Splitting a frame into a grid and combining again (c++, 2.4.11)

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Hi, new to this forum and new to OpenCV.

I'm using version 2.4.11, with Eclipse Neon C++

This is for a university assignment and is only the first step, main part of the assignment is scrambling the grid and applying different effects to them.

My problem is the webcam feed will not stay open for more than a split second, if I use waitKey(0) it will display a single frame but any other way and it just instantly closes, if anyone could help me get this working I'd be really grateful

    Mat frame, img_gry, img_gry_color;
    int a = 0;
    Mat imagesmall[100];
    vector<Mat> img_gry_vec;
    cv::Size sb_size(64,48);
    for(;;) {
        cap >> frame;
        if ( frame.empty()) {

            break;
        }
          for  ( int y = 0; y < 480; y += sb_size.height )

          {
            for  ( int  x= 0 ; x < 640; x += sb_size.width )
            {
                imagesmall[a] = Mat(frame, Rect(x,y,(sb_size.width), (sb_size.height))).clone();
                img_gry_vec.push_back(imagesmall[a]);
                a++;
            }
          }
        cv::Mat combined(480,640, img_gry_vec[0].type());
        a = 0;
          for  ( int y = 0; y < 480; y += sb_size.height )
          {
            for  ( int  x= 0 ; x < 640; x += sb_size.width )
            {
                  cv::Mat roi = combined(cv::Rect(x,y,sb_size.width,sb_size.height));
                  img_gry_vec[a].copyTo(roi);
                  a++;
            }
          }
        cv::imshow("combined" , combined);

        img_gry_vec.clear();
        if ( cv::waitKey(33) >= 0)
                        break;


    }

    return 0;