Display the bottom image of the line and cut the upper image using Opencv

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I am trying to crop the live video diagonally. With the help of cv.line, I have mentioned the dimensions and my goal is to display the video of the lower side of the line I have drawn, and the upper video should be cropped, As a beginner, I was just able to draw a line using the following code:

import cv2

cv2.namedWindow("preview")
vc = cv2.VideoCapture(0)

if vc.isOpened(): # try to get the first frame
    rval, frame = vc.read()
else:
    rval = False

while rval:
    cv2.imshow("preview", frame)
    rval, frame = vc.read()
    key = cv2.waitKey(20)
    if key == 27: # exit on ESC
        break
    else:
        cv2.line(img=frame, pt1=(700,5), pt2=(5, 450), color=(255, 0, 0), thickness=1, lineType=8, shift=0)

vc.release()
cv2.destroyWindow("preview")

Output:

Suggestion on this will be very helpful