From 2ac675ae100f612a422d0be53043f579dbd272b3 Mon Sep 17 00:00:00 2001
From: Brendan Zabarauskas <bjzaba@yahoo.com.au>
Date: Sun, 13 Oct 2013 06:54:24 +1100
Subject: [PATCH] Fix slerp

---
 AUTHORS                  |  1 +
 src/cgmath/quaternion.rs | 12 ++++++------
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index e834dec..3fa559d 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -11,6 +11,7 @@ Erick Tryzelaar
 Luqman Aden
 Maik Klein
 Mikko Perttunen
+Tomasz Stachowiak
 
 With thanks to:
 
diff --git a/src/cgmath/quaternion.rs b/src/cgmath/quaternion.rs
index 8de82ff..29cc10f 100644
--- a/src/cgmath/quaternion.rs
+++ b/src/cgmath/quaternion.rs
@@ -252,14 +252,14 @@ impl<S: Float> Quat<S> {
             // stay within the domain of acos()
             let robust_dot = dot.clamp(&-one::<S>(), &one::<S>());
 
-            let theta: Rad<S> = acos(robust_dot.clone());   // the angle between the quaternions
-            let theta: Rad<S> = theta.mul_s(amount);        // the fraction of theta specified by `amount`
+            let theta: Rad<S> = acos(robust_dot.clone());
 
-            let q = other.sub_q(&self.mul_s(robust_dot))
-                         .normalize();
+            let scale1 = sin(theta.mul_s(one::<S>() - amount));
+            let scale2 = sin(theta.mul_s(amount));
 
-            self.mul_s(cos(theta.clone()))
-                .add_q(&q.mul_s(sin(theta)))
+            self.mul_s(scale1)
+                .add_q(&other.mul_s(scale2))
+                .mul_s(sin(theta).recip())
         }
     }
 }