OpenCV VideoWriter won't set FPS correctly

In my program, I read a webcam or video file via OpenCV and display it via Qt. I get fps from video properties and set timer accordingly.

I have no problem reading the video, the fps calibration is good (since the webcam shows 0fps, I set it to 30) But when I record images, I set the fps output video the same as the original video, but when I read it in VLC or even Windows Media Player, the video is accelerated.

The most curious thing, when I play the recorded video in my program, the fps is good and the video is not accelerated.

This is how I do it:

Constructor()
{
    // Initializing the video resources
    cv::VideoCapture capture;
    cv::VideoWriter writer;
    cv::Mat frame;
    int fps;

    if (webcam)
    {
        capture.open(0);
        fps = 30;
    }
    else
    {
        capture.open(inputFilePath);
        fps = this->capture.get(CV_CAP_PROP_FPS);
    }
    // .avi
    writer = cv::VideoWriter(outputFilePath, CV_FOURCC('M', 'J', 'P', 'G'), fps, frame.size());

    // Defining the refresh timer;
    QTimer timer = new QTimer();
    connect(timer, SIGNAL(timeout()), this, SLOT(updateFeed()));
    this->timer->start(1000 / fps);
}

updateFeed()
{
    // ... display the image in a QLabel ... at a reasonnable speed.
    writer.write(frame);
}

      

        
        
        
      

    

Now am I doing something wrong? Thanks to