Drake

Modules  
Collision Filter Concepts  
Collision filters are a purely nonphysical concept.  
In the real world, we can rely on the axiom that no two objects can occupy the same space at the same time; the universe provides this constraint for free.
In simulation, this cannot be taken for granted. In fact, without active efforts to introduce this constraint, bodies in a simulated dynamical system would pass through each other without regard to this fundamental physical principle. Within the parameters of our models, we need to enforce the constraint that no two objects occupy the same space.
Collision detection consists of geometric techniques which allow us to detect when bodies occupy the same space and, to a certain extent, quantify the degree to which they invalidate the collision constraint.
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