--- a/inst/isgray.m
+++ b/inst/isgray.m
@@ -1,8 +1,9 @@
-## Copyright (C) 2000  Kai Habel
+## Copyright (C) 2000 Kai Habel <kai.habel@gmx.de>
+## Copyright (C) 2011 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 2 of the License, or
+## 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,
@@ -14,32 +15,45 @@
 ## along with this program; If not, see <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn {Function File} @var{bool}= isgray (@var{I})
-## Returns true for an gray-scale intensity image. An variable is a gray scale image
-## if it is 2-dimensional matrix, and
+## @deftypefn {Function File} @var{bool} = isgray (@var{img})
+## Return true if @var{img} is a grayscale image.
+##
+## A variable is considereed to be a gray scale image if it is 2-dimensional,
+## non-sparse matrix, and:
 ## @itemize @bullet
-## @item is of class double and all values are in the range [0, 1], or
+## @item is of class double and all values are in the range [0, 1];
 ## @item is of class uint8 or uint16.
 ## @end itemize
+##
+## Note that grayscale time-series image have 4 dimensions (NxMx1xtime) but
+## isgray will still return false.
+##
+## @seealso{isbw, isind, isrgb}
 ## @end deftypefn
 
-## Author:	Kai Habel <kai.habel@gmx.de>
-## Date:	20/03/2000
-
-function bool = isgray (I)
+function bool = isgray (img)
 
   if (nargin != 1)
     print_usage ();
   endif
 
   bool = false;
-  if (ismatrix(I) && ndims(I) == 2)
-    switch(class(I))
-    case "double"
-      bool = all((I(:) >= 0 & I(:) <= 1) | isnan(I(:)));
-    case {"uint8", "uint16"}
-      bool = true;
+  if (ismatrix (img) && ndims (img) == 2 && !issparse (img))
+    switch (class (img))
+      case "double"
+        ## to speed this up, we can look at a sample of the image first
+        bool = is_gray_double (img(1:ceil (rows (img) /100), 1:ceil (columns (img) /100)));
+        if (bool)
+          ## sample was true, we better make sure it's real
+          bool = is_gray_double (img);
+        endif
+      case {"uint8", "uint16"}
+        bool = true;
     endswitch
   endif
 
 endfunction
+
+function bool = is_gray_double (img)
+  bool = all ((img(:) >= 0 & img(:) <= 1) | isnan (img(:)));
+endfunction