1. Untangling Cloth David Baraff Andrew Witkin Michael Kass
Pixar Animation Studios, 2003

2. Introduction to Cloth Simulation
Dynamic physics simulation methods using mesh deformation
Reproduce behaviour of cloth-like soft-body objects
Production animation cloth simulation
Only last 5 years
Primarily concerned with character-fitted clothing
Cloth Simulation Fundamentals
Traditionally three-part process
Unconstrained cloth behaviour
Cloth-solid intersection
Cloth-cloth intersection
“Untangling Cloth” presents new approaches to the latter two tailored to production character animation

3. Cloth-Solid Intersection
Simulated soft-body intersecting with volumetric rigid body
Current algorithms
Test each cloth vertex against each solid
Apply force to vertices found to be inside an object
Production animation
Complex inescapably self-intersecting characters
Can cause fluttering, wiggling, snagging, scrunching

4. Collision Flypapering
No way of avoiding interpenetration
No way of correcting particle positions
Pinched cloth particles should stay motionless
Equidistant from intersecting objects
Relative motion to objects
Non-uniform surface weighting

5. Cloth-Cloth Intersection
Two simulated soft bodies in contact (or one self-intersecting)
Apply repulsive forces to both objects
Determine which particles are on “wrong” side
Current approaches use history
Check where particle was previously
Production animation
Any unresolved tangle in history-based method will escalate
New approach needed that compensates rather than attempts to avoid intersection

6. Global Intersection Analysis
Intersection algorithm designed to work at mesh-level, not particle-level
First find curves on meshes that bound intersections
Then determine which mesh segment is smaller
Flood-fill method
Apply forces to move meshes back outside
Quite computationally expensive (18k vertex garment mesh, 0.5 seconds/frame)

7. Further Examples
Flypapering used to influence motion: Boo rubbing stomach
Different influence weights on hand (0%, 50%, 100%)

8. Further Examples
Shirt tortured extensively (left), then GIA turned on, recovers quickly

9. Further Examples
Arm pulled in to side then moved away. Without GIA/flypapering, sleeve snags (c), with new algorithms, no snag (d)

10. Discussion References Possible use for techniques outside animation?
Real time character cloth animation - the next step?
Questions?
References
“Untangling Cloth”, Baraff, Witkins, Kass,
“Real Time Cloth Simulation”, Bargmann,