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

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

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

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

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

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)

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

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

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

Discussion References Possible use for techniques outside animation? Real time character cloth animation - the next step? Questions? References “Untangling Cloth”, Baraff, Witkins, Kass, 2003 www.pixar.com/companyinfo/research/deb/untangling.pdf “Real Time Cloth Simulation”, Bargmann, 2003 www.mpi-sb.mpg.de/~bargmann/research_cloth.htm