Conceptually, how does replay work in a game?

Question

I was kind of curious as to how replay might be implemented in a game.

Initially, I thought that there would be just a command list of every player/ai action that was t

Answer 1

NVidia PhysX (a physics simulation engine that is often used in games) is capable of recording the full state of the physical scene over time. This incorporates any driving inputs from the game engine, which means you don't need to track random number seeds as others have suggested. If you take this scene dump, you can replay it in an outside tool (provided by NVidia), which is very handy for tracking down problems with your physical models. However, you could also use the same physics stream to drive your graphics engine, which would then allow you to have normal camera control, since only the physics driving the graphics have been recorded. In many games, this includes the particle effects (PhysX includes some very sophisticated particle systems.) As for sound, I'm guessing that's recorded verbatim (as a sound stream), but I'm not sure.