transparently overlay two images with Java

edit

opencv is a computer-vision library, and usually ignores all things alpha (they're irrelevant)

you do not need opencv, for what you're trying, and you better should look for something else, like imageJ

It does however go to show that OpenCV handles 4-channel PNG images, with transparency intact. The blending between the two images is up to you.

I notice that although there's transparency data, the imshow function doesn't take the transparency data into account; it draws everything with 0 transparency by default. I used Pixelmator to create the file bg.png by setting the first layer's transparency to 50%.

Mat frame = imread("bg.png", CV_LOAD_IMAGE_UNCHANGED);

for (int j = 0; j < frame.rows; j++)
{
    for (int i = 0; i < frame.cols; i++)
    {
        Vec4b v = frame.at<Vec4b>(j, i);

        cout << (int)v[0] << endl; // 255
        cout << (int)v[1] << endl; // 255
        cout << (int)v[2] << endl; // 255
        cout << (int)v[3] << endl; // 127
        cout << endl;
    }
}

imshow("f", frame);

waitKey();

GIMP also exports 4-channel PNG files.

Your blending will probably look something like this:

unsigned char red = unsigned char((float(red1) + float(red2)*(alpha2/255.0f))/2.0f);

@sjhalayka -- why would any of this require using opencv ?

@berak If I were doing something like this, I'd use OpenCV simply for the ease of loading image files. I found this might work too:

https://www.learnopencv.com/alpha-ble...