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;}