如何在python opencv中使用阈值来识别视频的镜头边界？

import cv2
import numpy as np

video = cv2.VideoCapture('video.mp4')

fgbg = cv2.createBackgroundSubtractorMOG2()

while True:
    ret, frame = video.read()
    if not ret:
        break

    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    fgmask = fgbg.apply(gray)
    _, thresh = cv2.threshold(fgmask, 127, 255, cv2.THRESH_BINARY)

    contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
    for contour in contours:
        x, y, w, h = cv2.boundingRect(contour)
        cv2.rectangle(frame, (x, y), (x + w, y + h), (0, 255, 0), 2)

    cv2.imshow('Video', frame)
    if cv2.waitKey(1) == ord('q'):
        break

video.release()
cv2.destroyAllWindows()