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# Objective This PR extends and reworks the material from #15282 by allowing arbitrary curves to be used by the animation system to animate arbitrary properties. The goals of this work are to: - Allow far greater flexibility in how animations are allowed to be defined in order to be used with `bevy_animation`. - Delegate responsibility over keyframe interpolation to `bevy_math` and the `Curve` libraries and reduce reliance on keyframes in animation definitions generally. - Move away from allowing the glTF spec to completely define animations on a mechanical level. ## Solution ### Overview At a high level, curves have been incorporated into the animation system using the `AnimationCurve` trait (closely related to what was `Keyframes`). From the top down: 1. In `animate_targets`, animations are driven by `VariableCurve`, which is now a thin wrapper around a `Box<dyn AnimationCurve>`. 2. `AnimationCurve` is something built out of a `Curve`, and it tells the animation system how to use the curve's output to actually mutate component properties. The trait looks like this: ```rust /// A low-level trait that provides control over how curves are actually applied to entities /// by the animation system. /// /// Typically, this will not need to be implemented manually, since it is automatically /// implemented by [`AnimatableCurve`] and other curves used by the animation system /// (e.g. those that animate parts of transforms or morph weights). However, this can be /// implemented manually when `AnimatableCurve` is not sufficiently expressive. /// /// In many respects, this behaves like a type-erased form of [`Curve`], where the output /// type of the curve is remembered only in the components that are mutated in the /// implementation of [`apply`]. /// /// [`apply`]: AnimationCurve::apply pub trait AnimationCurve: Reflect + Debug + Send + Sync { /// Returns a boxed clone of this value. fn clone_value(&self) -> Box<dyn AnimationCurve>; /// The range of times for which this animation is defined. fn domain(&self) -> Interval; /// Write the value of sampling this curve at time `t` into `transform` or `entity`, /// as appropriate, interpolating between the existing value and the sampled value /// using the given `weight`. fn apply<'a>( &self, t: f32, transform: Option<Mut<'a, Transform>>, entity: EntityMutExcept<'a, (Transform, AnimationPlayer, Handle<AnimationGraph>)>, weight: f32, ) -> Result<(), AnimationEvaluationError>; } ``` 3. The conversion process from a `Curve` to an `AnimationCurve` involves using wrappers which communicate the intent to animate a particular property. For example, here is `TranslationCurve`, which wraps a `Curve<Vec3>` and uses it to animate `Transform::translation`: ```rust /// This type allows a curve valued in `Vec3` to become an [`AnimationCurve`] that animates /// the translation component of a transform. pub struct TranslationCurve<C>(pub C); ``` ### Animatable Properties The `AnimatableProperty` trait survives in the transition, and it can be used to allow curves to animate arbitrary component properties. The updated documentation for `AnimatableProperty` explains this process: <details> <summary>Expand AnimatableProperty example</summary An `AnimatableProperty` is a value on a component that Bevy can animate. You can implement this trait on a unit struct in order to support animating custom components other than transforms and morph weights. Use that type in conjunction with `AnimatableCurve` (and perhaps `AnimatableKeyframeCurve` to define the animation itself). For example, in order to animate font size of a text section from 24 pt. to 80 pt., you might use: ```rust #[derive(Reflect)] struct FontSizeProperty; impl AnimatableProperty for FontSizeProperty { type Component = Text; type Property = f32; fn get_mut(component: &mut Self::Component) -> Option<&mut Self::Property> { Some(&mut component.sections.get_mut(0)?.style.font_size) } } ``` You can then create an `AnimationClip` to animate this property like so: ```rust let mut animation_clip = AnimationClip::default(); animation_clip.add_curve_to_target( animation_target_id, AnimatableKeyframeCurve::new( [ (0.0, 24.0), (1.0, 80.0), ] ) .map(AnimatableCurve::<FontSizeProperty, _>::from_curve) .expect("Failed to create font size curve") ); ``` Here, the use of `AnimatableKeyframeCurve` creates a curve out of the given keyframe time-value pairs, using the `Animatable` implementation of `f32` to interpolate between them. The invocation of `AnimatableCurve::from_curve` with `FontSizeProperty` indicates that the `f32` output from that curve is to be used to animate the font size of a `Text` component (as configured above). </details> ### glTF Loading glTF animations are now loaded into `Curve` types of various kinds, depending on what is being animated and what interpolation mode is being used. Those types get wrapped into and converted into `Box<dyn AnimationCurve>` and shoved inside of a `VariableCurve` just like everybody else. ### Morph Weights There is an `IterableCurve` abstraction which allows sampling these from a contiguous buffer without allocating. Its only reason for existing is that Rust disallows you from naming function types, otherwise we would just use `Curve` with an iterator output type. (The iterator involves `Map`, and the name of the function type would have to be able to be named, but it is not.) A `WeightsCurve` adaptor turns an `IterableCurve` into an `AnimationCurve`, so it behaves like everything else in that regard. ## Testing Tested by running existing animation examples. Interpolation logic has had additional tests added within the `Curve` API to replace the tests in `bevy_animation`. Some kinds of out-of-bounds errors have become impossible. Performance testing on `many_foxes` (`animate_targets`) suggests that performance is very similar to the existing implementation. Here are a couple trace histograms across different runs (yellow is this branch, red is main). <img width="669" alt="Screenshot 2024-09-27 at 9 41 50 AM" src="https://github.com/user-attachments/assets/5ba4e9ac-3aea-452e-aaf8-1492acc2d7fc"> <img width="673" alt="Screenshot 2024-09-27 at 9 45 18 AM" src="https://github.com/user-attachments/assets/8982538b-04cf-46b5-97b2-164c6bc8162e"> --- ## Migration Guide Most user code that does not directly deal with `AnimationClip` and `VariableCurve` will not need to be changed. On the other hand, `VariableCurve` has been completely overhauled. If you were previously defining animation curves in code using keyframes, you will need to migrate that code to use curve constructors instead. For example, a rotation animation defined using keyframes and added to an animation clip like this: ```rust animation_clip.add_curve_to_target( animation_target_id, VariableCurve { keyframe_timestamps: vec![0.0, 1.0, 2.0, 3.0, 4.0], keyframes: Keyframes::Rotation(vec![ Quat::IDENTITY, Quat::from_axis_angle(Vec3::Y, PI / 2.), Quat::from_axis_angle(Vec3::Y, PI / 2. * 2.), Quat::from_axis_angle(Vec3::Y, PI / 2. * 3.), Quat::IDENTITY, ]), interpolation: Interpolation::Linear, }, ); ``` would now be added like this: ```rust animation_clip.add_curve_to_target( animation_target_id, AnimatableKeyframeCurve::new([0.0, 1.0, 2.0, 3.0, 4.0].into_iter().zip([ Quat::IDENTITY, Quat::from_axis_angle(Vec3::Y, PI / 2.), Quat::from_axis_angle(Vec3::Y, PI / 2. * 2.), Quat::from_axis_angle(Vec3::Y, PI / 2. * 3.), Quat::IDENTITY, ])) .map(RotationCurve) .expect("Failed to build rotation curve"), ); ``` Note that the interface of `AnimationClip::add_curve_to_target` has also changed (as this example shows, if subtly), and now takes its curve input as an `impl AnimationCurve`. If you need to add a `VariableCurve` directly, a new method `add_variable_curve_to_target` accommodates that (and serves as a one-to-one migration in this regard). ### For reviewers The diff is pretty big, and the structure of some of the changes might not be super-obvious: - `keyframes.rs` became `animation_curves.rs`, and `AnimationCurve` is based heavily on `Keyframes`, with the adaptors also largely following suite. - The Curve API adaptor structs were moved from `bevy_math::curve::mod` into their own module `adaptors`. There are no functional changes to how these adaptors work; this is just to make room for the specialized reflection implementations since `mod.rs` was getting kind of cramped. - The new module `gltf_curves` holds the additional curve constructions that are needed by the glTF loader. Note that the loader uses a mix of these and off-the-shelf `bevy_math` curve stuff. - `animatable.rs` no longer holds logic related to keyframe interpolation, which is now delegated to the existing abstractions in `bevy_math::curve::cores`. --------- Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com> Co-authored-by: aecsocket <43144841+aecsocket@users.noreply.github.com> |
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Bevy Math
