From 6cb43bf3e157f32418f3a8cde1ad357a401a6328 Mon Sep 17 00:00:00 2001
From: Teddy Tennant <teddytennant@icloud.com>
Date: Wed, 8 Jul 2026 12:50:07 -0400
Subject: [PATCH] fix: clamp acos argument in Rotation3::euler_angles_ordered
 to avoid NaN

Floating-point rounding in the chained matrix products computing `o_t`
can push `o_t.m33` slightly outside `[-1, 1]` (e.g. -1.0000000000000002
for a near-nadir orientation), so `acos` returns NaN and silently
poisons the whole returned angle array. Clamp the argument into the
valid range before calling `acos`, mirroring the existing handling in
`Matrix::angle`.

Fixes #1481.
---
 CHANGELOG.md                            |  6 ++++++
 src/geometry/rotation_specialization.rs |  2 +-
 tests/geometry/rotation.rs              | 23 +++++++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8d64160d3..0e34868c3 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/src/geometry/rotation_specialization.rs b/src/geometry/rotation_specialization.rs
index a2c5a5ad7..946794879 100644
--- a/src/geometry/rotation_specialization.rs
+++ b/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;
diff --git a/tests/geometry/rotation.rs b/tests/geometry/rotation.rs
index f461558aa..7c35353a9 100644
--- a/tests/geometry/rotation.rs
+++ b/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;