python - 无法在Raspberry Pi4上播放使用OpenCV 4.4.0录制的视频

Question

我在Raspberry Pi Model 4B上安装了OpenCV 4.4。我可以直接从相机播放视频。当我录制视频并想要播放文件时，则无法播放视频。我正在使用VLC播放器。这是我用来录制视频的代码

import numpy as np
import cv2

cap = cv2.VideoCapture(0)
if (cap.isOpened()==False) :
        print("error in opening  video stream")
        exit()

fourcc = cv2.VideoWriter_fourcc(*'mp4v')
out = cv2.VideoWriter('video.mp4',fourcc,20.0,(160,120),True)
while(cap.isOpened()):
        ret,frame = cap.read()
        if ret == True:
                frame = cv2.flip(frame,0)
                out.write(frame)
                cv2.imshow('frame',frame)
                # Press 'q' to quit
                if cv2.waitKey(25) & 0xFF == ord('q'):
                        break
                else:
                   break
cap.release()
out.release()
cv2.destroyAllWindows()

在运行上述脚本时，我得到以下警告

[ WARN:0] global /home/pi/opencv/modules/videoio/src/cap_gstreamer.cpp (935) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1

当我尝试使用ffmpeg转换录制的视频时，出现错误。
我用于转换的代码

ffmpeg -i video.mp4 -vcodec libx264 demo.mp4

错误详情

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf58.20.100
  Duration: 00:00:00.00, bitrate: N/A
Output #0, mp4, to 'demo.mp4':
Output file #0 does not contain any stream

似乎文件已创建，但缺少视频编解码器。寻求帮助。
谢谢

Answer 1

谢谢大家 !
我能够解决。就默认VideoCapture分辨率而言，我提供给VideoWriter的帧和高度似乎存在问题。下面是修改后的脚本

import numpy as np
import cv2

cap = cv2.VideoCapture(0)
cap.set(cv2.CAP_PROP_FRAME_WIDTH, 640)
cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 480)

if (cap.isOpened()==False) :
        print("error in opening  video stream")
        exit()

fourcc = cv2.VideoWriter_fourcc('M','J','P','G')
out = cv2.VideoWriter('video.avi',fourcc,10,(640,480)) 

while(cap.isOpened()):
        ret,frame = cap.read()
        if ret == True:
                frame = cv2.flip(frame,0)
                out.write(frame)
                cv2.imshow('frame',frame)
                # Press 'q' to quit
                if cv2.waitKey(25) & 0xFF == ord('q'):
                        break
        else:
          break
cap.release()
out.release()
cv2.destroyAllWindows()