Using openCV to overlay transparent image onto another image

Question

How can I overlay a transparent PNG onto another image without loosing it\'s transparency using openCV in python?

import cv2

background = cv2.imread(\'field         


        

Answer 1

You need to open the transparent png image using the flag IMREAD_UNCHANGED

Mat overlay = cv::imread("dice.png", IMREAD_UNCHANGED);

Then split the channels, group the RGB and use the transparent channel as an mask, do like that:

/**
 * @brief Draws a transparent image over a frame Mat.
 * 
 * @param frame the frame where the transparent image will be drawn
 * @param transp the Mat image with transparency, read from a PNG image, with the IMREAD_UNCHANGED flag
 * @param xPos x position of the frame image where the image will start.
 * @param yPos y position of the frame image where the image will start.
 */
void drawTransparency(Mat frame, Mat transp, int xPos, int yPos) {
    Mat mask;
    vector layers;

    split(transp, layers); // seperate channels
    Mat rgb[3] = { layers[0],layers[1],layers[2] };
    mask = layers[3]; // png's alpha channel used as mask
    merge(rgb, 3, transp);  // put together the RGB channels, now transp insn't transparent 
    transp.copyTo(frame.rowRange(yPos, yPos + transp.rows).colRange(xPos, xPos + transp.cols), mask);
}

Can be called like that:

drawTransparency(background, overlay, 10, 10);