if (cv2.__version__).split('.')[0] =='3':
_, contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
else:
contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
cnt = contours[np.argmax([cv2.contourArea(cnt) for cnt in contours])]