anchor.diff:           :accessor orientiation
            :documentation "the orientation of the tracker (usually a
            quaternion)")
  +   (iod
  +         :initform 0.063
  +         :accessor iod
  +         :documentation "Inter occular distance. Default = 0.063
  +         meters")
  +   (is-stereo
  +         :initform nil
  +         :accessor is-stereo
  +         :documentation "If the head (user) has stereo capability"))
     (:documentation "The tracked head. Returns a transformation matrix on run."))
   
   ;; ----------------------------------------------------------------------------
   ;; Screen to tracking base
   (defparameter *Screen* (make-instance 'Screen))
  -(defparameter *width* 2)
  -(defparameter *height* 2)
  +(defparameter *width* (width *Screen*))
  +(defparameter *height* (height *Screen*))
   (defparameter *Head* (make-instance 'Head))
  -(defparameter *IPD* 0.063)              ; This is in meters
  -(defparameter *STEREO* NIL)
  +(defparameter *IPD* (iod *Head*))
  +(defparameter *STEREO* (is-stereo *Head*))
   
   ;; ----------------------------------------------------------------------------
   (defmethod run ((self Screen))
  
positive.code:   (:documentation "Class to configure display coordinates wrt some
    base coordinates"))
  
  
  ;; ----------------------------------------------------------------------------
  ;; Until there is real head tracking this class is dummy. The run
  
negative.code:               (trans (sb-cga:translate center)))
            (sb-cga:matrix* trans rot))))))
  
  ;; ----------------------------------------------------------------------------
  (defmethod run ((self Head))
    "For now there is no head transformation. And we simply return the
  identity matrix"
  