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Another solution is to use hconcat and vconcat.

Demo code:

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>

using namespace cv;

int main() {
  Mat img11(240, 320, CV_8UC3, Scalar(0,0,255));
  Mat img12(240, 320, CV_8UC3, Scalar(0,255,0));
  Mat img21(240, 320, CV_8UC3, Scalar(255,0,0));
  Mat img22(240, 320, CV_8UC3, Scalar(255,0,255));

  Mat concat1;
  hconcat(img11, img12, concat1);

  Mat concat2;
  hconcat(img21, img22, concat2);

  Mat concat;
  vconcat(concat1, concat2, concat);

  imshow("Concat", concat);
  waitKey();

  return 0;
}

Another solution is to use hconcat and vconcat.. Will work for trivial dimensions.

Demo code:

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>

using namespace cv;

int main() {
  Mat img11(240, 320, CV_8UC3, Scalar(0,0,255));
  Mat img12(240, 320, CV_8UC3, Scalar(0,255,0));
  Mat img21(240, 320, CV_8UC3, Scalar(255,0,0));
  Mat img22(240, 320, CV_8UC3, Scalar(255,0,255));

  Mat concat1;
  hconcat(img11, img12, concat1);

  Mat concat2;
  hconcat(img21, img22, concat2);

  Mat concat;
  vconcat(concat1, concat2, concat);

  imshow("Concat", concat);
  waitKey();

  return 0;
}