Displaying multiple image on single window not working properly (C++ VS12) - c++

Question

I have tried this code I gotten here. Its for displaying multiple images on a single window for C++. I have included the opencv 3.0 library on the program as well. Below is the code. I am trying to load 2 images but only the first one (1.jpg) appears but when i put image2 to be equal to cv::imread("1.jpg"); two images of 1.jpg appears. I am really new to this and I dont understand where i am going wrong here. I hope someone can help me. Thank you.
int main(int argc, char *argv[])
{
// read an image
cv::Mat image1= cv::imread("1.jpg");
cv::Mat image2= cv::imread("2.jpg");
int dstWidth = image1.cols;
int dstHeight = image1.rows * 2;
cv::Mat dst = cv::Mat(dstHeight, dstWidth, CV_8UC3, cv::Scalar(0,0,0));
cv::Rect roi(cv::Rect(0,0,image1.cols, image1.rows));
cv::Mat targetROI = dst(roi);
image1.copyTo(targetROI);
targetROI = dst(cv::Rect(0,image1.rows,image1.cols, image1.rows));
image2.copyTo(targetROI);
// create image window named "My Image"
cv::namedWindow("OpenCV Window");
// show the image on window
cv::imshow("OpenCV Window", dst);
// wait key for 5000 ms
cv::waitKey(5000);
return 0;
}
This is the result of the program above

Answer 1

Your code works ok for me, if images have the same size. Otherwise, the call to
image2.copyTo(targetROI);
will copy image2 into a newly created image, not in dst as you would expect.
If you want to make it work in general, you should:
1) to set dstWidth and dstHeight like:
int dstWidth = max(image1.cols, image2.cols);
int dstHeight = image1.rows + image2.rows;
2) set the second ROI with the size of the second image:
targetROI = dst(cv::Rect(0, image1.rows, image2.cols, image2.rows));
// ^ ^
From the comments, to show 4 images disposed as 2x2, you need a little more work:
#include <opencv2\opencv.hpp>
#include <iostream>
using namespace cv;
using namespace std;
int main()
{
// read an image
cv::Mat image1 = cv::imread("path_to_image1");
cv::Mat image2 = cv::imread("path_to_image2");
cv::Mat image3 = cv::imread("path_to_image3");
cv::Mat image4 = cv::imread("path_to_image4");
//////////////////////
// image1 image2
// image3 image4
//////////////////////
int max13cols = max(image1.cols, image3.cols);
int max24cols = max(image2.cols, image4.cols);
int dstWidth = max13cols + max24cols;
int max12rows = max(image1.rows, image2.rows);
int max34rows = max(image3.rows, image4.rows);
int dstHeight = max12rows + max34rows;
cv::Mat dst = cv::Mat(dstHeight, dstWidth, CV_8UC3, cv::Scalar(0, 0, 0));
cv::Rect roi(cv::Rect(0, 0, image1.cols, image1.rows));
image1.copyTo(dst(roi));
roi = cv::Rect(max13cols, 0, image2.cols, image2.rows);
image2.copyTo(dst(roi));
roi = cv::Rect(0, max12rows, image3.cols, image3.rows);
image3.copyTo(dst(roi));
roi = cv::Rect(max13cols, max12rows, image4.cols, image4.rows);
image4.copyTo(dst(roi));
cv::imshow("OpenCV Window", dst);
cv::waitKey(0);
return 0;
}