smooth_follow/
smooth_follow.rs

1//! This example demonstrates how to use interpolation to make one entity smoothly follow another.
2
3use bevy::{
4    math::{prelude::*, vec3, NormedVectorSpace},
5    prelude::*,
6};
7use rand::SeedableRng;
8use rand_chacha::ChaCha8Rng;
9
10fn main() {
11    App::new()
12        .add_plugins(DefaultPlugins)
13        .add_systems(Startup, setup)
14        .add_systems(Update, (move_target, move_follower).chain())
15        .run();
16}
17
18// The sphere that the following sphere targets at all times:
19#[derive(Component)]
20struct TargetSphere;
21
22// The speed of the target sphere moving to its next location:
23#[derive(Resource)]
24struct TargetSphereSpeed(f32);
25
26// The position that the target sphere always moves linearly toward:
27#[derive(Resource)]
28struct TargetPosition(Vec3);
29
30// The decay rate used by the smooth following:
31#[derive(Resource)]
32struct DecayRate(f32);
33
34// The sphere that follows the target sphere by moving towards it with nudging:
35#[derive(Component)]
36struct FollowingSphere;
37
38/// The source of randomness used by this example.
39#[derive(Resource)]
40struct RandomSource(ChaCha8Rng);
41
42fn setup(
43    mut commands: Commands,
44    mut meshes: ResMut<Assets<Mesh>>,
45    mut materials: ResMut<Assets<StandardMaterial>>,
46) {
47    // A plane:
48    commands.spawn((
49        Mesh3d(meshes.add(Plane3d::default().mesh().size(12.0, 12.0))),
50        MeshMaterial3d(materials.add(Color::srgb(0.3, 0.15, 0.3))),
51        Transform::from_xyz(0.0, -2.5, 0.0),
52    ));
53
54    // The target sphere:
55    commands.spawn((
56        Mesh3d(meshes.add(Sphere::new(0.3))),
57        MeshMaterial3d(materials.add(Color::srgb(0.3, 0.15, 0.9))),
58        TargetSphere,
59    ));
60
61    // The sphere that follows it:
62    commands.spawn((
63        Mesh3d(meshes.add(Sphere::new(0.3))),
64        MeshMaterial3d(materials.add(Color::srgb(0.9, 0.3, 0.3))),
65        Transform::from_translation(vec3(0.0, -2.0, 0.0)),
66        FollowingSphere,
67    ));
68
69    // A light:
70    commands.spawn((
71        PointLight {
72            intensity: 15_000_000.0,
73            shadows_enabled: true,
74            ..default()
75        },
76        Transform::from_xyz(4.0, 8.0, 4.0),
77    ));
78
79    // A camera:
80    commands.spawn((
81        Camera3d::default(),
82        Transform::from_xyz(-2.0, 3.0, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
83    ));
84
85    // Set starting values for resources used by the systems:
86    commands.insert_resource(TargetSphereSpeed(5.0));
87    commands.insert_resource(DecayRate(2.0));
88    commands.insert_resource(TargetPosition(Vec3::ZERO));
89    commands.insert_resource(RandomSource(ChaCha8Rng::seed_from_u64(68941654987813521)));
90}
91
92fn move_target(
93    mut target: Single<&mut Transform, With<TargetSphere>>,
94    target_speed: Res<TargetSphereSpeed>,
95    mut target_pos: ResMut<TargetPosition>,
96    time: Res<Time>,
97    mut rng: ResMut<RandomSource>,
98) {
99    match Dir3::new(target_pos.0 - target.translation) {
100        // The target and the present position of the target sphere are far enough to have a well-
101        // defined direction between them, so let's move closer:
102        Ok(dir) => {
103            let delta_time = time.delta_secs();
104            let abs_delta = (target_pos.0 - target.translation).norm();
105
106            // Avoid overshooting in case of high values of `delta_time`:
107            let magnitude = f32::min(abs_delta, delta_time * target_speed.0);
108            target.translation += dir * magnitude;
109        }
110
111        // The two are really close, so let's generate a new target position:
112        Err(_) => {
113            let legal_region = Cuboid::from_size(Vec3::splat(4.0));
114            *target_pos = TargetPosition(legal_region.sample_interior(&mut rng.0));
115        }
116    }
117}
118
119fn move_follower(
120    mut following: Single<&mut Transform, With<FollowingSphere>>,
121    target: Single<&Transform, (With<TargetSphere>, Without<FollowingSphere>)>,
122    decay_rate: Res<DecayRate>,
123    time: Res<Time>,
124) {
125    let decay_rate = decay_rate.0;
126    let delta_time = time.delta_secs();
127
128    // Calling `smooth_nudge` is what moves the following sphere smoothly toward the target.
129    following
130        .translation
131        .smooth_nudge(&target.translation, decay_rate, delta_time);
132}
smooth_follow/smooth_follow.rs

smooth_follow/
smooth_follow.rs
