OpenCV VideoWriter断言失败img.cols ==宽度&& img.rows ==高度* 3

Question

I am trying to use the class VideoWriter(and VideoCapture of course) to use the camera and save a video, and if the video is longer than 10 seconds the last 10 seconds will be saved in the output file. There are two instances of VideoWriter in the code. However when running I always got the following error in the console window:
OpenCV Error: Assertion failed (img.cols == width && img.rows == height*3) in cv::mjpeg::MotionJpegWriter::write, file C:\\build\\master_winpack-build-win64- vc14\\opencv\\modules\\videoio\\src\\cap_mjpeg_encoder.cpp, line 842
Here is my code:(C++)

void main()
{
    VideoCapture vdc(0);
    VideoWriter vw("REC.avi", CV_FOURCC('M', 'J', 'P', 'G'), 100.0, Size(vdc.get(CAP_PROP_FRAME_WIDTH), vdc.get(CAP_PROP_FRAME_HEIGHT)),true);
    VideoWriter vw1("REC2.avi", CV_FOURCC('M', 'J', 'P', 'G'), 100.0, Size(vdc.get(CAP_PROP_FRAME_WIDTH), vdc.get(CAP_PROP_FRAME_HEIGHT)),true);
    char cmd = 'a';
    Mat frm;
    DWORD st, ed;
    st = GetTickCount();
    while (cmd != 27 && vdc.isOpened())
    {
        vdc >> frm;
        vw << frm;
        vw1 << frm;
        imshow("Camera", frm);
        cmd = waitKey(10);
    }
    ed = GetTickCount();
    destroyAllWindows();
    cout << ed - st << endl;
    vdc.release();
    vw.release();
    vw1.release();
    if (ed - st > 10000)
    {
        VideoCapture vdc2("REC2.avi");
        VideoWriter vw2("REC.avi", CV_FOURCC('M', 'J', 'P', 'G'), 100.0, Size(vdc2.get(CAP_PROP_FRAME_WIDTH), vdc2.get(CAP_PROP_FRAME_HEIGHT)),true);
        DWORD st2, ed2;
        st2 = getTickCount();
        Mat frame;
        while (vdc2.isOpened())
        {
            ed2 = getTickCount();
            vdc2 >> frame;
            if (ed2 - st2 > ed - st - 10000)
            {
                vw2 << frame;
                waitKey(10);
                if (vw2.get(CAP_PROP_POS_AVI_RATIO) == 1)break;
            }
        }
    }
}

I am sure the number of the channels is three and I met no problem with vw(In fact REC2.avi can be opened normally but REC.avi can't).
I use VS2017 and the latest version of OpenCV in Win10.
I am a complete novice at OpenCV and out of things to try.

Answer 1

There are a couple of problems that I notice:

1. You are opening the same file in both vw1 and vw2. I am not sure if vw1.release() closes the file properly.

2. Using the tick count to calculate the video duration of 10 seconds is a bit tricky. How about using a frame counter and the frame rate instead?

Answer 2

I am not sure but the dimensions of the frame and the size mentioned in vw2 does not match. Try resizing the frame and then writing it.