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Asked: 2026-06-06T18:04:49+00:00

I have just read this in the book OpenCV 2 Computer Vision Application Programming

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I have just read this in the book OpenCV 2 Computer Vision Application Programming Cookbook by R. Laganiere :

It is important to note that in order to open the specified video
file, your computer must have the corresponding codec installed,
otherwise cv::VideoCapture will not be able to understand the input
file. Normally, if you are able to open your video file with a video
player on your machine (such as the Windows Media Player), then OpenCV
should also be able to read this file.

Unfortunately, things aren’t that easy for me. Yes, I can read avi files on my video player, however it does not work with my OpenCV-Qt application. VideoCapture isOpen() method returns false, despite the fact that the path is correct, and every codec needed seem to be here. I tried several files, so it is not related to one particular format.

Does someone here has experience in opening avi files in Ubuntu using OpenCV ? I think this is a big issue, can’t find any trully relevant solution on the internet…

Thanks !!

[EDIT] Here is the function I am working on; some of the variables here are class members, so it may looks incomplete. However, it is this very piece of code that is not working. In particular, the line where I instanciate a new VideoCapture object.

void MainWindow::on_actionOuvrir_fichier_triggered()
{
    //mettre a -1 streamId
    streamId = -1;
    //get path to the avi file
    QString fileName = QFileDialog::getOpenFileName(this,tr("Ouvrir fichier video"),"/home", tr("Videos (*.avi)"));
    std::string utf8_text = fileName.toUtf8().constData();
    //open .avi
    capture = new VideoCapture(utf8_text);
    //check 
    if(!capture->isOpened())
        cout << "probleme ouverture fichier video" << endl;
    //delay between each frame in ms
    rate = capture->get(CV_CAP_PROP_FPS);
    delay = 1000 / rate;
    //start Qtimer recordId
    recordId = startTimer(delay);
    //capture first frame
    if(!capture->read(in))
        cout << "probleme lecture frame fichier video" << endl;
}

[EDIT 2] test on Windows 7

void MainWindow::on_actionOuvrir_fichier_triggered()
{
    //mettre a -1 streamId
    streamId = -1;
    //ouvrir fenetre navigation fichiers pour recuperer path vers .avi
    QString fileName = QFileDialog::getOpenFileName(this,tr("Ouvrir fichier video"),"/home",
                                                    tr("Videos (*.avi)"));
    //std::string utf8_text = fileName.toUtf8().constData();
    std::string current_locale_text = fileName.toLocal8Bit().constData();
    if(QDir().exists(current_locale_text.c_str())) std::cout << "Path is good!" << endl;
    //ouvrir .avi
    capture = new VideoCapture(current_locale_text);
    //check ouverture
    if(!capture->isOpened())
        cout << "probleme ouverture fichier video" << endl;
    //calculer delay between each frame in ms
//    rate = capture->get(CV_CAP_PROP_FPS);
//    delay = 1000 / rate;
    //demarrer timer recordId
    recordId = startTimer(100);
    //capture premiere frame
    if(!capture->read(in))
        cout << "probleme lecture frame fichier video" << endl;
}

With that code I was able to open SOME avi files, but not all (actually far from that). So I guess I definitely have a codec issue… does anyone can tell me how to fix this under Ubuntu ? Don’t let me go on bounty for that one ! Thank you very much.

[EDIT 3]
As suggested by Etienne, I followed instructions here and tried to convert my video to the I420 format supposedly supported by OpenCV on all platforms, using mencoder with the command line given. So I went from 24 bits RGB (RV24) codec to Planar 4:2:0 YUV (I420), according to VLC. Same behavior though, I’m still unable to instanciate the VideoCapture object.

THERE IS A LOT OF UNSOLVED CASES similar to mine on Stack Overflow…

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1 Answer

  1. Editorial Team

    I finally managed to solve my problem.

    What I did :

    • try to open some video to finally find one that would work. Fortunately it only took me 3 videos to find one that work.

    • check the codec with VLC. It is MPEG-4 XVID.

    • use mencoder to convert the file I wanted to read to XVID:

      sudo mencoder myFile.avi -ovc lavc vcodec=mpeg4:mbd=2:cbp:trell:vbitrate=300 -ffourcc XVID -o test.avi

    I also switched to old C function cvCaptureFromFile(). I will go back to C++ interface to make sure it was indeed a codec issue, but I am pretty sure it was ([EDIT] it was a codec issue).

    What did not work :

    • recompile OpenCV with Qt support enabled
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