non-normalized rotations

[matjlars]

I am getting an error in my game that seems like Avian is creating some non-normalized rotations. Specifically, bevy panics when calling transform.forward() when the rotation somehow becomes un-normalized. To explain what I mean, everywhere a rotation is set within Avian, if there was an assert!(rotation.is_normalized());, I think it may fail somewhere sometimes.

They key thing I wanted to demonstrate in this MRE below is that I am never setting a non-normalized rotation, and yet the rotations become non-normalized somehow somewhere.

I want to explain myself a little bit because it's tempting to think this is not a bug because I could just normalize the rotations, but that is not the case. I am making a spaceship rail shoot (like Star Fox for example) and my laser beams copy transform.forward() from the spaceship to know which direction to fly in. I have tried simply detecting non-normalized rotations and calling normalize() on those, and while this does resolve bevy panicking on transform.forward(), it also leads to about 5-10% of my lasers flying off in a random direction. Which, although funny, that does not work for the game :) I hope you understand. Thank you very much for making Avian!

use bevy::prelude::*;
use avian3d::prelude::*;

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

#[derive(Component)]
struct TestCube;

/// spawn a bunch of cubes to test
fn setup_scene(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
){
    commands.spawn(
        DirectionalLight::default(),
    );
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(50.0, 10.0, 50.0).looking_at(Vec3::new(50.0, 0.0, 0.0), Dir3::Y),
    ));

    // floor
    commands.spawn((
        RigidBody::Static,
        Collider::cylinder(100.0, 0.1),
        Mesh3d(meshes.add(Cylinder::new(100.0, 0.1))),
        MeshMaterial3d(materials.add(Color::WHITE)),
    ));

    // spawn some physics cubes
    for i in 0..100 {
        let x = i as f32;

        commands.spawn((
            Mesh3d(meshes.add(Cuboid::new(1.0, 1.0, 1.0))),
            Transform::from_xyz(x * 1.2, 0.0, -4.0),
            //RigidBody::Dynamic,  // NOTE: I never got a non-normalized rotation with a dynamic body. maybe this issue only exists for kinematic bodies?
            RigidBody::Kinematic,
            Collider::cuboid(1.0, 1.0, 1.0),
            MeshMaterial3d(materials.add(Color::WHITE)),
            TestCube,
        ));
    }
}


fn rotate_cubes(
    mut q: Query<&mut Transform, With<TestCube>>,
){
    for mut transform in q.iter_mut() {
        assert!(transform.rotation.is_normalized(), "A cube has a non-normalized rotation!");

        let pos = transform.translation;
        transform.rotation *= Quat::from_euler(EulerRot::XYZ, pos.x * 1.0, pos.x * 2.0, pos.x * 3.0);
        assert!(transform.rotation.is_normalized(), "I just set the rotation and it's not normalized. (This never happens)");
    }
}

(edit: simpler MRE)

[Jondolf]

Thanks for the great example! #620 seems to fix the problem at least for the MRE in my testing.

[matjlars]

Excellent! Thank you so much!!