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Comment by JoshTriplett | Hacker News Reader

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.

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> 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)

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.

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...

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