zw-xxx

I check the code in `core.falldetector.py` ``` math.sqrt(pow(abs(x-old_x), 2)+pow((y-old_y), 2)) >= 0.5*math.sqrt(pow(old_w, 2)+pow(old_h, 2)) ``` and the 'FALL' will be detected. but I have no idea where this method comes...