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