/******************************************************************************

   Use 'Quaternion' for optional rotation representation.

/******************************************************************************/
STRUCT(Quaternion , Vec4)
//{
   Quaternion& operator*=(  Flt         f);
   Quaternion& operator*=(C Quaternion &q) {return mul(q);}

   // transform
   void        mul     (C Quaternion &q, Quaternion &dest)C;                             // multiply by 'q' and store result in 'dest'
   Quaternion& mul     (C Quaternion &q                  )  {mul(q, T);       return T;} // multiply by 'q'
   void        inverse (  Quaternion &dest               )C;                             // inverse to 'dest'
   Quaternion& inverse (                                 )  {inverse(T);      return T;} // inverse
   Quaternion& identity(                                 )  {set(0, 0, 0, 1); return T;} // set identity

   Quaternion& setRotateX(              Flt angle); // set x     rotated identity
   Quaternion& setRotateY(              Flt angle); // set y     rotated identity
   Quaternion& setRotateZ(              Flt angle); // set z     rotated identity
   Quaternion& setRotate (Flt x, Flt y, Flt z    ); // set x-y-z rotated identity
   Quaternion& setRotate (C Vec &axis , Flt angle); // set axis  rotated identity

   // get
   Flt angle()C; // get rotation angle
   Vec axis ()C; // get rotation axis

   Quaternion() {}
   Quaternion(C Vec4    &v) : Vec4(v) {}
   Quaternion(C Matrix3 &m);
};
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Quaternion GetTangent(C Quaternion &prev, C Quaternion &cur, C Quaternion &next); // get tangent

Quaternion Slerp(C Quaternion &a   , C Quaternion &b                                         , Flt step); // spherical linear     interpolation, 'step'=0..1
Quaternion Squad(C Quaternion &from, C Quaternion &to, C Quaternion &tan0, C Quaternion &tan1, Flt step); // spherical quadrangle interpolation, 'step'=0..1
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