters – Dynamic Obstacles

27. Optimality / Speed Tradeoff A* search truncated A* inflated A* 2241 nodes 1977 nodes 1421 nodes faster than A* search by 6% 16%

28. “Anytime” Version of Algorithm initialize Tree and Queue while (!Q.empty() and t < Tmax) remove sbest if Goal reached return sbest if appropriate expand sbest end return no possible solution returns best “partial path” found so far

29. Navigating in uneven terrain Adjust relative costs of behaviors Add non-uniform terrain costs

30. Tuning Behavior Preferences

31. Re-use FSM for different characters

32. Skateboarder

33. Horses

34. 100 characters

35. Conclusion Behavior planning approach Organize the motion data into a data structure of high-level behaviors Planner performs global search of behaviors to synthesize motions