Interpenetration

A simulation or animation glitch where two objects that should block each other pass through one another instead.

Interpenetration (often called clipping) originated as gaming and animation jargon for models overlapping incorrectly; in robot simulation it means two collision bodies overlap — a finger poking through a tabletop, or a foot sinking into the ground. Physics engines tolerate a small amount of penetration by design: MuJoCo uses a soft-contact model that generates contact force based on penetration depth, and PhysX also treats penetration as a negative distance and progressively increases contact force to push objects apart. The problem is penetration that goes too deep. Common causes include too large a simulation timestep, too few solver iterations, collision geometry that is too thin or too coarse, or objects moving too fast (passing all the way through is called tunneling). Deep interpenetration distorts contact forces and grasp outcomes, and a policy can even learn to exploit it as a “cheat” that fails once deployed on a real robot. Fixes include shrinking the timestep, adding substeps and solver iterations, using convex decomposition for collision geometry, and enabling continuous collision detection. Feet sinking into the ground in motion-capture data is also called interpenetration.

ExampleWhen training a grasping policy in simulation, if a cup's collision geometry is a thin hollow mesh, a finger may pass straight through the cup wall; the simulator still counts this as a successful grasp, but the real robot cannot actually pick the cup up. Switching to convex-decomposed collision geometry and increasing solver iterations usually helps.

Also called

Penetration, Clipping

Related

Collision Geometry (Collider) · Soft Contact Model · Continuous Collision Detection · Simulation Timestep · Convex Decomposition · Solver Iteration Count (Position / Velocity Iterations)

Sources

MuJoCo Documentation: Computation (Soft contacts)
NVIDIA PhysX 5 Docs: Advanced Collision Detection