1cs426-winter-2008
NotesNotes
Example final exam up in Work section of website• Take with a grain of salt
Collision notes part 1 (primitive operations) up in Resources section
Review session coming up on Friday Final projects: if you need to do a demo• Email me your code by the deadline• Also email me to set up a time for demo,
either after class or the following Monday/Tuesday
2cs426-winter-2008
Motion CaptureMotion Capture
A lot of human motion is very subtle, and hard to nail down with the usual tools• E.g. shifting balance, complex joints,
personality… Motion capture (mocap) instead records
real motion from actors• E.g. Gollum, Polar Express, Beowulf, a lot of
TV shows, plenty of games Technical difficulties:• How do you record?• What do you do with the data?
3cs426-winter-2008
Mocap methodsMocap methods
Most common: “marker-based”• E.g. draw dots on the actor’s face, or dress in black
and attach retro-reflective balls in key places• Film from one or more cameras (preferably calibrated
and synchronized, preferably with a strobe light)• Reconstruct 3D positions of markers in each frame
through inverse solve Some “markerless” systems: rely on good
computer vision algorithms Some use direct or electromagnetic
measurement
4cs426-winter-2008
Skeleton modelingSkeleton modeling
Most systems only can measure path of points on skin/clothing
For full-body motion, usually want this instead in terms of global pose + joint angles in a skeletal model
Film actor in neutral reference pose first, set up correspondances
Another data fitting problem(and source of error)• Often allow for stretchable bones to correct joint
modeling errors
5cs426-winter-2008
FootskateFootskate
Almost inevitably stuck with one particularly bad error: “footskate”
Feet that should stay planted on the floor slide around (and up and down)• Enough to be noticeable!
Need to clean this up specially• E.g. Kovar, Gleicher & Schreiner SCA 2002• Detect footplants• Apply IK on legs and hips to keep feet in place• Smooth over a few frames to avoid pops
6cs426-winter-2008
What to do with the dataWhat to do with the data
Simplest: play it back• If it’s not what you want, record it again?
Directly edit in animation tool• No notion of layers though; very hard to change
without wiping out all the subtleties Motion warping• Witkin & Popovic, SIGGRAPH’95• Retime (use time as a motion curve)• Add smooth, slow components to motion curves to hit
desired key frames• Hope nobody notices!
7cs426-winter-2008
Move treesMove trees
Standard videogame solution Design a graph corresponding to available
player actions• E.g. walk forward, turn, jump, …
Design and record corresponding actions with mocap
Warp/retime/edit to make clips easily transition where needed
Note: in playback need to keep separate track of global position/orientation
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