Use OpenCV to achieve National Day gradient version of the avatar

During the National Day holiday, I found that many friends changed their profile pictures, adding the effect of the national flag, becoming more festive.

The effect looks easy and can be achieved with just a few dozen lines of OpenCV.

The simplest version of image fusion

For the simplest version of image fusion, find an image and a picture of a standard flag. Crop the image to match the size of the flag.

Then, the two images are superimposed

    cv::Mat image = cv::imread("/Users/tony/Downloads/icon.jpg");
    cv::Mat flag = cv::imread("/Users/tony/Downloads/flag.png");
    cv::Mat roi,dst;
    cv::imshow("image",image);
    cv::imshow("flag",flag);

    roi = image(cv::Rect(300.400, flag.cols, flag.rows));

    cv::addWeighted(roi, 0.8, flag, 0.35.0, dst);
    cv::imshow("dst",dst);
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The gradient version

Ok, so let’s try the gradient version again. The closer to the red flag, the greater the weight of the red flag. This will allow for a gradient effect.

    cv::Mat image = cv::imread("/Users/tony/Downloads/icon.jpg");
    cv::Mat flag = cv::imread("/Users/tony/Downloads/flag.png");

    int flag_width = flag.cols;
    int flag_height = flag.rows;

    Mat roi = image(cv::Rect(450.1200, flag_width, flag_height));

    int radius = 0;
    if (flag_width > flag_height) {
        radius = flag_width;
    } else {
        radius = flag_height;
    }

    for (int i=0; i<roi.rows; i++) {
        for (int j=0; j<roi.cols; j++) {int distance = std::sqrt(i*i+j*j);

            double alpha;
            if (distance > radius) {
                alpha =  1;
            }  else {
                alpha = (double) distance / radius;
            }

            Vec3b v1 = roi.at<Vec3b>(i,j);
            roi.at<Vec3b>(i,j)[0]=alpha*v1[0] + (1-alpha)*flag.at<Vec3b>(i,j)[0];
            roi.at<Vec3b>(i,j)[1]=alpha*v1[1] + (1-alpha)*flag.at<Vec3b>(i,j)[1];
            roi.at<Vec3b>(i,j)[2]=alpha*v1[2] + (1-alpha)*flag.at<Vec3b>(i,j)[2];
        }
    }

    cv::imshow("dst",roi);
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This time, the effect is much better than the first time :). BTW, HAPPY National Day to you all!