Why does cv2.VideoCapture.read() return the same frame from a camera

Python
1

Hi, I’m using python api to read images from an usb camera on win11. Here is my code:

import time

import cv2
import numpy as np
from multiprocessing import Queue, Process


def collect_frames(que):
    # camera settings
    cap = cv2.VideoCapture(0)
    cap.set(cv2.CAP_PROP_BUFFERSIZE, 0)
    cap.set(cv2.CAP_PROP_FRAME_WIDTH, 1280)
    cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 720)
    cap.set(cv2.CAP_PROP_FPS, 30)
    time.sleep(1)
    print(cap.get(cv2.CAP_PROP_FPS))

    while cap.isOpened():
        res, frame = cap.read()
        if res:
            que.put(frame)
            time.sleep(1 / 30)


if __name__ == '__main__':
    que = Queue()
    collect_process = Process(target=collect_frames, args=(que,))
    collect_process.start()

    prev_frame = None
    while True:
        if not que.empty():
            frame_que = que.get()
            if np.all(prev_frame == frame_que):
                print("get the same frame!")
            prev_frame = frame_que
            cv2.imshow("cam", frame_que)
            c = cv2.waitKey(1)
            if c == 27:
                break

    collect_process.join()

I expect cv2.VideoCapture.read() to return a new frame every time it gets called, but it turns out that a lot of frames are the same as the previous one. WHY?
Besides, multithreading implementation of above codes meets the same problem.
FYI, I’m trying to save specified number of frames from the camera, but I found there are same images saved. This is how I found this problem.