In this example, we'll create an image by swapping regions from an original one. For this, Regions of Interest (ROI) were created using rectangles with half the height and width of the original image. In sequence, using the Mat (const Mat &m, const Rect &roi) constructor, 4 images were created by using those rectangles and finally the CV::Mat::copyTo(OutPutArray m) method was used to perfom the swap of the 1-3 and 2-4 regions.

#include <iostream>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

int main()
{
  Mat image = imread("img/camera__.jpg",CV_LOAD_IMAGE_GRAYSCALE);

  if(!image.data)
    cout << "nao abriu camera__.jpg" << endl;

  namedWindow("janela",WINDOW_AUTOSIZE);

  imshow("janela", image);      
  waitKey();

  int x = 0, y = 0, width = image.rows/2, height = image.cols/2;
  Mat image2; 
  image.copyTo(image2);

 // creating Regions of Interest
  Rect rois[4] = { 
        Rect(     0,      0, width, height),
        Rect(width,      0, width, height),
        Rect(     0, height, width, height),
        Rect(width, height, width, height)
        };

  // copying regions and swapping them   
  Mat region1 (image, rois[3]);
  region1.copyTo(image2(rois[0]));

  Mat region2 (image, rois[2]);
  region2.copyTo(image2(rois[1]));

  Mat region3 (image, rois[1]);
  region3.copyTo(image2(rois[2]));

  Mat region4 (image, rois[0]);
  region4.copyTo(image2(rois[3]));

  imwrite("swapping.jpg", image2);
  imshow("janela", image2);  
  waitKey();

  return 0;
}