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Simulation of colliding constrained rigid bodies

Martin Kleppmann

University of Cambridge, Computer Laboratory, Tech Report UCAM-CL-TR-683, April 2007.

This paper received a Prize for best undergraduate dissertation.

This technical report is based on the dissertation for my final-year undergraduate project at university. The dissertation was marked as the best dissertation in its year and was awarded the AT&T prize. The code relating to the project is available on Github, and a video of simulations produced in this project is available on YouTube.

Abstract

I describe the development of a program to simulate the dynamic behaviour of interacting rigid bodies. Such a simulation may be used to generate animations of articulated characters in 3D graphics applications. Bodies may have an arbitrary shape, defined by a triangle mesh, and may be connected with a variety of different joints. Joints are represented by constraint functions which are solved at run-time using Lagrange multipliers. The simulation performs collision detection and prevents penetration of rigid bodies by applying impulses to colliding bodies and reaction forces to bodies in resting contact.

The simulation is shown to be physically accurate and is tested on several different scenes, including one of an articulated human character falling down a flight of stairs.

An appendix describes how to derive arbitrary constraint functions for the Lagrange multiplier method. Collisions and joints are both represented as constraints, which allows them to be handled with a unified algorithm. The report also includes some results relating to the use of quaternions in dynamic simulations.