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.
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.
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.
Very common that replay/demo uses the network stack of it's present in a game.
Nothing stops you from adding a PRNG seed parameter to initialize your deterministic game engine.
But if you're running a multiplayer game with random elements and aren't colluding, you don't have to let a malicious party set the RNG seed to whatever they like just because you agree on it at game start. There's any number of simple cryptographic protocols that allow each peer to contribute equally to the RNG state based on having a separate commitment phase. And it's a lot easier to run a quick cryptographic setup than it is to have constant input-driven adjustment.
DonHopkins on Feb 16, 2022 | parent | context | favorite | on: Don't use text pixelation to redact sensitive info...
When I implemented the pixelation censorship effect in The Sims 1, I actually injected some random noise every frame, so it made the pixels shimmer, even when time was paused. That helped make it less obvious that it wasn't actually censoring penises, boobs, vaginas, and assholes, because the Sims were actually more like smooth Barbie dolls or GI-Joes with no actual naughty bits to censor, and the players knowing that would have embarrassed the poor Sims.
The pixelized naughty bits censorship effect was more intended to cover up the humiliating fact that The Sims were not anatomically correct, for the benefit of The Sims own feelings and modesty, by implying that they were "fully functional" and had something to hide, not to prevent actual players from being shocked and offended and having heart attacks by being exposed to racy obscene visuals, because their actual junk that was censored was quite G-rated. (Or rather caste-rated.)
But when we later developed The Sims Online based on the original The Sims 1 code, its use of pseudo random numbers initially caused the parallel simulations that were running in lockstep on the client and headless server to diverge (causing terribly subtle hard-to-track-down bugs), because the headless server wasn't rendering the randomized pixelization effect but the client was, so we had to fix the client to use a separate user interface pseudo random number generator that didn't have any effect on the simulation's deterministic pseudo random number generator.
[4/6] The Sims 1 Beta clip ♦ "Dana takes a shower, Michael seeks relief" ♦ March 1999:
https://www.youtube.com/watch?v=ma5SYacJ7pQ
(You can see the shimmering while Michael holds still while taking a dump. This is an early pre-release so he doesn't actually take his pants off, so he's really just sitting down on the toilet and pooping his pants. Thank God that's censored! I think we may have actually shipped with that "bug", since there was no separate texture or mesh for the pants to swap out, and they could only be fully nude or fully clothed, so that bug was too hard to fix, closed as "works as designed", and they just had to crap in their pants.)
Will Wright on Sex at The Sims & Expansion Packs:
https://www.youtube.com/watch?v=DVtduPX5e-8
The other nasty bug involving pixelization that we did manage to fix before shipping, but that I unfortunately didn't save any video of, involved the maid NPC, who was originally programmed by a really brilliant summer intern, but had a few quirks:
A Sim would need to go potty, and walk into the bathroom, pixelate their body, and sit down on the toilet, then proceed to have a nice leisurely bowel movement in their trousers. In the process, the toilet would suddenly become dirty and clogged, which attracted the maid into the bathroom (this was before "privacy" was implemented).
She would then stroll over to toilet, whip out a plunger from "hammerspace" [1], and thrust it into the toilet between the pooping Sim's legs, and proceed to move it up and down vigorously by its wooden handle. The "Unnecessary Censorship" [2] strongly implied that the maid was performing a manual act of digital sex work. That little bug required quite a lot of SimAntics [3] programming to fix!
[1] Hammerspace: https://tvtropes.org/pmwiki/pmwiki.php/Main/Hammerspace
[2] Unnecessary Censorship: https://www.youtube.com/watch?v=6axflEqZbWU
[3] SimAntics: https://news.ycombinator.com/item?id=22987435 and https://simstek.fandom.com/wiki/SimAntics
Btw video number [2] is private.
It is fun to point at a chart and confidently state “We’re here! I reckon...”
I first learned of it in some writing about a 1997 multiplayer game called, heh, Dead Reckoning.
If you get the chance, you can see some of Harrison's chronometers at the Royal Observatory in London, though I don't know if they're always on display.
I'll add a recommendation for Sextant by David Barrie.
"On Trails" by Robert Moore that discusses how walking paths from the first peoples persist, grow and change over hundreds of years, along with advances in walking trail design in recent years to become a part time recreational activity vs the pure utility of terrain traversal as they first were. Covers how a trail is a "living thing", as it were, because any who tread on it help reinforce it. Covers non human trails like ants and their reenforcement via pheromones and the like.
Building a simulation that has perfect determinism is incredibly time consuming. Incredibly. Especially one that is identical across platforms, chipsets, and architectures.
Deterministic simulation replay also breaks anytime you change the simulation. Which is kind of obvious. But quite meaningful.
In any case, I’ve shipped games that use both solutions. And let me tell you, deterministic simulation from input is an order of magnitude more effort to build, test, and maintain!
https://github.com/ESWAT/john-carmack-plan-archive/blob/mast...
If you wanted to add random critical hits and random bullet spread based on the pixels in a live feed of a lava lamp cam, clients could still record .dem files and they would still work.
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Sure after you build a sophisticated the system that supports that, then you "just" do as you described. EASY!
Quake was completely different. The client/server term was aimed at describing that the game state is computed on the server, updated based on client inputs send to the server, and then the game state is sent from server to the clients for display. Various optimizations apply.
Deterministic/lockstep games more often used host/guest terminology to indicate that a machine was acting as coordinator/owner of the game, but none of them were serving state to others. This terminology is not strict and anyone could use those terms however they wanted, but it is a good ballpark.
Then you record the messages as they are recieved, and if networked, tx and rx the messages in the main pump loop.
If not networked, everything still works as normal: game engine itself never knows the difference.