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+## Copyright (C) 2009 Kai Habel
+##
+## 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} {} diffuse (@var{sx}, @var{sy}, @var{sz}, @var{l})
+## Calculate diffuse reflection strength of a surface defined by the normal
+## vector elements @var{sx}, @var{sy}, @var{sz}. 
+## The light vector can be specified using parameter @var{L}. It can be
+## given as 2-element vector [azimuth, elevation] in degrees or as 3-element
+## vector [lx, ly, lz]. 
+## @seealso{specular, surfl}
+## @end deftypefn
+
+## Author: Kai Habel <kai.habel@gmx.de>
+
+function retval = diffuse (sx, sy, sz, lv)
+
+  ## general checks
+  if (nargin != 4)
+    usage ("number of arguments must be 4")
+  endif
+
+  ## check for normal vector
+  if (!size_equal (sx, sy, sz))
+    usage ("SX, SY, and SZ must have same size")
+  endif
+  
+  ## check for light vector (lv) argument
+  if (length (lv) < 2 || length (lv) > 3)
+    usage ("light vector LV must be a 2- or 3-element vector");
+  elseif (length (lv) == 2)
+    [lv(1), lv(2), lv(3)] = sph2cart (lv(1) * pi/180, lv(2) * pi/180, 1.0);
+  endif
+
+  ## normalize view and light vector
+  if (sum (abs (lv)) > 0)
+    lv  /= norm (lv);
+  endif
+
+  ns = sqrt (sx.^2 + sy.^2 + sz.^2);
+  retval = (sx * lv(1) + sy * lv(2) + sz * lv(3)) ./ ns;
+  retval(retval < 0) = 0;
+  
+endfunction