Lecture 13: Motion Capture and Motion Editing

by Mike Gleicher on February 24, 2013

Motion Capture Technologies

  • Mechanical
  • Magnetic / Ultrasonic
  • Inertial
  • Passive Optical
    • resolution, framerate tradeoffs
  • Active Optical
  • Vision

Passive Optical Processing

  • Image issues (localization in 2D)
  • correspondance and matching (2D-2D –> 3D)
    • redunandant markers vs. fewer markers
  • occlusions and outliers
  • “tracking” (which 3D point is what body part – marker ID)
  • gap filling
  • skeletal solving
    • markers –> skeletons
    • issues: not an exact model, deal with noise, missing data, …
    • coherence issues
    • complicated IK problem
  • “traditional” pipeline
    • track/gap fill/denoise
    • skeletal solve
    • work with skeletons
    • skeleton as an accurate reconstruction of what the performer did
    • map skeleton to character
  • alternative pipeline
    • track / gap / solve
    • manipulate marker data
    • drive characters from marker data
    • who cares what the performer did – the character is different

Power of Mocap

  • anything you can get the actor to do
    • directors are good at working with actors
    • good actors are out there
  • easier to explain to an actor what you want than to try to understand it well enough to model it mathematically
  • not just “what makes a walk a walk”
    • like this character / person
    • in this mood
    • in this take
  • but that specificness is also the curse:
    • you get what you get – details and all
    • end of day story

What is the motion data?

  • sampled at “sample rate” (30hz, 60hz, 120hz, …)
  • separate measurement of pose at each time step
  • marker data – or skeleton data (angles)
  • usually, assume skeleton is fixed over motion
  • densely sampled signals

VS. Keyframe data

  • standard skeleton (not specialized rig)
  • dense vs. sparse
  • imperfect (not aligned) – vs. made to work with keyframes
    • staggered poses
  • keyframe reduction
    • not just trying to do spline fitting (but most work does that)
    • fitting splines creates more compact representations – not necessarily convenient keyframes
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