임계점 처리하기 참조 https://github.com/ndb796/Python-Data-Analysis-and-Image-Processing-Tutorial/blob/master/10.%20OpenCV%20%EC%9E%84%EA%B3%84%EC%A0%90%20%EC%B2%98%EB%A6%AC%ED%95%98%EA%B8%B0/OpenCV%20%EC%9E%84%EA%B3%84%EC%A0%90%20%EC%B2%98%EB%A6%AC%ED%95%98%EA%B8%B0.ipynb