Python - How to save the *exact* video stream that my USB cam is streaming live?

I am using OpenCV 4.5.0 to stream a video from my webcam using the VideoCapture method of OpenCV. Here is the snippet of how I am reading the frames, processing them and then writing them to a file.

import cv2
import numpy as np
dev_id = 0
stream = cv2.VideoCapture(dev_id, cv2.CAP_V4L2)
fourcc = cv2.VideoWriter_fourcc(*'FMP4')
vpath = 'test.mp4'
writer = cv2.VideoWriter(vpath, fourcc, 30, (640,480), isColor=True)

if stream.isOpened():
    rval, frame = stream.read()
else:
    print('Could not open the stream for reading')
if not writer.isOpened():
    print('Could not open the file for writing')

i = 0
while rval:
    i += 1
    with open('frame_{}.pkl'.format(i),'wb') as fout:
        np.save(fout, frame)

    writer.write(frame)

    rval, frame = stream.read()
    key = cv2.waitkey(1)
    if key == 27: # ESC
        break 

writer.release()

My goal is to then read the video file that was written above and reproduce the exact same frames that I got during the live stream. I have not been able to find a way to write the live video stream to a file such that the frame extracted from the written video file matches exactly with the frame stored in the numpy array. In addition to FMP4 as the FourCC, I tried with MJPG, MP4V, XVID, LAGS, H264, MPG4. No luck so far. What am I doing wrong?

Here is how I am reading the video and comparing the frames:

vc = cv2.VideoCapture(vpath)
if vc.isOpened():
    rval, frame_read = vc.read()
else:
    print('Could not read video from the file')

j = 0
while rval:
    j += 1
    with open('frame_{}.pkl'.format(j),'rb') as fin:
        frame_orig = np.load(fin)
    if np.array_equal(frame_read, frame_orig):
        print('same')
    else:
        print('different')

    rval, frame_read = vc.read()

I have also tested if this is simply the indexing mismatch between the video writer and the video reader. It is not. This really looks like the difference caused by the codec used to write the frame to a file. Is there no codec which writes the same exact raw frame that is streamed by the camera?

source https://stackoverflow.com/questions/68535374/python-how-to-save-the-exact-video-stream-that-my-usb-cam-is-streaming-live