OpenCV displaying 2 images adjacently in the same window - image-processing

Question

I am trying to display 2 images horizontally adjacent to each other in the same window using OpenCV.
I have tried using adjustROI function for this.Image 1 has 1088 pixels width and 2208 pixels height while Image 2 has 1280 pixels width and 2208 pixels height.Please suggest what could be wrong in the code below.All I am getting is an image of size Image2 with content from Image2 as well.
Mat img_matches=Mat(2208,2368,imgorig.type());//set size as combination of img1 and img2
img_matches.adjustROI(0,0,0,-1280);
imgorig.copyTo(img_matches);
img_matches.adjustROI(0,0,1088,1280);
imgorig2.copyTo(img_matches);

Answer 1

EDIT: Here's how I'd do what you want to do:
Mat left(img_matches, Rect(0, 0, 1088, 2208)); // Copy constructor
imgorig.copyTo(left);
Mat right(img_matches, Rect(1088, 0, 1280, 2208)); // Copy constructor
imgorig2.copyTo(right);
The copy constructors create a copy of the Mat header that points to the ROI defined by each of the Rects.
Full code:
#include <cv.h>
#include <highgui.h>
using namespace cv;
int
main(int argc, char **argv)
{
Mat im1 = imread(argv[1]);
Mat im2 = imread(argv[2]);
Size sz1 = im1.size();
Size sz2 = im2.size();
Mat im3(sz1.height, sz1.width+sz2.width, CV_8UC3);
Mat left(im3, Rect(0, 0, sz1.width, sz1.height));
im1.copyTo(left);
Mat right(im3, Rect(sz1.width, 0, sz2.width, sz2.height));
im2.copyTo(right);
imshow("im3", im3);
waitKey(0);
return 0;
}
Compiles with:
g++ foo.cpp -o foo.out `pkg-config --cflags --libs opencv`
EDIT2:
Here's how it looks with adjustROI:
#include <cv.h>
#include <highgui.h>
using namespace cv;
int
main(int argc, char **argv)
{
Mat im1 = imread(argv[1]);
Mat im2 = imread(argv[2]);
Size sz1 = im1.size();
Size sz2 = im2.size();
Mat im3(sz1.height, sz1.width+sz2.width, CV_8UC3);
// Move right boundary to the left.
im3.adjustROI(0, 0, 0, -sz2.width);
im1.copyTo(im3);
// Move the left boundary to the right, right boundary to the right.
im3.adjustROI(0, 0, -sz1.width, sz2.width);
im2.copyTo(im3);
// restore original ROI.
im3.adjustROI(0, 0, sz1.width, 0);
imshow("im3", im3);
waitKey(0);
return 0;
}
You have to keep track of what the current ROI is, and the syntax for moving the ROI around can be a little un-intuitive. The result is the same as the first block of code.

Answer 2

As the height (rows of Mat) of the images are same, function hconcat maybe used to horizontally concatenate two images (Mat) and thus can be used to display them side-by-side in the same window. OpenCV doc.
It works with both grayscale and color images. The number of color channels in the source matrices must be same.
Mat im1, im2; // source images im1 and im2
Mat newImage;
hconcat(im1, im2, newImage); // <---- place image side by side
imshow("Display side by side", newImage);
waitKey(0);
For the sake of completeness, vconcat can be similarly used for vertical concatenation.

Answer 3

Here's a solution inspired in #misha's answer.
#include <cv.h>
#include <highgui.h>
using namespace cv;
int
main(int argc, char **argv)
{
Mat im1 = imread(argv[1]);
Mat im2 = imread(argv[2]);
Size sz1 = im1.size();
Size sz2 = im2.size();
Mat im3(sz1.height, sz1.width+sz2.width, CV_8UC3);
im1.copyTo(im3(Rect(0, 0, sz1.width, sz1.height)));
im2.copyTo(im3(Rect(sz1.width, 0, sz2.width, sz2.height)));
imshow("im3", im3);
waitKey(0);
return 0;
}
Instead of using the copy constructor, this solution uses Mat::operator()(const Rect& roi). While both solutions are O(1), this solution seems cleaner.