--- a/inst/imadd.m
+++ b/inst/imadd.m
@@ -38,25 +38,8 @@
 
   if (nargin < 2 || nargin > 3)
     print_usage;
-  elseif ((!isnumeric (img) && !islogical (img)) || isempty (img) || issparse (img) || !isreal (img))
-    error ("first argument must be a numeric or logical, non-empty, non-sparse real matrix")
-  elseif ((!isnumeric (val) && !islogical (val)) || isempty (val) || issparse (val) || !isreal (val))
-    error ("second argument must be a numeric, non-empty, non-sparse real matrix")
-  elseif (!ischar (out_class))
-    error ("third argument must be a string that specifies the output class")
   endif
-
-  img_class = class (img);
-  if (all (size (img) == size (val)) && strcmpi (img_class, class (val)))
-    [img, val] = convert (out_class, img, val);
-  elseif (isscalar (val) && isfloat (val))
-    ## according to matlab's documentation, if val is not an image of same size
-    ## and class as img, then it must be a double scalar. But why not also support
-    ## a single scalar and use isfloat?
-    img = convert (out_class, img);
-  else
-    error ("second argument must either be of same class and size of the first or a floating point scalar")
-  end
+  [img, val] = imarithmetics ("imadd", img, val, out_class);
 
   ## output class is the same as input img, unless img is logical in which case
   ## it should be double. Tested in matlab by Freysh at ##matlab:
@@ -80,24 +63,3 @@
   endif
 
 endfunction
-
-function [a, b] = convert (out_class, a, b = 0)
-  ## in the case that we only want to convert one matrix, this subfunction is called
-  ## with 2 arguments only. Then, b takes the value of zero so that the call to the
-  ## functions that change the class is insignificant
-  if (!strcmpi (class (a), out_class))
-    switch tolower (out_class)
-      case {"logical"}  a = logical (a); b = logical (b);
-      case {"uint8"}    a = uint8   (a); b = uint8   (b);
-      case {"uint16"}   a = uint16  (a); b = uint16  (b);
-      case {"uint32"}   a = uint32  (a); b = uint32  (b);
-      case {"int8"}     a = int8    (a); b = int8    (b);
-      case {"int16"}    a = int16   (a); b = int16   (b);
-      case {"int32"}    a = int32   (a); b = int32   (b);
-      case {"double"}   a = double  (a); b = double  (b);
-      case {"single"}   a = single  (a); b = single  (b);
-      otherwise
-        error ("requested class '%s' for output is not supported")
-    endswitch
-  endif
-endfunction

new file mode 100644
--- /dev/null
+++ b/inst/private/imarithmetics.m
@@ -0,0 +1,68 @@
+## 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 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.
+##
+## 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} {} imarithmetics ()
+## This is a private function common to the likes of imadd, imsubtract, etc.
+##
+## First argument is the function name for the error message, while the others are
+## the same order as the original function. It returns the two first input of the
+## original function
+## @end deftypefn
+
+function [img, val] = imarithmetics (func, img, val, out_class)
+
+  is_valid = @(x) ((!isnumeric (x) && !islogical (x)) || isempty (x) || issparse (x) || !isreal (x));
+
+  if (is_valid (img))
+    error ("%s: first argument must be a numeric or logical, non-empty, non-sparse real matrix", func)
+  elseif (is_valid (img))
+    error ("%s: second argument must be a numeric, non-empty, non-sparse real matrix", func)
+  elseif (!ischar (out_class))
+    error ("%s: third argument must be a string that specifies the output class", func)
+  endif
+
+  if (all (size (img) == size (val)) && strcmpi (class (img), class (val)))
+    [img, val] = convert (out_class, img, val);
+  elseif (isscalar (val) && isfloat (val))
+    ## according to matlab's documentation, if val is not an image of same size
+    ## and class as img, then it must be a double scalar. But why not also support
+    ## a single scalar and use isfloat?
+    img = convert (out_class, img);
+  else
+    error ("second argument must either be of same class and size of the first or a floating point scalar")
+  end
+endfunction
+
+function [a, b] = convert (out_class, a, b = 0)
+  ## in the case that we only want to convert one matrix, this subfunction is called
+  ## with 2 arguments only. Then, b takes the value of zero so that the call to the
+  ## functions that change the class is insignificant
+  if (!strcmpi (class (a), out_class))
+    switch tolower (out_class)
+      case {"logical"}  a = logical (a); b = logical (b);
+      case {"uint8"}    a = uint8   (a); b = uint8   (b);
+      case {"uint16"}   a = uint16  (a); b = uint16  (b);
+      case {"uint32"}   a = uint32  (a); b = uint32  (b);
+      case {"int8"}     a = int8    (a); b = int8    (b);
+      case {"int16"}    a = int16   (a); b = int16   (b);
+      case {"int32"}    a = int32   (a); b = int32   (b);
+      case {"double"}   a = double  (a); b = double  (b);
+      case {"single"}   a = single  (a); b = single  (b);
+      otherwise
+        error ("requested class '%s' for output is not supported")
+    endswitch
+  endif
+endfunction