--- a/main/image/NEWS	Tue Mar 13 17:55:28 2012 +0000
+++ b/main/image/NEWS	Tue Mar 13 18:08:40 2012 +0000
@@ -46,6 +46,7 @@
       isgray
       isrgb
       isind
+      mat2gray
 
  ** `isbw' now defines a black-and-white image as a binary non-sparse
     matrix. This is compatible with matlab. To use the old behaviour,

--- a/main/image/inst/mat2gray.m	Tue Mar 13 17:55:28 2012 +0000
+++ b/main/image/inst/mat2gray.m	Tue Mar 13 18:08:40 2012 +0000
@@ -1,46 +1,75 @@
 ## Copyright (C) 1999,2000 Kai Habel <kai.habel@gmx.de>
-## Copyright (C) 2011 Carnë Draug <carandraug+dev@gmail.com>
+## Copyright (C) 2011, 2012 Carnë Draug <carandraug+dev@gmail.com>
 ##
-## This program 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.
+## This program 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.
 ##
-## This program 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.
+## This program 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 this program; If not, see <http://www.gnu.org/licenses/>.
+## You should have received a copy of the GNU General Public License along with
+## this program; if not, see <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn {Function File} @var{I}= mat2gray (@var{M},[min max])
+## @deftypefn {Function File} {@var{I} =} mat2gray (@var{M}, [@var{min} @var{max}])
 ## Converts a matrix to a intensity image.
-## @seealso{gray2ind, ind2gray, rgb2gray}
+## @seealso{gray2ind, ind2gray, rgb2gray, im2double, im2uin16, im2uint8, im2int16}
 ## @end deftypefn
 
-function I = mat2gray (M, scale)
+function out = mat2gray (in, scale)
 
   if (nargin < 1 || nargin > 2)
     print_usage;
-  elseif (!ismatrix (M))
-    error ("mat2gray(M,...) M must be a matrix");
+  elseif (!ismatrix (in) || ischar(in))
+    error ("mat2gray: first argument must be a matrix");
   elseif (nargin == 2 && (!isvector (scale) || numel (scale) != 2))
-    error ("mat2gray(M,scale) scale must be a vector with 2 elements");
+    error ("mat2gray: second argument must be a vector with 2 elements");
+  endif
+
+  if (nargin == 1)
+    out_min = min (in(:));
+    out_max = max (in(:));
+  else
+    ## see more at the end for the cases where max and min are swapped
+    out_min = min (scale (1), scale (2));
+    out_max = max (scale (1), scale (2));
   endif
 
-  ## what if the matrix has more than 2D?
-  if (nargin == 1)
-    Mmin = min (min (M));
-    Mmax = max (max (M));
-  else 
-    Mmin = min (scale (1), scale (2));
-    Mmax = max (scale (1), scale (2));
+  ## since max() and min() return a value of same class as input,
+  ## need to make this values double or the calculations later may fail
+  out_min = double (out_min);
+  out_max = double (out_max);
+
+  out = ones (size (in));
+  if (out_min == out_max)
+    ## if max and min are the same, matlab seems to simple return 1s, even
+    ## for values under the minimum/maximum. As such, we are done here
+    return
   endif
 
-  I = (M < Mmin) .* 0;
-  I = I + (M >= Mmin & M < Mmax) .* (1 / (Mmax - Mmin) * (M - Mmin));
-  I = I + (M >= Mmax);
+  out(in <= out_min) = 0;
+  ## no need to worry with values above or equal to out_max because
+  ## the output matrix was already generated with ones()
+
+  ## it's faster to get the index of values between max and min only once
+  ## than to have it calculated on both sides of the assignment
+  idx      = (in > out_min & in < out_max);
+  out(idx) =  (1/(out_max - out_min)) * (double(in(idx)) - out_min);
+
+  ## if the given min and max are in the inverse order...
+  if (nargin > 1 && scale(1) > scale (2))
+    ## matlab seems to allow setting the min higher than the max but not by
+    ## checking which one is actually correct. Seems to just invert it
+    out = abs (out - 1);
+  endif
 
 endfunction
+
+%!assert(mat2gray([1 2 3]), [0 0.5 1]);           # standard use
+%!assert(mat2gray(repmat ([1 2; 3 3], [1 1 3])), repmat ([0 0.5; 1 1], [1 1 3])); # setting min and max
+%!assert(mat2gray([1 2 3], [2 2]), [1 1 1]);      # equal min and max
+%!assert(mat2gray([1 2 3], [3 1]), [1 0.5 0]);    # max and min inverted