88 9. COLLISION DETECTION AND RESPONSE
We refer to Chapters 4 and 6 in Ericson [2004] for a thorough explanation and discussion.
9.2.2 BASIC PRIMITIVE TESTS
After pruning most of the possible intersections using the bounding volume hierarchy, we will
have to perform additional test to see whether the remaining geometry is colliding or intersect-
ing. ere are a number of primitive tests available for exactly this. For example, commonly used
intersection tests are
• closest point on plane to point;
• closest point on line segment to point;
• closest point on AABB to point;
• closest point on triangle to point;
• closest point of two line segments; and
• closest point of two triangles.
We refer the reader to Chapter 5 in Ericson [2004] for an in-detail explanation and ex-
ample code of all the different primitive tests.
9.3 COLLISION RESPONSE
Collision response can be treated separately for cloth-cloth and object-cloth collisions. We will
introduce an approach for each in the next two sections.
9.3.1 CLOTH-CLOTH COLLISION RESPONSE
A lot of cloth-cloth collisions, also named self-collisions, can be prevented by temporarily adding
a strongly damped repulsive spring to particles that are about to collide. is will accelerate the
particles away from each other, hopefully preventing the collision from happening. Baraff and
Witkin [1998] combine damped spring forces for self-collisions and constraints for object-cloth
collisions. Both are integrated in the implicit integration. e spring forces and their derivatives
are added to the linear system solve for stability.
Repulsion forces are essential to keep the number of collisions tractable but we will still
need to resolve some collisions that still occur by applying impulses to the particle velocities.
A more sophisticated method for cloth-cloth collision response was presented by Brid-
son et al. [2003]. Impulses are applied to instantly update the particle velocities to resolve the
collisions. ey also apply repulsive forces for when particles get too close together to prevent
the majority of possible collision events. More precisely, repulsion forces are added when the
particles are at a proximity similar in size to the cloth thickness.
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