MIN convert argument to np.uint8 in overlay()

The function is to be used to visualize results, so this is often the
desired result. The code already implicitly assumed np.uint8 as it used
255 as the highest value.

close #62

mahotas/mahotas_version.py

@@ -1 +1 @@
-__version__ = '1.4.0'
+__version__ = '1.4.0+git'

mahotas/stretch.py

@@ -176,24 +176,34 @@ def s(c):
     return np.dstack([s(r), s(g), s(b)])
 
 
-def overlay(gray, red=None, green=None, blue=None):
+def overlay(gray, red=None, green=None, blue=None, if_gray_dtype_not_uint8='stretch'):
     '''
     Create an image which is greyscale, but with possible boolean overlays.
 
     Parameters
     ----------
-    gray: ndarray
-        Should be a greyscale image
+    gray: ndarray of type np.uint8
+        Should be a greyscale image of type np.uint8
 
     red,green,blue : ndarray, optional
         boolean arrays
 
+    if_gray_dtype_not_uint8 : str, optional
+        What to do if ``gray`` is not of type ``np.uint8``, must be one of
+            'stretch' (default): the function ``stretch`` is called.
+            'error' : in this case, an error is raised
+
     Returns
     -------
     overlaid : ndarray
         Colour image
     '''
     _check_2(gray, 'overlay')
+    if gray.dtype != np.uint8:
+        if if_gray_dtype_not_uint8 == 'stretch':
+            gray = stretch(gray)
+        else:
+            raise ValueError('mahotas.overlay: first argument should be of dtype np.uint8')
     def _v(ch):
         if ch is None:
             return gray