alt Hacker NewsI don't see why it makes a difference for this purpose that you're replaying network packets or controller inputs or any other interface to the game engine. The important thing is that there is some well-defined interface. I guess designing for networked multiplayer does probably necessitate that, but if the engine isn't deterministic it still isn't going to work.
There was a twitter thread years ago (which appears to be long gone) about how the SNES Pilot Wings pre-game demo was just a recording of controller inputs. For cartridges manufactured later in the game's life, a plane in the demo crashes rather than landing gracefully, due to a revised version of a chip in the cartridge. The inputs for the demo were never re-recorded, so the behaviour was off.
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Look into dead reckoning vs lock step for networking. Lockstep requires determinism at the simulation layer, dead reckoning can be much more tolerant of differences and latency. Quake and most action games tend to be dead reckoning (with more modern ones including time rewind and some other neat tricks).
Very common that replay/demo uses the network stack of it's present in a game.
if its deterministic lockstep, then all you need to do is record inputs and replay the inputs, since the engine itself is guaranteed to behave the same. If it's client/server and non-deterministic, then you need to record the entire state of the system at every step (which you'll naturally receive from the server) to replay. The main difference would be in how large a replay file would get; and more dynamism naturally implies more information to record. Large unit quantities in e.g. an RTS behaves more sanely with deterministic replay.
the other negative with deterministic input-based replay is what you've said -- if the engine deviates in any manner, the replay becomes invalidated. You'd have to probably ship with every version of the engine, and the replay just runs on the relevant release. Just replaying and re-recording the inputs on the new version wouldn't do anything, because the outcome behavior would inevitably out of sync with the original.
I'm also not sure how one would support scrubbing, except by also having inverse operations defined for every action or by fully-capturing state at various snapshots and replaying forward at like 10x speed.
It does make quite a big difference. The network packets received from the server in Quake will tell you exactly what state the game is in at any point in time. They contain information about the position and state of every entity and their motion, compressed via delta encoding. That means there's very little room for misinterpretation on the client side that would lead to de-sync issues. In fact clients have quite a lot of freedom in how they want to represent said game state, and can for example add animation interpolation to smoothen things out.
The example you mention of demo playback de-syncing when the circumstances slightly change, that is exactly what you get when you only record inputs from the player. Doom actually did this too for its networking model and demo playback system. That relies much more on the engine being deterministic and the runtime environment behaving consistently, because each client that replays those inputs has to run the exact same game simulation, in order for the resulting game states to match.