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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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What is Bevy?
Bevy is a refreshingly simple data-driven game engine built in Rust. It is free and open-source forever!
WARNING
Bevy is still in the early stages of development. Important features are missing. Documentation is sparse. A new version of Bevy containing breaking changes to the API is released approximately once every 3 months. We provide migration guides, but we can't guarantee migrations will always be easy. Use only if you are willing to work in this environment.
MSRV: Bevy relies heavily on improvements in the Rust language and compiler. As a result, the Minimum Supported Rust Version (MSRV) is generally close to "the latest stable release" of Rust.
Design Goals
- Capable: Offer a complete 2D and 3D feature set
- Simple: Easy for newbies to pick up, but infinitely flexible for power users
- Data Focused: Data-oriented architecture using the Entity Component System paradigm
- Modular: Use only what you need. Replace what you don't like
- Fast: App logic should run quickly, and when possible, in parallel
- Productive: Changes should compile quickly ... waiting isn't fun
About
- Features: A quick overview of Bevy's features.
- News: A development blog that covers our progress, plans and shiny new features.
Docs
- Quick Start Guide: Bevy's official Quick Start Guide. The best place to start learning Bevy.
- Bevy Rust API Docs: Bevy's Rust API docs, which are automatically generated from the doc comments in this repo.
- Official Examples: Bevy's dedicated, runnable examples, which are great for digging into specific concepts.
- Community-Made Learning Resources: More tutorials, documentation, and examples made by the Bevy community.
Community
Before contributing or participating in discussions with the community, you should familiarize yourself with our Code of Conduct.
- Discord: Bevy's official discord server.
- Reddit: Bevy's official subreddit.
- GitHub Discussions: The best place for questions about Bevy, answered right here!
- Bevy Assets: A collection of awesome Bevy projects, tools, plugins and learning materials.
Contributing
If you'd like to help build Bevy, check out the Contributor's Guide. For simple problems, feel free to open an issue or PR and tackle it yourself!
For more complex architecture decisions and experimental mad science, please open an RFC (Request For Comments) so we can brainstorm together effectively!
Getting Started
We recommend checking out the Quick Start Guide for a brief introduction.
Follow the Setup guide to ensure your development environment is set up correctly. Once set up, you can quickly try out the examples by cloning this repo and running the following commands:
# Switch to the correct version (latest release, default is main development branch)
git checkout latest
# Runs the "breakout" example
cargo run --example breakout
To draw a window with standard functionality enabled, use:
use bevy::prelude::*;
fn main(){
App::new()
.add_plugins(DefaultPlugins)
.run();
}
Fast Compiles
Bevy can be built just fine using default configuration on stable Rust. However for really fast iterative compiles, you should enable the "fast compiles" setup by following the instructions here.
Bevy Cargo Features
This list outlines the different cargo features supported by Bevy. These allow you to customize the Bevy feature set for your use-case.
Thanks
Bevy is the result of the hard work of many people. A huge thanks to all Bevy contributors, the many open source projects that have come before us, the Rust gamedev ecosystem, and the many libraries we build on.
A huge thanks to Bevy's generous sponsors. Bevy will always be free and open source, but it isn't free to make. Please consider sponsoring our work if you like what we're building.
This project is tested with BrowserStack.
License
Bevy is free, open source and permissively licensed! Except where noted (below and/or in individual files), all code in this repository is dual-licensed under either:
- MIT License (LICENSE-MIT or http://opensource.org/licenses/MIT)
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
at your option. This means you can select the license you prefer! This dual-licensing approach is the de-facto standard in the Rust ecosystem and there are very good reasons to include both.
Some of the engine's code carries additional copyright notices and license terms due to their external origins.
These are generally BSD-like, but exact details vary by crate:
If the README of a crate contains a 'License' header (or similar), the additional copyright notices and license terms applicable to that crate will be listed.
The above licensing requirement still applies to contributions to those crates, and sections of those crates will carry those license terms.
The license field of each crate will also reflect this.
For example, bevy_mikktspace has code under the Zlib license (as well as a copyright notice when choosing the MIT license).
The assets included in this repository (for our examples) typically fall under different open licenses. These will not be included in your game (unless copied in by you), and they are not distributed in the published bevy crates. See CREDITS.md for the details of the licenses of those files.
Your contributions
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.