The difference between rotate_z and rotate_local_x in Transform

[zingxy]

I wrote a demo, but it seems there is no difference between the two

use std::f32::consts::PI;

use bevy::prelude::*;

#[derive(Component)]
struct Parent;

#[derive(Component)]
struct Child;

#[derive(Component)]
struct RotateLocal;
fn setup(
    mut command: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    command.spawn(Camera2d);

    command.spawn((
        Parent,
        Mesh2d(meshes.add(Rectangle::new(100.0, 100.0))),
        MeshMaterial2d(materials.add(Color::srgb(0.0, 1.0, 0.0))),
        Transform::from_xyz(-100.0, 100.0, 0.0),
        children![(
            Child,
            Mesh2d(meshes.add(Rectangle::new(20.0, 20.0))),
            MeshMaterial2d(materials.add(Color::srgb(1.0, 1.0, 0.0))),
            Transform::from_xyz(50.0, 50.0, 0.0),
        )],
    ));
}

fn rotate_child(mut transform: Single<&mut Transform, With<Child>>) {

    // are there any differencies?
    // transform.rotate_local_z(PI / 180.0);
    transform.rotate_z(PI / 180.0);
}

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

[Jondolf]

Here you're applying a rotation to the identity rotation (i.e. no initial rotation) and only rotating about the Z axis. In this case, the local Z axis and the global Z axis (or specifically the Z axis relative to the parent since this is a child entity) are always the same. If you make the initial Transform rotated about the X or Y axis by some amount, you'll see that rotate_z and rotate_local_z will then behave differently since the local and global Z axes would no longer be aligned.