Animation

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Animation

• Motion Capture

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Keyframing

• The development process
• Adjust trajectory
• Playback motion
• Parameters
• Locations
• Joint angles
• Shape -- flexible objects
• Material properties

• Camera Motion
• Lighting

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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
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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

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Motion Capture

• Goals
• Realistic motion
• Lots of different motions (300-1000)
• Contact
• Appropriate game genres
• Sports
• Fighting
• Human characters

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Applications
Movies, TV Video games Performance animation
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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

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Sensor Placement

• Place markers carefully
• Capture enough information
• Watch for marker movement
• Check data part way through shoot
• Videotape everything!

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Technology

• Numerous technologies
• Record energy transfer
• Light
• Electromagnetism
• Mechanical skeletons

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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

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Technology

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

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Technology

• Active light sources -- Optotrak
• Automatically digitized
• 256 markers
• 3500 marker/sec
• Real-time
• Specialized cameras

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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

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Technology

• Exoskeleton angle sensors
• Analogous
• Tethered
• No identification problem
• Realtime - 500Hz
• No range limit - Fit
• Rigid body approximation

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Technology
Mechanical motion capture

• Dataglove
• Low accuracy
• Focused resolution

• Monkey
• High accuracy
• High data rate
• Not realistic motion
• No paid actor

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Technology

• Technology issues
• Resolution/range of motion
• Calibration
• Accuracy
• Occlusion/Correspondence

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Animation Issues

• Style
• Scaling
• Generalization

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Resolution

• Positioning of camera

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Markers, Calibration

• Marker Placement
• Location should move rigidly with joint
• Stay away from bulging muscles, loose skin
• Shoulders Skeletal motion not closely tied to
  skin motion
• Calibration
• Zero position
• Fine calibration by hand

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Calibration

• Finding Joint Locations
• Move markers to joint centers
• Assume rigid links, rotary joints
• Shoulder?

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Calibration

• Extract best limb lengths
• Use estimator to compute limb length
• Minimize or reject outliers

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Calibration

• Example estimator
• 508 frames of walking
• 6 bad frames
• Collarbone to shoulder
• Hand editing 13.3cm
• Estimator 13.2cm
• Arithmetic mean 14.1cm

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Accuracy

• Marker movement
• Noise in sensor readings
• Skew in measurement time
• Environment restrictions
• Frame rate
• High frame rate allows good filtering

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Camera Calibration

• Internal camera parameters
• Optical distortion of lens
• External parameters
• Position and orientation
• Correlation between multiple cameras

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Model-Based Techniques

• Restricted search space for markers
• Dynamics (velocity integration)
• No infinite accelerations
• Model of behavior
• Model of bodies of occlusion
• Objects dont pass through each other

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Scaling Animation

• Contact
• Movement style
• Inverse kinematics

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Generalizating Animation

• Interpolation Synthesis for Articulated Figure
  Motion
• Wiley and Hahn
• IEEE CGA v176

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Generalizating Animation

• Keyframes as constraints in a smooth deformation
• Keyframe placing the ball on the racket at
  impact
• Motion Warping
• Witkin and Popovic, SIGGRAPH95

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Generalizating Animation

• Motion Editing With Spacetime Constraints
• Michael Gleicher
• 1997 Symposium on Interactive 3D Graphics

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Blending Animations

• Efficient Generation of Motion Transitions Using
  Spacetime Constraints
• Rose, Guenter, Bodenheimer, Cohen
• Siggraph 96