--- a/inst/isrgb.m
+++ b/inst/isrgb.m
@@ -1,8 +1,10 @@
-## Copyright (C) 2004 Josep Mones i Teixidor
+## Copyright (C) 2000 Kai Habel <kai.habel@gmx.de>
+## Copyright (C) 2004 Josep Mones i Teixidor <jmones@puntbarra.com>
+## 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,
@@ -14,67 +16,64 @@
 ## along with this program; If not, see <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn {Function File} {@var{flag} = } isrgb (@var{A})
-## Returns true if parameter is a RGB image.
-##
-## @code{flag=isrgb(A)} returns 1 if @var{A} is a RGB image and 0 if
-## not.
+## @deftypefn {Function File} {@var{bool} = } isrgb (@var{img})
+## Return true if @var{img} is a RGB image.
 ##
-## To the decide @code{isrgb} uses the follow algorithm:
+## A variable is considereed to be a RGB image if it is 3-dimensional,
+## non-sparse matrix, of size MxNx3 and:
 ## @itemize @bullet
-## @item
-## If @var{A} is of class double then it checks if all values are
-## between 0 and 1, and if size is m-by-n-by-3.
-## @item
-## If @var{A} is of class uint16, uint8 or logical then it checks is m-by-n-by-3.
+## @item is of class double and all values are in the range [0, 1];
+## @item is of class uint8 or uint16.
 ## @end itemize
 ##
-## @strong{Compatibility notes:}
+## Note that RGB time-series image have 4 dimensions (NxMx3xtime) but
+## isrgb will still return false.
 ##
-## Information needed on whether MATLAB accepts logical arrays as RGB
-## images (now this functions accepts them if they are m-by-n-by-3 arrays.
-##
+## @seealso{isbw, isgray, isind}
 ## @end deftypefn
 
-## TODO: Check if logical arrays should be considered RGB
-
-## Author:  Josep Mones i Teixidor <jmones@puntbarra.com>
+function bool = isrgb (img)
 
-function flag = isrgb(A)
-  if (nargin!=1)
-    usage("flag=isrgb(A)");
+  if (nargin != 1)
+    print_usage;
   endif
-  
-  if(ismatrix(A))
-    flag=1;
-    s=size(A);
-    if(length(s)!=3 || s(3)!=3)
-      flag=0; ## false if not m-by-n-by-3
-    elseif(strcmp(typeinfo(A),"matrix") && (any(A(:)<0) || any(A(:)>1)))
-      flag=0; ## false if class double but items are <0 or >1
-    endif
-  else
-    flag=0;
+
+  bool = false;
+  if (ismatrix (img) && ndims (img) == 3 && size (img, 3) == 3 && !issparse (img))
+    switch (class (img))
+      case "double"
+        ## to speed this up, we can look at a sample of the image first
+        bool = is_rgb_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_rgb_double (img);
+        endif
+      case {"uint8", "uint16"}
+        bool = true;
+    endswitch
   endif
+
 endfunction
 
+function bool = is_rgb_double (img)
+  bool = all ((img(:) >= 0 & img(:) <= 1) | isnan (img(:)));
+endfunction
 
 %!demo
 %! isrgb(rand(1,2,3))
 %! # A 1-by-2-by-3 double matrix with elements between 0 and 1 is a RGB image.
 
-
 %!# Non-matrix
-%!assert(isrgb("this is not a RGB image"),0);
+%!assert(isrgb("this is not a RGB image"),false);
 
 %!# Double matrix tests
-%!assert(isrgb(rand(5,5)),0);
-%!assert(isrgb(rand(5,5,1,5)),0);
-%!assert(isrgb(rand(5,5,3,5)),0);
-%!assert(isrgb(rand(5,5,3)),1);
-%!assert(isrgb(ones(5,5,3)),1);
-%!assert(isrgb(ones(5,5,3)+.0001),0);
-%!assert(isrgb(zeros(5,5,3)-.0001),0);
+%!assert(isrgb(rand(5,5)),false);
+%!assert(isrgb(rand(5,5,1,5)),false);
+%!assert(isrgb(rand(5,5,3,5)),false);
+%!assert(isrgb(rand(5,5,3)),true);
+%!assert(isrgb(ones(5,5,3)),true);
+%!assert(isrgb(ones(5,5,3)+.0001),false);
+%!assert(isrgb(zeros(5,5,3)-.0001),false);
 
-%!# Logical
-%!assert(isrgb(logical(round(rand(5,5,3)))),1);
+%!# Logical -- checked in matlab, should return false
+%!assert(isrgb(logical(round(rand(5,5,3)))),false);