patch division by zero (#122)

Author: qgeissmann

src/ethoscope/trackers/adaptive_bg_tracker.py

@@ -288,7 +288,10 @@ def _pre_process_input_minimal(self, img, mask, t, darker_fg=True):
         #
         mean = cv2.mean(self._buff_grey, mask)
 
-        scale = 128. / mean[0]
+        try:
+            scale = 128. / mean[0]
+        except ZeroDivisionError:
+            raise NoPositionError
 
         cv2.multiply(self._buff_grey, scale, dst = self._buff_grey)
 
@@ -508,4 +511,4 @@ def _track(self, img,  grey, mask,t):
 
 
         self._previous_shape=np.copy(hull)
-        return [out]
+        return [out]