One of the fun features that I developed for Warcraft (the RTS) was to fade the screen to grayscale when the game is paused.
Since the game uses a 256 color palette, it was only necessary to update a few bytes of data (3x256) instead of redrawing the whole screen, so the effect was quick.
I also used this trick when the game stalled due to missing network packets from other players. Initially the game would still be responsive when no messages were received so that you could still interact and send commands. After a few seconds the game would go into paused state with grayscale screen to signify the player that things were stuck. Then several seconds after that a dialog box would show allowing a player to quit the game.
This was much less disruptive than displaying a dialog box immediately on network stall.
One of my favourite things of being on HN is reading comments like this. Namely, devs who worked on games I played growing up. I absolutely love hearing stories from their past about little technical nuances like this comment. The more technical / specific, the better.
I'd honestly love to compile a book of "war stories" told by devs like netcoyote.
This is a great idea, but respectfully, if you're going to get traction you need to be the one instigating getting people to talk to you. Have a pitch, have an explicit ask, and be willing to put effort into making it happen.
Oh, and I forgot to mention that pause had to be synchronized across the network, so the pause button would pause for all players.
And in the "this is why we can't have nice things", that also introduced problems, because we didn't want a player who was losing to keep pausing the game until the winning player quit out of frustration, so I think we kept a per-player pause counter, which would only be restored if other players also paused? (I don't quite remember all the details, just that we had to prevent yet another abuse vector).
Omg I love this! I have been finding excuses to do little animation engine features that arent on the critical path of development for the sake of creative self-indulgence. Such features shipped was alpha channel based fading using the fundamental opengl fade parameter (under the hood its a linear interpolation of alpha values over 256, pieced together over a provided pair of timestamps).
I tell you what I'll do today on my dev time, I'll try implementing grayscale without aby research on pause and then compare notes (I'm assuming this wc code is available somewhere, which may be a bad assumption)
So I was able to create all the bits necessary to introduce the palette change in a similar manner (3x256 changes) on the triggerz and at the moment of truth instead of grey I got a GREEN and PURPLE fadeout (I wasnt sure if you meant rbg or rgb for the ratios so i tried both).
I also tried 128 across the board for grey, and it just made a dull fade which may be the best I can do with my method.
I think it may simply be because rather than have palletes controlled by rgb, I load predrawn sprites using sfml's sprite and texture classes. So the default rgba is 255,255,255,255 - so I have a sidequest to figure out the RIGHT WAY of applying rgb changes to predrawn sprites.
It may very well be a simple matter of "sfml does it differently" or perhaps having grey variants of all sprites and toggling. I feel there has to be a way to accomplish the fade to grey programmatically. Fun little dive tho! I'll have to post an update when I figure it out.
Hardware floating point was rare before the 486 DX and Pentiums. Not to mention that Integer<->FP conversion was slow. And division of any kind has always been slow. So you'd see a lot of fixed-point math approximations with power-of-two divisors so that you can shift-right.
I havent gotten behind the console, but thats like, exactly what I was gonna do, except precompute like 5 or 6 tween values for r,g,b between 255 and the target for greyscale.
But rather than do that and cache them for timing triggers, I kind of like the scaling down by multiplication approach.
Edit: manipulate the rgb values that is - I wouldnt have converged on those hard values on my own.
One of the things that impressed me in Quake (the first one) was the demo recording system. The system was deterministic enough that it could record your inputs/the game state and just play them back to get a gameplay video. Especially given that Quake had state of the art graphics at the time, and video playback on computers otherwise was a low-res, resource intensive affair at the time, it was way cool.
It always surprised me how few games had that feature - though a few important ones, like StarCraft, did - and it only became rarer over the years.
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.
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.
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.
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.
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...”
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.)
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.
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.
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.
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.
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.)
If you synchronize the seed at game start for speedruns, the seed is the same for everyone, and players can again manipulate their luck, so nothing was gained.
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:
(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.)
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!
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.
> 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!
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:
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.
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).
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.
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.
Instant replays that require long-term deterministic behavior have to be bit perfect in a way that is dramatically hard harder to implement if trying to also do network synchronization. The hard parts of each of those is fundamentally different and trying to do them at the same time terrifies me. I have shipped console games doing both (independently) and was responsible for de-bugging determinism.
Demo files work, but I'm talking about spectating live. The "Watch" tab was removed and the ability to just browse and spectate the top games currently being played.
I'm sure the technology still exists in the engine, but it's no longer the key feature it once was. HLTV/GOTV was launched with some fanfare back in the day.
Spectators don't watch the game on the same server that's hosting the game. The host server sends the traffic to a 'relay' on a delay, which spectators then connect to. Similarly for the HTTP streamed games, the game server is writing the data for spectators on a delay.
more specifically, the bug is due to a buffer overflow during paused chat, and presumably deviant undefined behavior between the original execution and the replayed execution.
and most importantly, it has nothing to do with the non-deterministic nature of the game -- non-determinism is trivial to account for, because PCRNGs are not actually non-deterministic; they're (pseudo)random, and deterministically so. They're just random enough to be equivalent to non-deterministic to the user.
As long as seeds are synced, it's trivially managed. But this kind of cause-for-desync is the reason deterministic engines are such a PITA to actually write. Any source of actual non-determinism wrecks everything, and there's always some miserably rare cases.
StarCraft Brood War replays would desync due to version issues, too. Any gameplay adjustments in a patch would affect the replay, often in subtle ways that would sometimes result in a valid and interesting game, just different outcome.
That's not the kind of nondeterminism that would cause replay divergence. The PRNG seed is stored in the replay (if it wasn't, almost every game would diverge very quickly. And since the multiplayer works the same basic way, the game would basically not function at all).
There was definitely sync bugs with replays at various points.
There was even desync bugs even in live multiplayer games; there was detection that it desynced which would end the game, which in turn meant exploits that would intentionally cause a desync (which would typically involve cancelled zerg buildings for some reason).
or, if you are replaying a single-player game that you saved+loaded (i.e. the replay only worked if the full game happened in one go without any loads).
This non-determinism would not and did not cause replays to diverge (the PRNG seed was most likely stored and would reproduce exactly the same results).
Checking in as a random indie developer who still prioritises determinism in my engine. I don't understand why so many games/engines sacrifice it when it has so much utility.
I think if it were as simple as "remember the RNG seed", game developers would do it every time. But determinism also means, for instance, running the physics engine at a deterministic timestep regardless of the frame rate, to avoid differences in accumulated error or collision detection. And that's something that needs designing in from day one.
> running the physics engine at a deterministic timestep
As well as using special library versions of floating-point functions which don't behave the same across different processors I suppose, if you want to be safe.
Eg cr-libm[1] or more modern alternatives like core-math[2].
It's desirable for arcade games, which have fixed hardware including the display. There's no possibility of upgrading for better framerate, and the game can be designed so slowdown is rare or non-existent. Tying the physics to the framerate gives you very low and very consistent input latency with minimum developer effort.
Right, all valid points, but consider the scale of a game like those coming out of rockstar. I'd understand for indie games and arcade games, but a single player rpg that will likely never be seen in arcade settings? Seems odd to me to see it here. Rockstar has the resources to do it properly, one would think, no?
Suppose you don't care as much about replays, and you're willing to use other tricks to "cheat" on multiplayer sync instead (because most AAA titles seem to have multiplayer these days). Suppose, instead, your top priority is visual fidelity and being perceived as having cutting-edge graphics. You want maximum computational effort going into letting the gamers with a top-of-the-line GPU render on their 360FPS monitor. And you want lots of objects and realistic physics.
If you run physics on a global timer, you could run it at a slower rate and try to fake some of those frames (extrapolating intermediate positions of objects), which is complex. Or you could run it at a faster rate, and every frame has real physics updates, and then it's taking time you could be using for graphics or something else that you think sells better. And there are ways around that, too, but they're complicated and your team is busy and they aren't what your engine gives you for free...
I think you mean timestep. The video frames get updated on one timestep (the so-called "frame rate" because it is the rate at which video frames get redrawn, the inverse of its timestep), physics gets updated on a separate timestep, and gameplay or input or network polling can be updated on its own timestep.
pretty much, over the dozen or so game and rendering engines I made over the decades name mutated from tick to timestep to frame (rate) to refresh rate (hz) to tick again.. it doesn't matter as long as every system is decoupled and rendering is unbounded (if hardware/display combo supports it). This needs thinking from day one. Cool stuff you can do then is determinism, you can do independent timers which go forward, halt, backward in time, different speed multipliers over those (so some things run slower, faster, everything goes slower / faster), etc.
Mostly because determinism is an all or nothing proposition. Either EVERYTHING in game logic is perfectly deterministic and isolated from everything else, or it pretty much as if nothing was. So if you want to commit to determinism, you have to be constantly vigilant and debugging these maddening types of bugs. Whether this investment is worth it or not is up to each dev.
Sometimes you can find small areas of the game that can be deterministic and worth it. In a basketball game I worked on in the 90s, I designed the ball physics to be deterministic (running at 100hz). The moment the ball left the player hands it ran deterministically; we knew if it was going to hit the shot and if not, where the rebound would go to.
Determinism isn't essential to achieve record/playback experiences. You could just record the transform of the player at some fixed interval and then replay it with interpolation. This is arguably more "deterministic" in a strategic sense of shipping a viable product.
The player is just one entity, you'd need to do the same to any other non-trivial entity. And you couldn't use fixed intervals and naive interpolation, otherwise you'd have entities clipping the ground when bouncing etc.
Bungies Marathon series (1994) had the same recording system, as other commenters mentioned due to networking multiplayer.
What's totally insane is that the modern engine rewrite Aleph One can also play back such old recordings, for M2 Durandal (1995) and Infinity (1996) at least.
I'm pretty sure it's because it's in fact 'just' a cool side effect to a common network architecture optimisation from the time where you could'nt send the 'state' of the entire game even with only delta modifiers and so you make the game detertministic to only synchronize inputs :) an exemple article I remember : https://www.gamedeveloper.com/programming/1500-archers-on-a-...
The main downside which probably caused the diseapearance is that any patch to the game will make the replay file unusable.
Also at the time (not sure for quake) there was often fixed framerate, today the upsides of using delta time based frame calculation AND multithreading/multi platform target probably make it harded to stay deterministic (specialy for game where you want to optimize input latency)
I think it's more the patching thing that made "collect and replay inputs" less common.
Networked games have a "tickrate", just for the networking/state aspect. For example, Counter-Strike 2 has a 64Hz tickrate by default. They also typically have a fixed time interval for physics engines. Both of these should be completely independent of framerate, because that's jittery and unpredictable.
Fun fact, overwatch must have done a similar things because they would let you play back games up until some release when you could no longer replay them unless you'd saved the render.
I think if I remember right there were also funny moments where things didn't look right after patches?
Overwatch also has “kill cams”, which basically create an entire alternate game state to show you how the enemy killed you, and they have the “Play of the Game” system that replays the coolest moment of the game at the end. It’s impressive tech.
One of the hardest determinism bugs I had to solve on the PlayStation three was that the PPU and the SPU actually used a different instruction set and had a different internal floating point register size. We had a multi threaded physics simulation, and during instant replay, we had to ensure that the job scheduler sent the exact same work to the correct cores or we got back subtly different floating point values, which of course, immediately caused major divergences.
Floating-point non-determinism issues usually come from one of these sources:
- Inconsistent use of floating-point precision, which can arise surprisingly easily if e.g. you had optional hardware acceleration support, but didn't truncate the intermediate results after every operation to match the smaller precision of the software version and other platforms.
- Treating floating-point arithmetic as associative, whether at the compiler level, the network level, or even the code level. Common violations here include: changing compiler optimization flags between versions (or similar effects from compiler bugs/changes), not annotating network messages or individual events with strictly consistent sequence numbers, and assuming it's safe to "batch" or "accumulate" deltas before applying them.
- Relying on hardware-accelerated trigonometric etc. functions, which are implemented slightly differently from FPU to FPU, since these, unlike basic arithmetic, are not strictly specified by IEEE. There are many reasons for them to vary, too, like: which argument ranges should be closest to correct, should a lookup table be used to save time or avoided to save space, after how many approximation iterations should the computation stop, etc.
In some games - most famously Doom - entire multiplayer is based on exchanging just the inputs and the games on all connected computers are deterministic enough to provide same outcome on all of them.
I am one of the authors of Fire Fight game (1996-ish) and we pulled the same stunt. It was actually easy, we just had to build our own "random number generator" and fix all bugs with uninitialized memory :-)
I love Fire Fight! I got the demo on a PC Gamer disc when I was a kid and played it forever. And couple of years ago when I built a retro PC, it was one of the first games I loaded up :)
Hey I am actually working on a browser game that is fully-deterministic (except for player inputs) and so I can basically replay games entirely. Think like a chess engine game.
I’ve been obsessing over this determinism and replayability for months, to the point where any game played is fully replay-able to the exact same events as the original game.
So you can play, then watch a recording and spectate your played games from different actors perspective (enemy perspective etc).
My rendering and game logic are fully decoupled.
I wrote the “engine” for the game from scratch. I think I only use one third party library currently.
Age of Empires 4 also does this. It's very cool and saves a lot of space, but it does have some significant downsides at least the way its implemented there - you can't rewind replays, and they become unwatchable when the game updates significantly.
The alternate to this was the first WORMS game, where, if I remember it properly, there were nondeterministic replays and the next turn picked up from the replay not the initial action
Demos were really useful for helping validate competitive play too. While certain anti-cheat programs were available such as PunkBuster (Quake 3), having gaming ladders request everyone records a demo and upload it from their POV was a very low friction way to deter cheating. The idea being, no one looked at them unless there was suspicion so it wasn't even a time sink for administrators.
No fancy kernel level anti-cheats. Just ensure matches were played on legitimate servers and demos were recorded.
Also, back then live streaming while playing was usually too much of a computational and network burden (56k modems), but casting was just coming around as being a thing and certain Quake 3 mods had spectator modes that let someone streaming spectate you from the first person live which also helped deter cheating. There was even split screen spectating modes so you can follow the action (useful for 4v4 games, etc.).
Carmack and team really made something special back then. The ideas they had and what they did with their tech on relatively low end hardware was remarkable.
Quake1 was my first love. From the old DOS version to the GLQuake to grappling hooks on Trinicom. I was amazed not only by said demo system but by QuakeC, and how heavily it was used, especially in college communities. I remember MIT and UWisc both being unreasonably productive modders in said language.
As a kid, I couldn't wait to see what came next. Sadly, Q1 was rather one of a kind, and it was many years until anything else like it showed up.
You could play back multiplayer Halo 3 matches in 3D, with a free camera. Was really interesting to see how matches played out, how you got killed and so on, and for taking cool screenshots.
Be nice if they fixed whatever bug they have on the Switch 2 that means every replay I previously downloaded[0] is unplayable and further that every time I now try and download a replay, the game crashes.
But then it'd also be nice if they fixed the "game crashes randomly when joining games" bug too.
(To give them credit, it doesn't now take 5 minutes after waking the Switch 2 before Rocket League reconnects me to the Epic servers like it did a couple of months ago...)
[0] Also the stupidly low limit on how many you can download - it's my storage cost, not yours, wtf.
Replays are very common in fighting games as well, rollback netcode gets you most of the way to a replay system already (replaying game state from inputs is a core requirement for online play)
It wasn't deterministic. It didn't record the inputs. It recorded the basic state of the objects you could see.
Deterministic game sync is a completely different approach more often used in RTS games. Quake had non-deterministic authoritative central server + clients getting an incomplete view of the world.
If memory serves well, that worked by replaying network packets, which is what some other games do as well, the problem with that approach is that for live service games unlike old games that were often "set in stone", the protocol always changes, so it's a huge maintenance burden. You either need to add conversion tools, keep maintaining backwards compatibility with older protocol versions, or you accept that replays quickly become outdated.
> ... or you accept that replays quickly become outdated
That's how Warcraft 3's fully deterministic save files would work. Old replay files would only work tied to one specific patch patch of the game.
But here's the thing: it's still a godsend while in development and it was still a godsend to players too. "Battlenet user spiritwolf beat me even though I had the upper hand, how did he do it? Let's check the replay immediately".
Also if you really think about it: if you plan for it from day one, there's not much preventing your game engine from having a pluggable system where you could have the various different patches of the game engine ship with every subsequent release of the game.
So when 1.03c is out but you want to play a replay meant for version 1.02b, the game automatically just use that version of the game engine.
The only case where this basically ain't working is if there's a patch for a security exploit: that'd probably need to be patched for good.
But for all other cases, backward compatibility for replay files / deterministic game engines is totally doable. It may not be how things are done, but it's totally doable.
> not much preventing your game engine from having a pluggable system where you could have the various different patches of the game engine ship with every subsequent release of the game
Starcraft 2 does that. It's still quite an achievement.
I worked on this for a pretty big game. We recorded the network traffic and played it back and simulated the game - so same problem with patches. It also has the awkward side effect of exposing a metric crap ton of “join in progress” style bugs because our game didn’t support JiP.
I had a puzzle game were all of the solutions it would show were playbacks of my keypresses as I solved it myself. As the puzzles got more difficult it got harder and harder to record a solution without having pauses to think about what to do next.
I don't mean this in an "I know better" way, just genuine curiosity: why couldn't you record a solution with pauses and then strip them from the replay file?
I always wondered how NES games, which were notoriously low memory, could have game simulation on the start screens. Think Super Mario Bros, but there are many others. If no input is received at the start menu, the game starts playing a demo run. You always see videos and posts about how developers were dissecting sprites and swapping color pallets to work around the small memory, so how in the heck did they manage having the gameplay demos?
I wrote about it here many times over the years but in 1991 I wrote a little DOS game (and I had a publisher and a deal but it never came out and yet it's how my career started but that's another story) and at some point I had an "impossible to find" bug because it was so hard to reproduce.
So I modified my game engine to be entirely deterministic: I'd record "random seed + player input + frame at which user(s) [two players but non-networked] input was happening". With that I could make tiny save files and replay (and I did find my "impossible to find" bug thanks to that).
First time I remember someone talking about it was a Gamasutra article by an Age of Empire dev (article which another poster already mentioned here in this thread): they had a 100% deterministic engine. FWIW I wrote an email to the author of that article back then and we discussed deterministic game engines.
Warcraft 3 definitely had a deterministic game engine: save files, even for 8 players (networked) games were tiny. But then you had another issue: when units, over different patches, would be "nerfed" to balance the game (or any other engine change really), your replay files wouldn't play correctly anymore. The game wouldn't bother shipping with older engines: no backward compatibility for replay files.
I had a fully deterministic game engine in 1991 and, funnily enough, a few days ago with the help of Claude Code CLI / Sonnet 4.6 I compiled that old game of mine again (I may put it on a public repo one day): I still had the source files and assets after all those years, but not the tooling anymore (no more MASM / no more linker) so I had to "fight" a bit (for example I had not one but two macros who now clashed with macros/functions used by the assembler: "incbin" and another one I forgot) to be able to compile it again (now using UASM, to compile for DOS but from Linux).
Another fun sidenote... A very good friends of mine wrote "World Rally Fever" (published by Team 17) and I was a beta tester of the game. Endless discussion with my friend because I was pissed off for his engine was so "non-deterministic" than hitting the Turbo button on my 486 (I think it was a 486) while I was playing the game would change the behavior of the (computer) opponents.
To me a deterministic game engine, unless you're a massively networked multi-player game, just makes sense.
Blizzard could do it for Warcraft 3 in 2002 for up to 8 players and hundreds of units. Several games had it already in the nineties.
It simplifies everything and I'd guesstimate something like 99% of all the game out there that don't do it could actually do it.
But it touches to something much more profound: state and how programmers think about state and reproducibility. Hint: most don't think about that at all.
Some do though: I was watching a Clojure conf vid the other day and they often keep hammering that "view is a function of state". And it is. That's how things are. It was true in 1991 when I wrote my DOS game, it was true for Age of Empire, Warcraft 3 and many other games. And it is still true today.
But we're in 2026 and there are still many devs insisting that "functional programming sucks" and that we should bow to the mutability gods for that is the only way and they'll fight you to death if you dare to say that "view <- fn(state)".
> I had a fully deterministic game engine in 1991 and, funnily enough, a few days ago with the help of Claude Code CLI / Sonnet 4.6 I compiled that old game of mine again (I may put it on a public repo one day): I still had the source files and assets after all those years, but not the tooling anymore (no more MASM / no more linker) so I had to "fight" a bit (for example I had not one but two macros who now clashed with macros/functions used by the assembler: "incbin" and another one I forgot) to be able to compile it again (now using UASM, to compile for DOS but from Linux).
Fun thing - I'm working on modernizing a legacy Fortran / Win32 application to something a bit more modern, and ran into similar issues with toolchain not being available anymore; and further some libraries where source is needed to compile, but only have binaries of the libraries.
Claude Code was amazing creating stubs by looking at function calls used and how, and then getting just enough in place to call existing binaries correctly; and further updating the code to be in alignment with Fortran specs that can compile on existing compilers - but it was a 'fight'.
The problem is that you have to recalculate the world at each update. In Clojure you have immutable datastructures that help. On the webstack you have React and reconciliation. All solutions come with a performance overhead. There are some interesting hybrid systems where you can try to isolate derived states and cache intermediate states (ex: Pathom for declarative state management and caching, and extension lifecycles in CLJFX for isolating UI subtrees)
Super Smash Bros Brawl does this too for replays. I remember being a child and just learning about how computers worked and being very confused at how such a long video (which I knew to be "big") could possibly fit in such a small number of "blocks" on the Wii while screenshots were larger. I think the newer games do this too but they have issues because the game can be updated and then the replays no longer work.
I’d love to hear about the 2020 release of Microsoft Flight Simulator, which had an “active pause” feature that they hyped as a big innovation for that release. You could pause and switch camera angles and see what was going on, then quickly resume. Pretty much the whole game was still interact-able, but with your plane’s position paused. It was supposed to be a nice user-friendly way to pause while you checked gauges or fiddled with cockpit settings or whatever.
It never worked. You’d pause, and the plane was frozen in place yes, but the instrument cluster would still animate and show your altitude/speed changing as if you never paused. But you couldn’t control anything until unpaused. So you’d resume, and your momentum would suddenly leap to where the accumulated deltas ended up. So if you active-paused at full throttle, you’d unpause and start going way too fast… if you active paused while stalling, you’d unpause and your speed would be near zero… you’d even consume fuel while paused.
It’s like they literally just froze the plane’s position and left every other aspect of the physics engine untouched, never tested it, shipped it, and even did a bunch of marketing at how great the feature was. When it was so obviously broken.
I came back to the game after a year or so of updates, and not a thing had improved, it was every bit as broken as when they shipped it.
The 2024 release seems to have largely fixed it though from what I can see. It’s just nuts they had such a clearly broken feature for that long.
I was going to comment on MSFS2020's Active Pause as well. It's a curious implementation, but I always assumed the behaviour was intentional. After all, there's a "regular" pause mode as well.
You gotta learn and understand its quirks, though. As long as your flight state is rock stable (e.g. on Auto Pilot) and/or you're not fiddling with the controls while paused, it's pretty much always worked fine for me.
I've also used its interactivity to my advantage and saved the plane from an otherwise unsaveable flight state, e.g. by gaining airspeed while paused.
> which had an “active pause” feature that they hyped as a big innovation for that release
So my memory might be playing me a trick, but wasn't that already the case in FS2002 and 2004? I seem to remember using the pause as a kid to look at my plane under every angle.
Too bad they didn't ask any VR developers. It's truly another beast, especially if you're developing with Unity for the Quest platform, since setting TimeScale to 0 in Unity effectively disables the physics engine, which means things like hands/controllers no longer work, which then breaks one of the requirements to even be able to release in the Meta store (handle pause state). The workaround used by Half-Life: Alyx (as told to me by the Hurricane VR developer when I asked years ago how to deal with pausing) is to clone your hands and disable/delete all physics-related stuff (e.g. Rigidbodies) on the new "paused" hands. If you are using laser pointers, then you'll also have to switch those out as well. If you have any active effects, particles or objects that obstruct the player's vision and/or visibility of the pause/resume UI, then you'll want to either disable those out or at least dim them substantially so the player can interact with the resume button e.g. with a laser pointer. You might also want to adjust the lighting to indicate that the user is paused.
Outside of VR, Unity offers a nice "AudioListener.pause" to pause all audio, but if you have any sound effects in your pause menu like when a user changes a setting, those no longer work, further requiring more hacky fun (or just set it to true, and ignore user-complaints about no audio on menus when paused).
On top of that, you have things like Animators for characters, which also have to be paused manually by setting speed to 0 (TimeScale = 0 doesn't seem to do it). Some particle systems also don't seem to be affected by TimeScale. If you have a networked game then you also have to (obviously) send the pause command to clients. If you have things in motion, pausing/restarting the physics engine might cause weird issues with (at least angular) velocity so you might have to save/restore those on any moving objects.
The strangest pause bug I know is in Mario Sunshine: pausing will misalign the collision logic (which runs 4 times per frame) and the main game loop. So certain specific physics interactions will behave differently depending on how many times the game has been paused modulo 4.
There is another great one in one of the Legend of Zelda games.
The game world is paused whenever Link pulls an item from a chest, but because his animation does not loop perfectly, because of a missing frame, he slowly slides across the ground and even through walls.
One of the minimum % speedrun abuses this by looping the animation for many hours in order to glitch through a wall, and not collect a progression item, which would count towards the collection percentages.
In the original (and maybe also DX) release of Link's Awakening, the game uses a top-down view with the world split up into tiles. Walking of the left side of a screen makes you end up on the right side of the next screen over.
What you could do is pause at the right frame on the screen transition, and you would end up on the new screen but link's position would not change. So you walk off the left side of a screen and end up on the left side of the new screen. Lots of fun to be had with skipping important stuff with that.
If it's the one in Pianta Village there is a well known glitch on that one to do with watersliding over it. I haven't heard of a general bridge glitch though.
Like a lot of issues in gamedev, pausing the game is a surprisingly difficult problem to solve.
It's especially difficult to provide a one size fits all solution which would obviously be desirable for the popular modern engines that try to be a general solution.
I see a lot of comments here saying something along the lines of "isn't it just a state in the state machine?" which isn't wrong, but is an extremely simplistic way of thinking about it.
In, say, 1983, you could get away with something like that:
- pause the game: write "PAUSED" to the tilemap
- paused main loop: check input to unpause
- unpause the game: erase the "PAUSED" / restore the previous tiles
But at that time you could already see the same sort of issues as today.
Something somewhat common in Famicom/NES games is the sprites disappearing when the game is paused.
Perhaps deliberate/desirable in some cases (e.g. Tetris) but a lot of the time, probably just a result of the 'is paused' conditional branch in the main loop skipping the sprite building code[0].
There's an extremely large problem space and ultimately, each game has its own way to define what "paused" actually means.
You might be interested in the features Godot provides[1] for this. Particularly, the thing that makes it interesting is the 'process mode' that each node in the scene tree has.
This gives the developer quite a lot of control over what pausing actually means for a given game.
It's not a complete solution, but a useful tool to help solve the various problems.
[0] Simplified description of course. Also, the sprite building code often ended up distributed throughout the various gameplay logic routines, which you don't want to run in the paused state.
[ed] Just adding that Tetris is only an example of a game where you might want that behaviour, not a comment about how any of the Tetris games were actually made.
I read your comment and the article and I’m still not really clear on why this isn’t as simple as saving the current state or pausing the render or time loop.
The short answer is: because of all the complexity that you're not seeing. I'll try to explain:
My '1983' example above is a simple way to implement a pause state -- when the game is paused, don't run the game logic.
The sprites disappearing that I described (a real quirk, if not a bug, of games of that era) is a side effect of that.
The sprites disappear because the set of objects to draw each frame is determined by the game logic, which is being skipped in order to pause the action.
Is it wrong that the game logic submits the drawables? I would say that it's inflexible, but not wrong.
After all, the sprites are representing the state of the game, so they must be derived from it in some way.
You can do more engineering to support a more robust & elegant pausing system. You can build that system such that pausing the game feels like simply "pausing the loop". But that's more code, more complexity, less simple.
I'm not exactly sure what "pausing the render loop" means to you, but let me ask you this: how do you render the pause menu? When there is no time passing, how does the little bouncy cursor in the menu continue to bounce?
You'd want sound effects to pause while the game is paused, and ideally resume from where they were when the game is unpaused. But does the pause menu make sounds? Is there music? Does the pause menu play different music or continue the in-game music?
And this is the most obvious, simplistic idea of what pausing the game is -- to show a pause menu. What about in-game cutscenes: do the NPCs in the background keep running around while you're talking to another one? Inventory management: the items the player has are a gameplay system but you access it while the game is paused?
This pattern applies to every system in the game.
You can't just cut out the xyz altogether, you need a sufficiently complicated xyz system in order to simply pause what you want to pause.
Saving the current state? What does that even mean? You don't save transient things like visual particle positions but it's something you expect to persist between pauses.
Pausing the render? But not the physics so you keep falling? You need to pause many systems. At the very least you'd want to pause gameplay, and sound and physics. You'd want to keep input and some of the UI unpaused. If you have a fancy UI with VFX, you need to make sure those are not using the paused game time. etc etc
And if you pause sound in Unity using AudioListener.pause = true, which is supposed to make life easier, but ends up being useless if changing settings/clicking buttons has audio feedback, or changing the volume has audible feedback to tell you how loud, or you allow to change voice style, and on and on.
Repeat that for every system - all those edge cases for each system can waste a lot of time and energy.
Against the Storm (and excellent rouguelite city-builder) does this in a really cool way. Pausing is a core mechanic of the game, and you frequently pause while you place building or things like that - and all the visual animations stop (fire, rain, trees swaying, etc).
But when you find a broken ancient seal in the forest, the giant creepy eyeball moving around in it keeps moving even when you pause the game, which helps emphasise how other-worldly it is.
I find it confusing: for me a clear indicator the game is paused is all animations also pausing. Some games do not pause in menu’s, for example. And some do, but not when in a multiplayer session
I think it really just depends on the game and what purposes “pausing” serves in that game. Take a game like solitaire, for example: there is no meaningful “pause” feature you could add, since the game state only advances in response to a user action.
Other pause some underlying simulation while still letting you modify the game state, as an expected part of gameplay, like a city builder. As the user might spend a significant amount of time in a paused state building things, it would be pretty visually unappealing to have the entire world completely frozen the whole time.
Others might pause all gameplay entirely, such as for displaying a menu, in which case pausing even environmental animations might make more sense since the user isn’t actively playing.
For the second type, I would much prefer some GUI element to indicate the simulation is paused rather than freezing the whole game world, such as a border around the screen or maybe a change of color theme of the GUI or similar.
Torch flames and trees swaying in the wind do not affect gameplay at all - they're most likely done in a shader and I think it's easier to keep updating a time uniform than to add extra conditions everywhere :D
That's usually because the system that runs those things is independent of the timing of the main game loop. And then when someone finally gets around to implementing the pause screen, they still run even with the main game time stopped. And you look at it and think "eh, you know what - looks cool - we'll leave it".
Pausing is unintuitive in Unity because you don't control the main loop - all active objects get updated every frame. The recommended way to do it is to set the "time scale" to zero and have menu animations use special timers that ignore time scale. If you control the game loop, you can usually just get away with an "if (paused)" [0].
ScriptX had a clock hierarchy, similar to a 2d or 3d transform hierarchy, but instead of each node just inheriting and modifying translation/scale/rotation, they also inherited and modified the "timebase" (time offset and scale).
DonHopkins on Aug 18, 2024 | root | parent | next [–]
The QuickTime API supported reverse play around 1993 or so, when I enjoyed using it for "back and forth" animated sprites, and playing music videos backwards to reveal the satanic messages with demonic backmasking.
Reverse / Backwards Moonwalk - Michael Jackson - Better Than The Original!!!
Here's the documentation and code I wrote when working at Kaleida Labs (a joint venture of Apple and IBM) on a ScriptX animation library that used QuickTime and other renderers, and depended on QuickTime's support for playing videos backwards, and single stepping forward and backwards of course:
ScriptX's QuickTime renderer would even play audio backwards and at whatever speed, too. ScriptX had excellent multimedia clock synchronization, and we would have considered it a terrible bug if ScriptX was not able to seamlessly and transparently synchronize video and audio with any arbitrary clock, time offset, and time scale, no matter what the speed and direction.
I'm sad that 31 years later, web browser video and audio players (not to mention VLC) don't support hierarchical clocks like ScriptX did for multimedia synchronization. (Each child clock can have a time offset and time scale relative to its parent, and everything's automatically kept in sync, from the simulation to the renderer.)
Here is Kaleida's (then Apple's) 1993 patent on "Synchronized clocks and media players", which has long since expired in 2013:
>Abstract: A media player and the clock which controls it are integrated into a single object. This integration may be achieved by the construct of inheritance between objects in an object oriented programming environment. A software class for player objects is established which inherits from a software class for clock objects. In this way, a player "is a" clock. This integration provides improved synchronization among different media, and simplifies design of applications which employ player objects and clock objects. Each object is synchronized to a RootClock object which operates at the speed of the fastest media player in the system. The RootClock may be separated into "low" order and "high" order components and a compare register in order to reduce interrupt overhead.
>[...] QuickTime essentially allows a multimedia player to process timebased data. Since the data are time based, QuickTime provides for the description of "time" (called time basis) for the data as well as a definition of the context for evaluating that "time." In QuickTime, a movie's or media's time basis is called its "timebase."
>A timebase is essentially a vector that defines the direction and velocity of time for a movie or media. The context for a timebase is called its time coordinate system. Conceptually, a time coordinate system provides an axis for measuring time. The time coordinate system, like a geometrical axis, is divided into units of measurement by a measurement system, called a time scale. [...]
DonHopkins on Dec 29, 2018 | parent | context | favorite | on: Steve Jobs hired a career juggler to teach program...
Oh, of course I wish Apple and IBM had made it free! But it included Apple's "crown jewels", the QuickTime player source code, and they were't going to give that away in 1995. Apple were even trepidatious about IBM having access to that source code.
Besides having proprietary decoders, it could also do some things you can't even do well with the Flash player or html video component today: like smoothly playing videos and music backwards!
Ever since the music industry switched from vinyl to CD, listening to demonic backmasking in Devil Music like the Beatles and Led Zeppelin's promotion of Satanism became much less convenient. ScriptX solved that important problem elegantly with its synchronized clock system, but today's HTML video player still hasn't, alas.
>Kaleida Lab's ScriptX (a multimedia programing language kinda like Dylan with classes) had built-in support for hierarchal clocks within the container (in the sense of "window" not "vm") hierarchy. The same way every window or node has a 2D or 3D transformation matrix, each clock has a time scale and offset relative to its parent, so anything that consumes time (like a QuickTime player, or a simulation) runs at the scaled and offset time that it inherits through its parent container. And you can move and scale each container around in time as necessary, to pause movies or simulations.) You could even set the scale to a negative number, and it played QuickTime movies backwards! (That was pretty cool in 1995 -- try playing a movie backwards in the web browser today!)
This is silly reporting with a couple of interesting stories. Forget about the technical ways of doing it. Doing it at all changes the game experience.
Pausing a game has a massive impact on the game experience. It lets you break the fourth wall experientially. Not wrong, but it changes the dynamic of the game.
Same as saving at any time does. As losing your loot or your life permanently does. Not wrong, but a hard choice that appeals to some players and not to others.
I used to pause pacman on my Atari 800 so I could run to church and sing in the choir or be an altar boy. Then I ran home and unpaused to continue. Sometimes in summer the computer over-heated and I lost everything while I was at church.
When I present TLA+ [0], I am referencing game pauses (pause buffer / item duplication Legend of Zelda exploit, Dark Souls menuing to cancel animations) and deliberate crashes as mechanics to exploit games, as those are still valid actions that can be modeled, and not doing that allows such exploits to happen.
A system is only correct relative to the transition system you wrote down. If the real system admits extra transitions that you care about (pause, crash, re-entry, partial commits), and you didn't model them, then you proved correctness of the wrong system.
Do you have any of these presentations available publicly? I'm always amused by the glitch names people come up with (force quit wrong warps, skirtless parry-walks...) and it'd be fun to see them in a TLA+ context.
While the game is paused, if a player were to click on the "level up" buttons for their skills, each click actually advanced the game by 1 frame - so it was possible for people to die etc. during a pause screen.
You also often need several tiers of pausing. For example when paused you want the current game time to be zero, but you also want to ignore only some user inputs. You don't want to disable the menu or pause/unpause actions along with player actions, also you might want to pause dialog but not music (although you might alter it for the menu).
Then there are others such as vfx that can have their own tiers, you might have something in the background that is difficult to pause, stop and/or resume.
Then there are other things such as timers etc. that will need special handling, graphics card interactions etc. When done with those you also need to be able to resume and ensure that all of the previous will continue as if nothing have ever happened.
In the same vein some other "simple" things like saving/loading the game is often anything but.
When I first played the NES the pause feature impressed me even more than did the graphics. Apparently Atari already had the feature on the 5200 console, but even as late as 1988 it felt like magic to hit a button, go and eat dinner, and an hour later resume my game with another press of the button.
Seems like a solved problem for consoles, at least. On the Nintendo switch you can "pause" any game regardless of if the devs implemented it by pressing the home button which suspends the entire game at the OS level
If by solved you mean it's a feature you're required to support... It can never be truly seamless when things like wall clock or device state (SD card is missing suddenly) or network connections disappear.
That is different, because you can't interact with the game anymore. In game pause can let you change your settings or map for example. Menus and map are still running in the game loop, so then you need to make they get the input events but not the gameplay part.
Nintendo, like all other platform-owners (e.g. Meta/Quest, Sony, Microsoft) is VERY strict about games released on their platform and have very strict requirements before anything is allowed to be sold with the Nintendo label. I very highly doubt they let devs NOT implement the pause ability. AFAIK you can't just OS-pause a game and expect it to run fine when it resumes, there are soooo many systems at play: animation, physics, sound, input, etc. that need to be cleanly stopped/resumed that I doubt it's as easy as just OS-pausing.
This used to be true, but one trip to any modern e-store front should dispell the notion. So much slop. Even for arguable non-slop, so much that just rapidly crashes and is unplayable. The extent of platform certifying these days for most titles seems to be: can launch, can back out to the console top level, and maybe doesn't crash if a controller is added/removed.
One of the things I was thinking about with regards to pause and or save games, is the need to control all aspects of real time logic, with possibly stopping/resuming, and saving it to disk is how our current ways of doing async is incredibly lacking.
Unity has introduced the idea of coroutines (which were essentially yield based generators), and people started using them, and immediately encountered problems with pausing/saves.
Internally these coroutines compile down to state machines with opaque internals which are not necessarily consistent across compilers/code changes, and its very difficult to accomodate needs like pausing when using them.
From what I've seen, the usual answer is that people go back to hand-written state machines, and go through the pain of essentially goto programming to fix these issues.
It depends. If you are using `yield return new WaitForSecondsRealtime(x)` then the coroutine keeps running when you set TimeScale to 0, but that's usually a feature and not a bug, since it lets you potentially use them when paused (since async/task C# stuff was usually been a bit sketchy, though it's definitely better now).
I would prefer to understand why a paused or backgrounded game still manages to consume a ton of CPU or GPU
Like, you're still just churning away at the main game loop while literally nothing else is happening except for you waiting for me to unpause it?
Because THAT would be an actual achievement. Hell, I can suspend any process from the Unixy side of things by sending a SIGSTOP signal, for a far more perfect "pause".
If I was a game dev, I would not settle for "looks like paused, but still burning down the environment and heating up my home as a side effect"
Because the engine is still running, even in a paused state, the game still has to show something and process input. Sometimes there is a menu too, sometimes the game is not completely frozen: flames may flicker for instance.
In the article, there is a case where the game takes a screenshot and disable costly rendering, presumably to deal with this problem. But the thing is that most games are designed to to be actively played, not paused for extended periods of time and having an active pause for a couple of minutes isn't going to destroy the environment.
For backgrounding, is is common for games to run at a much slower framerate to save resources.
Early versions of Unreal Engine had these animated procedural textures that would produce sparks, fire effects, etc. The odd part is that when you paused the game, the animated textures would still animate. Presumably, the game would pause its physics engine or set the timestep to 0, but the texture updater didn't pause. I suspect it was part of the core render loop and each new iteration of the texture was some sort of filtered version of the previous frame's texture. Arguably a very early version of GPU physics.
Modern games can have the same issue. Even taking a capture of the exact graphics commands and repeating them, you'll sometimes see animated physics effects like smoke and raindrops. They're doing the work on the GPU where it's not necessarily tied to any traditional physics timestep.
I only know pausing games is funky because the highest my playstation fans ever go is pausing some games. Quite weird pausing is not just a feature of the game engine or runtime, especially as the menu and settings system seem to be totally separate in most cases anyways.
The ability to pause is extremely important in games (at least single player ones).
I hate when games are into multiplayer modes even when played in single-player campaign (e.g.: Generation Zero) and thus cannot be paused.
Another thing that I hate in this regard are unpausable cutscenes. I remember when I was playing The Witcher 3, that at last there was some cutscene advancing some plot point, and right into the middle of it The Wife™ would barge in telling me something important that would require my attention... but I cannot pause that scene so I had to miss it while I listened to her. Why, oh why, devs hate pausing cutscenes so much??
You still have to be careful about how you implement "gameplay", though. For example if at any point you read the 'system clock' to do time-based stuff like animations or physics, then when you unpause you suddenly will have a couple minutes of advance in a place where you expect fractions of a second.
the notion of using time directly in gamedev has always confused me; it feels like the correct answer should be to simply have a notion of turns, and a real-time game simply iterates over turns automatically. And then so many turns are executed per second
It's a simulation; why should clock time be involved to begin with? The only things that should care about clock time are those that exist outside the sim, e.g audio
Simulation can proceed in logical time but animation is necessarily tied to wall clock time in order to be perceived as smooth and consistent by a human viewer.
In that case, animation belongs to the same category as audio — it exists outside the simulation state.
But I’m not 100% that’s even true; in the context of replay, I imagine it’d be more appropriately part of the sim for it to scrub properly.
In the context of networked games with client-side prediction, I think it’d probably be key frames tied to logical time and intermediate frames tied to wallclock
This gets bugged if it takes longer than one tick to run an iteration of game loop. Afaik the typical solution is to have a maximum number of iterations that you'll run and reset accum if it exceeds that.
I find the notion odd that this is even a problem to be solved.
It suggests a level of control way below what I would ordinarily consider required for game development.
I have made maybe around 50 games, and I think the level of control of time has only ever gone up. Starting at move one step when I say, to move a non-integer amount when I say, to (when network stuff comes into play) return to time X and then move forward y amount.
Total self brag, one of the key foundations of my game engine is that every single instance of any object has an anchor to a timing system, and pausing can be propogated on the same cycle as the input-capture at the most granular level as desired.
for our game dev project in undergrad we built an rts game as our capstone, and we used the "slow stuff down" trick - except different systems had different clock/time systems! and so some things were still running while it was paused, which led to some weird side effects (eg money glitch)
i did the thing you're "not supposed to do" and attached everything to a world clock so everything ran at like 60fps in terms of events, so it was a real-time "turn based" system
Pausing a game is a source of many bugs. In many games some subsystems aren't paused, like particles or animated materials. In some games I even saw that the character moves forward if the forward moving button was pressed at the moment when the pause button was hit.
It surprises me, that it's so hard to do properly.
I am recently working on a "realtime with pause" style grand strategy game using my own engine (think Europa Universalis, Crusader King, Hearts of Iron).
The trick is to separate the logic simulation from other game loops (rendering, UI, input, sound, etc). So when a player pauses the game, everything else still more or less works. And the logic simulation should be able to take user "command" while being paused.
Most commands should mutate the game state and reflect in the UI immediately. A few commands that have to wait until the next tick should at least acknowledge the action result.
Hey, I'm working on a grand strategy game as well based on a fictional setting, would love to chat and compare notes if you're open to it, my email is in my profile
For my game (custom engine) I had a way to stop the game clock from advancing, while the input and draw loop kept running. It would also put the game into the "pause" input state during which only the resume button would be active.
The console cert ones are interesting but all the others are just Unity/Gamemaker/Unreal not allowing the developers to write normal code? The nonzero timestep thing is very strange
I would expect pausing to bring a game’s CPU/GPU usage down to near-zero, which won’t happen if the game keeps redundantly rendering the exact same frame. A game engine can optimize this by special casing time scale zero to simply render a single textured quad under the pause menu (which is probably what one of the commenters in TFA referred to).
You would expect it to do that, and I'd say that's a desirable behaviour, but it's not really that simple and you certainly don't get that for free.
Typically any of the common modern engines with a "time scale" variable like that are not at all optimising anything in that way.
It's likely that the physics engine won't be stepped with a zero delta time, which will reduce the time spent on physics, but that's more of a byproduct of how physics engines work[0] than an optimisation.
You would have to go out of your way to capture the scene and display it "under the pause menu" in that way.
Not saying nobody does that, just that it's not something the engine is giving you for free nor is it related to the time scale variable.
Further, doing that won't necessarily reduce resource usage.
For example, if there isn't some sleep time inserted in the main loop when in a menu, or v-sync[1] to limit the framerate, the result of the simplified scene (just the menu and the quad with the captured scene) is an extremely high framerate, which may or may not cook the hardware more than the in-game load.
[0] Typical rigidbody physics engines are only (what I'll call) predictably stable with a constant delta time (same dt every tick). And a common way to manage this is with a time accumulator main loop, that only steps physics for whole units of dt.
[1] And v-sync isn't a silver bullet. consider refresh rates, different hardware, different drivers, driver overrides.
From a lot of game jam games with custom engines, I have a pretty standard approach: I set the game clock to zero timestep, and push a pause gamestate onto the gamestate stack, where top gamestate gets all input. The trick is that you need many clocks, clocks for the gameplay/game animations, HUD animations, menu animations.
> when it was time to ship, we’d read the [Technical Requirements Checklists] and have to go back and add a special pause for when you unplug the controller
article confirms my early theory I formed when reading the title about why would pause be complicated
One game that I thought had a great "pause" feature was Dragon's Dogma, you can pause the game mid fight to use items that heal or remove status effects. At first I felt this was kind of cheesy, but due to the games inventory weight limits it actually made traveling long distance or tackling dungeons more strategic because you had to bring the appropriate items for the enemies you'd be facing and use them sparingly or you'd be in trouble. Further into the game it actually felt like it required more strategy and planning, I thought it was pretty great game design and really liked it.
Paused game does not equal paused process. It means the gameplay clock has paused.
The rendering loop continues to run. The GUI is immediate mode and is still rendered. In some games visual effects continue to be rendered. If it’s a networked game the network code will continue to run.
A paused game is not like a paused video where there is no processing going on. A paused game still has all the rendering usually going on (unless the developers got time to implement something custom), audio still needs to continue, the only thing actually paused in the game logic and gameplay elements.
Given this the game needs to slow down and lock its frame rate while paused and in menus otherwise it will run at full speed and burn a lot of CPU & GPU time. Though I would say this is not usually an issue because it gets fixed pretty quickly during development.
Now more work can be done by the developers to make the paused state this efficient, but it is all a matter of priorities, and in all cases the developers would rather spend their time making the gameplay better and fixing crash bugs. If the player wants their game to run more efficiently then they should reduce the processing demands by limiting the frame rate and lowering quality settings.
Obviously not every game is doing that level of styling, but often a lot of stuff might still be happening when a game is "paused" for various reasons e.g. aesthetics, or handling input for pause menus, which usually offer you the ability to change all the games settings, e.g. video resolution, volume levels, etc. and also provide you with audio/visual feedback when you make choices, which requires keeping the audio engine running as well as rendering to the screen @ 60 fps.
Games are multithreaded and have loads of objects running everywhere. If you're using anything that's not a custom game engine, there really isn't a single main() function that you can plop an if statement like that into.
Slowing down time applies it universally. Otherwise you're going to need that condition to every single object in the game.
It's usually not as simple as that. You'd still want to at least keep the UI alive, and you also need to continue rendering while the game is paused because the swapchain surfaces might lose their content (window resizing, changing the display mode, alt-tabbing to the desktop etc).
E.g. when you open the ingame menu, the inventory (etc) you usually want to pause the gameplay, but still want to interact with the UI. Sometimes that means that at least also some of the gameplay logic needs to remain alive (inventory management, crafting, levelling up, etc...).
There are also a lot of games which need some sort of 'active pause', e.g. the gameplay needs to stop while the user can issue commands to units (traditional example: real-time combat with pause like in Baldurs-Gate-style RPGs).
Sometimes the underlying engine also doesn't properly separate gameplay logic from rendering, e.g. you can't skip one without also skipping the other (which is an engine design bug, but similar situations may also happen up in gameplay code).
Finally: pausing and the save-game-implementation is often an afterthought, but really should be implemented as the very first thing. It's quite easy to run into the trap that a frame also needs to advance time. If the game has the concept of a fixed-duration 'game logic tick' which is independent from the frame rate you're already halfway there though, but many games simply use a variable-length game tick which is identical with the frame duration.
Yep, this comment does a good job of illustrating the (surprising) complexity of this.
I'll add that the notion of the "time scale" variable as mentioned in the article is something that's only solidified/codified since Unity and the like came about.
And at the same time, the way Unity et al. works[0] doesn't really encourage thinking about what I'd call "main loop logic" in the bottom-up way that's required to build a really robust system for managing states.
You can do it, of course, (you can write as much code as you want) but by default, everything in the scene is "active" and somewhat independent from everything else, and you don't have direct control over the various major systems.
[0] I guess I should say "worked" -- I mostly used 3.x and a little bit of early version 4 -- I'm sure it's improved but I wouldn't expect anything drastically different.
The slowing down thing sounds like a hack needed for engines that don’t give you control over the main loop.
I haven’t tried this yet, but for a custom engine I would introduce a second delta time that is set to 0 in the paused state. Multiplying with the paused-dt „bakes in“ the pause without having to sprinkle ifs everywhere. Multiplying with the conventional dt makes the thing happen even when paused (debug camera, UI animations).
I don't think it's a hack necessarily - as a well implemented time system would produce the same results at game rate = 0 as with pause.
Also there's a need for different time domains - like imagine, in a paused state the menu animations still need to play, or if the player enters a conversation, the game logic needs to pasue (depending on designer intent etc.)
Unity does this. You get scaledDeltaTime (when you set game speed to 0, it'll return 0) and unscaledDeltaTime (returns time between frames ignoring game speed). Pauseable logic uses the former. Pause menus use the latter.
A surprising number of systems break down if you pause time by passing a 0 time delta, hence why slowing down time is usually the thing that gets done. The alternative (which you imply with your suggestion) is to pass the many paused states down to every system which further complicates their implementation and is a major source of bugs.
The added fact is that many systems still need to continue running, such as audio, video, and input otherwise the program doesn't appear to respond to input and so isn't in a useable state any more.
It depends on how your timers are implemented. If they are implemented as a "rendez-vous" absolute date (as in UTC for instance - not in local time unless you want to add "eastern eggs" related to daylight saving time...), this will cause bugs. If you implement your own in-game monotonic clock that you can stop, it should be ok.
>If they are implemented as a "rendez-vous" absolute date
Do people actually do that? What's the plan for when the user sleeps their machine? All the events just inexplicably happen all at once when they wake it?
I've implemented timers that had timeouts using unix timestamps, but only for multiplayer - when a player's attempt to connect to the server times out, etc.
Inside the game loop, we would keep the global tick counter that incremented on every tick, and timeouts would be based on that rather than on UTC.
The tick counter was updated only when the game logic was actually running. Our approach to pausing was to not run the functions that handled frame updates or physics updates, and to only run the rendering functions.
Generally we would never care about actual world time other than for some timeouts like for network (as the time passes for everyone), or for easter eggs like changing the tree models for Christmas or so.
I don't think anyone serious would implement event timers based on real time.
That's sort of the point TFA. You make implementation choices that feel OK and then the time comes to implement the "trivial" pause function...
In other domains, adding the delta time of your main loop to your timers can cause (logical) clock drifts in the long term because of resolution errors.
I was suspicious of those random game developers getting quoted, and this is the pinned post of the one giving this cute silly story about slowing down game speed time:
"Announcing TORMENT HEXUS, a match-3 roguelike about putting technofascist CEOs on the wrong side of skyscraper windows!
[...]
And remember: they SHOULD be afraid of us.
#indiedev #indiegame"
I'm not sure if you're saying that your suspicions were valid or not.
Suspicious of what, exactly?
Like, yes, the most likely people to respond to such a call for "stuff to put in an article on Kotaku" are probably developers that want some publicity.
But this is hardly surprising.
Eh, I remember the myriad of both "Torture Bin Laden" as well as "Torture George Bush" flash games in the early/mid 2000s. I think it's very on brand for indie developers.
There's probably something eloquent by Hannah Arendt about how 190,000 Americans killed by health insurance companies goes unnoticed while one person killing a CEO becomes a spectre of "left wing extremism" held up as an example.
Or was it by The Joker from Batman?
Or was it when protesters in Latin America sat down blocking a road to protest environmental destruction and an American driver was so angry that he was mildly inconvenienced that he got out of his car and murdered one of them with his gun. And Joe Rogan's podcast commentary was "what did they expect?", more annoyed at the inconvenience to drivers than the murder of a human.
Or maybe when Just Stop Oil protestors threw soup and mashed potato on the glass in front of a painting, with the idea "look how angry you are at the damage to a valuable and irreplacable object, this is how angry you should be at the damage to the valuable and irreplacable environment which keeps all humans alive" and Fox News laughed at them for both damaging something important and not causing any real damage so they were ineffective. Then the judge gave them 2 years in prison on the grounds that throwing a can of soup at someone's face would be violence, so throwing it at a painting is violence. But no oil executives overseeing the Exxon Valdez disaster or the Gulf of Mexico disaster faced any jail time at all.
Or when the suffragette movement cut a painting of Venus de Milo to protest against Emmeline Pankhurst being arrested and rough-handled, and people were angrier about the harm to the painting of a woman than about the harm to a real woman.
Or when Fox News says "they aren't protesting the right way" so Kapaernik asked actual verterans how to peacefully protest respectfully and they told him to kneel during the national anthem, and the complainers didn't care a whit and said that was still the wrong way and disrespectable, and he lost his job and the president tweeted rude things about him personally, and the national football thingy made that kind of protest forbidden, almost as if the objection "protesting the wrong way" was all bullshit.
Yes, probably Hannnah Arendt could put it eloquently.
But you're right, murder is wrong, and that's all there is to it.
> There's probably something eloquent by Hannah Arendt about how 190,000 Americans killed by health insurance companies
Health insurance companies don't kill people, quite the opposite. If it weren't for health insurance, a lot more people would die. Murdering their CEOs is crazy extremism.
Physicians For A National Health Program put the figure at 200,000 people annually[1]. What's your source for saying the number is zero? When they deny claims, people die. When they override medical doctor recommendations and insist on cheaper treatments, people die. When they tangle up customers with paperwork and bureaucracy, that some people can't access the health insurance they pay for. When they take money out of the system as profit, that money isn't helping the sick. When United Healthcare spends $12M/year on lobbying[1] it isn't doing that to improve patient care.
> Murdering their CEOs is crazy extremism.
When a system doesn't have a pressure release valve, the pressure doesn't go away. When a system blocks or ignores peaceful protest, the pressure doesn't go away. The thread running through my comment is that harming humans is wrong, yes murder is wrong - but sticking a label on it and saying "leftist extremism" and then denying real issues is not helping. The system needs ways to hear people saying "things aren't fine" before those people go crazy extremist, not after.
> Physicians For A National Health Program put the figure at 200,000 people annually[1]. What's your source for saying the number is zero?
I'm not saying the number is zero. I'm saying the number is vastly negative. They are overall saving a lot of people rather than killing them. Health insurance companies are hugely net-positive.
> but sticking a label on it and saying "leftist extremism" and then denying real issues is not helping.
Talking about murdering CEOs is helping far, far less.
Imagine I believe that the Democrats are net-negative. Would this justify people saying that Democrat leaders should be murdered? Or that labelling these justifications of murder as "rightist extremism" is "not helping"?
Compared to no healthcare at all, yes, but similar could be said of Crassus' firefighting service in ancient Rome. He brought his slaves to your burning property and they stood around outside while you negotiated selling your property to Crassus at a bargain price. If you agreed, he ordered his slaves to fight the fire and you got some money. If you didn't, they let it burn and you got nothing. Crassus would be there to buy the ruins for even less if you couldn't afford to rebuild. That's a net positive for Rome compared to no fire service - fires don't spread to other buildings as often, people get something instead of nothing - but it's hardly a ringing endorsement, and it could be better.
Observation 1: you are bothered by the murder of the CEO. You dismiss the business-as-usual harms to hundreds of thousands of poor people. You consider yourself to have a good grasp of what is crazy.
Observation 2: when faced with claims that insurance companies kill people, you turn to dreaming of a world where you can talk of killing Democrat leaders. You still consider yourself to have a good grasp of what is crazy.
Complaints, letters to the editor, letters to congresspersons, achieved nothing; the murder of a CEO has achived nothing; what size event would make you notice?
> "Imagine I believe that the Democrats are net-negative"
Just feels important to say, for the record, that facts don't support that position; the Economic Policy Institute[1], and the Senate Joint Economic Committee[2] found that since 1949 the economy performs better under Democrat administrations than under Republican administrations. Job growth is greater. GDP growth is faster. Unemployment is lower. Small business creation is higher. Manufacturing investment is higher. Stock market returns are higher. Wage growth is faster. Recessions start less often.
> "Would this justify people saying that Democrat leaders should be murdered?"
First problem here is your implication that I would support the Democrats being awful and not be on the side of people objecting [although not calling for murder]. Second is the implication that I would want to silence your free speech instead of, say, supporting your right to say things I disagree with, or sarcastically mocking you. Third (or really, first) problem is that you're replying to claims that insurance company behaviour causes humans to die with "Left bad".
Pausing a game is not as simple as freezing its entire state (and saving is also not as easy as dumping the entire game state to disc, at the least you'd end up with gigabyte-sized savegames).
Many game systems need to remain active in paused state (the UI needs to remain working for example, and actions in the UI may also manipulate game state (inventory, crafting, levelling up...). There are also plenty of games with 'active-pause' where the user can issue commands to units while in paused state.
Live migration boils down to copy memory over the network, stream the page faults till you converge enough, and resume execution on the other host. It’s not a hard problem but a precise and tedious one.
Pausing a game might involve a lot of GPU contexts to freeze, network resources to pause, storage streams to pause, input handling, sound, etc. Add to that physics engine that may be tied deeply in the system and you end up with a hard problem.
What a VM does is not the role of the hypervisor, thus it can apply its hammer that works in pretty much all cases, and VMs are pretty much all the same. On the other hand, all games are bespoke with custom plugins and custom integrations, which make them the opposite of "generic pause implementation".
One of the fun features that I developed for Warcraft (the RTS) was to fade the screen to grayscale when the game is paused.
Since the game uses a 256 color palette, it was only necessary to update a few bytes of data (3x256) instead of redrawing the whole screen, so the effect was quick.
I also used this trick when the game stalled due to missing network packets from other players. Initially the game would still be responsive when no messages were received so that you could still interact and send commands. After a few seconds the game would go into paused state with grayscale screen to signify the player that things were stuck. Then several seconds after that a dialog box would show allowing a player to quit the game.
This was much less disruptive than displaying a dialog box immediately on network stall.
One of my favourite things of being on HN is reading comments like this. Namely, devs who worked on games I played growing up. I absolutely love hearing stories from their past about little technical nuances like this comment. The more technical / specific, the better.
I'd honestly love to compile a book of "war stories" told by devs like netcoyote.
Maybe I will.
Net, if you're interested, hit me up.
This is a great idea, but respectfully, if you're going to get traction you need to be the one instigating getting people to talk to you. Have a pitch, have an explicit ask, and be willing to put effort into making it happen.
Fantastic idea though, you should do it.
There are a few of these floating around for older games, but the world needs more:
Ara technica has a war stories feature on game development.
https://arstechnica.com/video/series/war-stories
For apple 2 games John Romero did a podcast. It’s decent but he seems to have stopped doing them.
https://appletimewarp.libsyn.com/ Or YouTube
Ted dabney experience has a lot of interesting interviews with older arcade game designers:
https://www.teddabneyexperience.com/episodes
Wow, thank you for sharing. If I could upvote your comment twice, I would. I'm going to enjoy this series.
Gamasutra's "Postmortem" series was great: https://web.archive.org/web/20210823172711/https://www.gamas...
Sid Meier's Memoir! is exactly that, Sid Meier wrote a memoir which is indeed mostly war stories of his involvement in making games.
Oh, and I forgot to mention that pause had to be synchronized across the network, so the pause button would pause for all players.
And in the "this is why we can't have nice things", that also introduced problems, because we didn't want a player who was losing to keep pausing the game until the winning player quit out of frustration, so I think we kept a per-player pause counter, which would only be restored if other players also paused? (I don't quite remember all the details, just that we had to prevent yet another abuse vector).
I remember something like this back when playing StarCraft (maybe Brood War, it's been a minute) online.
Palette rotation was also heavily used by Ultima & Origin games up to U8 - Pagan
Omg I love this! I have been finding excuses to do little animation engine features that arent on the critical path of development for the sake of creative self-indulgence. Such features shipped was alpha channel based fading using the fundamental opengl fade parameter (under the hood its a linear interpolation of alpha values over 256, pieced together over a provided pair of timestamps).
I tell you what I'll do today on my dev time, I'll try implementing grayscale without aby research on pause and then compare notes (I'm assuming this wc code is available somewhere, which may be a bad assumption)
WC code is likely not (legally) available, but Wolfenstein and Doom both did similar palette tricks and are documented in the Black Books for each - https://fabiensanglard.net/three_books_update/index.html
Code for those is available.
Oh rad! Thanks for the heads up. I'll do a post-dev comparison to see what I land on and what was done here.
While I don't have the original code, it's something along the lines of this:
So I was able to create all the bits necessary to introduce the palette change in a similar manner (3x256 changes) on the triggerz and at the moment of truth instead of grey I got a GREEN and PURPLE fadeout (I wasnt sure if you meant rbg or rgb for the ratios so i tried both).
I also tried 128 across the board for grey, and it just made a dull fade which may be the best I can do with my method.
I think it may simply be because rather than have palletes controlled by rgb, I load predrawn sprites using sfml's sprite and texture classes. So the default rgba is 255,255,255,255 - so I have a sidequest to figure out the RIGHT WAY of applying rgb changes to predrawn sprites.
It may very well be a simple matter of "sfml does it differently" or perhaps having grey variants of all sprites and toggling. I feel there has to be a way to accomplish the fade to grey programmatically. Fun little dive tho! I'll have to post an update when I figure it out.
From my recollection of doing fun palette stuff back in the DOS VGA days, I'm betting it was more like:
Hardware floating point was rare before the 486 DX and Pentiums. Not to mention that Integer<->FP conversion was slow. And division of any kind has always been slow. So you'd see a lot of fixed-point math approximations with power-of-two divisors so that you can shift-right.I havent gotten behind the console, but thats like, exactly what I was gonna do, except precompute like 5 or 6 tween values for r,g,b between 255 and the target for greyscale.
But rather than do that and cache them for timing triggers, I kind of like the scaling down by multiplication approach.
Edit: manipulate the rgb values that is - I wouldnt have converged on those hard values on my own.
This was a neat design choice I remember it well.
And also that my “sound card works perfectly!”
One of the things that impressed me in Quake (the first one) was the demo recording system. The system was deterministic enough that it could record your inputs/the game state and just play them back to get a gameplay video. Especially given that Quake had state of the art graphics at the time, and video playback on computers otherwise was a low-res, resource intensive affair at the time, it was way cool.
It always surprised me how few games had that feature - though a few important ones, like StarCraft, did - and it only became rarer over the years.
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.
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.
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.
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.
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...”
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.)
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.
Thanks for the recommendation, I'll add it to the shortlist!
What other books do you like?
Pick a subject I guess, that's a really hard question
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.
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.
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.
> a lockstep solution which only considers inputs
Nothing stops you from adding a PRNG seed parameter to initialize your deterministic game engine.
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.
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.)
That's not more of a problem than synchronizing the player names at game start. It's table stakes for an online game.
If you synchronize the seed at game start for speedruns, the seed is the same for everyone, and players can again manipulate their luck, so nothing was gained.
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
I love stories like these.
Btw video number [2] is private.
Quite fitting then that it is called "unnecessary censorship".
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.
Using a client-server model for single-player allows it to lean on that system for replaying state changes.
HN post linking to that Twitter thread!
https://news.ycombinator.com/item?id=19883714
> 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!
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...
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.
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.
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.
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).
"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!
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.
League of Legends is using the same system
It's just capturing inputs and replaying them.
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.
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Instant replays that require long-term deterministic behavior have to be bit perfect in a way that is dramatically hard harder to implement if trying to also do network synchronization. The hard parts of each of those is fundamentally different and trying to do them at the same time terrifies me. I have shipped console games doing both (independently) and was responsible for de-bugging determinism.
> The system was deterministic enough that it could record your inputs/the game state and just play them back to get a gameplay video.
NOT how demos work in Quake. It’s more like Quake uses a client/server architecture, and the demo is a capture of the messages.
https://www.gamers.org/dEngine/quake/Qdem/dem-1.0.2.html
Related to that is the ability to watch games using the game-client too.
This used to be a promoted feature in CS, with "HLTV/GOTV", but sadly disappeared when they moved to CS2.
Spectating in-client is such as powerful way to learn what people are doing that you can't always see even from a recording from their perspective.
> Related to that is the ability to watch games using the game-client too.
Halo 3's in-engine replay system was the high water mark of gaming for me.
Also allowed to watch games _live_! Long before streaming videos was a reality.
Ah, the good old days of watching live competition of quake through the game itself, chatting with others basically through the game console.
Pretty cool system.
I think some games allow this, I remember watching DotA 2 torunaments this way
The game engine, Source, is also using client-server architecture
Also allows maphacks, not cool.
This absolutely still exists - I have a library for reading Source 2 (CS2, Deadlock etc) demo files and streams (HTTP ones like CSTV).
https://github.com/saul/demofile-net
Demo files work, but I'm talking about spectating live. The "Watch" tab was removed and the ability to just browse and spectate the top games currently being played.
I'm sure the technology still exists in the engine, but it's no longer the key feature it once was. HLTV/GOTV was launched with some fanfare back in the day.
Guessing too much potential for abuse if the same server was handling both match and spectating.
Spectators don't watch the game on the same server that's hosting the game. The host server sends the traffic to a 'relay' on a delay, which spectators then connect to. Similarly for the HTTP streamed games, the game server is writing the data for spectators on a delay.
Absolutely crazy they haven’t revived this yet given the popularity of streaming.
Interesting you mention StarCraft. The replay feature could diverge off due to the non deterministic nature of the game.
https://news.ycombinator.com/item?id=21920508
The comment you linked to doesn't know what they are talking. (Edit: given the context, they know what they're talking about, but you don't)
A game having random mechanisms has absolutely nothing to do with whether it's deterministic.
Slay the spire is 100% deterministic, gameplay-wise. All the online poker games too.
Good call, thnaks for the correction. But there definitely was a sync bug.
Here's an old forum post discussing it
https://tl.net/forum/brood-war/352588-my-replays-are-broken
Here's a hacky patch that also documents some of the possible issues
https://liquipedia.net/starcraft/Replay_Fix_for_1.16.1
It seems like the bug is related to pausing the game.
more specifically, the bug is due to a buffer overflow during paused chat, and presumably deviant undefined behavior between the original execution and the replayed execution.
and most importantly, it has nothing to do with the non-deterministic nature of the game -- non-determinism is trivial to account for, because PCRNGs are not actually non-deterministic; they're (pseudo)random, and deterministically so. They're just random enough to be equivalent to non-deterministic to the user.
As long as seeds are synced, it's trivially managed. But this kind of cause-for-desync is the reason deterministic engines are such a PITA to actually write. Any source of actual non-determinism wrecks everything, and there's always some miserably rare cases.
StarCraft Brood War replays would desync due to version issues, too. Any gameplay adjustments in a patch would affect the replay, often in subtle ways that would sometimes result in a valid and interesting game, just different outcome.
That's not the kind of nondeterminism that would cause replay divergence. The PRNG seed is stored in the replay (if it wasn't, almost every game would diverge very quickly. And since the multiplayer works the same basic way, the game would basically not function at all).
The way I remember it was that replay playback would only break if you played a replay with a different game version than it was recorded with.
There was definitely sync bugs with replays at various points.
There was even desync bugs even in live multiplayer games; there was detection that it desynced which would end the game, which in turn meant exploits that would intentionally cause a desync (which would typically involve cancelled zerg buildings for some reason).
or, if you are replaying a single-player game that you saved+loaded (i.e. the replay only worked if the full game happened in one go without any loads).
You misinterpreted the comment you are citing.
This non-determinism would not and did not cause replays to diverge (the PRNG seed was most likely stored and would reproduce exactly the same results).
Checking in as a random indie developer who still prioritises determinism in my engine. I don't understand why so many games/engines sacrifice it when it has so much utility.
I think if it were as simple as "remember the RNG seed", game developers would do it every time. But determinism also means, for instance, running the physics engine at a deterministic timestep regardless of the frame rate, to avoid differences in accumulated error or collision detection. And that's something that needs designing in from day one.
Thank you for still prioritizing it.
> running the physics engine at a deterministic timestep
As well as using special library versions of floating-point functions which don't behave the same across different processors I suppose, if you want to be safe.
Eg cr-libm[1] or more modern alternatives like core-math[2].
[1]: https://github.com/SixTrack/crlibm
[2]: https://core-math.gitlabpages.inria.fr/
Tying physics to framerate at all is a mistake. Like, should be filed as a bug mistake.
There's no scenario in which that's desirable.
And yet even Rockstar gets it wrong. (GTA V has several framerate dependent bugs)
It's desirable for arcade games, which have fixed hardware including the display. There's no possibility of upgrading for better framerate, and the game can be designed so slowdown is rare or non-existent. Tying the physics to the framerate gives you very low and very consistent input latency with minimum developer effort.
Right, all valid points, but consider the scale of a game like those coming out of rockstar. I'd understand for indie games and arcade games, but a single player rpg that will likely never be seen in arcade settings? Seems odd to me to see it here. Rockstar has the resources to do it properly, one would think, no?
Suppose you don't care as much about replays, and you're willing to use other tricks to "cheat" on multiplayer sync instead (because most AAA titles seem to have multiplayer these days). Suppose, instead, your top priority is visual fidelity and being perceived as having cutting-edge graphics. You want maximum computational effort going into letting the gamers with a top-of-the-line GPU render on their 360FPS monitor. And you want lots of objects and realistic physics.
If you run physics on a global timer, you could run it at a slower rate and try to fake some of those frames (extrapolating intermediate positions of objects), which is complex. Or you could run it at a faster rate, and every frame has real physics updates, and then it's taking time you could be using for graphics or something else that you think sells better. And there are ways around that, too, but they're complicated and your team is busy and they aren't what your engine gives you for free...
I completely agree, but it's an easy mistake to make.
not framerate of rendering but physics running at (its own) fixed frame rate.
Every game logic update, not only physics, should run on a timer that's fully independent from the frame rate.
The only place where that doesn't matter is fixed hardware - i.e. old generation consoles, before they started to make "pro" upgrades.
> i.e. old generation consoles, before they started to make "pro" upgrades.
And before it was realistically possible to port a game to run on multiple consoles without a complete rewrite.
I think you mean timestep. The video frames get updated on one timestep (the so-called "frame rate" because it is the rate at which video frames get redrawn, the inverse of its timestep), physics gets updated on a separate timestep, and gameplay or input or network polling can be updated on its own timestep.
pretty much, over the dozen or so game and rendering engines I made over the decades name mutated from tick to timestep to frame (rate) to refresh rate (hz) to tick again.. it doesn't matter as long as every system is decoupled and rendering is unbounded (if hardware/display combo supports it). This needs thinking from day one. Cool stuff you can do then is determinism, you can do independent timers which go forward, halt, backward in time, different speed multipliers over those (so some things run slower, faster, everything goes slower / faster), etc.
Mostly because determinism is an all or nothing proposition. Either EVERYTHING in game logic is perfectly deterministic and isolated from everything else, or it pretty much as if nothing was. So if you want to commit to determinism, you have to be constantly vigilant and debugging these maddening types of bugs. Whether this investment is worth it or not is up to each dev.
Sometimes you can find small areas of the game that can be deterministic and worth it. In a basketball game I worked on in the 90s, I designed the ball physics to be deterministic (running at 100hz). The moment the ball left the player hands it ran deterministically; we knew if it was going to hit the shot and if not, where the rebound would go to.
Determinism isn't essential to achieve record/playback experiences. You could just record the transform of the player at some fixed interval and then replay it with interpolation. This is arguably more "deterministic" in a strategic sense of shipping a viable product.
The player is just one entity, you'd need to do the same to any other non-trivial entity. And you couldn't use fixed intervals and naive interpolation, otherwise you'd have entities clipping the ground when bouncing etc.
Probably (armchair HN reader, not a game developer here) due to dealing with multiplayer latency and / or performance / multithreading / scalability.
Bungies Marathon series (1994) had the same recording system, as other commenters mentioned due to networking multiplayer.
What's totally insane is that the modern engine rewrite Aleph One can also play back such old recordings, for M2 Durandal (1995) and Infinity (1996) at least.
I'm pretty sure it's because it's in fact 'just' a cool side effect to a common network architecture optimisation from the time where you could'nt send the 'state' of the entire game even with only delta modifiers and so you make the game detertministic to only synchronize inputs :) an exemple article I remember : https://www.gamedeveloper.com/programming/1500-archers-on-a-...
The main downside which probably caused the diseapearance is that any patch to the game will make the replay file unusable. Also at the time (not sure for quake) there was often fixed framerate, today the upsides of using delta time based frame calculation AND multithreading/multi platform target probably make it harded to stay deterministic (specialy for game where you want to optimize input latency)
I think it's more the patching thing that made "collect and replay inputs" less common.
Networked games have a "tickrate", just for the networking/state aspect. For example, Counter-Strike 2 has a 64Hz tickrate by default. They also typically have a fixed time interval for physics engines. Both of these should be completely independent of framerate, because that's jittery and unpredictable.
Fun fact, overwatch must have done a similar things because they would let you play back games up until some release when you could no longer replay them unless you'd saved the render.
I think if I remember right there were also funny moments where things didn't look right after patches?
Overwatch also has “kill cams”, which basically create an entire alternate game state to show you how the enemy killed you, and they have the “Play of the Game” system that replays the coolest moment of the game at the end. It’s impressive tech.
Quake 1 ran game logic and model animation at 10 ticks per second (a reasonable choice when PCs running Quake at 20fps were impressive).
Camera and linear motion of objects were interpolated to the framerate.
You don't need to tun the whole game at a fixed framerate, only the physics. That's actually common practice.
The bigger problem is that floating point math isn't deterministic. So replays need to save key frames to avoid drift.
Quake used fixed point math.
Quake needs a FPU; if that was true it would run on a 486 SX.
You're right, I must have gotten that mixed up. Sorry.
I guess floats are still mostly deterministic if you use the exact same machine code on every PC.
One of the hardest determinism bugs I had to solve on the PlayStation three was that the PPU and the SPU actually used a different instruction set and had a different internal floating point register size. We had a multi threaded physics simulation, and during instant replay, we had to ensure that the job scheduler sent the exact same work to the correct cores or we got back subtly different floating point values, which of course, immediately caused major divergences.
> I guess floats are still mostly deterministic if you use the exact same machine code on every PC.
Nope, they are not. Part of the problem is that "mostly deterministic" is a synonym for "non-deterministic".
Floating-point non-determinism issues usually come from one of these sources:
- Inconsistent use of floating-point precision, which can arise surprisingly easily if e.g. you had optional hardware acceleration support, but didn't truncate the intermediate results after every operation to match the smaller precision of the software version and other platforms.
- Treating floating-point arithmetic as associative, whether at the compiler level, the network level, or even the code level. Common violations here include: changing compiler optimization flags between versions (or similar effects from compiler bugs/changes), not annotating network messages or individual events with strictly consistent sequence numbers, and assuming it's safe to "batch" or "accumulate" deltas before applying them.
- Relying on hardware-accelerated trigonometric etc. functions, which are implemented slightly differently from FPU to FPU, since these, unlike basic arithmetic, are not strictly specified by IEEE. There are many reasons for them to vary, too, like: which argument ranges should be closest to correct, should a lookup table be used to save time or avoided to save space, after how many approximation iterations should the computation stop, etc.
In some games - most famously Doom - entire multiplayer is based on exchanging just the inputs and the games on all connected computers are deterministic enough to provide same outcome on all of them.
I am one of the authors of Fire Fight game (1996-ish) and we pulled the same stunt. It was actually easy, we just had to build our own "random number generator" and fix all bugs with uninitialized memory :-)
I love Fire Fight! I got the demo on a PC Gamer disc when I was a kid and played it forever. And couple of years ago when I built a retro PC, it was one of the first games I loaded up :)
Hey I am actually working on a browser game that is fully-deterministic (except for player inputs) and so I can basically replay games entirely. Think like a chess engine game.
I’ve been obsessing over this determinism and replayability for months, to the point where any game played is fully replay-able to the exact same events as the original game. So you can play, then watch a recording and spectate your played games from different actors perspective (enemy perspective etc).
My rendering and game logic are fully decoupled.
I wrote the “engine” for the game from scratch. I think I only use one third party library currently.
Cool to see this discussion
Age of Empires 4 also does this. It's very cool and saves a lot of space, but it does have some significant downsides at least the way its implemented there - you can't rewind replays, and they become unwatchable when the game updates significantly.
You can still rewind by storing checkpoints, resuming at the most recent before the seek time and fast forwarding from there.
The updates thing is a shame. You can store multiple configuration files for balance patches, but executable code is much harder.
The alternate to this was the first WORMS game, where, if I remember it properly, there were nondeterministic replays and the next turn picked up from the replay not the initial action
Supreme Commander 2 savefiles appear to be a list of timestamped user interface inputs and unit commands
Yeah they used determinism, few bit can be read about here:
https://gafferongames.com/post/floating_point_determinism/ https://www.forrestthewoods.com/blog/synchronous_rts_engines...
Demos were really useful for helping validate competitive play too. While certain anti-cheat programs were available such as PunkBuster (Quake 3), having gaming ladders request everyone records a demo and upload it from their POV was a very low friction way to deter cheating. The idea being, no one looked at them unless there was suspicion so it wasn't even a time sink for administrators.
No fancy kernel level anti-cheats. Just ensure matches were played on legitimate servers and demos were recorded.
Also, back then live streaming while playing was usually too much of a computational and network burden (56k modems), but casting was just coming around as being a thing and certain Quake 3 mods had spectator modes that let someone streaming spectate you from the first person live which also helped deter cheating. There was even split screen spectating modes so you can follow the action (useful for 4v4 games, etc.).
Carmack and team really made something special back then. The ideas they had and what they did with their tech on relatively low end hardware was remarkable.
Quake1 was my first love. From the old DOS version to the GLQuake to grappling hooks on Trinicom. I was amazed not only by said demo system but by QuakeC, and how heavily it was used, especially in college communities. I remember MIT and UWisc both being unreasonably productive modders in said language.
As a kid, I couldn't wait to see what came next. Sadly, Q1 was rather one of a kind, and it was many years until anything else like it showed up.
Factorio follows that method also, given its complexity it's quite an achievement
You could play back multiplayer Halo 3 matches in 3D, with a free camera. Was really interesting to see how matches played out, how you got killed and so on, and for taking cool screenshots.
Rocket League is a relatively recent game that allows match recording. It’s nice.
Be nice if they fixed whatever bug they have on the Switch 2 that means every replay I previously downloaded[0] is unplayable and further that every time I now try and download a replay, the game crashes.
But then it'd also be nice if they fixed the "game crashes randomly when joining games" bug too.
(To give them credit, it doesn't now take 5 minutes after waking the Switch 2 before Rocket League reconnects me to the Epic servers like it did a couple of months ago...)
[0] Also the stupidly low limit on how many you can download - it's my storage cost, not yours, wtf.
also trackmania, and it's a common way they use to catch cheaters as pb on leaderboard have inputs
This[0] is a good intro to the topic (see also [1] for the written report)
[0] https://www.youtube.com/watch?v=yDUdGvgmKIw
[1] https://donadigo.com/tmx1
Replays are very common in fighting games as well, rollback netcode gets you most of the way to a replay system already (replaying game state from inputs is a core requirement for online play)
It wasn't deterministic. It didn't record the inputs. It recorded the basic state of the objects you could see.
Deterministic game sync is a completely different approach more often used in RTS games. Quake had non-deterministic authoritative central server + clients getting an incomplete view of the world.
If memory serves well, that worked by replaying network packets, which is what some other games do as well, the problem with that approach is that for live service games unlike old games that were often "set in stone", the protocol always changes, so it's a huge maintenance burden. You either need to add conversion tools, keep maintaining backwards compatibility with older protocol versions, or you accept that replays quickly become outdated.
Or you bundle a copy of the engine and game content with every recording…
> ... or you accept that replays quickly become outdated
That's how Warcraft 3's fully deterministic save files would work. Old replay files would only work tied to one specific patch patch of the game.
But here's the thing: it's still a godsend while in development and it was still a godsend to players too. "Battlenet user spiritwolf beat me even though I had the upper hand, how did he do it? Let's check the replay immediately".
Also if you really think about it: if you plan for it from day one, there's not much preventing your game engine from having a pluggable system where you could have the various different patches of the game engine ship with every subsequent release of the game.
So when 1.03c is out but you want to play a replay meant for version 1.02b, the game automatically just use that version of the game engine.
The only case where this basically ain't working is if there's a patch for a security exploit: that'd probably need to be patched for good.
But for all other cases, backward compatibility for replay files / deterministic game engines is totally doable. It may not be how things are done, but it's totally doable.
> not much preventing your game engine from having a pluggable system where you could have the various different patches of the game engine ship with every subsequent release of the game
Starcraft 2 does that. It's still quite an achievement.
You might find this talk interesting: https://youtu.be/W20t1zCZv8M
It’s one of my favourites
I worked on this for a pretty big game. We recorded the network traffic and played it back and simulated the game - so same problem with patches. It also has the awkward side effect of exposing a metric crap ton of “join in progress” style bugs because our game didn’t support JiP.
The best replay feature was in "Heroes of Newerth." (DotA 1.5 in 2009)
Warcraft 3 replays couldn't jump in time, just forward very fast. HoN could do that. It was amazing.
For a few months they even made ALL replays searchable on a website. Every game of HoN played globally.
I had a puzzle game were all of the solutions it would show were playbacks of my keypresses as I solved it myself. As the puzzles got more difficult it got harder and harder to record a solution without having pauses to think about what to do next.
I don't mean this in an "I know better" way, just genuine curiosity: why couldn't you record a solution with pauses and then strip them from the replay file?
I tried but the change in behaviour immediately before and after the pause could be seen in the playback.
It's the time it takes to go "uhh, I'm stuck, I'd better pause" and then the bit before your brain kicks in following a pause.
saving rocket league replays to watch yourself play from your opponent's perspective was super helpful in 1v1
I always wondered how NES games, which were notoriously low memory, could have game simulation on the start screens. Think Super Mario Bros, but there are many others. If no input is received at the start menu, the game starts playing a demo run. You always see videos and posts about how developers were dissecting sprites and swapping color pallets to work around the small memory, so how in the heck did they manage having the gameplay demos?
The demo playback on 8-bit games was rarely more than a few seconds long, and it's just a capture of input data.
Here's Super Mario Bros's demo replay data: https://gist.github.com/1wErt3r/4048722#file-smbdis-asm-L108...
21 bytes of joypad input and 21 bytes of input timings
I wish I kept my demo files!
> The system was deterministic enough ...
I wrote about it here many times over the years but in 1991 I wrote a little DOS game (and I had a publisher and a deal but it never came out and yet it's how my career started but that's another story) and at some point I had an "impossible to find" bug because it was so hard to reproduce.
So I modified my game engine to be entirely deterministic: I'd record "random seed + player input + frame at which user(s) [two players but non-networked] input was happening". With that I could make tiny save files and replay (and I did find my "impossible to find" bug thanks to that).
First time I remember someone talking about it was a Gamasutra article by an Age of Empire dev (article which another poster already mentioned here in this thread): they had a 100% deterministic engine. FWIW I wrote an email to the author of that article back then and we discussed deterministic game engines.
Warcraft 3 definitely had a deterministic game engine: save files, even for 8 players (networked) games were tiny. But then you had another issue: when units, over different patches, would be "nerfed" to balance the game (or any other engine change really), your replay files wouldn't play correctly anymore. The game wouldn't bother shipping with older engines: no backward compatibility for replay files.
I had a fully deterministic game engine in 1991 and, funnily enough, a few days ago with the help of Claude Code CLI / Sonnet 4.6 I compiled that old game of mine again (I may put it on a public repo one day): I still had the source files and assets after all those years, but not the tooling anymore (no more MASM / no more linker) so I had to "fight" a bit (for example I had not one but two macros who now clashed with macros/functions used by the assembler: "incbin" and another one I forgot) to be able to compile it again (now using UASM, to compile for DOS but from Linux).
Another fun sidenote... A very good friends of mine wrote "World Rally Fever" (published by Team 17) and I was a beta tester of the game. Endless discussion with my friend because I was pissed off for his engine was so "non-deterministic" than hitting the Turbo button on my 486 (I think it was a 486) while I was playing the game would change the behavior of the (computer) opponents.
https://youtu.be/NhRQWNqbvTk
To me a deterministic game engine, unless you're a massively networked multi-player game, just makes sense.
Blizzard could do it for Warcraft 3 in 2002 for up to 8 players and hundreds of units. Several games had it already in the nineties.
It simplifies everything and I'd guesstimate something like 99% of all the game out there that don't do it could actually do it.
But it touches to something much more profound: state and how programmers think about state and reproducibility. Hint: most don't think about that at all.
Some do though: I was watching a Clojure conf vid the other day and they often keep hammering that "view is a function of state". And it is. That's how things are. It was true in 1991 when I wrote my DOS game, it was true for Age of Empire, Warcraft 3 and many other games. And it is still true today.
But we're in 2026 and there are still many devs insisting that "functional programming sucks" and that we should bow to the mutability gods for that is the only way and they'll fight you to death if you dare to say that "view <- fn(state)".
This explains that.
> I had a fully deterministic game engine in 1991 and, funnily enough, a few days ago with the help of Claude Code CLI / Sonnet 4.6 I compiled that old game of mine again (I may put it on a public repo one day): I still had the source files and assets after all those years, but not the tooling anymore (no more MASM / no more linker) so I had to "fight" a bit (for example I had not one but two macros who now clashed with macros/functions used by the assembler: "incbin" and another one I forgot) to be able to compile it again (now using UASM, to compile for DOS but from Linux).
Fun thing - I'm working on modernizing a legacy Fortran / Win32 application to something a bit more modern, and ran into similar issues with toolchain not being available anymore; and further some libraries where source is needed to compile, but only have binaries of the libraries.
Claude Code was amazing creating stubs by looking at function calls used and how, and then getting just enough in place to call existing binaries correctly; and further updating the code to be in alignment with Fortran specs that can compile on existing compilers - but it was a 'fight'.
The problem is that you have to recalculate the world at each update. In Clojure you have immutable datastructures that help. On the webstack you have React and reconciliation. All solutions come with a performance overhead. There are some interesting hybrid systems where you can try to isolate derived states and cache intermediate states (ex: Pathom for declarative state management and caching, and extension lifecycles in CLJFX for isolating UI subtrees)
Super Smash Bros Brawl does this too for replays. I remember being a child and just learning about how computers worked and being very confused at how such a long video (which I knew to be "big") could possibly fit in such a small number of "blocks" on the Wii while screenshots were larger. I think the newer games do this too but they have issues because the game can be updated and then the replays no longer work.
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I’d love to hear about the 2020 release of Microsoft Flight Simulator, which had an “active pause” feature that they hyped as a big innovation for that release. You could pause and switch camera angles and see what was going on, then quickly resume. Pretty much the whole game was still interact-able, but with your plane’s position paused. It was supposed to be a nice user-friendly way to pause while you checked gauges or fiddled with cockpit settings or whatever.
It never worked. You’d pause, and the plane was frozen in place yes, but the instrument cluster would still animate and show your altitude/speed changing as if you never paused. But you couldn’t control anything until unpaused. So you’d resume, and your momentum would suddenly leap to where the accumulated deltas ended up. So if you active-paused at full throttle, you’d unpause and start going way too fast… if you active paused while stalling, you’d unpause and your speed would be near zero… you’d even consume fuel while paused.
It’s like they literally just froze the plane’s position and left every other aspect of the physics engine untouched, never tested it, shipped it, and even did a bunch of marketing at how great the feature was. When it was so obviously broken.
I came back to the game after a year or so of updates, and not a thing had improved, it was every bit as broken as when they shipped it.
The 2024 release seems to have largely fixed it though from what I can see. It’s just nuts they had such a clearly broken feature for that long.
I was going to comment on MSFS2020's Active Pause as well. It's a curious implementation, but I always assumed the behaviour was intentional. After all, there's a "regular" pause mode as well.
You gotta learn and understand its quirks, though. As long as your flight state is rock stable (e.g. on Auto Pilot) and/or you're not fiddling with the controls while paused, it's pretty much always worked fine for me.
I've also used its interactivity to my advantage and saved the plane from an otherwise unsaveable flight state, e.g. by gaining airspeed while paused.
> which had an “active pause” feature that they hyped as a big innovation for that release
So my memory might be playing me a trick, but wasn't that already the case in FS2002 and 2004? I seem to remember using the pause as a kid to look at my plane under every angle.
Too bad they didn't ask any VR developers. It's truly another beast, especially if you're developing with Unity for the Quest platform, since setting TimeScale to 0 in Unity effectively disables the physics engine, which means things like hands/controllers no longer work, which then breaks one of the requirements to even be able to release in the Meta store (handle pause state). The workaround used by Half-Life: Alyx (as told to me by the Hurricane VR developer when I asked years ago how to deal with pausing) is to clone your hands and disable/delete all physics-related stuff (e.g. Rigidbodies) on the new "paused" hands. If you are using laser pointers, then you'll also have to switch those out as well. If you have any active effects, particles or objects that obstruct the player's vision and/or visibility of the pause/resume UI, then you'll want to either disable those out or at least dim them substantially so the player can interact with the resume button e.g. with a laser pointer. You might also want to adjust the lighting to indicate that the user is paused.
Outside of VR, Unity offers a nice "AudioListener.pause" to pause all audio, but if you have any sound effects in your pause menu like when a user changes a setting, those no longer work, further requiring more hacky fun (or just set it to true, and ignore user-complaints about no audio on menus when paused).
On top of that, you have things like Animators for characters, which also have to be paused manually by setting speed to 0 (TimeScale = 0 doesn't seem to do it). Some particle systems also don't seem to be affected by TimeScale. If you have a networked game then you also have to (obviously) send the pause command to clients. If you have things in motion, pausing/restarting the physics engine might cause weird issues with (at least angular) velocity so you might have to save/restore those on any moving objects.
The strangest pause bug I know is in Mario Sunshine: pausing will misalign the collision logic (which runs 4 times per frame) and the main game loop. So certain specific physics interactions will behave differently depending on how many times the game has been paused modulo 4.
There is another great one in one of the Legend of Zelda games.
The game world is paused whenever Link pulls an item from a chest, but because his animation does not loop perfectly, because of a missing frame, he slowly slides across the ground and even through walls.
One of the minimum % speedrun abuses this by looping the animation for many hours in order to glitch through a wall, and not collect a progression item, which would count towards the collection percentages.
My favorite as a kid was also in a Zelda game.
In the original (and maybe also DX) release of Link's Awakening, the game uses a top-down view with the world split up into tiles. Walking of the left side of a screen makes you end up on the right side of the next screen over.
What you could do is pause at the right frame on the screen transition, and you would end up on the new screen but link's position would not change. So you walk off the left side of a screen and end up on the left side of the new screen. Lots of fun to be had with skipping important stuff with that.
really? is one state the one where you fall through bridges? I can't play Sunshine because of that.
If it's the one in Pianta Village there is a well known glitch on that one to do with watersliding over it. I haven't heard of a general bridge glitch though.
Like a lot of issues in gamedev, pausing the game is a surprisingly difficult problem to solve. It's especially difficult to provide a one size fits all solution which would obviously be desirable for the popular modern engines that try to be a general solution.
I see a lot of comments here saying something along the lines of "isn't it just a state in the state machine?" which isn't wrong, but is an extremely simplistic way of thinking about it. In, say, 1983, you could get away with something like that:
- pause the game: write "PAUSED" to the tilemap
- paused main loop: check input to unpause
- unpause the game: erase the "PAUSED" / restore the previous tiles
But at that time you could already see the same sort of issues as today. Something somewhat common in Famicom/NES games is the sprites disappearing when the game is paused. Perhaps deliberate/desirable in some cases (e.g. Tetris) but a lot of the time, probably just a result of the 'is paused' conditional branch in the main loop skipping the sprite building code[0].
There's an extremely large problem space and ultimately, each game has its own way to define what "paused" actually means.
You might be interested in the features Godot provides[1] for this. Particularly, the thing that makes it interesting is the 'process mode' that each node in the scene tree has. This gives the developer quite a lot of control over what pausing actually means for a given game. It's not a complete solution, but a useful tool to help solve the various problems.
[0] Simplified description of course. Also, the sprite building code often ended up distributed throughout the various gameplay logic routines, which you don't want to run in the paused state.
[1] https://docs.godotengine.org/en/stable/tutorials/scripting/p...
[ed] Just adding that Tetris is only an example of a game where you might want that behaviour, not a comment about how any of the Tetris games were actually made.
I read your comment and the article and I’m still not really clear on why this isn’t as simple as saving the current state or pausing the render or time loop.
The short answer is: because of all the complexity that you're not seeing. I'll try to explain:
My '1983' example above is a simple way to implement a pause state -- when the game is paused, don't run the game logic. The sprites disappearing that I described (a real quirk, if not a bug, of games of that era) is a side effect of that. The sprites disappear because the set of objects to draw each frame is determined by the game logic, which is being skipped in order to pause the action.
Is it wrong that the game logic submits the drawables? I would say that it's inflexible, but not wrong. After all, the sprites are representing the state of the game, so they must be derived from it in some way.
You can do more engineering to support a more robust & elegant pausing system. You can build that system such that pausing the game feels like simply "pausing the loop". But that's more code, more complexity, less simple.
I'm not exactly sure what "pausing the render loop" means to you, but let me ask you this: how do you render the pause menu? When there is no time passing, how does the little bouncy cursor in the menu continue to bounce?
You'd want sound effects to pause while the game is paused, and ideally resume from where they were when the game is unpaused. But does the pause menu make sounds? Is there music? Does the pause menu play different music or continue the in-game music?
And this is the most obvious, simplistic idea of what pausing the game is -- to show a pause menu. What about in-game cutscenes: do the NPCs in the background keep running around while you're talking to another one? Inventory management: the items the player has are a gameplay system but you access it while the game is paused?
This pattern applies to every system in the game. You can't just cut out the xyz altogether, you need a sufficiently complicated xyz system in order to simply pause what you want to pause.
Saving the current state? What does that even mean? You don't save transient things like visual particle positions but it's something you expect to persist between pauses.
Pausing the render? But not the physics so you keep falling? You need to pause many systems. At the very least you'd want to pause gameplay, and sound and physics. You'd want to keep input and some of the UI unpaused. If you have a fancy UI with VFX, you need to make sure those are not using the paused game time. etc etc
And if you pause sound in Unity using AudioListener.pause = true, which is supposed to make life easier, but ends up being useless if changing settings/clicking buttons has audio feedback, or changing the volume has audible feedback to tell you how loud, or you allow to change voice style, and on and on.
Repeat that for every system - all those edge cases for each system can waste a lot of time and energy.
It’s only simple if your state machine has zero implicit state and all transitions are perfectly and precisely articulated. Good luck with that!
P.s. And once you are done achieving the above, you can then make sure you haven’t caused performance issues :)
But yes, conceptually, it’s a relatively simple idea. The devil is always in the details.
I quite like when games keep playing some visual-only animations when paused.
Like torch flames and trees swaying in the wind.
Against the Storm (and excellent rouguelite city-builder) does this in a really cool way. Pausing is a core mechanic of the game, and you frequently pause while you place building or things like that - and all the visual animations stop (fire, rain, trees swaying, etc).
But when you find a broken ancient seal in the forest, the giant creepy eyeball moving around in it keeps moving even when you pause the game, which helps emphasise how other-worldly it is.
I find it confusing: for me a clear indicator the game is paused is all animations also pausing. Some games do not pause in menu’s, for example. And some do, but not when in a multiplayer session
I think it really just depends on the game and what purposes “pausing” serves in that game. Take a game like solitaire, for example: there is no meaningful “pause” feature you could add, since the game state only advances in response to a user action.
Other pause some underlying simulation while still letting you modify the game state, as an expected part of gameplay, like a city builder. As the user might spend a significant amount of time in a paused state building things, it would be pretty visually unappealing to have the entire world completely frozen the whole time.
Others might pause all gameplay entirely, such as for displaying a menu, in which case pausing even environmental animations might make more sense since the user isn’t actively playing.
For the second type, I would much prefer some GUI element to indicate the simulation is paused rather than freezing the whole game world, such as a border around the screen or maybe a change of color theme of the GUI or similar.
Torch flames and trees swaying in the wind do not affect gameplay at all - they're most likely done in a shader and I think it's easier to keep updating a time uniform than to add extra conditions everywhere :D
I'm the opposite, it drives me crazy! Along with sound effects / music playing.
That's usually because the system that runs those things is independent of the timing of the main game loop. And then when someone finally gets around to implementing the pause screen, they still run even with the main game time stopped. And you look at it and think "eh, you know what - looks cool - we'll leave it".
It's nice when a bug manifests as a feature.
Pausing is unintuitive in Unity because you don't control the main loop - all active objects get updated every frame. The recommended way to do it is to set the "time scale" to zero and have menu animations use special timers that ignore time scale. If you control the game loop, you can usually just get away with an "if (paused)" [0].
[0] https://github.com/rameshvarun/marble-mouse/blob/8b25684a815...
ScriptX had a clock hierarchy, similar to a 2d or 3d transform hierarchy, but instead of each node just inheriting and modifying translation/scale/rotation, they also inherited and modified the "timebase" (time offset and scale).
https://en.wikipedia.org/wiki/ScriptX
So time scale 0 meant paused relative to the parent time scale, and time scale -1 meant time flowed backwards, which was actually quite useful!
https://news.ycombinator.com/item?id=41283223
DonHopkins on Aug 18, 2024 | root | parent | next [–]
The QuickTime API supported reverse play around 1993 or so, when I enjoyed using it for "back and forth" animated sprites, and playing music videos backwards to reveal the satanic messages with demonic backmasking.
Reverse / Backwards Moonwalk - Michael Jackson - Better Than The Original!!!
https://www.youtube.com/watch?v=IT_Aq2FjJo0
Here's the documentation and code I wrote when working at Kaleida Labs (a joint venture of Apple and IBM) on a ScriptX animation library that used QuickTime and other renderers, and depended on QuickTime's support for playing videos backwards, and single stepping forward and backwards of course:
ScriptX Animation Library:
https://www.donhopkins.com/home/catalog/lang/scriptx/anim.ht...
ScriptX Animation Implementation Module:
https://donhopkins.com/home/archive/scriptx/animimp.sx
ScriptX's QuickTime renderer would even play audio backwards and at whatever speed, too. ScriptX had excellent multimedia clock synchronization, and we would have considered it a terrible bug if ScriptX was not able to seamlessly and transparently synchronize video and audio with any arbitrary clock, time offset, and time scale, no matter what the speed and direction.
I'm sad that 31 years later, web browser video and audio players (not to mention VLC) don't support hierarchical clocks like ScriptX did for multimedia synchronization. (Each child clock can have a time offset and time scale relative to its parent, and everything's automatically kept in sync, from the simulation to the renderer.)
Here is Kaleida's (then Apple's) 1993 patent on "Synchronized clocks and media players", which has long since expired in 2013:
https://patents.google.com/patent/US5452435A/en
>Abstract: A media player and the clock which controls it are integrated into a single object. This integration may be achieved by the construct of inheritance between objects in an object oriented programming environment. A software class for player objects is established which inherits from a software class for clock objects. In this way, a player "is a" clock. This integration provides improved synchronization among different media, and simplifies design of applications which employ player objects and clock objects. Each object is synchronized to a RootClock object which operates at the speed of the fastest media player in the system. The RootClock may be separated into "low" order and "high" order components and a compare register in order to reduce interrupt overhead.
>[...] QuickTime essentially allows a multimedia player to process timebased data. Since the data are time based, QuickTime provides for the description of "time" (called time basis) for the data as well as a definition of the context for evaluating that "time." In QuickTime, a movie's or media's time basis is called its "timebase."
>A timebase is essentially a vector that defines the direction and velocity of time for a movie or media. The context for a timebase is called its time coordinate system. Conceptually, a time coordinate system provides an axis for measuring time. The time coordinate system, like a geometrical axis, is divided into units of measurement by a measurement system, called a time scale. [...]
https://news.ycombinator.com/item?id=18781950
DonHopkins on Dec 29, 2018 | parent | context | favorite | on: Steve Jobs hired a career juggler to teach program...
Oh, of course I wish Apple and IBM had made it free! But it included Apple's "crown jewels", the QuickTime player source code, and they were't going to give that away in 1995. Apple were even trepidatious about IBM having access to that source code.
Besides having proprietary decoders, it could also do some things you can't even do well with the Flash player or html video component today: like smoothly playing videos and music backwards!
Ever since the music industry switched from vinyl to CD, listening to demonic backmasking in Devil Music like the Beatles and Led Zeppelin's promotion of Satanism became much less convenient. ScriptX solved that important problem elegantly with its synchronized clock system, but today's HTML video player still hasn't, alas.
https://en.wikipedia.org/wiki/Backmasking
https://www.youtube.com/watch?v=5BDh_j5qAJ4
Here's a description of ScriptX's clock system:
https://news.ycombinator.com/item?id=18350511
>Kaleida Lab's ScriptX (a multimedia programing language kinda like Dylan with classes) had built-in support for hierarchal clocks within the container (in the sense of "window" not "vm") hierarchy. The same way every window or node has a 2D or 3D transformation matrix, each clock has a time scale and offset relative to its parent, so anything that consumes time (like a QuickTime player, or a simulation) runs at the scaled and offset time that it inherits through its parent container. And you can move and scale each container around in time as necessary, to pause movies or simulations.) You could even set the scale to a negative number, and it played QuickTime movies backwards! (That was pretty cool in 1995 -- try playing a movie backwards in the web browser today!)
Is it possible to play HTML5 video in reverse?
https://stackoverflow.com/questions/5277293/is-it-possible-t...
Kinda, but it's not smooth enough to sing along while dancing backwards and worshiping Satan to:
https://codepen.io/adrianparr/pen/qmCek
This is silly reporting with a couple of interesting stories. Forget about the technical ways of doing it. Doing it at all changes the game experience.
Pausing a game has a massive impact on the game experience. It lets you break the fourth wall experientially. Not wrong, but it changes the dynamic of the game.
Same as saving at any time does. As losing your loot or your life permanently does. Not wrong, but a hard choice that appeals to some players and not to others.
I used to pause pacman on my Atari 800 so I could run to church and sing in the choir or be an altar boy. Then I ran home and unpaused to continue. Sometimes in summer the computer over-heated and I lost everything while I was at church.
Lessons learnt? None, I think :)
When I present TLA+ [0], I am referencing game pauses (pause buffer / item duplication Legend of Zelda exploit, Dark Souls menuing to cancel animations) and deliberate crashes as mechanics to exploit games, as those are still valid actions that can be modeled, and not doing that allows such exploits to happen.
A system is only correct relative to the transition system you wrote down. If the real system admits extra transitions that you care about (pause, crash, re-entry, partial commits), and you didn't model them, then you proved correctness of the wrong system.
[0] https://lamport.azurewebsites.net/video/videos.html
Do you have any of these presentations available publicly? I'm always amused by the glitch names people come up with (force quit wrong warps, skirtless parry-walks...) and it'd be fun to see them in a TLA+ context.
There used to be a funny bug in Dota 2:
While the game is paused, if a player were to click on the "level up" buttons for their skills, each click actually advanced the game by 1 frame - so it was possible for people to die etc. during a pause screen.
You also often need several tiers of pausing. For example when paused you want the current game time to be zero, but you also want to ignore only some user inputs. You don't want to disable the menu or pause/unpause actions along with player actions, also you might want to pause dialog but not music (although you might alter it for the menu).
Then there are others such as vfx that can have their own tiers, you might have something in the background that is difficult to pause, stop and/or resume.
Then there are other things such as timers etc. that will need special handling, graphics card interactions etc. When done with those you also need to be able to resume and ensure that all of the previous will continue as if nothing have ever happened.
In the same vein some other "simple" things like saving/loading the game is often anything but.
When I first played the NES the pause feature impressed me even more than did the graphics. Apparently Atari already had the feature on the 5200 console, but even as late as 1988 it felt like magic to hit a button, go and eat dinner, and an hour later resume my game with another press of the button.
Seems like a solved problem for consoles, at least. On the Nintendo switch you can "pause" any game regardless of if the devs implemented it by pressing the home button which suspends the entire game at the OS level
If by solved you mean it's a feature you're required to support... It can never be truly seamless when things like wall clock or device state (SD card is missing suddenly) or network connections disappear.
That is different, because you can't interact with the game anymore. In game pause can let you change your settings or map for example. Menus and map are still running in the game loop, so then you need to make they get the input events but not the gameplay part.
Nintendo, like all other platform-owners (e.g. Meta/Quest, Sony, Microsoft) is VERY strict about games released on their platform and have very strict requirements before anything is allowed to be sold with the Nintendo label. I very highly doubt they let devs NOT implement the pause ability. AFAIK you can't just OS-pause a game and expect it to run fine when it resumes, there are soooo many systems at play: animation, physics, sound, input, etc. that need to be cleanly stopped/resumed that I doubt it's as easy as just OS-pausing.
This used to be true, but one trip to any modern e-store front should dispell the notion. So much slop. Even for arguable non-slop, so much that just rapidly crashes and is unplayable. The extent of platform certifying these days for most titles seems to be: can launch, can back out to the console top level, and maybe doesn't crash if a controller is added/removed.
One of the things I was thinking about with regards to pause and or save games, is the need to control all aspects of real time logic, with possibly stopping/resuming, and saving it to disk is how our current ways of doing async is incredibly lacking.
Unity has introduced the idea of coroutines (which were essentially yield based generators), and people started using them, and immediately encountered problems with pausing/saves.
Internally these coroutines compile down to state machines with opaque internals which are not necessarily consistent across compilers/code changes, and its very difficult to accomodate needs like pausing when using them.
From what I've seen, the usual answer is that people go back to hand-written state machines, and go through the pain of essentially goto programming to fix these issues.
I understand why coroutines are an issue for saving, but why are they an issue for pausing? Don't you just not resume the coroutines while paused?
It depends. If you are using `yield return new WaitForSecondsRealtime(x)` then the coroutine keeps running when you set TimeScale to 0, but that's usually a feature and not a bug, since it lets you potentially use them when paused (since async/task C# stuff was usually been a bit sketchy, though it's definitely better now).
Some more info here (and in general about pausing in Unity): https://gamedevbeginner.com/the-right-way-to-pause-the-game-...
I would prefer to understand why a paused or backgrounded game still manages to consume a ton of CPU or GPU
Like, you're still just churning away at the main game loop while literally nothing else is happening except for you waiting for me to unpause it?
Because THAT would be an actual achievement. Hell, I can suspend any process from the Unixy side of things by sending a SIGSTOP signal, for a far more perfect "pause".
If I was a game dev, I would not settle for "looks like paused, but still burning down the environment and heating up my home as a side effect"
Because the engine is still running, even in a paused state, the game still has to show something and process input. Sometimes there is a menu too, sometimes the game is not completely frozen: flames may flicker for instance.
In the article, there is a case where the game takes a screenshot and disable costly rendering, presumably to deal with this problem. But the thing is that most games are designed to to be actively played, not paused for extended periods of time and having an active pause for a couple of minutes isn't going to destroy the environment.
For backgrounding, is is common for games to run at a much slower framerate to save resources.
But if you were a game dev, you would understand why it‘s not as easy as it seems to outsiders. :)
I'm not saying it would be trivial, but I bet that once you figure out a workable pattern, you could replicate that on other games.
One idea that might be relatively easy to implement- Slow the framerate down to something super slow instead of fully stopping
Ah, but now your pause menu feels like total garbage to use!
Early versions of Unreal Engine had these animated procedural textures that would produce sparks, fire effects, etc. The odd part is that when you paused the game, the animated textures would still animate. Presumably, the game would pause its physics engine or set the timestep to 0, but the texture updater didn't pause. I suspect it was part of the core render loop and each new iteration of the texture was some sort of filtered version of the previous frame's texture. Arguably a very early version of GPU physics.
Modern games can have the same issue. Even taking a capture of the exact graphics commands and repeating them, you'll sometimes see animated physics effects like smoke and raindrops. They're doing the work on the GPU where it's not necessarily tied to any traditional physics timestep.
I only know pausing games is funky because the highest my playstation fans ever go is pausing some games. Quite weird pausing is not just a feature of the game engine or runtime, especially as the menu and settings system seem to be totally separate in most cases anyways.
It is a feature of most game engines. Unreal has a standard SetGamePaused function, for example.
But like most things in game development there is no solution that fits every use case.
The ability to pause is extremely important in games (at least single player ones).
I hate when games are into multiplayer modes even when played in single-player campaign (e.g.: Generation Zero) and thus cannot be paused.
Another thing that I hate in this regard are unpausable cutscenes. I remember when I was playing The Witcher 3, that at last there was some cutscene advancing some plot point, and right into the middle of it The Wife™ would barge in telling me something important that would require my attention... but I cannot pause that scene so I had to miss it while I listened to her. Why, oh why, devs hate pausing cutscenes so much??
So the simple case is using some sort of state variable:
switch(game_state):
You still have to be careful about how you implement "gameplay", though. For example if at any point you read the 'system clock' to do time-based stuff like animations or physics, then when you unpause you suddenly will have a couple minutes of advance in a place where you expect fractions of a second.This is why delta time accumulator is preferred over using clocks, something like this would be best:
the notion of using time directly in gamedev has always confused me; it feels like the correct answer should be to simply have a notion of turns, and a real-time game simply iterates over turns automatically. And then so many turns are executed per second
It's a simulation; why should clock time be involved to begin with? The only things that should care about clock time are those that exist outside the sim, e.g audio
Simulation can proceed in logical time but animation is necessarily tied to wall clock time in order to be perceived as smooth and consistent by a human viewer.
In that case, animation belongs to the same category as audio — it exists outside the simulation state.
But I’m not 100% that’s even true; in the context of replay, I imagine it’d be more appropriately part of the sim for it to scrub properly.
In the context of networked games with client-side prediction, I think it’d probably be key frames tied to logical time and intermediate frames tied to wallclock
That's exactly how I do it. Makes the most sense to me.
This gets bugged if it takes longer than one tick to run an iteration of game loop. Afaik the typical solution is to have a maximum number of iterations that you'll run and reset accum if it exceeds that.
Suddenly I realize why so many games didn't have a real pause feature, but a view maps feature that did the same.
I find the notion odd that this is even a problem to be solved.
It suggests a level of control way below what I would ordinarily consider required for game development.
I have made maybe around 50 games, and I think the level of control of time has only ever gone up. Starting at move one step when I say, to move a non-integer amount when I say, to (when network stuff comes into play) return to time X and then move forward y amount.
Total self brag, one of the key foundations of my game engine is that every single instance of any object has an anchor to a timing system, and pausing can be propogated on the same cycle as the input-capture at the most granular level as desired.
I really need to start blogging my notebook
for our game dev project in undergrad we built an rts game as our capstone, and we used the "slow stuff down" trick - except different systems had different clock/time systems! and so some things were still running while it was paused, which led to some weird side effects (eg money glitch)
i did the thing you're "not supposed to do" and attached everything to a world clock so everything ran at like 60fps in terms of events, so it was a real-time "turn based" system
Pausing a game is a source of many bugs. In many games some subsystems aren't paused, like particles or animated materials. In some games I even saw that the character moves forward if the forward moving button was pressed at the moment when the pause button was hit.
It surprises me, that it's so hard to do properly.
I am recently working on a "realtime with pause" style grand strategy game using my own engine (think Europa Universalis, Crusader King, Hearts of Iron).
The trick is to separate the logic simulation from other game loops (rendering, UI, input, sound, etc). So when a player pauses the game, everything else still more or less works. And the logic simulation should be able to take user "command" while being paused.
Most commands should mutate the game state and reflect in the UI immediately. A few commands that have to wait until the next tick should at least acknowledge the action result.
Hey, I'm working on a grand strategy game as well based on a fictional setting, would love to chat and compare notes if you're open to it, my email is in my profile
For my game (custom engine) I had a way to stop the game clock from advancing, while the input and draw loop kept running. It would also put the game into the "pause" input state during which only the resume button would be active.
The console cert ones are interesting but all the others are just Unity/Gamemaker/Unreal not allowing the developers to write normal code? The nonzero timestep thing is very strange
I would expect pausing to bring a game’s CPU/GPU usage down to near-zero, which won’t happen if the game keeps redundantly rendering the exact same frame. A game engine can optimize this by special casing time scale zero to simply render a single textured quad under the pause menu (which is probably what one of the commenters in TFA referred to).
You would expect it to do that, and I'd say that's a desirable behaviour, but it's not really that simple and you certainly don't get that for free.
Typically any of the common modern engines with a "time scale" variable like that are not at all optimising anything in that way. It's likely that the physics engine won't be stepped with a zero delta time, which will reduce the time spent on physics, but that's more of a byproduct of how physics engines work[0] than an optimisation.
You would have to go out of your way to capture the scene and display it "under the pause menu" in that way. Not saying nobody does that, just that it's not something the engine is giving you for free nor is it related to the time scale variable.
Further, doing that won't necessarily reduce resource usage. For example, if there isn't some sleep time inserted in the main loop when in a menu, or v-sync[1] to limit the framerate, the result of the simplified scene (just the menu and the quad with the captured scene) is an extremely high framerate, which may or may not cook the hardware more than the in-game load.
[0] Typical rigidbody physics engines are only (what I'll call) predictably stable with a constant delta time (same dt every tick). And a common way to manage this is with a time accumulator main loop, that only steps physics for whole units of dt.
[1] And v-sync isn't a silver bullet. consider refresh rates, different hardware, different drivers, driver overrides.
Speaking of "capturing the scene and display it under the pause menu"....
acerola on YouTube has an excellent 23 minute frame rendering analysis video about what goes into drawing just the "pause menu" in Persona 3 Reload:
https://youtu.be/dVWkPADNdJ4?t=19m10s
From a lot of game jam games with custom engines, I have a pretty standard approach: I set the game clock to zero timestep, and push a pause gamestate onto the gamestate stack, where top gamestate gets all input. The trick is that you need many clocks, clocks for the gameplay/game animations, HUD animations, menu animations.
> when it was time to ship, we’d read the [Technical Requirements Checklists] and have to go back and add a special pause for when you unplug the controller
article confirms my early theory I formed when reading the title about why would pause be complicated
More people should play Lobotomy Inc. Coolest pause mechanism I've ever seen.
One game that I thought had a great "pause" feature was Dragon's Dogma, you can pause the game mid fight to use items that heal or remove status effects. At first I felt this was kind of cheesy, but due to the games inventory weight limits it actually made traveling long distance or tackling dungeons more strategic because you had to bring the appropriate items for the enemies you'd be facing and use them sparingly or you'd be in trouble. Further into the game it actually felt like it required more strategy and planning, I thought it was pretty great game design and really liked it.
If your game uses more than 1% CPU and 1% GPU when paused you're doing something wrong.
A damn blurred screenshot should not make the GPU consume hundreds of Watts.
Paused game does not equal paused process. It means the gameplay clock has paused.
The rendering loop continues to run. The GUI is immediate mode and is still rendered. In some games visual effects continue to be rendered. If it’s a networked game the network code will continue to run.
I think the parent comment still ought to stand. Unless you're saying it's literally infeasible for some reason, which I find hard to believe?
A paused game is not like a paused video where there is no processing going on. A paused game still has all the rendering usually going on (unless the developers got time to implement something custom), audio still needs to continue, the only thing actually paused in the game logic and gameplay elements.
Given this the game needs to slow down and lock its frame rate while paused and in menus otherwise it will run at full speed and burn a lot of CPU & GPU time. Though I would say this is not usually an issue because it gets fixed pretty quickly during development.
Now more work can be done by the developers to make the paused state this efficient, but it is all a matter of priorities, and in all cases the developers would rather spend their time making the gameplay better and fixing crash bugs. If the player wants their game to run more efficiently then they should reduce the processing demands by limiting the frame rate and lowering quality settings.
Yes, I already understood all that.
> unless the developers got time to implement something custom
As I said: the point nevertheless ought to stand.
See this video: https://youtu.be/dVWkPADNdJ4?t=19m10s
Taken from this comment: https://news.ycombinator.com/item?id=47825155
Obviously not every game is doing that level of styling, but often a lot of stuff might still be happening when a game is "paused" for various reasons e.g. aesthetics, or handling input for pause menus, which usually offer you the ability to change all the games settings, e.g. video resolution, volume levels, etc. and also provide you with audio/visual feedback when you make choices, which requires keeping the audio engine running as well as rendering to the screen @ 60 fps.
Not sure if slowing down time is the right approach.
The best approach would be using something like if(game_is_paused) return; in the game loops.
Games are multithreaded and have loads of objects running everywhere. If you're using anything that's not a custom game engine, there really isn't a single main() function that you can plop an if statement like that into.
Slowing down time applies it universally. Otherwise you're going to need that condition to every single object in the game.
It's usually not as simple as that. You'd still want to at least keep the UI alive, and you also need to continue rendering while the game is paused because the swapchain surfaces might lose their content (window resizing, changing the display mode, alt-tabbing to the desktop etc).
E.g. when you open the ingame menu, the inventory (etc) you usually want to pause the gameplay, but still want to interact with the UI. Sometimes that means that at least also some of the gameplay logic needs to remain alive (inventory management, crafting, levelling up, etc...).
There are also a lot of games which need some sort of 'active pause', e.g. the gameplay needs to stop while the user can issue commands to units (traditional example: real-time combat with pause like in Baldurs-Gate-style RPGs).
Sometimes the underlying engine also doesn't properly separate gameplay logic from rendering, e.g. you can't skip one without also skipping the other (which is an engine design bug, but similar situations may also happen up in gameplay code).
Finally: pausing and the save-game-implementation is often an afterthought, but really should be implemented as the very first thing. It's quite easy to run into the trap that a frame also needs to advance time. If the game has the concept of a fixed-duration 'game logic tick' which is independent from the frame rate you're already halfway there though, but many games simply use a variable-length game tick which is identical with the frame duration.
Yep, this comment does a good job of illustrating the (surprising) complexity of this.
I'll add that the notion of the "time scale" variable as mentioned in the article is something that's only solidified/codified since Unity and the like came about. And at the same time, the way Unity et al. works[0] doesn't really encourage thinking about what I'd call "main loop logic" in the bottom-up way that's required to build a really robust system for managing states. You can do it, of course, (you can write as much code as you want) but by default, everything in the scene is "active" and somewhat independent from everything else, and you don't have direct control over the various major systems.
[0] I guess I should say "worked" -- I mostly used 3.x and a little bit of early version 4 -- I'm sure it's improved but I wouldn't expect anything drastically different.
The slowing down thing sounds like a hack needed for engines that don’t give you control over the main loop.
I haven’t tried this yet, but for a custom engine I would introduce a second delta time that is set to 0 in the paused state. Multiplying with the paused-dt „bakes in“ the pause without having to sprinkle ifs everywhere. Multiplying with the conventional dt makes the thing happen even when paused (debug camera, UI animations).
I don't think it's a hack necessarily - as a well implemented time system would produce the same results at game rate = 0 as with pause.
Also there's a need for different time domains - like imagine, in a paused state the menu animations still need to play, or if the player enters a conversation, the game logic needs to pasue (depending on designer intent etc.)
Unity does this. You get scaledDeltaTime (when you set game speed to 0, it'll return 0) and unscaledDeltaTime (returns time between frames ignoring game speed). Pauseable logic uses the former. Pause menus use the latter.
A surprising number of systems break down if you pause time by passing a 0 time delta, hence why slowing down time is usually the thing that gets done. The alternative (which you imply with your suggestion) is to pass the many paused states down to every system which further complicates their implementation and is a major source of bugs.
The added fact is that many systems still need to continue running, such as audio, video, and input otherwise the program doesn't appear to respond to input and so isn't in a useable state any more.
It depends on how your timers are implemented. If they are implemented as a "rendez-vous" absolute date (as in UTC for instance - not in local time unless you want to add "eastern eggs" related to daylight saving time...), this will cause bugs. If you implement your own in-game monotonic clock that you can stop, it should be ok.
>If they are implemented as a "rendez-vous" absolute date
Do people actually do that? What's the plan for when the user sleeps their machine? All the events just inexplicably happen all at once when they wake it?
I've implemented timers that had timeouts using unix timestamps, but only for multiplayer - when a player's attempt to connect to the server times out, etc.
Inside the game loop, we would keep the global tick counter that incremented on every tick, and timeouts would be based on that rather than on UTC.
The tick counter was updated only when the game logic was actually running. Our approach to pausing was to not run the functions that handled frame updates or physics updates, and to only run the rendering functions.
Generally we would never care about actual world time other than for some timeouts like for network (as the time passes for everyone), or for easter eggs like changing the tree models for Christmas or so.
I don't think anyone serious would implement event timers based on real time.
That's sort of the point TFA. You make implementation choices that feel OK and then the time comes to implement the "trivial" pause function...
In other domains, adding the delta time of your main loop to your timers can cause (logical) clock drifts in the long term because of resolution errors.
I was suspicious of those random game developers getting quoted, and this is the pinned post of the one giving this cute silly story about slowing down game speed time:
"Announcing TORMENT HEXUS, a match-3 roguelike about putting technofascist CEOs on the wrong side of skyscraper windows!
[...]
And remember: they SHOULD be afraid of us. #indiedev #indiegame"
Weird times.
I'm not sure if you're saying that your suspicions were valid or not. Suspicious of what, exactly?
Like, yes, the most likely people to respond to such a call for "stuff to put in an article on Kotaku" are probably developers that want some publicity. But this is hardly surprising.
Eh, I remember the myriad of both "Torture Bin Laden" as well as "Torture George Bush" flash games in the early/mid 2000s. I think it's very on brand for indie developers.
Bluesky has a lot of left-wing extremists where things like murdering CEOs is cool because they are allegedly "technofascist".
"Evil megacorp" media is hardly new.
Murdering CEOs is extremism, saying something is an "evil megacorp" is totally different.
There's probably something eloquent by Hannah Arendt about how 190,000 Americans killed by health insurance companies goes unnoticed while one person killing a CEO becomes a spectre of "left wing extremism" held up as an example.
Or was it by The Joker from Batman?
Or was it when protesters in Latin America sat down blocking a road to protest environmental destruction and an American driver was so angry that he was mildly inconvenienced that he got out of his car and murdered one of them with his gun. And Joe Rogan's podcast commentary was "what did they expect?", more annoyed at the inconvenience to drivers than the murder of a human.
Or maybe when Just Stop Oil protestors threw soup and mashed potato on the glass in front of a painting, with the idea "look how angry you are at the damage to a valuable and irreplacable object, this is how angry you should be at the damage to the valuable and irreplacable environment which keeps all humans alive" and Fox News laughed at them for both damaging something important and not causing any real damage so they were ineffective. Then the judge gave them 2 years in prison on the grounds that throwing a can of soup at someone's face would be violence, so throwing it at a painting is violence. But no oil executives overseeing the Exxon Valdez disaster or the Gulf of Mexico disaster faced any jail time at all.
Or when the suffragette movement cut a painting of Venus de Milo to protest against Emmeline Pankhurst being arrested and rough-handled, and people were angrier about the harm to the painting of a woman than about the harm to a real woman.
Or when Fox News says "they aren't protesting the right way" so Kapaernik asked actual verterans how to peacefully protest respectfully and they told him to kneel during the national anthem, and the complainers didn't care a whit and said that was still the wrong way and disrespectable, and he lost his job and the president tweeted rude things about him personally, and the national football thingy made that kind of protest forbidden, almost as if the objection "protesting the wrong way" was all bullshit.
Yes, probably Hannnah Arendt could put it eloquently.
But you're right, murder is wrong, and that's all there is to it.
Name-dropping Hannah Arendt does not rescue this from being an elaborate equivocation about murder.
> There's probably something eloquent by Hannah Arendt about how 190,000 Americans killed by health insurance companies
Health insurance companies don't kill people, quite the opposite. If it weren't for health insurance, a lot more people would die. Murdering their CEOs is crazy extremism.
Physicians For A National Health Program put the figure at 200,000 people annually[1]. What's your source for saying the number is zero? When they deny claims, people die. When they override medical doctor recommendations and insist on cheaper treatments, people die. When they tangle up customers with paperwork and bureaucracy, that some people can't access the health insurance they pay for. When they take money out of the system as profit, that money isn't helping the sick. When United Healthcare spends $12M/year on lobbying[1] it isn't doing that to improve patient care.
> Murdering their CEOs is crazy extremism.
When a system doesn't have a pressure release valve, the pressure doesn't go away. When a system blocks or ignores peaceful protest, the pressure doesn't go away. The thread running through my comment is that harming humans is wrong, yes murder is wrong - but sticking a label on it and saying "leftist extremism" and then denying real issues is not helping. The system needs ways to hear people saying "things aren't fine" before those people go crazy extremist, not after.
[1] https://pnhp.org/news/estimated-us-deaths-associated-with-he...
[2] https://www.opensecrets.org/federal-lobbying/industries/summ...
> Physicians For A National Health Program put the figure at 200,000 people annually[1]. What's your source for saying the number is zero?
I'm not saying the number is zero. I'm saying the number is vastly negative. They are overall saving a lot of people rather than killing them. Health insurance companies are hugely net-positive.
> but sticking a label on it and saying "leftist extremism" and then denying real issues is not helping.
Talking about murdering CEOs is helping far, far less.
Imagine I believe that the Democrats are net-negative. Would this justify people saying that Democrat leaders should be murdered? Or that labelling these justifications of murder as "rightist extremism" is "not helping"?
Compared to no healthcare at all, yes, but similar could be said of Crassus' firefighting service in ancient Rome. He brought his slaves to your burning property and they stood around outside while you negotiated selling your property to Crassus at a bargain price. If you agreed, he ordered his slaves to fight the fire and you got some money. If you didn't, they let it burn and you got nothing. Crassus would be there to buy the ruins for even less if you couldn't afford to rebuild. That's a net positive for Rome compared to no fire service - fires don't spread to other buildings as often, people get something instead of nothing - but it's hardly a ringing endorsement, and it could be better.
Observation 1: you are bothered by the murder of the CEO. You dismiss the business-as-usual harms to hundreds of thousands of poor people. You consider yourself to have a good grasp of what is crazy.
Observation 2: when faced with claims that insurance companies kill people, you turn to dreaming of a world where you can talk of killing Democrat leaders. You still consider yourself to have a good grasp of what is crazy.
Complaints, letters to the editor, letters to congresspersons, achieved nothing; the murder of a CEO has achived nothing; what size event would make you notice?
> "Imagine I believe that the Democrats are net-negative"
Just feels important to say, for the record, that facts don't support that position; the Economic Policy Institute[1], and the Senate Joint Economic Committee[2] found that since 1949 the economy performs better under Democrat administrations than under Republican administrations. Job growth is greater. GDP growth is faster. Unemployment is lower. Small business creation is higher. Manufacturing investment is higher. Stock market returns are higher. Wage growth is faster. Recessions start less often.
> "Would this justify people saying that Democrat leaders should be murdered?"
First problem here is your implication that I would support the Democrats being awful and not be on the side of people objecting [although not calling for murder]. Second is the implication that I would want to silence your free speech instead of, say, supporting your right to say things I disagree with, or sarcastically mocking you. Third (or really, first) problem is that you're replying to claims that insurance company behaviour causes humans to die with "Left bad".
[1] https://www.epi.org/press/new-report-finds-that-the-economy-...
[2] https://www.jec.senate.gov/public/index.cfm/democrats/2024/1...
[3] https://en.wikipedia.org/wiki/U.S._economic_performance_by_p...
Log off and go watch Robocop, jesus
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This reminds me of the Action Replay device you could get for the Amiga 500 in the ‘90s.
You could use a knob to slow down any game to a stop. You could also press a button to go to a console that let you change memory.
It would even figure out which bit of memory kept the number of lives of you deliberately lost a life and it could see what decremented.
There was some cheat software that did the same back in the windows 9x/XP days. Dont remember the name but there were many I am sure.
Cheat Engine
https://github.com/cheat-engine/cheat-engine
Can we just link to the twitter thread the article copies content from?
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How difficult can it be when Cloud providers are able to do live migration of VM from one bare metal server to another?
Pausing a game is not as simple as freezing its entire state (and saving is also not as easy as dumping the entire game state to disc, at the least you'd end up with gigabyte-sized savegames).
Many game systems need to remain active in paused state (the UI needs to remain working for example, and actions in the UI may also manipulate game state (inventory, crafting, levelling up...). There are also plenty of games with 'active-pause' where the user can issue commands to units while in paused state.
Likely more difficult.
Live migration boils down to copy memory over the network, stream the page faults till you converge enough, and resume execution on the other host. It’s not a hard problem but a precise and tedious one.
Pausing a game might involve a lot of GPU contexts to freeze, network resources to pause, storage streams to pause, input handling, sound, etc. Add to that physics engine that may be tied deeply in the system and you end up with a hard problem.
What a VM does is not the role of the hypervisor, thus it can apply its hammer that works in pretty much all cases, and VMs are pretty much all the same. On the other hand, all games are bespoke with custom plugins and custom integrations, which make them the opposite of "generic pause implementation".
that's exactly what i wanted to know, trick works: be confidently wrong on web and get the right answer :D