Bounce!

examples/games/contributors.rs

@@ -58,7 +58,7 @@ struct Velocity {
     rotation: f32,
 }
 
-const GRAVITY: f32 = -9.821 * 100.0;
+const GRAVITY: f32 = 9.821 * 100.0;
 const SPRITE_SIZE: f32 = 75.0;
 
 const SATURATION_DESELECTED: f32 = 0.3;
@@ -243,7 +243,7 @@ fn velocity_system(time: Res<Time>, mut velocity_query: Query<&mut Velocity>) {
     let delta = time.delta_seconds();
 
     for mut velocity in &mut velocity_query {
-        velocity.translation += Vec3::new(0.0, GRAVITY * delta, 0.0);
+        velocity.translation.y -= GRAVITY * delta;
     }
 }
 
@@ -256,16 +256,24 @@ fn collision_system(
     windows: Res<Windows>,
     mut query: Query<(&mut Velocity, &mut Transform), With<Contributor>>,
 ) {
-    let mut rng = rand::thread_rng();
-
-    let window = windows.primary();
+    let window = if let Some(window) = windows.get_primary() {
+        window
+    } else {
+        println!("The window was closed. Bye!");
+        return;
+    };
 
     let ceiling = window.height() / 2.;
     let ground = -(window.height() / 2.);
 
     let wall_left = -(window.width() / 2.);
     let wall_right = window.width() / 2.;
 
+    // The maximum height the birbs should try to reach is one birb below the top of the window.
+    let max_bounce_height = window.height() - SPRITE_SIZE * 2.0;
+
+    let mut rng = rand::thread_rng();
+
     for (mut velocity, mut transform) in &mut query {
         let left = transform.translation.x - SPRITE_SIZE / 2.0;
         let right = transform.translation.x + SPRITE_SIZE / 2.0;
@@ -275,11 +283,16 @@ fn collision_system(
         // clamp the translation to not go out of the bounds
         if bottom < ground {
             transform.translation.y = ground + SPRITE_SIZE / 2.0;
-            // apply an impulse upwards
-            velocity.translation.y = rng.gen_range(700.0..1000.0);
+
+            // How high this birb will bounce.
+            let bounce_height = rng.gen_range((max_bounce_height * 0.4)..max_bounce_height);
+
+            // Apply the velocity that would bounce the birb up to bounce_height.
+            velocity.translation.y = (bounce_height * GRAVITY * 2.).sqrt();
         }
         if top > ceiling {
             transform.translation.y = ceiling - SPRITE_SIZE / 2.0;
+            velocity.translation.y *= -1.0;
         }
         // on side walls flip the horizontal velocity
         if left < wall_left {