--- /tmp/tmplrsu9G-meld/Eigen/src/Geometry/Quaternion.h
+++ /home/strasdat/src/ScaViSLAM/EXTERNAL/eigen3.1/Eigen/src/Geometry/Quaternion.h
@@ -545,8 +545,8 @@
   return derived();
 }
 
-/** Convert the quaternion to a 3x3 rotation matrix. The quaternion is required to
-  * be normalized, otherwise the result is undefined.
+/** Convert the quaternion to a 3x3 rotation matrix.
+  * The quaternion is not required to be normalized.
   */
 template<class Derived>
 inline typename QuaternionBase<Derived>::Matrix3
@@ -558,9 +558,17 @@
   // it has to be inlined, and so the return by value is not an issue
   Matrix3 res;
 
-  const Scalar tx  = Scalar(2)*this->x();
-  const Scalar ty  = Scalar(2)*this->y();
-  const Scalar tz  = Scalar(2)*this->z();
+  Scalar n2 = this->squaredNorm();
+  if (n2 == 0)
+  {
+    res.setIdentity();
+    return res;
+  }
+
+  const Scalar two_by_squarednorm = 2./n2;
+  const Scalar tx  = two_by_squarednorm*this->x();
+  const Scalar ty  = two_by_squarednorm*this->y();
+  const Scalar tz  = two_by_squarednorm*this->z();
   const Scalar twx = tx*this->w();
   const Scalar twy = ty*this->w();
   const Scalar twz = tz*this->w();