From d6b93f052e50613c1a1259f0abed6177d28d9c4b Mon Sep 17 00:00:00 2001
From: Sundaram Ramaswamy <legends2k@yahoo.com>
Date: Thu, 6 May 2021 23:38:26 +0530
Subject: [PATCH] Add nlerp for quaternions

Normalized linear interpolation for quaterions are a cheaper
alternative to slerp.  This PR adds nlerp operaiton for quaternions.
---
 include/cglm/quat.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/cglm/quat.h b/include/cglm/quat.h
index 8560ec8..97b27ed 100644
--- a/include/cglm/quat.h
+++ b/include/cglm/quat.h
@@ -38,6 +38,7 @@
    CGLM_INLINE void glm_quat_lerp(versor from, versor to, float t, versor dest);
    CGLM_INLINE void glm_quat_lerpc(versor from, versor to, float t, versor dest);
    CGLM_INLINE void glm_quat_slerp(versor q, versor r, float t, versor dest);
+   CGLM_INLINE void glm_quat_nlerp(versor q, versor r, float t, versor dest);
    CGLM_INLINE void glm_quat_look(vec3 eye, versor ori, mat4 dest);
    CGLM_INLINE void glm_quat_for(vec3 dir, vec3 fwd, vec3 up, versor dest);
    CGLM_INLINE void glm_quat_forp(vec3 from,
@@ -628,6 +629,29 @@ glm_quat_lerpc(versor from, versor to, float t, versor dest) {
   glm_vec4_lerpc(from, to, t, dest);
 }
 
+/*!
+ * @brief interpolates between two quaternions
+ *        taking the shortest rotation path using
+ *        normalized linear interpolation (NLERP)
+ *
+ * @param[in]   from  from
+ * @param[in]   to    to
+ * @param[in]   t     interpolant (amount)
+ * @param[out]  dest  result quaternion
+ */
+CGLM_INLINE
+void
+glm_quat_nlerp(versor from, versor to, float t, versor dest) {
+  float dot = glm_vec4_dot(from, to);
+  versor target;
+  if (dot >= 0)
+    glm_quat_copy(to, target);
+  else
+    glm_vec4_negate_to(to, target);
+  glm_quat_lerp(from, target, t, dest);
+  glm_quat_normalize(dest);
+}
+
 /*!
  * @brief interpolates between two quaternions
  *        using spherical linear interpolation (SLERP)