From 3765631cd7bda540fdc03bf6716bb457ad83f4e0 Mon Sep 17 00:00:00 2001
From: Robert Osfield <robert@openscenegraph.com>
Date: Tue, 25 Jun 2002 20:36:17 +0000
Subject: [PATCH] Fix to Quat::slerp so that it interpolates along the shortest
 path.

---
 src/osg/Quat.cpp | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/osg/Quat.cpp b/src/osg/Quat.cpp
index f17d5c31e..ddafb2e48 100644
--- a/src/osg/Quat.cpp
+++ b/src/osg/Quat.cpp
@@ -151,9 +151,17 @@ void Quat::slerp( const float t, const Quat& from, const Quat& to )
 {
     const double epsilon = 0.00001;
     double omega, cosomega, sinomega, scale_from, scale_to ;
-
-                                 // this is a dot product
-    cosomega = from.asVec4() * to.asVec4() ;
+    
+    osg::Quat quatTo(to);
+    // this is a dot product
+    
+    cosomega = from.asVec4() * to.asVec4();
+    
+    if ( cosomega <0.0 )
+    { 
+        cosomega = -cosomega; 
+        quatTo.set(-to._fv);
+    }
 
     if( (1.0 - cosomega) > epsilon )
     {
@@ -175,7 +183,8 @@ void Quat::slerp( const float t, const Quat& from, const Quat& to )
     }
 
                                  // use Vec4 arithmetic
-    _fv = (from._fv*scale_from) + (to._fv*scale_to);
+    _fv = (from._fv*scale_from) + (quatTo._fv*scale_to);
+    
     // so that we get a Vec4
 }