Fix apparent jitter in player movement

Remember: `time.delta_secs()` needs to be included in ALL movement calculations!!! This one was pretty well hidden because the player movement was properly interpolated and the camera lerped after it, but it was still barely noticeable!

src/game/runtime.rs

@@ -1,6 +1,7 @@
 use avian2d::prelude::*;
 use bevy::{
     input::mouse::{AccumulatedMouseScroll, MouseScrollUnit},
+    math::curve::EaseFunction::SmoothStep,
     prelude::*,
 };
 
@@ -14,19 +15,19 @@ pub fn move_player(
 ) -> Result {
     let acceleration = 500.0;
 
-    let delta_time = time.delta_secs();
+    let delta_v = acceleration * time.delta_secs();
 
     if keyboard_input.any_pressed([KeyCode::KeyW, KeyCode::ArrowUp]) {
-        velocity.y += acceleration * delta_time;
+        velocity.y += delta_v;
     }
     if keyboard_input.any_pressed([KeyCode::KeyS, KeyCode::ArrowDown]) {
-        velocity.y -= acceleration * delta_time;
+        velocity.y -= delta_v;
     }
     if keyboard_input.any_pressed([KeyCode::KeyA, KeyCode::ArrowLeft]) {
-        velocity.x -= acceleration * delta_time;
+        velocity.x -= delta_v;
     }
     if keyboard_input.any_pressed([KeyCode::KeyD, KeyCode::ArrowRight]) {
-        velocity.x += acceleration * delta_time;
+        velocity.x += delta_v;
     }
 
     Ok(())
@@ -39,10 +40,14 @@ pub fn quit(mut exit: EventWriter<AppExit>) {
 
 /// Follow the player character with the camera
 pub fn move_camera(
+    time: Res<Time>,
     mut camera: Single<&mut Transform, (Without<Player>, With<IsDefaultUiCamera>)>,
     player: Single<&Transform, (With<Player>, Without<IsDefaultUiCamera>)>,
 ) {
-    camera.translation = camera.translation.lerp(player.translation, 0.05);
+    camera.translation = camera.translation.lerp(
+        player.translation,
+        SmoothStep.sample_clamped(time.delta_secs() * 15.0),
+    );
 }
 
 /// Adjust the camera zoom based on the scroll wheel input