Ggez: How to change main loop fps?

[TakWolf]

I use the follow code to measuring fps.

impl event::EventHandler for App {

    fn update(&mut self, _ctx: &mut Context, _dt: Duration) -> GameResult<()> {
        Ok(())
    }

    fn draw(&mut self, ctx: &mut Context) -> GameResult<()> {
        graphics::clear(ctx);

        println!("FPS: {}", ggez::timer::get_fps(ctx));

        graphics::present(ctx);
        Ok(())
    }

}

FPS: 1228.6385824459492
FPS: 1118.5031743120085
FPS: 1028.1549964220205
....... about 150 frames .......
FPS: 60.05427825777494
FPS: 60.05327566297165
FPS: 60.04459271722737
FPS: 60.057084258587786
FPS: 60.060687721301115
FPS: 60.05724656742807
FPS: 60.051757408675385
FPS: 60.04946755037865
FPS: 60.04679927441849
FPS: 60.05704819007574
FPS: 60.063858693285525
FPS: 60.054094326005085
FPS: 60.06029452967834
FPS: 60.047177866706306
FPS: 59.91234823453287
FPS: 60.053614666101595
FPS: 60.060756259817296
FPS: 60.06152823075014
FPS: 60.06709855310973
FPS: 60.07317152584533
FPS: 60.07846608141028
FPS: 60.060972698263825
FPS: 60.06805470325659
..........

After stabilization, fps about for 60.

Can I change this fps ?
Is there a setting?

[icefoxen]

If you are using the released crate, there is vsync flag on Conf that tells it to set the display rate to your monitor's refresh rate; in this case, 60 hz. This flag is on by default because it's usually a good idea anyway.

In the current git master, the flag has moved to conf.window_mode.vsync.

ggez::timer::get_fps() averages over the last couple hundred frames, so it takes a second or two for it to "warm up" and give consistent results.

[icefoxen]

If you have any further questions please feel free to re-open this.

[TakWolf]

Frame rate not only affect the display, sometimes it will also affect the game logic.
I hope there can be a way to set the frame rate, especially if the vsync is not enabled.

pub fn run(ctx, state) -> Result {
    loop {
        if (ticks_and_check(ctx)) {
            sdl_events_handle();
            state.update(ctx);
            state.draw(ctx);
        }
   }
}

fn ticks_and_check(ctx) {
    let delay_time = now_time - last_time;
    if ctx.vsync || delay_time >= 1.0 / ctx.fps {
        true
    } else {
        false
    }
}

I am now no way to deal with this elegance, because event handle is wrapped.
I can only do this in an hack way.
I hope the engine can provide a feature.

[icefoxen]

The various ways of doing this, with pros and cons, are described well here: https://gafferongames.com/post/fix_your_timestep/

AstroBlasto and a couple of the other examples show how to do this well with ggez:

impl EventHandler for MainState {
    fn update(&mut self, ctx: &mut Context) -> GameResult<()> {
        const DESIRED_FPS: u32 = 60;

        while timer::check_update_time(ctx, DESIRED_FPS) {
            ...
        }
    }
    ....
}

This is basically the setup described by "Free the physics" in that article. timer::check_update_time() keeps track of how much time has passed since the last frame, and will decrement it in DESIRED_FPS-sized chunks and return true as long as you should keep running physics updates. It correctly keeps track of the remainder of the time so you don't get round-off errors between frames.

Doing interpolation for the remaining timestep, as described in "The Final Step", is left as an exercise to the reader. ;-)