树莓派 pi4 opencv 和 USB 网络摄像头 python3 我已经安装了 opencv

如何解决树莓派 pi4 opencv 和 USB 网络摄像头 python3 我已经安装了 opencv

在人脸识别中，我写了下面的代码，但我仍然遇到无法在新窗口中打开 USB 网络摄像头的问题，我尝试输入 -1 和 1 但它不起作用

import cv2
cam = cv2.VideoCapture(0)

cv2.namedWindow("press space to take a photo",cv2.WINDOW_norMAL)
cv2.resizeWindow("press space to take a photo",500,300)

img_counter = 0

while True:
    ret,frame = cam.read()
    if not ret:
        print("Failed to grab frame")
        break
    cv2.imshow("press space to take a photo",frame)

error message
 [ WARN:0] global /tmp/pip-wheel-qd18ncao/opencv-python/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video0): cant open camera by index
Failed to grab frame