Smooth Transition between Animations (bevyengine#6922)

# Objective

- Fixes bevyengine#6338

This PR allows for smooth transitions between different animations.

## Solution

- This PR uses very simple linear blending of animations.
- When starting a new animation, you can give it a duration, and throughout that duration, the previous and the new animation are being linearly blended, until only the new animation is running.
- I'm aware of bevyengine/rfcs#49 and bevyengine/rfcs#51, which are more complete solutions to this problem, but they seem still far from being implemented. Until they're ready, this PR allows for the most basic use case of blending, i.e. smoothly transitioning between different animations.

## Migration Guide

- no bc breaking changes

crates/bevy_animation/src/lib.rs

@@ -3,12 +3,13 @@
 #![warn(missing_docs)]
 
 use std::ops::Deref;
+use std::time::Duration;
 
 use bevy_app::{App, CoreStage, Plugin};
 use bevy_asset::{AddAsset, Assets, Handle};
 use bevy_core::Name;
 use bevy_ecs::{
-    change_detection::DetectChanges,
+    change_detection::{DetectChanges, Mut},
     entity::Entity,
     prelude::Component,
     query::With,
@@ -114,22 +115,18 @@ impl AnimationClip {
     }
 }
 
-/// Animation controls
-#[derive(Component, Reflect)]
-#[reflect(Component)]
-pub struct AnimationPlayer {
-    paused: bool,
+#[derive(Reflect)]
+struct PlayingAnimation {
     repeat: bool,
     speed: f32,
     elapsed: f32,
     animation_clip: Handle<AnimationClip>,
     path_cache: Vec<Vec<Option<Entity>>>,
 }
 
-impl Default for AnimationPlayer {
+impl Default for PlayingAnimation {
     fn default() -> Self {
         Self {
-            paused: false,
             repeat: false,
             speed: 1.0,
             elapsed: 0.0,
@@ -139,33 +136,106 @@ impl Default for AnimationPlayer {
     }
 }
 
+/// An animation that is being faded out as part of a transition
+struct AnimationTransition {
+    /// The current weight. Starts at 1.0 and goes to 0.0 during the fade-out.
+    current_weight: f32,
+    /// How much to decrease `current_weight` per second
+    weight_decline_per_sec: f32,
+    /// The animation that is being faded out
+    animation: PlayingAnimation,
+}
+
+/// Animation controls
+#[derive(Component, Default, Reflect)]
+#[reflect(Component)]
+pub struct AnimationPlayer {
+    paused: bool,
+
+    animation: PlayingAnimation,
+
+    // List of previous animations we're currently transitioning away from.
+    // Usually this is empty, when transitioning between animations, there is
+    // one entry. When another animation transition happens while a transition
+    // is still ongoing, then there can be more than one entry.
+    // Once a transition is finished, it will be automatically removed from the list
+    #[reflect(ignore)]
+    transitions: Vec<AnimationTransition>,
+}
+
 impl AnimationPlayer {
     /// Start playing an animation, resetting state of the player
+    /// This will use a linear blending between the previous and the new animation to make a smooth transition
     pub fn start(&mut self, handle: Handle<AnimationClip>) -> &mut Self {
-        *self = Self {
+        self.animation = PlayingAnimation {
             animation_clip: handle,
             ..Default::default()
         };
+
+        // We want a hard transition.
+        // In case any previous transitions are still playing, stop them
+        self.transitions.clear();
+
+        self
+    }
+
+    /// Start playing an animation, resetting state of the player
+    /// This will use a linear blending between the previous and the new animation to make a smooth transition
+    pub fn start_with_transition(
+        &mut self,
+        handle: Handle<AnimationClip>,
+        transition_duration: Duration,
+    ) -> &mut Self {
+        let mut animation = PlayingAnimation {
+            animation_clip: handle,
+            ..Default::default()
+        };
+        std::mem::swap(&mut animation, &mut self.animation);
+
+        // Add the current transition. If other transitions are still ongoing,
+        // this will keep those transitions running and cause a transition between
+        // the output of that previous transition to the new animation.
+        self.transitions.push(AnimationTransition {
+            current_weight: 1.0,
+            weight_decline_per_sec: 1.0 / transition_duration.as_secs_f32(),
+            animation,
+        });
+
         self
     }
 
     /// Start playing an animation, resetting state of the player, unless the requested animation is already playing.
+    /// If `transition_duration` is set, this will use a linear blending
+    /// between the previous and the new animation to make a smooth transition
     pub fn play(&mut self, handle: Handle<AnimationClip>) -> &mut Self {
-        if self.animation_clip != handle || self.is_paused() {
+        if self.animation.animation_clip != handle || self.is_paused() {
             self.start(handle);
         }
         self
     }
 
+    /// Start playing an animation, resetting state of the player, unless the requested animation is already playing.
+    /// This will use a linear blending between the previous and the new animation to make a smooth transition
+    pub fn play_with_transition(
+        &mut self,
+        handle: Handle<AnimationClip>,
+        transition_duration: Duration,
+    ) -> &mut Self {
+        if self.animation.animation_clip != handle || self.is_paused() {
+            self.start_with_transition(handle, transition_duration);
+        }
+        self
+    }
+
     /// Set the animation to repeat
     pub fn repeat(&mut self) -> &mut Self {
-        self.repeat = true;
+        self.animation.repeat = true;
         self
     }
 
     /// Stop the animation from repeating
     pub fn stop_repeating(&mut self) -> &mut Self {
-        self.repeat = false;
+        self.animation.repeat = false;
         self
     }
 
@@ -186,23 +256,23 @@ impl AnimationPlayer {
 
     /// Speed of the animation playback
     pub fn speed(&self) -> f32 {
-        self.speed
+        self.animation.speed
     }
 
     /// Set the speed of the animation playback
     pub fn set_speed(&mut self, speed: f32) -> &mut Self {
-        self.speed = speed;
+        self.animation.speed = speed;
         self
     }
 
     /// Time elapsed playing the animation
     pub fn elapsed(&self) -> f32 {
-        self.elapsed
+        self.animation.elapsed
     }
 
     /// Seek to a specific time in the animation
     pub fn set_elapsed(&mut self, elapsed: f32) -> &mut Self {
-        self.elapsed = elapsed;
+        self.animation.elapsed = elapsed;
         self
     }
 }
@@ -283,37 +353,119 @@ pub fn animation_player(
     mut animation_players: Query<(Entity, Option<&Parent>, &mut AnimationPlayer)>,
 ) {
     animation_players.par_for_each_mut(10, |(root, maybe_parent, mut player)| {
-        let Some(animation_clip) = animations.get(&player.animation_clip) else { return };
-        // Continue if paused unless the `AnimationPlayer` was changed
-        // This allow the animation to still be updated if the player.elapsed field was manually updated in pause
-        if player.paused && !player.is_changed() {
-            return;
-        }
-        if !player.paused {
-            player.elapsed += time.delta_seconds() * player.speed;
+        update_transitions(&mut player, &time);
+        run_animation_player(
+            root,
+            player,
+            &time,
+            &animations,
+            &names,
+            &transforms,
+            maybe_parent,
+            &parents,
+            &children,
+        );
+    });
+}
+
+#[allow(clippy::too_many_arguments)]
+fn run_animation_player(
+    root: Entity,
+    mut player: Mut<AnimationPlayer>,
+    time: &Time,
+    animations: &Assets<AnimationClip>,
+    names: &Query<&Name>,
+    transforms: &Query<&mut Transform>,
+    maybe_parent: Option<&Parent>,
+    parents: &Query<(Option<With<AnimationPlayer>>, Option<&Parent>)>,
+    children: &Query<&Children>,
+) {
+    let paused = player.paused;
+    // Continue if paused unless the `AnimationPlayer` was changed
+    // This allow the animation to still be updated if the player.elapsed field was manually updated in pause
+    if paused && !player.is_changed() {
+        return;
+    }
+
+    // Apply the main animation
+    apply_animation(
+        1.0,
+        &mut player.animation,
+        paused,
+        root,
+        time,
+        animations,
+        names,
+        transforms,
+        maybe_parent,
+        parents,
+        children,
+    );
+
+    // Apply any potential fade-out transitions from previous animations
+    for AnimationTransition {
+        current_weight,
+        animation,
+        ..
+    } in &mut player.transitions
+    {
+        apply_animation(
+            *current_weight,
+            animation,
+            paused,
+            root,
+            time,
+            animations,
+            names,
+            transforms,
+            maybe_parent,
+            parents,
+            children,
+        );
+    }
+}
+
+#[allow(clippy::too_many_arguments)]
+fn apply_animation(
+    weight: f32,
+    animation: &mut PlayingAnimation,
+    paused: bool,
+    root: Entity,
+    time: &Time,
+    animations: &Assets<AnimationClip>,
+    names: &Query<&Name>,
+    transforms: &Query<&mut Transform>,
+    maybe_parent: Option<&Parent>,
+    parents: &Query<(Option<With<AnimationPlayer>>, Option<&Parent>)>,
+    children: &Query<&Children>,
+) {
+    if let Some(animation_clip) = animations.get(&animation.animation_clip) {
+        if !paused {
+            animation.elapsed += time.delta_seconds() * animation.speed;
         }
-        let mut elapsed = player.elapsed;
-        if player.repeat {
+        let mut elapsed = animation.elapsed;
+        if animation.repeat {
             elapsed %= animation_clip.duration;
         }
         if elapsed < 0.0 {
             elapsed += animation_clip.duration;
         }
-        if player.path_cache.len() != animation_clip.paths.len() {
-            player.path_cache = vec![Vec::new(); animation_clip.paths.len()];
+        if animation.path_cache.len() != animation_clip.paths.len() {
+            animation.path_cache = vec![Vec::new(); animation_clip.paths.len()];
         }
-        if !verify_no_ancestor_player(maybe_parent, &parents) {
+        if !verify_no_ancestor_player(maybe_parent, parents) {
             warn!("Animation player on {:?} has a conflicting animation player on an ancestor. Cannot safely animate.", root);
             return;
         }
+
         for (path, bone_id) in &animation_clip.paths {
-            let cached_path = &mut player.path_cache[*bone_id];
+            let cached_path = &mut animation.path_cache[*bone_id];
             let curves = animation_clip.get_curves(*bone_id).unwrap();
-            let Some(target) = find_bone(root, path, &children, &names, cached_path) else { continue };
+            let Some(target) = find_bone(root, path, children, names, cached_path) else { continue };
             // SAFETY: The verify_no_ancestor_player check above ensures that two animation players cannot alias
             // any of their descendant Transforms.
-            // 
-            // The system scheduler prevents any other system from mutating Transforms at the same time, 
+            //
+            // The system scheduler prevents any other system from mutating Transforms at the same time,
             // so the only way this fetch can alias is if two AnimationPlayers are targetting the same bone.
             // This can only happen if there are two or more AnimationPlayers are ancestors to the same
             // entities. By verifying that there is no other AnimationPlayer in the ancestors of a
@@ -327,11 +479,16 @@ pub fn animation_player(
                 // Some curves have only one keyframe used to set a transform
                 if curve.keyframe_timestamps.len() == 1 {
                     match &curve.keyframes {
-                        Keyframes::Rotation(keyframes) => transform.rotation = keyframes[0],
+                        Keyframes::Rotation(keyframes) => {
+                            transform.rotation = transform.rotation.slerp(keyframes[0], weight);
+                        }
                         Keyframes::Translation(keyframes) => {
-                            transform.translation = keyframes[0];
+                            transform.translation =
+                                transform.translation.lerp(keyframes[0], weight);
+                        }
+                        Keyframes::Scale(keyframes) => {
+                            transform.scale = transform.scale.lerp(keyframes[0], weight);
                         }
-                        Keyframes::Scale(keyframes) => transform.scale = keyframes[0],
                     }
                     continue;
                 }
@@ -362,24 +519,31 @@ pub fn animation_player(
                             rot_end = -rot_end;
                         }
                         // Rotations are using a spherical linear interpolation
-                        transform.rotation =
-                            rot_start.normalize().slerp(rot_end.normalize(), lerp);
+                        let rot = rot_start.normalize().slerp(rot_end.normalize(), lerp);
+                        transform.rotation = transform.rotation.slerp(rot, weight);
                     }
                     Keyframes::Translation(keyframes) => {
                         let translation_start = keyframes[step_start];
                         let translation_end = keyframes[step_start + 1];
                         let result = translation_start.lerp(translation_end, lerp);
-                        transform.translation = result;
+                        transform.translation = transform.translation.lerp(result, weight);
                     }
                     Keyframes::Scale(keyframes) => {
                         let scale_start = keyframes[step_start];
                         let scale_end = keyframes[step_start + 1];
                         let result = scale_start.lerp(scale_end, lerp);
-                        transform.scale = result;
+                        transform.scale = transform.scale.lerp(result, weight);
                     }
                 }
             }
         }
+    }
+}
+
+fn update_transitions(player: &mut AnimationPlayer, time: &Time) {
+    player.transitions.retain_mut(|animation| {
+        animation.current_weight -= animation.weight_decline_per_sec * time.delta_seconds();
+        animation.current_weight > 0.0
     });
 }
 

examples/animation/animated_fox.rs

@@ -1,6 +1,7 @@
 //! Plays animations from a skinned glTF.
 
 use std::f32::consts::PI;
+use std::time::Duration;
 
 use bevy::prelude::*;
 
@@ -122,7 +123,10 @@ fn keyboard_animation_control(
         if keyboard_input.just_pressed(KeyCode::Return) {
             *current_animation = (*current_animation + 1) % animations.0.len();
             player
-                .play(animations.0[*current_animation].clone_weak())
+                .play_with_transition(
+                    animations.0[*current_animation].clone_weak(),
+                    Duration::from_millis(250),
+                )
                 .repeat();
         }
     }