## Copyright (C) 1998, 2000, 2005, 2007
##               Auburn University.  All rights reserved.
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or (at
## your option) any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} qinv (@var{q})
## Return the inverse of a quaternion.
##
## @example
## q = [w, x, y, z] = w*i + x*j + y*k + z
## qmult (q, qinv (q)) = 1 = [0 0 0 1]
## @end example
## @end deftypefn

## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## Adapted-By: jwe

function retval = qinv (q)

  if (norm (q) != 0)
    retval = qconj (q) / sum (q .* q);
  else
    error ("qinv: zero quaternion passed!");
  endif

endfunction