C++ Place an image on top of another image in a certain location

Question

I'm looking for a way to place on image on top of another image at a set location.

I have been able to place images on top of each other using cv::addWeighted but when I searched for this particular problem, there wasn't any posts that I could find relating to C++.

Quick Example:

200x200 Red Square & 100x100 Blue Square

&

Blue Square on the Red Square at 70x70 (From top left corner Pixel of Blue Square)

Answer 1

You can also create a Mat that points to a rectangular region of the original image and copy the blue image to that:

Mat bigImage = imread("redSquare.png", -1);
Mat lilImage = imread("blueSquare.png", -1);

Mat insetImage(bigImage, Rect(70, 70, 100, 100));
lilImage.copyTo(insetImage);

imshow("Overlay Image", bigImage);

Answer 2

Building from beaker answer, and generalizing to any input images size, with some error checking:

cv::Mat bigImage = cv::imread("redSquare.png", -1);
const cv::Mat smallImage = cv::imread("blueSquare.png", -1);

const int x = 70;
const int y = 70;
cv::Mat destRoi;
try {
    destRoi = bigImage(cv::Rect(x, y, smallImage.cols, smallImage.rows));
}  catch (...) {
    std::cerr << "Trying to create roi out of image boundaries" << std::endl;
    return -1;
}
smallImage.copyTo(destRoi);

cv::imshow("Overlay Image", bigImage);

Check cv::Mat::operator()

Note: Probably this will still fail if the 2 images have different formats, e.g. if one is color and the other grayscale.