[Rapier] [3D] Is it possible to have a collider attached directly to a RigidBody?

[ThibaultLemaire]

Alt. title: Why is Rapier messing with my meshes??

Mesh entity has RigidBody::Dynamic, this is what I do not want (// 1 commented out, // 2 uncommented, see code snippets below):

Parent entity has RigidBody::Dynamic, this is what I want (// 1 uncommented, // 2 commented out):

And for completeness, parent entity does not have a RigidBody (// 1 commented out, // 2 commented out):

I am loading a gltf made with Blender and building colliders from its meshes. The tiles that you see on screen are 2 entities each: The Blender object with its Transform and custom Tile marker component, and its child, the Blender mesh (with a Mesh3d component).

When I attach the RigidBody directly to the child mesh entity I get the screwed up rendering in the first picture. When I attach it to the parent entity however it produces the expected result. So far, I have been working with that last setup, but it's tedious and error-prone to always work on the parent from the collider when e.g. handling events.

Plus, I don't understand why inserting the collider would translate the mesh like that, or indeed change it at all.

---

Relevant code snippets: (The project is libre software, so you can always ask for more snippets)

# Cargo.toml
[dependencies]
bevy = { version = "0.16.1", features = ["jpeg", "wayland"] }
bevy_rapier3d = "0.31.0" 

// main.rs
fn main() {
    let mut app = App::new();
    app
        .add_systems(
            OnExit(GameState::Loading),
            (
                make_tiles_interactible,
                build_convex_hull_colliders_from_meshes.after(make_tiles_interactible),
            ),
        );
}

fn make_tiles_interactible(mut commands: Commands, tiles: Query<(Entity, &Children), With<Tile>>) {
    for (tile, meshes) in tiles {
        commands.entity(tile).insert(RigidBody::Dynamic); // 1
        commands
            .entity(
                *meshes
                    .first()
                    .expect("A tile has a single child and that is its mesh"),
            )
            .insert((
                RigidBody::Dynamic, // 2
                ConvexHullCollider { remove_mesh: false },
            ));
    }
}

#[derive(Component, Reflect, Default)]
#[reflect(Component)]
struct ConvexHullCollider {
    pub remove_mesh: bool,
}

fn build_convex_hull_colliders_from_meshes(
    collider_meshes: Query<(Entity, &Mesh3d, &ConvexHullCollider)>,
    mesh_data: Res<Assets<Mesh>>,
    mut commands: Commands,
) {
    for (collider, mesh, &ConvexHullCollider { remove_mesh }) in collider_meshes {
        let mesh = mesh_data.get(mesh).unwrap();

        // https://github.com/Defernus/bevy_gltf_collider/blob/7242bb856bfb87774e36674df43ddc327a868c8e/src/mesh_collider.rs#L56
        let vertices: Vec<_> = match mesh.attribute(Mesh::ATTRIBUTE_POSITION) {
            Some(VertexAttributeValues::Float32x3(positions)) => positions
                .into_iter()
                .map(|&a| Vec3::from_array(a))
                .collect(),
            unexpected => panic!("Unimplemented VertexAttributeValues: {:?}", unexpected),
        };

        let mut entity_cmds = commands.entity(collider);
        entity_cmds
            .insert((
                Collider::convex_hull(&vertices).expect("The mesh must be a convex hull"),
            ))
            .remove::<ConvexHullCollider>();
        if remove_mesh {
            entity_cmds
                .remove::<Mesh3d>()
                .remove::<MeshMaterial3d<StandardMaterial>>();
        }
    }
}

[ThibaultLemaire]

Upon further investigation, the issue turns out not to be with colliders, but simply with attaching a RigidBody to a child of an entity with a non-zero Transform...

So this is my new code:

fn make_tiles_interactible(
    mut commands: Commands,
    tiles: Query<(&Children, &Transform), With<Tile>>,
) {
    for (meshes, parent_transform) in tiles {
        commands
            .entity(
                *meshes
                    .first()
                    .expect("A tile has a single child and that is its mesh"),
            )
            // Unparent and copy parent transform because Rapier is very confused when a RigidBody's parent as a non-zero transform
            .remove::<ChildOf>()
            .insert((
                RigidBody::Dynamic,
                *parent_transform,
                ConvexHullCollider { remove_mesh: false },
            ));
    }
}