From a78d4b0387e3aea8f6dbf7eead1dd792c3782e29 Mon Sep 17 00:00:00 2001
From: hut <hut@hut.pm>
Date: Mon, 1 Apr 2024 20:38:28 +0200
Subject: [PATCH] handle parents in `position_to_transform`, like in bevy_xpbd

---
 src/world.rs | 43 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/src/world.rs b/src/world.rs
index b204f9e..722b1e4 100644
--- a/src/world.rs
+++ b/src/world.rs
@@ -272,20 +272,55 @@ fn handle_cheats(
 // A variant of bevy_xpbd_3d::plugins::position_to_transform that adjusts
 // the rendering position to center entities at the player camera.
 // This avoids rendering glitches when very far away from the origin.
-//
-// NOTE: This currently does not support parent objects.
 pub fn position_to_transform(
     q_player: Query<&Position, With<actor::PlayerCamera>>,
-    mut q_trans: Query<(&mut Transform, &Position, &Rotation)>,
+    mut q_trans: Query<(&'static mut Transform, &'static Position, &'static Rotation, Option<&'static Parent>)>,
+    parents: Query<(&'static GlobalTransform, Option<&'static Position>, Option<&'static Rotation>), With<Children>>,
 ) {
     if let Ok(player_pos) = q_player.get_single() {
-        for (mut transform, pos, rot) in &mut q_trans {
+        for (mut transform, pos, rot, parent) in &mut q_trans {
+            if let Some(parent) = parent {
+                if let Ok((parent_transform, parent_pos, parent_rot)) = parents.get(**parent) {
+                    // Compute the global transform of the parent using its Position and Rotation
+                    let parent_transform = parent_transform.compute_transform();
+                    let parent_pos = parent_pos.map_or(parent_transform.translation, |pos| {
+                        pos.as_vec3()
+                            // NOTE: I commented out this because it turns a vec3 to a vec4,
+                            // and I don't understand why bevy_xpbd would do that.
+                            //.extend(parent_transform.translation.z)
+                    });
+                    let parent_rot = parent_rot.map_or(parent_transform.rotation, |rot| {
+                        rot.as_quat()
+                    });
+                    let parent_scale = parent_transform.scale;
+                    let parent_transform = Transform::from_translation(parent_pos)
+                        .with_rotation(parent_rot)
+                        .with_scale(parent_scale);
+
+                    // The new local transform of the child body,
+                    // computed from the its global transform and its parents global transform
+                    let new_transform = GlobalTransform::from(
+                        Transform::from_translation(
+                            pos.as_vec3()
+                                // NOTE: I commented out this because it turns a vec3 to a vec4,
+                                // and I don't understand why bevy_xpbd would do that.
+                                //.extend(parent_pos.z + transform.translation.z * parent_scale.z),
+                        )
+                        .with_rotation(rot.as_quat()),
+                    )
+                    .reparented_to(&GlobalTransform::from(parent_transform));
+
+                    transform.translation = new_transform.translation;
+                    transform.rotation = new_transform.rotation;
+                }
+            } else {
                 transform.translation = Vec3::new(
                     (pos.x - player_pos.x) as f32,
                     (pos.y - player_pos.y) as f32,
                     (pos.z - player_pos.z) as f32,
                 );
                 transform.rotation = rot.as_quat();
+            }
         }
     }
 }