Generate transparent shape on image

Here is an example code to generate transparent shape on image. Need to pay attention can not use cv::Mat mask(mat) to create the mask image, because mask will be just a shallow copy of mat.

int GenerateTransparentMask()
{
    Mat mat = imread("test.jpg");
    if (mat.empty())
        return -1;

    Size size = mat.size();
    cv::Mat mask ( size, mat.type() );
    mat.copyTo(mask);

    //cv::Mat mask(mat);    //Can not work, becuase mask will be just a copy of mat, if draw on mask, means draw on mat too

    Rect rect(100, 100, 100, 100);
    rectangle(mask, rect, Scalar(0, 0, 255), CV_FILLED, CV_AA, 0);

    double alpha = 0.3;
    double beta = 1.0 - alpha;
    cv::addWeighted(mask, alpha, mat, beta, 0.0, mat);

    cvNamedWindow("Result");
    imshow("Result", mat);

    cvWaitKey(0); //wait for a key press
    return 0;
}

 The result will as below: 

If you need to generate complicated mask on an image, need to use a little different method. Below is the example code.

int GenerateTransparentMask()
{
    const Mat mat = imread("DetectingContours.jpg");
    if (mat.empty())
        return -1;

    imshow("origianl", mat);

    Size size = mat.size();
    cv::Mat copy ( size, mat.type() );
    mat.copyTo ( copy );

    //cv::Mat mask(mat);    //Can not work, becuase mask will be just a copy of mat, draw on mask, means draw on mat too

    cv::Mat mask(size, CV_8U);　　//mask must be CV_8U
    mask.setTo(Scalar(0));
    Rect rect(200, 100, 200, 200);
    rectangle(mask, rect, Scalar(255, 255, 255), CV_FILLED, CV_AA, 0);
    rectangle(mask, Rect(280, 120, 40, 40), Scalar(0, 0, 0), CV_FILLED);
    circle(mask, Point(300, 220), 30, Scalar(0), CV_FILLED);

    copy.setTo(Scalar(0, 0, 255), mask);
    //imshow("copy", copy);

    double alpha = 0.5;
    double beta = 1.0 - alpha;

    cv::Mat result(size, mat.type());
    cv::addWeighted(copy, alpha, mat, beta, 0.0, result);

    cvNamedWindow("Result");
    imshow("Result", result);

    cvWaitKey(0); //wait for a key press
    return 0;
}