Production Physics

Real engines:

Box2D (Erin Catto, 2007):

• 2D rigid body.
• Used in: Angry Birds, Crayon Physics, Limbo.
• Simple API, great docs.

Bullet (Erwin Coumans):

• 3D rigid body + soft body + cloth.
• Used in: Grand Theft Auto, Red Dead Redemption, films.
• Open source.

PhysX (NVIDIA):

• 3D + GPU acceleration.
• Used in: Unreal Engine, many AAA games.
• Free for game devs.

Havok (Microsoft):

• AAA commercial.
• Used in: Halo, Skyrim, GTA.

Rapier (Rust):

• Modern, deterministic, parallel.
• Used in: Bevy game engine.

Chipmunk2D:

• 2D rigid body, simpler than Box2D.
• Used in: many indie games.

Liquid simulation:

• SPH (smoothed particle hydrodynamics).
• PIC/FLIP.
• Used in films, VFX.

Cloth:

• Mass-spring system.
• Position-based dynamics (PBD).

Soft body:

• Tetrahedron mesh + spring + bending forces.

Performance budgets (60 FPS = 16 ms per frame):

• AAA game: 4-8 ms physics, 2 ms broad-phase, 2 ms solver.
• Optimization is critical.

Game engine integration:

• Unity: built-in (PhysX or Box2D).
• Unreal: built-in (Chaos or PhysX).
• Godot: built-in.
• Custom engines: link Box2D / Bullet.

Common bugs:

• Tunneling.
• Tower of boxes vibrating.
• Friction applied wrong direction.
• Frame-rate-dependent behavior.
• Numerical drift in long simulations.

Don't roll your own for production. Use Box2D / Bullet / PhysX. Build a toy to learn.

Modern trends:

• GPU-accelerated physics.
• Differentiable physics (for AI training).
• Real-time fluid + cloth at AAA quality.
• Procedural destruction (Chaos in Unreal).