Untangling cloth

Deficient cloth-to-cloth collision response is the most serious shortcoming of most cloth simulation systems. Past approaches to cloth-cloth collision have used history to decide whether nearby cloth regions have interpenetrated. The biggest pitfall of history-based methods is that an error anywhere along the way can give rise to persistent tangles. This is a particularly serious issue for production character animation, because characters' bodies routinely self-intersect, for instance in the bend of an elbow or knee, or where the arm or hand rests against the body. Cloth that becomes pinched in these regions is often forced into jagged self-intersections that defeat history-based methods, leaving a tangled mess when the body parts separate. This paper describes a history-free cloth collision response algorithm based on global intersection analysis of cloth meshes at each simulation step. The algorithm resolves tangles that arise during pinching as soon as the surrounding geometry permits, and also resolves tangled initial conditions. The ability to untangle cloth after pinching is not sufficient, because standard cloth-solid collision algorithms handle pinches so poorly that they often give rise to visible flutters and other simulation artifacts during the pinch. As a companion to the global intersection analysis method, we present a cloth-solid collision algorithm called collision flypapering, that eliminates these artifacts. The two algorithms presented have been used together extensively and successfully in a production animation environment.