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  1. Animation Low-Level behaviors Overview Keyframing Motion Capture Simulation CS4455

  2. Low-Level Behaviors • Keyframing • Motion Capture • Simulation CS 4455

  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 CS 4455

  4. Keyframe Example CS 4455

  5. Keyframing • Fine level of control • Quality of motion depends on skill of animator CS 4455

  6. Motion Capture • Natural-looking motion • Hard to generalize motions • Registration is difficult • “Weightless” according to professional animators CS 4455

  7. Motion CaptureImages courtesy Microsoft Motion Capture Group CS 4455

  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 CS 4455

  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 CS 4455

  10. Keyframing CS 4455

  11. Keyframing CS 4455

  12. Keyframe Example CS 4455

  13. Keyframing • Fine level of control • Quality of motion depends on skill of animator CS 4455

  14. Hand Drawn Animation -- 2D • Sketches • Pencil tests • Inking • Coloring • Digitize to sprites CS 4455

  15. Computer Animation: 2D or 3D • Sketches • Models and materials • Key configurations • Playback of motion or render to sprites Improv Ken Perlin, NYU CS 4455

  16. The development process: Adjust trajectory Playback motion Parameters: Locations Joint angles Shape -- flexible objects Material properties Camera Motion Lighting Keyframing CS 4455

  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 CS 4455

  18. Interpolation Example CS 4455

  19. Key Frames 1 2 3 CS 4455

  20. Key Frames • Timeline • 1 2 3 3 1 CS 4455

  21. Inbetweening • Frame dependent (“wrong”) slow-in/out • Iterate once per frame • This is a variant on the Infinite Impulse Response (IIR) filter: CS 4455

  22. Inbetweening • Frame-Independent (“right”) slow in/out • Compute acceleration a Time of last frame Time Tend CS 4455

  23. Spline-driven Animation y x,y = Q(u) for u:[0,1] • Equal arc lengths • Equal spacing in u x CS 4455

  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) CS 4455

  25. Keyframing -- Constraints • Joint limits • Position limits • Inverse kinematics CS 4455

  26. Keyframing -- Constraints CS 4455

  27. Coordinate Systems CS 4455

  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 CS 4455

  29. Inverse Kinematics CS 4455

  30. Inverse Kinematics CS 4455

  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 CS 4455

  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 (4th derivative) CS 4455

  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 CS 4455

  34. Motion Capture • Goals: • Realistic motion • Lots of different motions (300-1000) • Contact • Appropriate game genres • Sports • Fighting • Human characters CS 4455

  35. Applications Movies, TV Video games Performance animation CS 4455

  36. CS 4455

  37. CS 4455

  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 CS 4455

  39. Sensor Placement • Place markers carefully • Capture enough information • Watch for marker movement • Check data part way through shoot • Videotape everything! CS 4455

  40. CS 4455

  41. Technology • Numerous technologies • Record energy transfer • Light • Electromagnetism • Mechanical skeletons CS 4455

  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 CS 4455

  43. Technology • Passive reflection --Acclaim, Motion Analysis • Automatically digitized • 240Hz • Not real-time, Correspondence • 3+ markers/body part • 2+ cameras for 3D position data CS 4455

  44. Technology • Vicon Motion Systems • Retroreflective paint on reflectors • Lights on camera • Very high contrast markers CS 4455

  45. Technology • Active light sources -- Optotrak • Automatically digitized • 256 markers • 3500 marker/sec • Real-time • Specialized cameras CS 4455

  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 CS 4455

  47. Technology • Exoskeleton + angle sensors • Analogous • Tethered • No identification problem • Realtime - 500Hz • No range limit - Fit • Rigid body approximation CS 4455

  48. Dataglove Low accuracy Focused resolution Monkey High accuracy High data rate Not realistic motion No paid actor Technology Mechanical motion capture CS 4455

  49. Technology • Technology issues • Resolution/range of motion • Calibration • Accuracy • Occlusion/Correspondence CS 4455

  50. Animation Issues • Style • Scaling • Generalization CS 4455