Resolve #867 - Make deltatime use the lowest delta of several frames to avoid 'teleport' effect after single-frame lag spike

common/src/clock.rs

@@ -54,6 +54,7 @@ pub struct ClockStats {
 }
 
 const NUMBER_OF_OLD_DELTAS_KEPT: usize = 100;
+const NUMBER_OF_DELTAS_COMPARED: usize = 5;
 
 impl Clock {
     pub fn new(target_dt: Duration) -> Self {
@@ -75,6 +76,24 @@ impl Clock {
 
     pub fn dt(&self) -> Duration { self.last_dt }
 
+    pub fn get_stable_dt(&self) -> Duration {
+        if self.last_dts.len() < NUMBER_OF_DELTAS_COMPARED {
+            self.last_dt
+        } else {
+            let stable_dt = Duration::from_secs_f32(
+                self.last_dts
+                    .iter()
+                    .skip(self.last_dts.len() - NUMBER_OF_DELTAS_COMPARED)
+                    .min()
+                    .map_or(self.last_dt.as_secs_f32(), |t| t.into_inner()),
+            );
+            if self.last_dt > 2 * stable_dt {
+                tracing::debug!(?self.last_dt, ?self.total_tick_time, "lag spike detected, unusually slow tick");
+            }
+            stable_dt
+        }
+    }
+
     /// Do not modify without asking @xMAC94x first!
     pub fn tick(&mut self) {
         span!(_guard, "tick", "Clock::tick");

voxygen/src/session.rs

@@ -634,7 +634,7 @@ impl PlayState for SessionState {
                     let right = self.scene.camera().right();
                     let dir = right * axis_right + forward * axis_up;
 
-                    let dt = global_state.clock.dt().as_secs_f32();
+                    let dt = global_state.clock.get_stable_dt().as_secs_f32();
                     if self.freefly_vel.magnitude_squared() > 0.01 {
                         let new_vel = self.freefly_vel
                             - self.freefly_vel.normalized() * (FREEFLY_DAMPING * dt);
@@ -671,7 +671,11 @@ impl PlayState for SessionState {
             // Runs if either in a multiplayer server or the singleplayer server is unpaused
             if !global_state.paused() {
                 // Perform an in-game tick.
-                match self.tick(global_state.clock.dt(), global_state, &mut outcomes) {
+                match self.tick(
+                    global_state.clock.get_stable_dt(),
+                    global_state,
+                    &mut outcomes,
+                ) {
                     Ok(TickAction::Continue) => {}, // Do nothing
                     Ok(TickAction::Disconnect) => return PlayStateResult::Pop, // Go to main menu
                     Err(err) => {
@@ -740,7 +744,7 @@ impl PlayState for SessionState {
                 global_state,
                 &debug_info,
                 &self.scene.camera(),
-                global_state.clock.dt(),
+                global_state.clock.get_stable_dt(),
                 HudInfo {
                     is_aiming,
                     is_first_person: matches!(