From 88c844ae2f3db7f87cab441e186cbdd2f9c7dcc3 Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Wed, 4 Mar 2015 17:03:13 +0100
Subject: [PATCH] bug #824: improve accuracy of Quaternion::angularDistance
 using atan2 instead of acos. (grafted from
 2dc968e453e347966b7a45c9c497c6b1d3845f80 )

---
 Eigen/src/Geometry/Quaternion.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/Eigen/src/Geometry/Quaternion.h b/Eigen/src/Geometry/Quaternion.h
index f06653f1c..270059bcd 100644
--- a/Eigen/src/Geometry/Quaternion.h
+++ b/Eigen/src/Geometry/Quaternion.h
@@ -667,12 +667,10 @@ template <class OtherDerived>
 inline typename internal::traits<Derived>::Scalar
 QuaternionBase<Derived>::angularDistance(const QuaternionBase<OtherDerived>& other) const
 {
-  using std::acos;
+  using std::atan2;
   using std::abs;
-  Scalar d = abs(this->dot(other));
-  if (d>=Scalar(1))
-    return Scalar(0);
-  return Scalar(2) * acos(d);
+  Quaternion<Scalar> d = (*this) * other.conjugate();
+  return Scalar(2) * atan2( d.vec().norm(), abs(d.w()) );
 }