README.md

bevy_xpbd_3d_parenting

Allows children of a bevy_xpbd_3d RigidBody to exert forces on their parents.

Installation

# Use the latest release of bevy_xpbd_3d_parenting
[dependencies.bevy_xpbd_3d_parenting]
version = "0.2"
default-features = false

Theoretical usage

This library exports a single Plugin, ParentingPlugin, which must be added to the app with the same Schedule as bevy_xpbd_3d's PhysicsPlugin.

Then, any systems that mutate these InternalForces should be schedule before:

bevy_xpbd_3d_parenting::ParentingSystemSet::ManuallyClearForces;

Parents must have:

• RigidBody
  • RigidBody::Dynamic or nothing will move
  • Collider so that bevy_xpbd works
• ExternalForce with persistence set to false (will warn using tracing if not upheld)
• TransformBundle for position in space:
  • Transform
  • GlobalTransform

Children must have:

• RigidBody, see parent
• TransformBundle, see parent
• InternalForce to exert forces on the parent

Types of Internal Forces

Internal forces come in two flavours, InternalForce::Global and InternalForce::Local. Local InternalForces are in the local space of the parent, while global InternalForces are in the global space. This means local InternalForces will rotate with the parent, while global InternalForces will not.

Check out the global_versus_local example for a demonstration.

Quick usage example:

See the examples for complete examples.

// Shows a basic usage of [InternalForce].

use bevy::prelude::*;
use bevy_xpbd_3d::prelude::*;
use bevy_xpbd_3d_parenting::InternalForce;

fn main() {
	App::new()
		.add_plugins((
			DefaultPlugins,
			PhysicsPlugins::new(Update),
			bevy_xpbd_3d_parenting::prelude::ParentingPlugin::new(Update),
		))
		.add_systems(Startup, setup)
		.run();
}

fn setup(
	mut commands: Commands,
	mut meshs: ResMut<Assets<Mesh>>,
	mut materials: ResMut<Assets<StandardMaterial>>,
) {
	// cube
	let mut cube = commands.spawn((
		PbrBundle {
			mesh: meshs.add(Mesh::from(Cuboid::default())),
			transform: Transform::from_xyz(0., 5., 0.),
			..default()
		},
		RigidBody::Dynamic,
		// IMPORTANT: parent's external force must be non-persistent
		// so that each frame this library can update it
		ExternalForce::ZERO.with_persistence(false),
		// Exact collider is arbitrary
		Collider::capsule(1.0, 1.0),
	));

	// sphere child: using internal force
	cube.with_children(|cube| {
		cube.spawn((
			PbrBundle {
				mesh: meshs.add(Mesh::from(Sphere::new(0.5).mesh().uv(16, 32))),
				// to the right a bit
				transform: Transform::from_xyz(3.0, 0.0, 0.0),
				..default()
			},
			// no rigidbody
			// no external force
			// internal force pushes downwards, which should rotate clockwise
			InternalForce::new_local(Vec3::new(0., -3., 0.)),
		));
	});
}

Running examples

Run:

cargo r --example rotating --features bevy_xpbd_3d/async-collider
cargo r --example global_versus_local --features bevy_xpbd_3d/async-collider

Compatibility table

Bevy	Bevy XPBD	Bevy XPBD 3D Parenting
0.12	0.3.3	0.1.0
0.13	0.4.2	0.2.0

Developing notes

cargo t for testing, which runs:

• Every combination of feature flags
• A variety of proptests