example on how to create an animation in code (bevyengine#4399)

# Objective

- Animation can be used outside of glTF, but are not obvious on how to create them

## Solution

- Add an example creating an animation

Cargo.toml

@@ -241,6 +241,10 @@ path = "examples/3d/wireframe.rs"
 name = "animated_fox"
 path = "examples/animation/animated_fox.rs"
 
+[[example]]
+name = "animated_transform"
+path = "examples/animation/animated_transform.rs"
+
 [[example]]
 name = "custom_skinned_mesh"
 path = "examples/animation/custom_skinned_mesh.rs"

examples/README.md

@@ -123,6 +123,7 @@ Example | File | Description
 Example | File | Description
 --- | --- | ---
 `animated_fox` | [`animation/animated_fox.rs`](./animation/animated_fox.rs) | Plays an animation from a skinned glTF.
+`animated_transform` | [`animation/animated_transform.rs`](./animation/animated_transform.rs) | Create and play an animation defined by code that operates on the `Transform` component.
 `custom_skinned_mesh` | [`animation/custom_skinned_mesh.rs`](./animation/custom_skinned_mesh.rs) | Skinned mesh example with mesh and joints data defined in code.
 `gltf_skinned_mesh` | [`animation/gltf_skinned_mesh.rs`](./animation/gltf_skinned_mesh.rs) | Skinned mesh example with mesh and joints data loaded from a glTF file.
 

examples/animation/animated_transform.rs

@@ -0,0 +1,141 @@
+use std::f32::consts::{FRAC_PI_2, PI};
+
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .insert_resource(AmbientLight {
+            color: Color::WHITE,
+            brightness: 1.0,
+        })
+        .add_startup_system(setup)
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    mut animations: ResMut<Assets<AnimationClip>>,
+) {
+    // Camera
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..default()
+    });
+
+    // The animation API uses the `Name` component to target entities
+    let planet = Name::new("planet");
+    let orbit_controller = Name::new("orbit_controller");
+    let satellite = Name::new("satellite");
+
+    // Creating the animation
+    let mut animation = AnimationClip::default();
+    // A curve can modify a single part of a transform, here the translation
+    animation.add_curve_to_path(
+        EntityPath {
+            parts: vec![planet.clone()],
+        },
+        VariableCurve {
+            keyframe_timestamps: vec![0.0, 1.0, 2.0, 3.0, 4.0],
+            keyframes: Keyframes::Translation(vec![
+                Vec3::new(1.0, 0.0, 1.0),
+                Vec3::new(-1.0, 0.0, 1.0),
+                Vec3::new(-1.0, 0.0, -1.0),
+                Vec3::new(1.0, 0.0, -1.0),
+                // in case seamless looping is wanted, the last keyframe should
+                // be the same as the first one
+                Vec3::new(1.0, 0.0, 1.0),
+            ]),
+        },
+    );
+    // Or it can modify the rotation of the transform.
+    // To find the entity to modify, the hierarchy  will be traversed looking for
+    // an entity with the right name at each level
+    animation.add_curve_to_path(
+        EntityPath {
+            parts: vec![planet.clone(), orbit_controller.clone()],
+        },
+        VariableCurve {
+            keyframe_timestamps: vec![0.0, 1.0, 2.0, 3.0, 4.0],
+            keyframes: Keyframes::Rotation(vec![
+                Quat::from_axis_angle(Vec3::Y, 0.0),
+                Quat::from_axis_angle(Vec3::Y, FRAC_PI_2),
+                Quat::from_axis_angle(Vec3::Y, PI),
+                Quat::from_axis_angle(Vec3::Y, 3.0 * FRAC_PI_2),
+                Quat::from_axis_angle(Vec3::Y, 0.0),
+            ]),
+        },
+    );
+    // If a curve in an animation is shorter than the other, it will not repeat
+    // until all other curves are finished. In that case, another animation should
+    // be created for each part that would have a different duration / period
+    animation.add_curve_to_path(
+        EntityPath {
+            parts: vec![planet.clone(), orbit_controller.clone(), satellite.clone()],
+        },
+        VariableCurve {
+            keyframe_timestamps: vec![0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0],
+            keyframes: Keyframes::Scale(vec![
+                Vec3::splat(0.8),
+                Vec3::splat(1.2),
+                Vec3::splat(0.8),
+                Vec3::splat(1.2),
+                Vec3::splat(0.8),
+                Vec3::splat(1.2),
+                Vec3::splat(0.8),
+                Vec3::splat(1.2),
+                Vec3::splat(0.8),
+            ]),
+        },
+    );
+    // There can be more than one curve targeting the same entity path
+    animation.add_curve_to_path(
+        EntityPath {
+            parts: vec![planet.clone(), orbit_controller.clone(), satellite.clone()],
+        },
+        VariableCurve {
+            keyframe_timestamps: vec![0.0, 1.0, 2.0, 3.0, 4.0],
+            keyframes: Keyframes::Rotation(vec![
+                Quat::from_axis_angle(Vec3::Y, 0.0),
+                Quat::from_axis_angle(Vec3::Y, FRAC_PI_2),
+                Quat::from_axis_angle(Vec3::Y, PI),
+                Quat::from_axis_angle(Vec3::Y, 3.0 * FRAC_PI_2),
+                Quat::from_axis_angle(Vec3::Y, 0.0),
+            ]),
+        },
+    );
+
+    // Create the animation player, and set it to repeat
+    let mut player = AnimationPlayer::default();
+    player.play(animations.add(animation)).repeat();
+
+    // Create the scene that will be animated
+    // First entity is the planet
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Icosphere::default())),
+            material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
+            ..default()
+        })
+        // Add the Name component, and the animation player
+        .insert_bundle((planet, player))
+        .with_children(|p| {
+            // This entity is just used for animation, but doesn't display anything
+            p.spawn_bundle(TransformBundle { ..default() })
+                // Add the Name component
+                .insert(orbit_controller)
+                .with_children(|p| {
+                    // The satellite, placed at a distance of the planet
+                    p.spawn_bundle(PbrBundle {
+                        transform: Transform::from_xyz(1.5, 0.0, 0.0),
+                        mesh: meshes.add(Mesh::from(shape::Cube { size: 0.5 })),
+                        material: materials.add(Color::rgb(0.3, 0.9, 0.3).into()),
+                        ..default()
+                    })
+                    // Add the Name component
+                    .insert(satellite);
+                });
+        });
+}