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## -*- texinfo -*-
## @deftypefn {} {@var{d} =} diffuse (@var{sx}, @var{sy}, @var{sz}, @var{lv})
## Calculate the diffuse reflection strength of a surface defined by the normal
## vector elements @var{sx}, @var{sy}, @var{sz}.
##
## The light source location vector @var{lv} can be given as a 2-element vector
## [azimuth, elevation] in degrees or as a 3-element vector [x, y, z].
## @seealso{specular, surfl}
## @end deftypefn

function d = diffuse (sx, sy, sz, lv)

  if (nargin != 4)
    print_usage ();
  endif

  ## Check normal vectors
  if (! size_equal (sx, sy, sz))
    error ("diffuse: SX, SY, and SZ must be the same size");
  endif

  ## Check light vector (lv) argument
  if (! isvector (lv) || numel (lv) < 2 || numel (lv) > 3)
    error ("diffuse: light vector LV must be a 2- or 3-element vector");
  elseif (numel (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);
  d = (sx * lv(1) + sy * lv(2) + sz * lv(3)) ./ ns;
  d(d < 0) = 0;

endfunction
