How to overlay an PNG image with alpha channel to another PNG?

edit

Looking at this post: http://jepsonsblog.blogspot.be/2012/1...

taking Max5684 comment

// http://jepsonsblog.blogspot.be/2012/10/overlay-transparent-image-in-opencv.html
// https://gist.github.com/maximus5684/082f8939edb6aed7ba0a

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

void overlayImage(Mat* src, Mat* overlay, const Point& location)
{
    for (int y = max(location.y, 0); y < src->rows; ++y)
    {
        int fY = y - location.y;

        if (fY >= overlay->rows)
            break;

        for (int x = max(location.x, 0); x < src->cols; ++x)
        {
            int fX = x - location.x;

            if (fX >= overlay->cols)
                break;

            double opacity = ((double)overlay->data[fY * overlay->step + fX * overlay->channels() + 3]) / 255;

            for (int c = 0; opacity > 0 && c < src->channels(); ++c)
            {
                unsigned char overlayPx = overlay->data[fY * overlay->step + fX * overlay->channels() + c];
                unsigned char srcPx = src->data[y * src->step + x * src->channels() + c];
                src->data[y * src->step + src->channels() * x + c] = srcPx * (1. - opacity) + overlayPx * opacity;
            }
        }
    }
}

int main( int argc, char** argv )
{
    Mat underlay = imread("14447300874345549.png",IMREAD_UNCHANGED);
    Mat overlay = imread("14447300692773615.png",IMREAD_UNCHANGED);
    Mat test = imread("1429209553641108.png",IMREAD_UNCHANGED);

    if( underlay.empty() || overlay.empty() || test.empty() )
    {
        cout << "Could not read input image files " << endl;
        return -1;
    }

    Mat rgba[4];
    split(underlay,rgba);
    imshow("alpha1.png",rgba[3]);
    imwrite("alpha1.png",rgba[3]);

    split(overlay,rgba);
    imshow("alpha2.png",rgba[3]);
    imwrite("alpha2.png",rgba[3]);

    overlayImage( &underlay, &overlay, Point() );
    overlayImage( &test, &underlay, Point(120,180) );

    split(underlay,rgba);
    imshow("alpha3.png",rgba[3]);
    imwrite("alpha3.png",rgba[3]);

    imshow("result1",underlay);
    imwrite("result1.png",underlay);
    imshow("result2",test);
    imwrite("result2.png",test);

    waitKey();

    return 0;
}

Test Images :

Result Images:

Alpha channels before:

Alpha channel of result:

The output image will always be of the same size as the background image. The position parameter determines how the foreground is placed on top of the background. A position of (100, -50) will move the foreground 100 pixels to the right and 50 pixels up.