I don't understand some aspect in optical flow

Python
1

Hi, everyone!
I’ve been coding an optical flow algorithm and have mentioned some interesting issue. The number of vectors returned by this part of code is equal to the y-size of an image:

import cv2 as cv
cap = cv.VideoCapture(0)
ret, frame1 = cap.read()
print(frame1.shape)
prvs = cv.cvtColor(frame1, cv.COLOR_BGR2GRAY)
while True:
    _, frame2 = cap.read()
    next = cv.cvtColor(frame2, cv.COLOR_BGR2GRAY)
    flow = cv.calcOpticalFlowFarneback(prvs, next, None, 0.5, 3, 15, 3, 5, 1.2, 0)
    mag = cv.cartToPolar(flow[..., 0], flow[..., 1], angleInDegrees=True)[0]
    ang = cv.cartToPolar(flow[..., 0], flow[..., 1], angleInDegrees=True)[1]
    print(len(mag))
    prvs = next

the result of the print is always 480 and I don’t understand why. Could anyone please explain me, why is it so?