Box2D Physics Engine

Box2D is a 2D rigid body simulation physics library for games, originally written in C++ by Erin Catto in 2007 and used in countless games including Angry Birds and Crayon Physics. Key features: continuous collision detection CCD for fast-moving objects preventing tunneling through walls and other bodies. Rigid body dynamics with circles, convex polygons, capsules, and chains (edge shapes) for collision shapes. Joint system including revolute (hinge), prismatic (slider), distance, pulley, gear, mouse, weld, and wheel joints for mechanical simulations. Contact filtering via collision groups and categories for precise control over which bodies interact. Broadphase using dynamic AABB trees for efficient collision pair detection with thousands of bodies. Solver using sequential impulse with constraint stabilization and warm starting for stable stacks. Buoyancy and fluid effects via particle system (liquid simulation) in newer versions. Island solving for parallel processing of independent body groups. Gravity and force application including linear and angular damping. Ray casting for line-of-sight, sensor detection, and projectile trajectory queries. Time stepping with fixed or variable timestep and sub-stepping for stability. Recent version 3.x rewritten in C with improved performance, better broadphase, and simplified API. Used in Unity, Cocos2d, Defold, LÖVE, Godot, and many game engines via bindings.