; docformat = 'rst'
;+
;Construct a quaternion from an axis and angle returns a 4-element array with
;the vector part of the quaternion first, followed by the scalar part, to
;correspond to the quaternion library.
;
; :Examples:
;   quaternion = axis_angle_to_quat(axis, angle)
;-
;+
; :Params:
;   axis : in, required, type=int or double vector of int or double
;     The axis that will be used to construct the quaternion.
;   angle : in, required, type=int or double
;     The angle from the axis used to construct the quaternion.
;     
; :Returns:
;   Returns a 4-element array with the vector part of the quaternion first, 
;   followed by the scalar part, to correspond to the quaternion library.
;-
function axis_angle_to_quat,axis,angle
  if (~real_numeric_type(axis) or ~real_numeric_type(angle)) then begin
    doc_library, 'axis_angle_to_quat'
    return, -1
  endif
  return,[sin(angle/2.0)*axis,cos(angle/2.0)]
end