fix: clamp acos argument in Rotation3::euler_angles_ordered to avoid NaN (#1481)

CHANGELOG.md

@@ -8,6 +8,12 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 **nalgebra-lapack change log** is found [here](https://github.com/dimforge/nalgebra/blob/main/nalgebra-lapack/CHANGELOG.md)
 starting with `nalgebra-lapack` version `0.27.0`.
 
+## Unreleased
+
+### Fixed
+
+- Clamp the `acos` argument in `Rotation3::euler_angles_ordered` so that rounding in the intermediate matrix products can no longer push the middle angle to `NaN` [#1481](https://github.com/dimforge/nalgebra/issues/1481).
+
 ## [0.35.0] (24 May 2026)
 
 ### Added

src/geometry/rotation_specialization.rs

@@ -1098,7 +1098,7 @@ impl<T: SimdRealField> Rotation3<T> {
             c1,
         );
         let o_t = c * self.matrix() * (c.transpose() * r1l);
-        angles[1] = o_t.m33.acos();
+        angles[1] = o_t.m33.clamp(-T::one(), T::one()).acos();
 
         let safe1 = angles[1].abs() >= eps;
         let safe2 = (angles[1] - T::pi()).abs() >= eps;

tests/geometry/rotation.rs

@@ -85,6 +85,29 @@ fn quaternion_euler_angles_issue_494() {
     assert_eq!(angs.2, 0.0);
 }
 
+#[test]
+fn euler_angles_ordered_issue_1481() {
+    // A valid unit quaternion can yield a rotation matrix whose diagonal
+    // entry is slightly outside [-1, 1] due to rounding; feeding it to acos
+    // produced NaN before the clamp fix.
+    let q = UnitQuaternion::new_normalize(Quaternion::<f64>::new(
+        0.1045170328727042,
+        -0.6993398242910853,
+        0.10451703287270421,
+        0.6993398242910854,
+    ));
+    let seq = [
+        UnitVector3::new_normalize(Vector3::z()),
+        UnitVector3::new_normalize(Vector3::y()),
+        UnitVector3::new_normalize(Vector3::x()),
+    ];
+    let (angles, _observable) = q.to_rotation_matrix().euler_angles_ordered(seq, false);
+    assert!(
+        angles.iter().all(|a| a.is_finite()),
+        "euler angles must be finite, got {angles:?}"
+    );
+}
+
 #[cfg(feature = "proptest-support")]
 mod proptest_tests {
     use approx::AbsDiffEq;