CS-184: Computer GraphicsLecture #20: Motion CaptureProf. James O’BrienUniversity of California, BerkeleyV2011-S-20-1.02Today•Motion Capture3Motion Capture•Record motion from physical objects•Use motion to animate virtual objectsSimplified Pipeline:Setup and calibrate equipmentRecord performanceProcess motion dataGenerate animation4Basic PipelineFrom Rose, et al., 1998SetupRecordProcessAnimation5What types of objects?•Human, whole body•Portions of body•Facial animation•Animals•Puppets•Other objects6Capture Equipment•Passive Optical•Reflective markers•IR (typically) illumination•Special cameras •Fast, high res., filters•Triangulate for positions Images from Motion Analysis7Capture Equipment•Passive Optical Advantages•Accurate•May use many markers•No cables•High frequency•Disadvantages•Requires lots of processing •Expensive systems•Occlusions•Marker swap •Lighting / camera limitationsCapture EquipmentPassive Optical AdvantagesAccurateMay use many markersNo cablesHigh frequencyDisadvantagesRequires lots of processingExpensive (>$100K)OcclusionsLighting/camera limitationsMarker Swap8Capture Equipment•Active Optical •Similar to passive but uses LEDs•Blink IDs, no marker swap•Number of markers trades off w/ frame ratePhoenix Technology Phase Space9Capture Equipment•Magnetic Trackers•Transmitter emits field•Trackers sense field•Trackers report position and orientationCapture EquipmentMagnetic TrackersTransmitter emits fieldTrackers sense fieldTrackers report locationand orientationControlMay be wireless10Capture Equipment•Electromagnetic Advantages•6 DOF data•No occlusions•Less post processing•Cheaper than optical•Disadvantages•Cables•Problems with metal objects•Low(er) frequency•Limited range•Limited number of trackers11Capture Equipment•ElectromechanicalAnalogus12Capture Equipment•PuppetsDigital Image Design13Performance Capture•Many studios regard Motion Capture as evil•Synonymous with low quality motion•No directive / creative control•Cheap•Performance Capture is different•Use mocap device as an expressive input device•Similar to digital music and MIDI keyboards14Manipulating Motion Data•Basic tasks•Adjusting•Blending•Transitioning•Retargeting•Building graphs15Nature of Motion DataAdjustingWhy is this task not trivial?From Witkin and Popovic, SIGGRAPH 95Witkin and Popovic, 1995Subset of motion curves from captured walking motion.16Adjusting•IK on single frames will not workAdjustingIK on single frames will not workFrom Gleicher, SIGGRAPH 98Gleicher, SIGGRAPH 9817Adjusting•Define desired motion function in partsAdjustingDefine desired function withResult after adjustmentInital sampled dataAdjustment18Adjusting•Select adjustment function from “some nice space”•Example C2 B-splines•Spread modification over reasonable period of time•User selects support radius19AdjustingWitkin and Popovic SIGGRAPH 95IK uses control points of the B-spline nowExample: position racket fix right foot fix left toes balance20AdjustingWitkin and Popovic SIGGRAPH 95What if adjustment periods overlap?21Blending•Given two motions make a motion that combines qualities of both•Assume same DOFs•Assume same parameter mappingsBlendingIf given two motions, can we blend themto find a motion 1/2 between them?Assume same DOFsAssume same parameter mappings22Blending•Consider blending slow-walk and fast-walkBruderlin and Williams, SIGGRAPH 9523Blending•Define timewarp functions to align features in motionDefine timewarp functionsBlendingNormalized time is w24Blending•Blend in normalized time•Blend playback rateBlend in normalized timeBlendingBlend playback rateBlend in normalized timeBlendingBlend playback rate25Blending•Blending may still break features in original motionsBlendingBlending may still break "features" inoriginal motionsTouchdown for RunTouchdown for WalkBlend misses ground and floats26BlendingAdd explicit constraints to key pointsTouchdown for RunTouchdown for WalkBlending•Add explicit constrains to key points•Enforce with IK over time27Blending / Adjustment•Short edits will tend to look acceptable•Longer ones will often exhibit problems•Optimize to improve blends / adjustments•Add quality metric on adjustment•Minimize accelerations / torques•Explicit smoothness constraints•Other criteria...28Multivariate Blending•Extend blending to multivariate interpolationBlendingExtend to multivariate interpolation"Hippiness""Speed"Weights are now barycentric coordiantes“Speed”“Happiness”29BlendingExtend to multivariate interpolation"Hippiness""Speed"If we have other examplesplace them in the space alsoBecomes standard interpolation problem...Multivariate Blending•Extend blending to multivariate interpolation“Speed”“Happiness”Use standard scattered-data interpolation methods30Transitions•Transition from one motion to anotherTransitioningTransition from motion A to motion BPerform blend in overlapregion31Cyclification•Special case of transitioning•Both motions are the same•Need to modify beginning and end of a motion simultaneously32Transition GraphsTransition GraphsFlipStandRunWalkSitTripDance33Motion Graphs•Hand build motion graphs often used in games•Significant amount of work required•Limited transitions by design•Motion graphs can also be built automaticallyTransition GraphsFlipStandRunWalkSitTripDance34Motion Graphs•Similarity metric•Measurement of how similar two frames of motion are•Based on joint angles or point positions•Must include some measure of velocity•Ideally independent of capture setup and skeleton•Capture a “large” database of motions35Motion Graphs•Compute similarity metric between all pairs of frames•Maybe expensive•Preprocessing step•There may be too many good edgesTo appear in the ACM SIGGRAPH conference proceedings2. The motion should not penetrate any objects in the environ-ment.3. The body should be at a particular position and orientation ata particular time.4. A particular joint should be at a particular position (andmaybe having a specific velocity) at a specific time.5. The motion should have a specified style (such as happy orenergetic) at a particular time.Finding paths in the motion graph that satisfy the hard con-straints and optimize soft constraints involves a graph search. Un-fortunately, for even a small collection of motions, the graph G hasa large
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