What is required to get Heirarchical transforms to work?

[sdfgeoff]

Originally I was going to post this as an issue, but it's probably more likely to be an issue on my side. But I've still followed the issue template.

Bevy version

Recent git:
source = "git+https://github.com/bevyengine/bevy?branch=main#d9fb61d474518eba06d0c3dce5cb81db9f7ed66a"

Operating system & version

Linux

What you did

Set up a bunch of heirarchies of objects and meshes, loaded from scenes

What you expected to happen

When you transform a parent, the children should also be transformed such that they act like a single "unit"

What actually happened

The results are inconsistent. Sometimes no objects are updated, sometimes some objects are updated

Additional information

Consider this scene file:

[
    (entity: 0, components:[
        {"type":"blend_loader::blend_mesh::BlendMeshLoader","struct":{"path":"Meshes/26ac592701faecdf9c9ba136dbe087b2.mesh"}},
        {"type":"blend_loader::blend_label::BlendLabel","struct":{"name":"Ship"}},
        {"type":"bevy_transform::components::transform::Transform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,0.0,0.0)},"rotation":{"type":"glam::quat::Quat","value":(0.0,0.0,0.0,1.0)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::global_transform::GlobalTransform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,0.0,0.0)},"rotation":{"type":"glam::quat::Quat","value":(0.0,0.0,0.0,1.0)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::children::Children","tuple_struct":[{"type":"smallvec::SmallVec<[bevy_ecs::core::entities::Entity; 1]>","list":[{"type":"bevy_ecs::core::entities::Entity","value":1}]}]}
    ]),
    (entity: 1, components:[
        {"type":"blend_loader::blend_mesh::BlendMeshLoader","struct":{"path":"Meshes/26ac592701faecdf9c9ba136dbe087b2.mesh"}},
        {"type":"blend_loader::blend_label::BlendLabel","struct":{"name":"Cube.002"}},
        {"type":"bevy_transform::components::transform::Transform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,-5.0,0.0)},"rotation":{"type":"glam::quat::Quat","value":(-0.7071068286895752,0.0,0.0,0.7071068286895752)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::global_transform::GlobalTransform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,-5.0,0.0)},"rotation":{"type":"glam::quat::Quat","value":(-0.7071068286895752,0.0,0.0,0.7071068286895752)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::children::Children","tuple_struct":[{"type":"smallvec::SmallVec<[bevy_ecs::core::entities::Entity; 1]>","list":[{"type":"bevy_ecs::core::entities::Entity","value":2}]}]},
        {"type":"bevy_transform::components::parent::Parent","tuple_struct":[{"type":"bevy_ecs::core::entities::Entity","value":0}]}
    ]),
    (entity: 2, components:[
        {"type":"blend_loader::blend_mesh::BlendMeshLoader","struct":{"path":"Meshes/26ac592701faecdf9c9ba136dbe087b2.mesh"}},
        {"type":"blend_loader::blend_label::BlendLabel","struct":{"name":"Cube.003"}},
        {"type":"bevy_transform::components::transform::Transform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,-5.0,0.0)},"rotation":{"type":"glam::quat::Quat","value":(0.0,0.0,0.0,1.0)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::global_transform::GlobalTransform","struct":{"translation":{"type":"glam::vec3::Vec3","value":(0.0,-5.0,5.0)},"rotation":{"type":"glam::quat::Quat","value":(-0.7071068286895752,0.0,0.0,0.7071068286895752)},"scale":{"type":"glam::vec3::Vec3","value":(1.0,1.0,1.0)}}},
        {"type":"bevy_transform::components::parent::Parent","tuple_struct":[{"type":"bevy_ecs::core::entities::Entity","value":1}]}
    ])
]

There are three objects with parent/child relations set up between them ("Cube.003" is child of "Cube.002" is child of "Ship"). I have a system that rotates any entity called "Ship" - which for the above scene is the root of the hierarchy:

fn test_motion_system(mut query: Query<(Mut<Transform>, &BlendLabel)>) {
    for (mut transform, label) in query.iter_mut() {
        if label.name == "Ship" {
            println!("label: {}", label.name);
            transform.rotate(Quat::from_axis_angle(Vec3::new(0.0, 0.5, 0.866), 0.05))
        }
    }
}

My expectation is that, given that scene file, the objects would all rotate. But:

simple.mp4

However, that isn't to say it doesn't work. Another scene file - a much larger one, produces this result when rotated:

complex.mp4

As you can see, many of the child entities are being rotated, but some are not. Also, of the ones that are being rotated, some of them are in the wrong location. The asset should look like:

My guess at the moment is that something is wrong with my scene files and it's messing with the transforms. As far as I can guess, all I need is Transform, Parent and Child components but perhaps there is another component I need?

[cart]

If you switch to Bevy 0.4 (instead of main), do you still have issues? This might be related to our new parallel system executor we added on main, which works as expected so far, but requires some "order of operations" fixes in internal bevy plugins. We're in the process of fixing those now.

[Davier]

In my tests on main, when loading a hierarchy with both Parent and Children components set, I get this panic:

thread 'Compute Task Pool (0)' panicked at 'children already added', /home/user/.cargo/git/checkouts/bevy-f7ffde730c324c74/f61e44d/crates/bevy_transform/src/hierarchy/hierarchy_maintenance_system.rs:53:13

If the scene only sets Children components, Parent components are not added automatically.
Finally, if the scene only sets Parent components, the Children components are added correctly.

[sdfgeoff]

In my tests on main, when loading a hierarchy with both Parent and Children components set, I get this panic:

thread 'Compute Task Pool (0)' panicked at 'children already added', /home/user/.cargo/git/checkouts/bevy-f7ffde730c324c74/f61e44d/crates/bevy_transform/src/hierarchy/hierarchy_maintenance_system.rs:53:13

If the scene only sets Children components, Parent components are not added automatically.
Finally, if the scene only sets Parent components, the Children components are added correctly.

I updated main again, and had the same result. After removing the Children components from the scene file, it loaded and worked fine. I guess I just got a bad commit 😕

Now I just need to figure out how to spawn a dynamic scene as a child and I'll have everything I need to actually start making my game.

[Davier]

I discovered meanwhile that the issue with having Children can also be avoided by adding both Parent and PreviousParent.

Now I just need to figure out how to spawn a dynamic scene as a child and I'll have everything I need to actually start making my game.

One way is to convert it to a Scene and use SceneSpawner::spawn_as_child() as discussed in #1446.

[sdfgeoff]

One way is to convert it to a Scene and use SceneSpawner::spawn_as_child() as discussed in #1446.

Unfortunately current master doesn't seem to have a DynamicScene::get_scene() method anymore....

[Davier]

I missed that. I think you can call DynamicScene::write_to_world() on a new Scene's world to get the same effect.

[sdfgeoff]

Updating to a newer main fixed it.