[ndepoel]

It wasn't really that much to do with determinism. Quake uses a client-server network model all the time, even when you're only playing a local single-player game. What the demo recording system does is capture all of the network packets that are being sent from the server to the client. When playing back a demo, all the game has to do is run a client and replay the packets that it originally received from the server. It's a very elegant system that naturally flows out of the rather forward-looking decision to build the entire engine around a robust networking model.

[anonymous_sorry]

I 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.

[ndepoel]

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.

[vvanders]

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.

[mjlee]

I used to be a professional sailor, and love finding nautical terminology in programming. At sea dead reckoning is navigating using the speed and direction of the ship, and adding tide and wind to calculate a fix based on the last known position. The term dates back to the 1600s.

It is fun to point at a chart and confidently state “We’re here! I reckon...”

[drzaiusx11]

There's a book I read a while back named "Longitude" that maps the storied quest in science to improve upon dead reckoning by devising greater and greater accuracy in time pieces used on ships. Iirc it was a fun read if anyone else finds that sort of thing interesting (as I do.)

[mjlee]

It's a great read! A story of how the scientific elite stalled progress because the right answer wasn't the one they hoped it would be, and didn't come from the sort of person they thought it should.

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.

[drzaiusx11]

Thanks for the recommendation, I'll add it to the shortlist!

[sebg]

What other books do you like?

[drzaiusx11]

Pick a subject I guess, that's a really hard question

[Jare]

I think the introduction of the term in networking simulations and games came with SIMNET https://en.wikipedia.org/wiki/SIMNET and continued more widely in the DIS https://en.wikipedia.org/wiki/Distributed_Interactive_Simula...

I first learned of it in some writing about a 1997 multiplayer game called, heh, Dead Reckoning.

[cubefox]

An interesting thing about the a lockstep solution which only considers inputs is that any RNG required in the game must be generated from the input history somehow. This could lead to players being able to manipulate their luck with extremely precise inputs.

[mackman]

The other interesting trick is you need a separate RNG for visual only affects such as particles than the one you use for the physics simulation. Depending on the game during replays, you could position the camera differently and then particle effects would render differently depend, depending on what’s on screen. Obviously that shouldn’t affect the way objects decide to break during the physics simulation.

[10000truths]

> a lockstep solution which only considers inputs

Nothing stops you from adding a PRNG seed parameter to initialize your deterministic game engine.

[cubefox]

That could lead to other subtle problems elsewhere though, because it requires synchronizing the seed. If you can't do that, it could lead to problems. E.g. when comparing offline speedruns where everyone would have a different seed. Then some players could have more luck than others even with the same inputs, which would be unfair. (Though I can't think of anything else at the moment.)

[chowells]

That's not more of a problem than synchronizing the player names at game start. It's table stakes for an online game.

[Jare]

Typical deterministic game engines will do this, send it to every machine as part of the initial game state, and also check the seed across machines on every simulation frame (or periodically) to detect desyncs.

[DonHopkins]

https://news.ycombinator.com/item?id=30359560

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

[dormento]

I love stories like these.

Btw video number [2] is private.

[cubefox]

Quite fitting then that it is called "unnecessary censorship".

[forrestthewoods]

> I don't see why it makes a difference for this purpose that you're replaying network packets or controller input

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!

[setr]

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.

[DANmode]

HN post linking to that Twitter thread!

https://news.ycombinator.com/item?id=19883714

[vor_]

Using a client-server model for single-player allows it to lean on that system for replaying state changes.

[_jackdk_]

Carmack wrote a really interesting .plan about this. It seems to be written between Q2 and Q3A, and cites the Windows message queue as a big inspiration:

https://github.com/ESWAT/john-carmack-plan-archive/blob/mast...

[Narishma]

I'm not sure that's the reason since Doom and Wolfenstein 3d before it also had such demo systems but they didn't use a client/server model.

[Jare]

Doom and Wolf3d and many other multiplayer games of the 90s (including some I worked on) were deterministic/lockstep and machines only needed to exchange inputs (in a deterministic manner ofc).

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.

[lelanthran]

It doesn't need a client server model, but it does need a message pump design.

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.

[trashface]

The engine needs to save the RNG seed too and various other details, the goal is definitely to make it as deterministic as possible (and yes saving the packets is part of that).

[syspec]

"All you have the game has to do is run the client and replay the packets"

---

Sure after you build a sophisticated the system that supports that, then you "just" do as you described. EASY!

[Rohansi]

It sounds like it would be complicated but it's really not! The server should already be sending a snapshot of the world when you connect and then stream deltas after that. If you capture all of the packets the server sends you can mock the connection to the server and it should just work because the client renders everything based on that data. You'll only need to do a bit of work to disable client input etc.

[vidalee]

League of Legends is using the same system

[alfg]

It's just capturing inputs and replaying them.

[furyofantares]

That's not true though, is what they're saying. Quake demo files are server-to-client packets, results of the simulation, not client-to-server packets, the inputs.

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.