环境

Ubuntu 18.04 aarch64
Python 3.7.15
opencv-python 4.6.0

问题现象

插入USB摄像头后，/dev/video0会正常出现，使用fswebcam也能正常拍摄照片。但运行opencv-python的视频拍摄例程时出错，例程如下。
如果例程正确运行，屏幕窗口中将显示灰度处理后的摄像头实时视频。

import numpy as np
import cv2 as cv
cap = cv.VideoCapture(0)
if not cap.isOpened():
    print("Cannot open camera")
    exit()
while True:
    # Capture frame-by-frame
    ret, frame = cap.read()
    # if frame is read correctly ret is True
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break
    # Our operations on the frame come here
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
    # Display the resulting frame
    cv.imshow('frame', gray)
    if cv.waitKey(1) == ord('q'):
        break
# When everything done, release the capture
cap.release()
cv.destroyAllWindows()

报错信息如下

[ WARN:0@10.618] global /io/opencv/modules/videoio/src/cap_v4l.cpp (1013) tryIoctl VIDEOIO(V4L2:/dev/video0): select() timeout.
Can't receive frame (stream end?). Exiting ...

问题分析

单步调试后，发现出错语句在于

ret, frame = cap.read()

执行该语句后，终端会出现警告信息[ WARN:0@10.618] global /io/opencv/modules/videoio/src/cap_v4l.cpp (1013) tryIoctl VIDEOIO(V4L2:/dev/video0): select() timeout.；并且语句的返回值ret为False，代表执行失败。

解决问题

见招拆招，接下来谷歌这个警告，有如下收获

https://forum.opencv.org/t/videoio-v4l2-dev-video0-select-timeout/8822/5

Hey, I got simmilar issue and solve it with setting format as MJPG int the 4-character code of codec.

from cv2 import cv2

camera = cv2.VideoCapture(0)
camera.set(cv2.CAP_PROP_FOURCC, cv2.VideoWriter_fourcc('M', 'J', 'P', 'G'))
status, image = camera.read()
camera.release()

我们在初始化cap对象后，给它额外设置属性”MJPG int 编解码器的 4 字符代码”，即可解决该问题。原理尚不清楚，先记录下过程。

修改后的例程如下

import numpy as np
import cv2 as cv
cap = cv.VideoCapture(0)
cap.set(cv.CAP_PROP_FOURCC, cv.VideoWriter_fourcc('M', 'J', 'P', 'G'))

if not cap.isOpened():
    print("Cannot open camera")
    exit()
while True:
    # Capture frame-by-frame
    ret, frame = cap.read()
    # if frame is read correctly ret is True
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break
    # Our operations on the frame come here
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
    # Display the resulting frame
    cv.imshow('frame', gray)
    if cv.waitKey(1) == ord('q'):
        break
# When everything done, release the capture
cap.release()
cv.destroyAllWindows()

此时，屏幕窗口中应当正确显示灰度处理后的摄像头实时视频。