Bullet is an open source software multi-threaded 3D Collision Detection, Soft Body and Rigid Body Dynamics Library (physics engine). It is published under the zlib license making it free for commercial use. Its main author, Erwin Coumans, previously worked for the Havok project. The Bullet physics engine is used by many game companies in AAA titles on PC, XBox 360, PlayStation 3 and Wii. Bullet provides rigid body dynamics for the Blender 3-D modeling, rendering, and animation package. The Bullet website also hosts a Physics Forum for general discussion around Physics Simulation for Games and Animation.