Aabb wrt centre/issue #155 (#182)

* non-functional halfway modification, read the docs

* Working and with a single unit test to verify

* feat: simplify implementation of Aabb::scaled_wrt_center

* chore: move Aabb::scaled_wirt_center test to the test dir.

* chore: update changelog

---------

Co-authored-by: whatf0xx <[email protected]>
Co-authored-by: Sébastien Crozet <[email protected]>

CHANGELOG.md

@@ -11,6 +11,10 @@
 
 - Fix a crash in `Qbvh::refit` that results from the QBVH tree becoming increasingly imbalanced.
 
+### Added
+
+- Add `Aabb::scaled_wrt_center` to scale an AABB while keeping its center unchanged.
+
 ## v0.13.6
 
 ### Fixed

crates/parry2d/tests/geometry/aabb_scale.rs

@@ -0,0 +1,16 @@
+use na::{Point2, Vector2};
+use parry2d::bounding_volume::Aabb;
+
+#[test]
+fn test_aabb_scale_wrt_center() {
+    let aabb = Aabb::from_half_extents(Point2::new(1.0, 2.0), Vector2::new(4.0, 5.0));
+    let scale = Vector2::new(10.0, -20.0);
+    let scaled_aabb = aabb.scaled_wrt_center(&scale);
+    let scaled_aabb_neg = aabb.scaled_wrt_center(&-scale);
+    let scaled_aabb_abs = aabb.scaled_wrt_center(&scale.abs());
+
+    assert_eq!(&scaled_aabb, &scaled_aabb_neg);
+    assert_eq!(&scaled_aabb, &scaled_aabb_abs);
+    assert_eq!(aabb.center(), scaled_aabb.center());
+    assert_eq!(scaled_aabb.half_extents(), Vector2::new(40.0, 100.0));
+}

crates/parry2d/tests/geometry/mod.rs

@@ -1,3 +1,4 @@
+mod aabb_scale;
 mod ball_ball_toi;
 mod ball_cuboid_contact;
 mod epa2;

crates/parry3d/tests/geometry/aabb_scale.rs

@@ -0,0 +1,16 @@
+use na::{Point3, Vector3};
+use parry3d::bounding_volume::Aabb;
+
+#[test]
+fn test_aabb_scale_wrt_center() {
+    let aabb = Aabb::from_half_extents(Point3::new(1.0, 2.0, 3.0), Vector3::new(4.0, 5.0, 6.0));
+    let scale = Vector3::new(10.0, -20.0, 50.0);
+    let scaled_aabb = aabb.scaled_wrt_center(&scale);
+    let scaled_aabb_neg = aabb.scaled_wrt_center(&-scale);
+    let scaled_aabb_abs = aabb.scaled_wrt_center(&scale.abs());
+
+    assert_eq!(&scaled_aabb, &scaled_aabb_neg);
+    assert_eq!(&scaled_aabb, &scaled_aabb_abs);
+    assert_eq!(aabb.center(), scaled_aabb.center());
+    assert_eq!(scaled_aabb.half_extents(), Vector3::new(40.0, 100.0, 300.0));
+}

crates/parry3d/tests/geometry/mod.rs

@@ -1,3 +1,4 @@
+mod aabb_scale;
 mod ball_ball_toi;
 mod ball_triangle_toi;
 mod convex_hull;

src/bounding_volume/aabb.rs

@@ -176,6 +176,23 @@ impl Aabb {
         }
     }
 
+    /// Returns an AABB with the same center as `self` but with extents scaled by `scale`.
+    ///
+    /// # Parameters
+    /// - `scale`: the scaling factor. It can be non-uniform and/or negative. The AABB being
+    ///            symmetric wrt. its center, a negative scale value has the same effect as scaling
+    ///            by its absolute value.
+    #[inline]
+    #[must_use]
+    pub fn scaled_wrt_center(self, scale: &Vector<Real>) -> Self {
+        let center = self.center();
+        // Multiply the extents by the scale. Negative scaling might modify the half-extent
+        // sign, so we take the absolute value. The AABB being symmetric that absolute value
+        // is  valid.
+        let half_extents = self.half_extents().component_mul(scale).abs();
+        Self::from_half_extents(center, half_extents)
+    }
+
     /// The smallest bounding sphere containing this `Aabb`.
     #[inline]
     pub fn bounding_sphere(&self) -> BoundingSphere {