8 ms·

First, even at a constant framerate, different players will have different experiences with the game due to different amounts of latency in their peripherals, w

by wareya 10y ago

First, even at a constant framerate, different players will have different experiences with the game due to different amounts of latency in their peripherals, weird CPU bugs, very very minor floating point behavior deviations, etc. The important part of making something framerate independent is to make the effects of framerate independence /smaller enough/ than other factors that make player experiences. It's entirely possible to not have gameplay change at different tickrates. There are three usual types of problem that cause gameplay to change at different tickrates: 1) Events that happen at specific times If events can only happen "in sync" with ticks, then running at different framerates will make them happen at different times. Say you have a machine gun that fires once every 1/10th of a second. If you're running at 24fps, then not only do you need to make sure that you correctly output a bullet every two-or-three frames, you also have to make it so that they /act like/ they came out at a time between frames, presumably by simulating them a little extra bit or a little less depending on what's necessary to make each bullet a fixed distance apart (for example). You could also use a continuous interaction system here but that's really hardcore and doesn't matter in 99.9%+ of cases. 2) Treating curved motions as linear motions This isn't a problem in 99% of cases, but it ~can~ be a problem. If you can't represent your physics curves in closed form, it's effectively impossible to trace them in a framerate-independent way. The only thing you can do for these things is to "fix your timestep", but "fix your timestep" should be used with extreme caution and not be applied to action games because it adds input latency and hides frame-specific-timing information from players. Recommendation: use simpler game physics. Constant acceleration everywhere you can, special-purpose closed form functions with the right curves where you can't, and avoid tight curves. This makes it less of an issue. You might also want a collision system that allows curved paths, but if you have enough control over your game's environments, and you don't expect people to run at SUPER low framerates, this isn't an issue. 3) Running different "integral" parts of the simulation out of sync with eachother. This isn't literally about code order execution, it's about the behavior of different pieces of code that "add up" "over time" (if you're into calculus, think antiderivatives) If you add n to your position every frame, that's all fine and good, as long as it always starts at exactly the right time. If you add "five pixels per second" to your position, then you need to make sure that always causes the same amount of distance travelled. Quake 3 handles this by giving subframe timings to things like pressing and releasing movement keys. Another example is gravity. Normally, game code adds a specific amount of gravity every frame, possibly adjusted for time, either before or after motion. This isn't good enough, because if you add gravity after moving, very low framerates will have a higher initial frame of gravitational movement; essentially, low framerates will skip "outside" the ideal jump arc rather than tracing it. If you add gravity before moving, the opposite happens, low framerates trace "inside" the ideal jump arc. The correct thing to do is to calculate motion with around half the added velocity from all accelerations for that frame, which basically means the average of accelerating before or after. You can also trace a hermite spline, which is more flexible (you will have perfect framerate independence for any acceleration values that only change along line segments, i.e. "constant jerk", as long as you handle start/stop conditions for changes in acceleration with a correct continuous interaction simulation), but harder to implement. The above is only 100% ideally possible for things you can represent closed form, and in practice, people will only implement something correctly if it's not just closed form but /simple/. Many games stop short of correct jump arcs, but it's entirely possible. Gang Garrison 2 was just changed to have a correct framerate-independent jump arc.

CJefferson 10y ago
I don't know what kind of CPU bugs or float point behaviour deviations you think modern cpus have that will effect a game.. such things would cause most online games to immediately go out-of-sync, and in practice they don't, and while latency might effect player's enjoyment, it won't effect how fast they run / how high they jump. The hardest part is the curved motions -- and also collision detection. Without ticks it's extremely hard to make collision detection repeatable. I'm unaware of any recent notable multi-player game which don't run on a tick-based physics engine (I'd be interested if you could point me to some) -- that suggests to me that tick-based is necessary. Sure, you can get things "closer" in tickless physics, but I've never seen anything non-trivial which can produce exactly the same results, for reasons of floating point: when (A+B)+C isn't the same as A+(B+C), it's very hard to make your game produce exactly the same results. As you say, with a lot of work, you can get a 95% repeatable result with a tickless engine, but you can easily get 100% repeatable with a fixed time-step engine, and in practice it seems to make things work well enough, and is VASTLY easier.
- wareya 10y ago
  Online games avoid that problem by having an authoritative networking architecture, not by avoiding minor deviations in CPU behavior between players. Games that use synchronized lockstep that use floating point math always desync, because floating point math differs slightly between different processors. Curved motions are easy as long as you have simple polygons and only one of them is accelerating and is accelerating in a simple way. You remove the acceleration by skewing the polygon you're going to collide with and then you have a linear path of motion again. (if the acceleration is not constant for the duration of that frame, then skewing doesn't give the right result; as long as you have a closed form representation, though, it's entirely possible to get it right) "Don't run on a tick-based physics engine" is a misconception. Interactions are the borders between ticks. It's just dynamic. As long as your tick doesn't contain interaction changes inside it, you can simulate that tick with 100% tickrate independence with 100% certainty. Floating point differences between cpus are Much Larger, especially if your code uses hardware-accelerated trig at all. Games networking is authoritative for this reason. You can get a 100% repeatable result with a tickless engine. Fixed timestep doesn't solve the problem. If the user can't run the simulation at 60 ticks per second, they're still going to slow down, period. All you're doing is separating simulation from rendering, which basically every modern FPS under the sun already tries to do in a different way than fixed timestep does. If you run the simulation slow enough that nobody will have performance problems with it, you just added tons of input latency. Thanks a lot, sincerely, someone with dysgraphia.

by wareya 10y ago

CJefferson 10y ago

I don't know what kind of CPU bugs or float point behaviour deviations you think modern cpus have that will effect a game.. such things would cause most online games to immediately go out-of-sync, and in practice they don't, and while latency might effect player's enjoyment, it won't effect how fast they run / how high they jump. The hardest part is the curved motions -- and also collision detection. Without ticks it's extremely hard to make collision detection repeatable. I'm unaware of any recent notable multi-player game which don't run on a tick-based physics engine (I'd be interested if you could point me to some) -- that suggests to me that tick-based is necessary. Sure, you can get things "closer" in tickless physics, but I've never seen anything non-trivial which can produce exactly the same results, for reasons of floating point: when (A+B)+C isn't the same as A+(B+C), it's very hard to make your game produce exactly the same results. As you say, with a lot of work, you can get a 95% repeatable result with a tickless engine, but you can easily get 100% repeatable with a fixed time-step engine, and in practice it seems to make things work well enough, and is VASTLY easier.

wareya 10y ago
Online games avoid that problem by having an authoritative networking architecture, not by avoiding minor deviations in CPU behavior between players. Games that use synchronized lockstep that use floating point math always desync, because floating point math differs slightly between different processors. Curved motions are easy as long as you have simple polygons and only one of them is accelerating and is accelerating in a simple way. You remove the acceleration by skewing the polygon you're going to collide with and then you have a linear path of motion again. (if the acceleration is not constant for the duration of that frame, then skewing doesn't give the right result; as long as you have a closed form representation, though, it's entirely possible to get it right) "Don't run on a tick-based physics engine" is a misconception. Interactions are the borders between ticks. It's just dynamic. As long as your tick doesn't contain interaction changes inside it, you can simulate that tick with 100% tickrate independence with 100% certainty. Floating point differences between cpus are Much Larger, especially if your code uses hardware-accelerated trig at all. Games networking is authoritative for this reason. You can get a 100% repeatable result with a tickless engine. Fixed timestep doesn't solve the problem. If the user can't run the simulation at 60 ticks per second, they're still going to slow down, period. All you're doing is separating simulation from rendering, which basically every modern FPS under the sun already tries to do in a different way than fixed timestep does. If you run the simulation slow enough that nobody will have performance problems with it, you just added tons of input latency. Thanks a lot, sincerely, someone with dysgraphia.