From 3860a0bc8f6af25d9115a66db1bdd3d7cad9b99a Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Thu, 29 Sep 2016 23:23:35 +0200
Subject: [PATCH] bug #1312: Quaternion to AxisAngle conversion now ensures the
 angle will be in the range [-pi,pi]. This also increases accuracy when q.w is
 negative.

---
 Eigen/src/Geometry/AngleAxis.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/Eigen/src/Geometry/AngleAxis.h b/Eigen/src/Geometry/AngleAxis.h
index 7fdb8ae83..99f3c3a66 100644
--- a/Eigen/src/Geometry/AngleAxis.h
+++ b/Eigen/src/Geometry/AngleAxis.h
@@ -158,7 +158,8 @@ typedef AngleAxis<float> AngleAxisf;
 typedef AngleAxis<double> AngleAxisd;
 
 /** Set \c *this from a \b unit quaternion.
-  * The resulting axis is normalized.
+  *
+  * The resulting axis is normalized, and the the computed angle is in the [-pi,pi] range.
   * 
   * This function implicitly normalizes the quaternion \a q.
   */
@@ -167,12 +168,16 @@ template<typename QuatDerived>
 AngleAxis<Scalar>& AngleAxis<Scalar>::operator=(const QuaternionBase<QuatDerived>& q)
 {
   using std::atan2;
+  using std::abs;
   Scalar n = q.vec().norm();
   if(n<NumTraits<Scalar>::epsilon())
     n = q.vec().stableNorm();
-  if (n > Scalar(0))
+
+  if (n != Scalar(0))
   {
-    m_angle = Scalar(2)*atan2(n, q.w());
+    m_angle = Scalar(2)*atan2(n, std::abs(q.w()));
+    if(q.w() < 0)
+      n = -n;
     m_axis  = q.vec() / n;
   }
   else