ftvf

ftvf is a crate for carrying out game logic the One True Way: Fixed Tickrate, Variable Framerate. By having your game logic in strictly fungible ticks, rather than having it vary based on framerate, you gain many advantages:

• Repeatability: the same inputs will have the same outputs, period.
• Framerate independence: no issues like Quake had where your exact jump height depends on how fast your computer is.
• Satisfaction: knowing that you made the morally correct choice. :)

Bonus: If you know your refresh rate, ftvf can help you render frames at exactly that rate, jitter-free.

To get started, add ftvf to your dependencies in Cargo.toml:

ftvf = "0.6"

then initialize yourself a Metronome:

let mut metronome = Metronome::new(
  RealtimeNowSource::new(),
  // want 30 ticks per 1 second
  Rate::per_second(30, 1),
  // accept being up to 5 ticks behind
  5,
);

And then your game loop looks like this:

while !world.should_quit() {
  world.handle_input();
  for reading in metronome.sample(Mode::UnlimitedFrames) {
    match reading {
      Reading::Tick => world.perform_tick(),
      Reading::Frame{phase} => world.render(phase),
      Reading::TimeWentBackwards
        => eprintln!("Warning: time flowed backwards!"),
      Reading::TicksLost
        => eprintln!("Warning: we're too slow, lost some ticks!"),
      // Mode::UnlimitedFrames never returns Idle, but other modes can, and
      // this is one way to handle it.
      Reading::Idle{duration} => std::thread::sleep(duration),
    }
  }
}

Your logic ticks operate in discrete, fixed time intervals. Then, when it comes time to render, you render a frame which represents time some portion of the way between two ticks, represented by its phase. Your rendering process should render an interpolated state between the previous tick and the current tick, based on the value of phase. Simple example:

self.render_at(self.previous_position
               + (self.current_position - self.previous_position) * phase);

Changes

Since 0.5.0

• ftvf no longer depends on std. You can use the no_std feature flag to make the std dependency go away, at the cost of not being able to use the built-in RealtimeNowSource.
• Mode::MaxOneFramePerTick has been renamed to Mode::OneFramePerTick.
• metronome.sample() now returns an iterator directly, instead of making you repeatedly call metronome.status() in a disciplined way.
• Rates are now passed using the new Rate structure, instead of as tuples.
• Timing is now perfectly accurate, instead of "only" having nanosecond precision. (Nanosecond precision is still used for frame phase calculation, and changing tick-/framerates at runtime also discards sub-nanosecond components.)
• Status has been renamed to Reading.
• Reading::Idle now directly gives you the wait time as a Duration, instead of making you go indirectly through the metronome.
• Mode::TargetFramesPerSecond added.
• Tickrate can now be changed at any time, with no temporal anomaly—apart from up to one nanosecond of one-time temporal error per change.
• NowSource::sleep removed.
• NowSource no longer implies Copy.
• There is now a blanket NowSource implementation for all Deref<Target=RefCell<NowSource>> types, including &RefCell<NowSource> and Box<RefCell<NowSource>>. This makes fake NowSources a little more ergonomic.
• There is now a FakeNowSource, available with or without no_std, which you can use in any situation where real time is not a factor, such as unit tests or rendering replays to disk.

License

ftvf is distributed under the zlib license. The complete text is as follows:

Copyright (c) 2019, 2023 Solra Bizna

This software is provided "as-is", without any express or implied warranty. In no event will the author be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgement in the product documentation would be appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.