--- a/scripts/image/ind2gray.m	Mon Nov 26 12:09:09 2012 -0500
+++ b/scripts/image/ind2gray.m	Tue Nov 27 16:38:13 2012 -0800
@@ -17,11 +17,20 @@
 ## <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn  {Function File} {} ind2gray (@var{x})
-## @deftypefnx {Function File} {} ind2gray (@var{x}, @var{map})
-## Convert a color indexed image to a gray scale intensity image.
+## @deftypefn  {Function File} {@var{I} =} ind2gray (@var{x})
+## @deftypefnx {Function File} {@var{I} =} ind2gray (@var{x}, @var{map})
+## Convert a color indexed image to a grayscale intensity image.
 ## If @var{map} is omitted, the current colormap is used to determine the
 ## intensities.
+##
+## The output @var{I} is of the same class as the input @var{x} and may be
+## one of @code{uint8}, @code{uint16}, @code{single}, or @code{double}.
+##
+## Implementation Note: There are several ways of converting colors to
+## grayscale intensities.  This functions uses the luminance value obtained
+## from @code{rgb2ntsc} which is @code{I = 0.299*R + 0.587*G + 0.114*B}.  
+## Other possibilities include the value component from @code{rgb2hsv} or
+## using a single color channel from @code{ind2rgb}.
 ## @seealso{gray2ind, ind2rgb}
 ## @end deftypefn
 
@@ -29,22 +38,62 @@
 ## Created: July 1994
 ## Adapted-By: jwe
 
-function y = ind2gray (x, map)
+function I = ind2gray (x, map)
 
   if (nargin < 1 || nargin > 2)
     print_usage ();
-  elseif (nargin == 1)
+  endif
+
+  if (! isreal (x) || issparse (x)
+      || (isfloat (x) && (any (x(:) < 1 || any (x(:) != fix (x(:)))))))
+    error ("ind2gray: X must be an indexed image");
+  endif
+  cls = class (x);
+  if (! any (isa (x, {"logical", "uint8", "uint16", "single", "double"})))
+    error ("ind2gray: invalid data type '%s'", cls);
+  endif
+
+  if (nargin == 1)
     map = colormap ();
+  elseif (! iscolormap (map))
+    error ("ind2gray: MAP must be a valid colormap");
+  endif
+
+  ## Convert colormap to luminance intensity values 
+  map *= [0.299; 0.587; 0.114];
+
+  ## Convert colormap to same class as that of input so that
+  ## indexing in colormap will produce output of the same type as the input.
+  if (isinteger (x)) 
+    map *= intmax (cls);
+  elseif (strcmp (cls, "single"))
+    map = single (map);
   endif
 
-  [rows, cols] = size (x);
-
-  ## Convert colormap to intensity values (the first column of the
-  ## result of the call to rgb2ntsc) and then replace indices in
-  ## the input matrix with indexed values in the output matrix (indexed
-  ## values are the result of indexing the intensity values by the
-  ## elements of x(:)).
-
-  y = reshape (((rgb2ntsc (map))(:,1))(x(:)), rows, cols);
+  ## Replace indices in the input matrix with the indexed luminance value.
+  I = reshape (map(x(:)), size (x));
 
 endfunction
+
+
+%!test
+%! i2g = ind2gray (1:100, gray (100));
+%! assert (i2g, 0:1/99:1, eps);
+%! g2i = gray2ind (i2g, 100);
+%! assert (g2i, uint8 (1:100));
+
+%%test input validation
+%!error ind2gray ()
+%!error ind2gray (1,2,3)
+%!error <X must be an indexed image> ind2gray ({1})
+%!error <X must be an indexed image> ind2gray (1+i)
+%!error <X must be an indexed image> ind2gray (sparse (1))
+%!error <X must be an indexed image> ind2gray (0)
+%!error <X must be an indexed image> ind2gray (1.1)
+%!error <MAP must be a valid colormap> ind2gray (1, {1})
+%!error <MAP must be a valid colormap> ind2gray (1, 1+i)
+%!error <MAP must be a valid colormap> ind2gray (1, ones (2,2,2))
+%!error <MAP must be a valid colormap> ind2gray (1, ones (2,4))
+%!error <MAP must be a valid colormap> ind2gray (1, [-1])
+%!error <MAP must be a valid colormap> ind2gray (1, [2])
+