Image of a layer with opacity on top of another image. - OpenCV

Question

Edit

Anyone who has a similar problem, I found another answer here with a great python solution that uses NumPy speed.

Please consider the following problem:

I have two images of the same size. One of them is a red square with various levels of opacity:

And the second, a blue square, is smaller than red, without transparency, but white, surrounding it.

I am using Python bindings for OpenCV for this project so far (after reading about watermarks here I have this:

redSquare = cv2.imread('redSquare.png', cv2.IMREAD_UNCHANGED)
(rH, rW) = redSquare.shape[:2]

blueSquare = cv2.imread('blueSquare.png')
(h, w) = blueSquare.shape[:2]

blueSquare = np.dstack([blueSquare, np.ones((h,w), dtype = 'uint8') * 255])
overlay = np.zeros((h,w,4), dtype = 'uint8')
overlay[0:rH, 0:rW] = redSquare
output = blueSquare .copy()
cv2.addWeighted(overlay, 0.5, output, 0.5, 0, output)

cv2.imwrite('imageAdded.png', output)

Which produces the following output:

However, the desired effect:

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Answer 1

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// 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 overlay = imread("ZuWDz.png",IMREAD_UNCHANGED);
    Mat underlay = imread("CtBAe.png",IMREAD_UNCHANGED);

    if( underlay.empty() || overlay.empty() )
    {
        return -1;
    }

    overlayImage( &underlay, &overlay, Point() );
    imshow("underlay result",underlay);

    waitKey();

    return 0;
}