Summary Baraff's course notes discuss the problem of preventing rigid bodies from inter-penetrating as they move about an environment and addresses the issues of how to model instantaneous changes in velocity of a rigid body and how to calculate the point of contact using information that only indicates a penetration after it has already occurred. Key Ideas * There are two main problems that need to be solved when dealing with object collision: computing velocity changes for colliding contact, and computing the contact forces that prevent inter-penetration. *Penetration is detected using a system driven by separating planes controlled by the complex polyhedra that represent the objects. Bounding boxes are used to minimize the number of pairwise collision/contact determinations necessary. * Bisection can be used to solve for the time that an object collides given some initial time and the time-step that leads to state where inter-penetration is detected. This does not find the exact value for the time of collision, but can determine a value within a certain numerical tolerance. * The simulator described in the paper runs the simulation backward to determine the point of contact once inter-penetration occurs. A force impulse is then added directly to the velocity of the object at that point, and then the simulation continues as normal.