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

Throw my two pence in.

Depends on what you want, replay may be accomplished via

1. Recording video buffer and replaying later,
2. Capturing object state every frame and replaying later,

Most of the time, people want an interactive replay, so 2. is the way to go. Then depending on your constraints there are a number of ways to optimize this process

• ensure system is a deterministic simulation *, such that every input generates a consistent and expected output
• if randomicity is required, ensure random numbers may be reproduced exactly at a later time [look at seeding with Pseudo Random Number Generators PRNG, or use canned random sets]
• divide game elements into "mechanic" and "aesthetic" elements. mechanic elements affect outcome [eg column falling over and blocking path], aesthetic elements are for show and do not influence any decision making process in the system [eg visual particle effects like sparks].

It really is a fascinating topic. I remember that one launch title for original Xbox Wreckless had a good playback feature. Unfortunately, on more than one occasion the replay would screw up ;)

oh yeah, how could anyone forget Blinx Time Sweeper! great interactive replay that was incorporated into actual game mechanic!

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*= seems there are some comments regarding time stepping. i am using "simulation" here to capture this feature. at the core, your engine needs to be able to produce discrete frames of time. even if a replay frame takes longer or shorter to process than the original, the system must perceive that the same time delta has passed. this means recording the frame time-step with each recorded input, and supplying this delta to your engine clock.