How to grab video frames in Qt?

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I am new to Qt, I only know the basics: create interfaces and connect slots. In a few words, my knowledge is not deep at all.

I need to open a video file and capture all of its frames to get the R, G, B channels and, later on, process optical flow (this is already done) frame to frame to finally represent it on a window.

Is it possible to get the video frames with Qt? I have researched a lot but not found anything conclusive.

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0
Leonardo Lanchas On BEST ANSWER

I do not know why I could not include the necessary Qt headers to process frames (they seemed to always have unresolved dependencies and some did not exist) so I turned to OpenCV 3.0 and did it this way:

cv::VideoCapture cap(videoFileName);

if(!cap.isOpened())  // check if we succeeded
    return;

while (cap.isOpened())
{
    cv::Mat frame;
    cap >> frame;
    cv::flip(frame, frame, -1);
    cv::flip(frame, frame, 1);

    // get RGB channels
    w = frame.cols;
    h = frame.rows;
    int size          = w * h * sizeof(unsigned char);
    unsigned char * r = (unsigned char*) malloc(size);
    unsigned char * g = (unsigned char*) malloc(size);
    unsigned char * b = (unsigned char*) malloc(size);

    for(int y = 0; y < h;y++)
    {
        for(int x = 0; x < w; x++)
        {
            // get pixel
            cv::Vec3b color = frame.at<cv::Vec3b>(cv::Point(x,y));
            r[y * w + x] = color[2];
            g[y * w + x] = color[1];
            b[y * w + x] = color[0];
        }
    }
}

cap.release();

It has worked perfectly for my purpose so I did not continue researching.

Thanks anyway.

2
Rudolfs Bundulis On

You can use QMediaPlayer to achieve this.

  1. Instantiate the QMediaPlayer.
  2. Subclass the QAbstractVideoSurface.
  3. Set your implementation as the output for the media player via QMediaPlayer::setVideoOutput.
  4. Feed the media player the needed file and eventually it will start calling QAbstractVideoSurface::present(const QVideoFrame & frame) on your implementation of QAbstractVideoSurface if the video was loaded successfully. Then you can access the channels and everything from the QVideoFrame and draw the frame on a widget.