fix(physics): collider auto-fit no longer inflates rotated shapes on load

RELEASE_NOTES.md

@@ -1,5 +1,8 @@
 # Renzora Engine `r1-alpha8`
 
+## 2026-10-07 21:03
+- fix(physics): reopening a scene no longer resizes the colliders of rotated objects
+
 ## 2026-09-18 16:27
 - fix(game-ui): an `expand` canvas previewed over the viewport lays out like the shipped game, so edge-anchored UI reaches the edge
 - fix(game-ui): a canvas root no longer keeps a preview scale that was saved into a scene, which shrank the UI in an exported game

crates/renzora_physics/src/auto_fit.rs

@@ -54,31 +54,15 @@ pub fn auto_fit_collision_shapes(
             commands.entity(entity).remove::<PendingAutoFit>();
             continue;
         }
-        let Some((min, max)) = combined_world_aabb(entity, &aabbs, &children) else {
-            // Mesh not ready yet — try again next frame.
+        // Bounds are taken in the collider's own local space, corner by corner.
+        // Fitting a world-axis box first and mapping that back inflated every
+        // rotated collider (a 15 degree Y platform grew to 1.4x1.6 of its mesh).
+        let inv = entity_gt.affine().inverse();
+        let Some((local_min, local_max)) = combined_local_aabb(entity, inv, &aabbs, &children) else {
+            // Mesh not ready yet: try again next frame.
             continue;
         };
 
-        let inv = entity_gt.affine().inverse();
-        let corners = [
-            Vec3::new(min.x, min.y, min.z),
-            Vec3::new(max.x, min.y, min.z),
-            Vec3::new(min.x, max.y, min.z),
-            Vec3::new(max.x, max.y, min.z),
-            Vec3::new(min.x, min.y, max.z),
-            Vec3::new(max.x, min.y, max.z),
-            Vec3::new(min.x, max.y, max.z),
-            Vec3::new(max.x, max.y, max.z),
-        ];
-
-        let mut local_min = Vec3::splat(f32::INFINITY);
-        let mut local_max = Vec3::splat(f32::NEG_INFINITY);
-        for c in corners {
-            let lp = inv.transform_point3(c);
-            local_min = local_min.min(lp);
-            local_max = local_max.max(lp);
-        }
-
         let half = (local_max - local_min) * 0.5;
         let center = (local_max + local_min) * 0.5;
 
@@ -104,13 +88,26 @@ pub fn auto_fit_collision_shapes(
             CollisionShapeType::Mesh => unreachable!(),
         }
 
-        commands.entity(entity).insert(new_shape);
+        // A default-valued shape on a unit mesh already fits; rewriting it only
+        // injects float noise (0.5 -> 0.49999997) and dirties a loaded scene.
+        if !shape_close(shape, &new_shape) {
+            commands.entity(entity).insert(new_shape);
+        }
         commands.entity(entity).remove::<PendingAutoFit>();
     }
 }
 
-fn combined_world_aabb(
+fn shape_close(a: &CollisionShapeData, b: &CollisionShapeData) -> bool {
+    const EPS: f32 = 1e-4;
+    a.offset.abs_diff_eq(b.offset, EPS)
+        && a.half_extents.abs_diff_eq(b.half_extents, EPS)
+        && (a.radius - b.radius).abs() <= EPS
+        && (a.half_height - b.half_height).abs() <= EPS
+}
+
+fn combined_local_aabb(
     root: Entity,
+    to_local: bevy::math::Affine3A,
     aabbs: &Query<(&Aabb, &GlobalTransform)>,
     children: &Query<&Children>,
 ) -> Option<(Vec3, Vec3)> {
@@ -127,9 +124,9 @@ fn combined_world_aabb(
                 for sy in [-1.0, 1.0] {
                     for sz in [-1.0, 1.0] {
                         let local = center + half * Vec3::new(sx, sy, sz);
-                        let world = gt.transform_point(local);
-                        min = min.min(world);
-                        max = max.max(world);
+                        let root_local = to_local.transform_point3(gt.transform_point(local));
+                        min = min.min(root_local);
+                        max = max.max(root_local);
                     }
                 }
             }

crates/renzora_physics/tests/auto_fit_rotated.rs

@@ -0,0 +1,44 @@
+//! Auto-fit must be rotation-invariant. A unit cube mesh under a rotated,
+//! non-uniformly scaled entity already fits the default 0.5 box exactly; the old
+//! fit took a world-axis AABB and mapped it back, which grew a 15 degree Y
+//! platform's collider to roughly 0.70 x 0.5 x 0.81 every time the scene loaded.
+
+use bevy::camera::primitives::Aabb;
+use bevy::math::Vec3A;
+use bevy::prelude::*;
+use renzora_physics::auto_fit::{auto_fit_collision_shapes, mark_new_collision_shapes};
+use renzora_physics::CollisionShapeData;
+
+#[test]
+fn rotated_unit_cube_keeps_default_extents() {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins)
+        .add_systems(Update, (mark_new_collision_shapes, auto_fit_collision_shapes).chain());
+
+    let tf = Transform {
+        translation: Vec3::new(7.0, 1.0, 0.0),
+        rotation: Quat::from_rotation_y(15f32.to_radians()),
+        scale: Vec3::new(5.0, 0.5, 4.0),
+    };
+    let e = app
+        .world_mut()
+        .spawn((
+            tf,
+            GlobalTransform::from(tf),
+            Aabb { center: Vec3A::ZERO, half_extents: Vec3A::splat(0.5) },
+            CollisionShapeData::default(),
+        ))
+        .id();
+
+    for _ in 0..3 {
+        app.update();
+    }
+
+    let shape = app.world().get::<CollisionShapeData>(e).unwrap();
+    assert!(
+        shape.half_extents.abs_diff_eq(Vec3::splat(0.5), 1e-4),
+        "half extents changed to {:?}",
+        shape.half_extents
+    );
+    assert!(shape.offset.abs_diff_eq(Vec3::ZERO, 1e-4), "offset changed to {:?}", shape.offset);
+}