Bevy XPBD

Bevy XPBD is a 2D and 3D physics engine based on Extended Position Based Dynamics (XPBD) for the Bevy game engine.

Design

Below are some of the core design principles used in Bevy XPBD.

• Made with Bevy, for Bevy. No wrappers around existing engines.
• Provide an ergonomic and familiar API. Ergonomics is key for a good experience.
• Utilize the ECS as much as possible. The engine should feel like a part of Bevy, and it shouldn't need to maintain a separate physics world.
• Use a highly modular plugin architecture. Users should be able to replace parts of the engine with their own implementations.
• Have good documentation. A physics engine is pointless if you don't know how to use it.

Features

Below are some of the current features of Bevy XPBD.

• Dynamic, kinematic and static rigid bodies
  • Linear and angular velocity
  • External forces, torque and impulses
  • Gravity and gravity scale
  • Linear and angular damping
  • Locking translational and rotational axes
  • Rigid body dominance
  • Automatic deactivation with sleeping
• Collision detection powered by Parry
  • Colliders with configurable collision layers, density, material properties and more
  • Collision events
  • Access to colliding entities
  • Filtering and modifying collisions with custom systems
  • Manual contact queries and intersection tests
• Constraints and joints
  • Flexible API for creating position-based constraints
  • Several built-in joint types: fixed, distance, prismatic, revolute, spherical
  • Support for custom joints and other constraints
• Spatial queries
  • Raycasting, shapecasting, point projection and intersection tests
  • Ergonomic component-based API for raycasts and shapecasts
  • Flexible SpatialQuery system parameter
  • Spatial query filters
• Debug rendering for colliders, AABBs, contacts, joints, sleeping, axes and spatial queries
• Configurable scheduling and high customizability
• Highly modular plugin architecture, freely extend and replace parts of the engine
• f32/f64 precision (f32 by default)

You can find a more complete list along with documentation in the Table of contents on docs.rs.

Documentation

• 2D documentation
• 3D documentation

Usage example

First, add bevy_xpbd_2d or bevy_xpbd_3d to your dependencies in Cargo.toml:

# For 2D applications:
[dependencies]
bevy_xpbd_2d = "0.3"

# For 3D applications:
[dependencies]
bevy_xpbd_3d = "0.3"

# If you want to use the most up-to-date version, you can follow the main branch:
[dependencies]
bevy_xpbd_3d = { git = "https://github.com/Jondolf/bevy_xpbd", branch = "main" }

Below is a very simple example where a box with initial angular velocity falls onto a plane. This is a modified version of Bevy's 3d_scene example.

use bevy::prelude::*;
use bevy_xpbd_3d::prelude::*;

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, PhysicsPlugins::default()))
        .add_systems(Startup, setup)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // Plane
    commands.spawn((
        RigidBody::Static,
        Collider::cuboid(8.0, 0.002, 8.0),
        PbrBundle {
            mesh: meshes.add(Mesh::from(shape::Plane::from_size(8.0))),
            material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
            ..default()
        },
    ));
    // Cube
    commands.spawn((
        RigidBody::Dynamic,
        AngularVelocity(Vec3::new(2.5, 3.4, 1.6)),
        Collider::cuboid(1.0, 1.0, 1.0),
        PbrBundle {
            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
            material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
            transform: Transform::from_xyz(0.0, 4.0, 0.0),
            ..default()
        },
    ));
    // Light
    commands.spawn(PointLightBundle {
        point_light: PointLight {
            intensity: 1500.0,
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..default()
    });
    // Camera
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-4.0, 6.5, 8.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

3d_scene.mp4

More examples

You can find lots of 2D and 3D examples in /crates/bevy_xpbd_2d/examples and /crates/bevy_xpbd_3d/examples respectively.

The examples support both f32 and f64 precisions, so the code contains some feature-dependent types like Scalar and Vector. In actual usage these are not needed, so you can just use f32 or f64 types depending on the features you have chosen.

By default the examples use f32. To run the f64 versions, you need to disable default features and manually choose the dimension and precision:

cargo run --example cubes --no-default-features --features "3d f64"

Supported Bevy versions

Bevy	Bevy XPBD
0.12	0.3
0.11	0.2
0.10	0.1

Future features

• Joint motors
• Articulations, aka. multibody joints
• Continuous collision detection (CCD)
• Per-entity collision hooks or callbacks
• Flags for what types of collisions are active, like collisions against specific rigid body types, sensors or parents
• Performance optimization (better broad phase, parallel solver...)
• Proper cross-platform determinism
• Soft bodies (cloth and deformable solids)
• Maybe fluid simulation

Contributing

If you encounter any problems, feel free to open issues or create pull requests. For larger changes and additions, it's better to open an issue or ask me for input before making a pull request.

You can also ask for help or ask questions on the Bevy Discord server's bevy_xpbd thread in #crate-help. My username on the Discord is Jondolf (@jondolfdev).

Acknowledgements

Huge thanks to the entire Bevy community for the incredible support! All of your contributions, insight and requests are a massive help in driving the state of physics in Bevy forward, and it's what keeps me motivated to build the best engine I can.

I would also like to give a special thanks to Johan Helsing for inspiring this project and helping me significantly in the early stages. His original tutorial series is the reason bevy_xpbd exists in the first place, and without his support and contributions, the project wouldn't be anywhere near where it is today.

License

Bevy XPBD is free and open source. 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.