Animation

1. Animation Low-Level behaviors Overview Keyframing Motion Capture Simulation IAT 410

2. Low-Level Behaviors • Keyframing • Motion Capture • Simulation IAT 410

3. Generating Motion • What matters? • Quality of motion appropriate for rendering style and frame rate • Controllable from UI • Controllable from AI • Personality of the animated character IAT 410

4. Keyframe Example IAT 410

5. Keyframing • Fine level of control • Quality of motion depends on skill of animator IAT 410

6. Motion Capture • Natural-looking motion • Hard to generalize motions • Registration is difficult • “Weightless” according to professional animators IAT 410

7. Motion CaptureImages courtesy Microsoft Motion Capture Group IAT 410

8. Simulation (Broadly Defined) • Physics is hard to simulate • Pseudo-physics is somewhat hard • Control is very hard • Gives Generalization + Interactivity Desired Behavior Forces and Torques Model Numerical Integrator User/AI Control State Graphics IAT 410

9. When to Use What Method? • Keyframing • Sprites and other simple animations • Non-human characters • Coarse collision detection • Motion Capture • Human figures • Subtle motions, long motions • Simulation • Passive simulations • When interactivity w/ motion is important IAT 410

10. Keyframing IAT 410

11. Keyframing IAT 410

12. Keyframe Example IAT 410

13. Keyframing • Fine level of control • Quality of motion depends on skill of animator IAT 410

14. Hand Drawn Animation -- 2D • Sketches • Pencil tests • Inking • Coloring • Digitize to sprites IAT 410

15. Computer Animation: 2D or 3D • Sketches • Models and materials • Key configurations • Playback of motion or render to sprites IAT 410

16. The development process: Adjust trajectory Playback motion Parameters: Locations Joint angles Shape -- flexible objects Material properties Camera Motion Lighting Keyframing IAT 410

17. Keyframing Interpolation • Inbetweening v 1,2,3 4 5 6 7,8,9… Linear time v 1,2,3 4 5 6 7,8,9… Slow in, Slow out time IAT 410

18. Interpolation Example IAT 410

19. Key Frames 1 2 3 IAT 410

20. Key Frames • Timeline • 1 2 3 3 1 IAT 410

21. Inbetweening • Frame dependent (“wrong”) slow-in/out • Iterate once per frame • This is a variant on the Infinite Impulse Response (IIR) filter: IAT 410

22. Inbetweening • Frame-Independent (“right”) slow in/out • Compute acceleration a Time of last frame Time Tend IAT 410

23. Spline-driven Animation y x,y = Q(u) for u:[0,1] • Equal arc lengths • Equal spacing in u x IAT 410

24. Reparameterize Arc Length • S= A(u) = arc length • Reparam: • Find: Bisection search for a value of u where A(u) = S with a numerical evaluation of A(u) (Details in Watt & Watt) IAT 410

25. Keyframing -- Constraints • Joint limits • Position limits • Inverse kinematics IAT 410

26. Keyframing -- Constraints IAT 410

27. Coordinate Systems IAT 410

28. Kinematics • The study of motion without regard to the forces that cause it Draw graphics Specify fewer Degrees Of Freedom (DOF) More intuitive control of DOF Pull on hand Glue feet to ground IAT 410

29. Inverse Kinematics IAT 410

30. Inverse Kinematics IAT 410

31. What makes IK Hard? • Many DOF -- non-linear transcendental equations • Redundancies • Choose a solution that is “closest” to the current configuration • Move outermost links the most • Energy minimization • Minimum time IAT 410

32. IK Difficulties • Singularities • Equations are ill-conditioned near singularities • High state-space velocities for low Cartesian velocities • Goal of “Natural Looking” motion • Minimize jerk (3rd derivative) IAT 410

33. Motion Capture • What do we need to know? • X, Y, Z • Roll, Pitch, Yaw • Errors cause • Joints to come apart • Links grow/shrink • Bad contact points • Sampling Rate and Accuracy IAT 410

34. Motion Capture • Goals: • Realistic motion • Lots of different motions (300-1000) • Contact • Appropriate game genres • Sports • Fighting • Human characters IAT 410

35. Applications Movies, TV Video games Performance animation IAT 410

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38. Plan out Shoots Carefully • Know needed actions (80-100 takes/day) • Bridges between actions • Speed of actions • Starting/ending positions • Hire the right actor • Watch for idiosyncrasies in motion • Good match in proportions IAT 410

39. Sensor Placement • Place markers carefully • Capture enough information • Watch for marker movement • Check data part way through shoot • Videotape everything! IAT 410

40. IAT 410

41. Technology • Numerous technologies • Record energy transfer • Light • Electromagnetism • Mechanical skeletons IAT 410

42. Technology • Passive reflection – Peak Performance Tech • Hand or semi-automatically digitized • Video • Time consuming • Issues • No glossy or reflective materials • Tight clothing • Marker occlusion by props • High frames/sec IAT 410

43. Technology • Passive reflection --Acclaim, Motion Analysis • Automatically digitized • 240Hz • Not real-time, Correspondence • 3+ markers/body part • 2+ cameras for 3D position data IAT 410

44. Technology • Vicon Motion Systems • Retroreflective paint on reflectors • Lights on camera • Very high contrast markers IAT 410

45. Technology • Active light sources -- Optotrak • Automatically digitized • 256 markers • 3500 marker/sec • Real-time • Specialized cameras IAT 410

46. Technology • Electromagnetic Transducers • Ascension Flock of Birds, etc • Polhemus Fastrak, etc • Limited range/resolution • Tethered (cables to box) • Metal in environment (treadmill, Rebar!) • No identification problem • 6DOF Realtime • 30-144 Hz 13-18 markers IAT 410

47. Technology • Exoskeleton + angle sensors • Analogous • Tethered • No identification problem • Realtime - 500Hz • No range limit - Fit • Rigid body approximation IAT 410

48. Dataglove Low accuracy Focused resolution Monkey High accuracy High data rate Not realistic motion No paid actor Technology Mechanical motion capture IAT 410

49. Technology • Technology issues • Resolution/range of motion • Calibration • Accuracy • Occlusion/Correspondence IAT 410

50. Animation Issues • Style • Scaling • Generalization IAT 410