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

Your original idea is right, and for the really complex effects, they aren't remembered exclusively. For example, the Warcraft 3 replay system doesn't store the state of animations, or particle effects in the case of random effects, etc. Besides, MOST things can be computationally computed from a starting point in a deterministic way, so for most systems that use random variables (a particle explosion that gives a random offset, for example), all you would need is the time of the effect, and the random seed. You could then re-generate the effect without really knowing what it will end up looking like.. knowing that it is going through a deterministic code path.

Thinking of it purely conceptually, to replay a timeline of events, all you need are the user actions. The program will react exactly the same way, except in the case of random variables. In this scenario, you could either ignore the randomness (does it REALLY matter if the effects look EXACTLY the same, or can they be randomly re-generated), or store the seed value and fake the randomness.