Chapter 8 Contact and Collision

ArtiSynth supports contact and collisions between various collidable bodies, including rigid bodies and FEM models, through a collision handling mechanism build into MechModel. Collisions are disabled by default, but can be enabled between specific pairs of bodies, or between general categories of rigid and deformable bodies (Section 8.1). Collisions are supported between any body that implements the interface Collidable.

Collision detection works by finding the intersecting regions between the collision meshes of collidable bodies, and then using the intersection information to determine contact points and their associated constraints (Section 8.4). Collision meshes must be instances of PolygonalMesh and must be triangular; mesh requirements are described in more detail in Section 8.3. Typically, a collision mesh is the same as the body’s surface mesh, but other meshes (or collections of meshes) can be specified instead (Section 8.3).

Detailed aspects of the collision behavior, such as friction, optional compliance and damping terms, methods used to determine contacts, and various tolerances, can be controlled using CollisionBehavior objects (Section 8.2). Collisions can be visualized by rendering contact normals, forces, intersection contours, and contact pressures (Section 8.5), and information about specific collisions, including contact data, forces, and pressures, can also be monitored as the simulation proceeds (Section 8.8).

It is also possible to explicitly control contact forces by making them an explicit function of penetration depth (Section 8.7); in particular, this capability is used to support elastic foundation contact (Section 8.7.3).

It should be understood that collision handling can be computationally expensive and, due to its discontinuous nature, may be less accurate than other aspects of the simulation. ArtiSynth therefore provides a number of ways to selectively control collision handling between different pairs of bodies. Collision handling is also challenging because if collisions are enabled among objects, then one needs to be able to easily specify the characteristics of up to collision interactions, while also managing the fact that these interactions are highly transient. In ArtiSynth, collision handling is managed by a CollisionManager that is a subcomponent of each MechModel. The collision manager maintains default collision behaviors among certain groups of collidable objects, while also allowing the user to override the default behaviors by setting specific behaviors for any given pair of collidables.

Within ArtiSynth, the terms collision and contact are used somewhat interchangeably, although we acknowledge that in the literature, collision is generally understood to refer to the transient process that occurs when bodies first come into contact, while contact refers to the more persistent situation as the bodies remain together.