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• OpenCV : Cannot open camera with CAP_FFMPEG even though ffmpeg is installed

  24 février 2021, par cildiracagim

  Initializing a camera video capture using CAP_FFMPEG fails even though I have ffmpeg installed :

  


  cv::VideoCapture vcap;
vcap.open(0, cv::CAP_FFMPEG);

// After these, vcap.isOpened() returns false.


  


  However, it works perfectly when I open a file in the same way :

  


  cv::VideoCapture vcap;
vcap.open(argv[1], cv::CAP_FFMPEG);

/* Correctly opens the video file at path argv[1],
vcap.isOpened() returns true. */


  


  The problem occurs in both OpenCV versions installed in my system : 4.5.1 and 3.2.0. Version 3.2.0 came installed in my Linux distribution but I have compiled 4.5.1 myself. During compilation, I have verified that CMake succesfully recognized the ffmpeg installation in my system. I guess the problem is not related to OpenCV not recognizing ffmpeg since I can open and work on video files without any problems on both OpenCV versions.

  


  Any help is greatly appreciated.

  


• FFMPEG : audio unsynchronized, losing framerate and strange behavior when pavucontrol is open

  10 mars 2021, par Rik Ferreira

  I'm trying to record my desktop screen with internal audio and mic, but I'm noticing some issues with the output, according to this empirical scale :

  


  

  • If I record only the desktop, the output is GOOD.

  


  • If I record the desktop + mic, the output is GOOD.

  


  • If I record the desktop + internal audio only, the output is BAD.

  


  • If I record the desktop + mic + internal audio, the output is BAD.

  


  • If I record the desktop + mic + internal audio + open pavucontrol, the output is GOOD.

  


  


  Good means the two audio sources and the video are acceptably synchronized and bad means not synchronized at all.

  


  I recorded some samples to show what is going on :

  


  

  • Screen + Mic + Internal audio

  


  • Screen + Mic

  


  • Screen + Mic + Internal audio + pavucontrol

  


  


  I'm snapping my fingers near the microphone when the Beep! message appears along with the sound.

  


  I'm using a simple script to play a sound and show a message in the terminal instead of using pavucontrol because it masks the problem.

  


  FFMPEG call

  


  Although the entire script is visible in the videos, here is the ffmpeg call in plain text :

  


  Dual audio

  


  ffmpeg -y \
    -f x11grab -s 1920x1080 -i :0.0 \
    -f pulse -i 0 \
    -f pulse -i 1 \
    -filter_complex amix=inputs=2 \
    output.mp4


  


  Only microphone

  


  ffmpeg -y \
    -f x11grab -s 1920x1080 -i :0.0 \
    -f pulse -i 1 \
    output.mp4


  


  Other info

  


  SO : Arch Linux

  


  When pavucontrol isn't open, pacmd list-source-outputs returns : 0 source output(s) available., but when pavucontrol is open :

  


  pacmd list-source-outputs | grep source

2 source output(s) available.
        source: 0 
                module-stream-restore.id = "source-output-by-application-id:org.PulseAudio.pavucontrol"
        source: 1 
                module-stream-restore.id = "source-output-by-application-id:org.PulseAudio.pavucontrol"


  


  What am I missing ? Why the ffmpeg works fine when pavucontrol is open ?

  


• OpenCV4 (python 3.9) VideoCapture failing to open .mp4 file on osx

  15 juin 2021, par Ender

  I am trying to read an mp4 video frame-wise with OpenCV4 installed with brew. The following snippet, though, fails to open the file :

  


  v_path = os.path.join(v_dir, v_title + ".mp4")
v = cv2.VideoCapture(v_path)


  


  resulting in the following error :

  


  OpenCV: Couldn't read video stream from file []


  


  v_path is an absolute path. Some advise searching for the opencv_ffmpeg.dll file and copying it to the root python folder but its not in /usr/local/Cellar/opencv/4.5.2_4/lib and I would rather not download it directly from a site. If this is crucial, how should I reinstall opencv (ideally still using brew) ? The thing is brew include ffmpeg with the opencv installation. Others say converting to .avi with ffmpeg is the trick but there should be a solution for .mp4, right ? Other questions focus on issues where VideoCapture opens the video but is unable to load any frames, this seems like a less severe problem.

  


  brew doctor


  


  reveals no serious issues.