move attributes from Contrast class to subclass

only two for testing reason
subclassing a subcalls of numpy was hard due to the use of __new__ and
__array_finalize__
max recursion errors appear now after multiple use of derive. Do I need
numpy's __del__ method?

itten/contrasts.py

@@ -28,8 +28,6 @@ class Contrast(np.ndarray):
         obj._frames = frames
         obj._channel = 2
         obj._frm_step = 50
-        obj._bins = 16
-        obj._threshold = 60000
         obj._save = False
         return obj
 
@@ -46,12 +44,24 @@ class Contrast(np.ndarray):
         return np.ndarray.__array_wrap__(self, out_arr, context)
 
 
-class LightDark(Contrast):
-    def __init__(self, arg):
-        super(LightDark, self).__init__()
+# subclassing subclass of numpy http://stackoverflow.com/questions/7342637/how-to-subclass-a-subclass-of-numpy-ndarray
+# TODO es gibt noch das Problem, dass numpy nach mehreren Berechnungen von drive eine max recursion Warnung ausgiebt, warum? Brauche ich __del__
+class VHistStack(Contrast):
+    def __new__(cls, frames, input_array=None):
+        obj = Contrast.__new__(cls, frames, input_array)
+        obj._bins = 16
+        obj._threshold = 60000
+        return obj
+
+    def __array_finalize__(self, obj):
+        if obj is None: return
+        Contrast.__array_finalize__(self, obj)
+        self._bins = getattr(obj, '_bins', None)
+        self._threshold = getattr(obj, '_threshold', None)
+
 
     # TODO jetzt ausschließlich mit self numpy rechnen statt mit contrast_points liste
-    def hist_vstack(self):
+    def derive(self):
         contrast_points = []
         # pwd list sollte in Frames sein und hier nur durchlaufen werden
         for frm_nr in range(self._frames.start, self._frames.end, self._frm_step):