1. Adapting Simulated Behaviors For New Characters Jessica K. Hodgins and Nancy S. Pollard presentation by Barış Aksan

2. Introduction To make human-like characters be useful in animations and virtual environments, we must be able to create new, appealing characters easily. Appealing human motion has several components:  The kinematics and dynamics of the figure must be physically correct  The control algorithms must make the figure perform in ways that appear natural and are stylistically

3. Dynamic Simulation  Animated motions in the paper are computed using dynamic simulation.  Each simulation contains:  The equations of motion for rigid-body model  Constraint equations for the interaction with the ground  Parametrized control algorithms  A graphical image  A user interface for changing the parameters

4. Dynamic Simulation (cont.)  During each simulation time step:  Control algorithm computes desired positions and velocities of joints  Proportional-Derivative servos compute joint torques  Equations of motion is integrated with internal joint torques and external forces and torques from ground or other objects

5. Graphical Models  Either modeled or purchased  Intermediate models for the morph scenes can be constructed using squares and cylinders

6. Dynamic Models  Derived from graphical models.  Mass and moment of inertia of each body part is computed using density.  There are various joints and certain controlled degrees of freedom of joints in the models.  Each internal joint has a torque source.  Equations of motion were generated using SD/FAST.  Points of contact with ground are modeled using constraints with Baumgarte stabilization.

7. THE CONTROLLED DEGREES OF FREEDOM FOR THE DYNAMIC MODEL OF THE RUNNER.

8. Running Control Algorithms  Running is a cyclic behavior consisting different phases for legs:  Flight  Heel contact  Heel and metatarsus contact  Metatarsus contact  Since different phases need different control actions, a state machine is used to select the active control actions.

9. Running Control Algorithms (cont.)  To generate steady-state running;  Forward speed,  Flight duration,  Balance must be maintained by the control system. Those are adjusted in the search step.

10. Running Control Algorithms (cont.)  Foot is positioned at touchdown to correct errors in forward speed and balance  To reduce disturbances at touchdown, ground speed matching is used.  To control flight duration, ankle and knee joints are extended.  Proportional-Derivative servos are used to compute torquesa and cause the body move to desired roll(0), pitch(slightly forward), yaw(0) values.

11. Running Control Algorithms (cont.) k and k v are proportional and derivative gains. d: desired For each internal joint, the control equation:

12. Bicycling Control Algorithms  The goal is controlling:  Balance  Speed  Facing direction  The rider;  navigates by applying forces to the handlebars,  controls speed by applying forces to the pedals.

13. Scaling A control system tuned for one dynamic model, will not work on a different model. Images showing the result of using the control system designed for the The running motion for a model that is halfway between the man and the woman.

14. Geometric Scaling Assumes;  Uniform scaling in all directions,  Gravity is same for both characters.  Geometric scaling is applied to:  The state of the system  Gains for PD joint servos  Values and constants used to control motion (desired values)  Integration time step

15. Geometric Scaling (cont.)  Simple and straightforward  Only requires control parameters and the new model  Scaling factor is important  leg length is a good one for running  wheel radius is a good one for bicycling

16. MEASUREMENTS OF MASS AND SIZE OF BODIES AND BICYCLE PARTS

17. Mass Scaling  Adapts the control system to differences not captured with geometric scaling.  Scales masses and moments of inertia  A system with same link lengths has k’ gains,  If link lengths also differ,

18. Mass Scaling (cont.)  Requires selecting relevant body segments for each gain  Still an approximation because;  Depend on a single ratio,  Each gain is scaled based on a subset of body parts  Moment of inertia is assumed to be constant despite changing angles

19. SCALING RULES

21. Tuning the Motion This step is a search over high-level parameters. Must result in stable and repetable motion. Different tunings presented in paper: Running Bicycling Metamorphosis

22. Running The search is restricted to five high-level control parameters: Ground speed matching (affects running speed) Pitch angle Timing of thrust (affects duration of flight) Extension of ankle (affects duration of flight) Extension of knee (affects duration of flight)

23. Running (cont.)  The tuning process requires several stages  Intermediate characters are used  An intermediate model that runs approximately 10 seconds with geometric and mass scaling is worth tuning.

24. Evaluation Function  Values assigned to search parameters define a control system.  Runner is commanded to run for a fixed duration (15 seconds in the paper)  Contains penalties for:  Falling  Errors in velocity  Head acceleration  Deviations in roll, pitch, yaw between strides

25. Bicycling The search parameters for tuning bicycling behavior:  Stiffness of arms and shoulders  Control of roll and yaw of the bicycle  Control of the handlebars and pedals Different characters may require new bicycle designs.

26. Metamorphosis  The algorithm can be used for online metamorphosis from one model to another  Equally spaced models between the two models are needed  Has problems because changes in a way that violates physical laws. (change of mass)

27. Discussion  A hybrid approach is selected  Would the same approach work for new control systems? Diving Vaulting  Is it adaptable to a wide variation of models Toe-strike runners instead of heel-strike runners  Kinematic constraints such as ground or bicycle contact for different link lengths need to be resolved

28. A COMPARISON OF DATA FROM THE BIOMECHANICAL LITERATURE WITH DATA RECORDED FROM THE SIMULATED RUNNERS.