Render multiple images into a single image

C++
1

I want to add multiple images to a single texture. Example background size 1024x1024.
I want to resize and put some images on 1024x1024 texture to specific position. I can blend and even offset images on each other when their sizes are same. When different I get error on vs2019.

I did something like this:

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgcodecs/imgcodecs.hpp>
#include <opencv2/imgproc.hpp>
#include "imageOperations.h"
#include <list>
using namespace cv;
using namespace std;

struct imageStruct {
	string path;
	string name;
	short channels;
	bool isPowerOfTwo;
	Mat image;
	Size dimensions;

};
//this works for offseting when blending images with same size
void OffsetImage(Mat& image, cv::Scalar bordercolour, int xoffset, int yoffset)
{
	int xoffset_ = xoffset * -1;
	int yoffset_ = yoffset * -1;
	Mat offsetImage = Mat::zeros(image.size(), image.type());
	Mat padded = Mat(image.rows + 2 * abs(yoffset_), image.cols + 2 * abs(xoffset_), CV_8UC3, bordercolour);
	image.copyTo(padded(Rect(abs(xoffset_), abs(yoffset_), image.cols, image.rows)));
	image = Mat(padded, Rect(abs(xoffset_) + xoffset_, abs(yoffset_) + yoffset_, image.cols, image.rows));
}

...
int main()
{
	std::list<imageStruct>imageList;

	imageStruct img;
	img.image = imread("C:/Users/gomi/source/repos/UEFlipbookPacker/Debug/cover.jpg");
	Size imgsize = img.image.size();
	img.dimensions = imgsize;
	INT2 new_img_size = resize(ImageResizeMethod::width, 512, INT2{ img.dimensions.width,img.dimensions.height });
	resize(img.image, img.image, { new_img_size.x, new_img_size.y },0,0,cv::INTER_NEAREST);
	
	imageStruct img2;
	img2.image = imread("C:/Users/gomi/source/repos/UEFlipbookPacker/Debug/cover_blend.jpg");
	Size img2size = img2.image.size();
	img2.dimensions = img2size;
	INT2 new_img2_size = resize(ImageResizeMethod::width, 512, INT2{ img2.dimensions.width,img2.dimensions.height });
	resize(img2.image, img2.image, { new_img2_size.x, new_img2_size.y }, 0, 0, cv::INTER_NEAREST);

	imageList.push_back(img);
	imageList.push_back(img2);

	//imshow("Display Window", ???final_img???);

	waitKey(0);
	return 0;

}

I produce many textures into one texture and in UE4 that looks like this:
Soviet_Starx
First image is the thing I want to do with openCV

I manually do this process I produce many textures so I need to put them to gether into power of two textures.

I do not know how to put two or more different images in different sizes onto a third texture with different size. How to do it?

UPDATE 1:
After searching for an hour I found a solution I manipulated pixel data like this:

struct imageStruct {
	string path;
	string name;
	short channels;
	bool isPowerOfTwo;
	Mat image;
	Size dimensions;

};

bool OverlayTwoImages(imageStruct src_img, imageStruct target_img, INT2 offset)
{

	for (int x = 0; x < src_img.image.cols; x++)
	{
		for (int y = 0; y < src_img.image.rows; y++)
		{
			if (x < target_img.image.cols && y < target_img.image.rows) {
				src_img.image.at<Vec3b>(Point(x + offset.x, y + offset.y))[2] = target_img.image.at<Vec3b>(Point(x, y))[2];
				src_img.image.at<Vec3b>(Point(x + offset.x, y + offset.y))[1] = target_img.image.at<Vec3b>(Point(x, y))[1];
				src_img.image.at<Vec3b>(Point(x + offset.x, y + offset.y))[0] = target_img.image.at<Vec3b>(Point(x, y))[0];
			}
		}
	}
	return true;
}

Usage:
OverlayTwoImages(img, img2, { 100,100 });