I want to get the bright area of my picture(opencv3 Blob Detection)

edit

• I am using opencv3,
• and I want to "circle" the bright area of the picture I read by using
• Blob Detection

Be more specific, I want to "circle" the biggest red area.

• I checked this document

http://docs.opencv.org/master/d0/d7a/...

• But I really don't know how to write the code...

Thank you very much.

• this is my code

 #include <opencv2\opencv.hpp>
    #include <iostream>

        using namespace cv;
        using namespace std;


    int main()
    {
        Mat image = imread("reid.jpg");

        namedWindow("img");

        imshow("img", image);

        Mat dstImage;
        threshold(image, dstImage, 200, 255, 3);

        Mat element = getStructuringElement(MORPH_RECT, Size(15, 15));
        Mat out;
        erode(dstImage, out, element);

        Ptr<cv::SimpleBlobDetector> detector = cv::SimpleBlobDetector::create();
        std::vector<KeyPoint> keyImg;
        detector->detect(out, keyImg);

        Mat result;
        drawKeypoints(out, keyImg, result);

        int i = 0;
        for (vector<KeyPoint>::iterator k = keyImg.begin(); k != keyImg.end(); ++k, ++i)
            circle(result, k->pt, (int)k->size, Scalar(0,0,0)[i % 65536]);

        namedWindow("pic", WINDOW_AUTOSIZE);
        imshow("pic", result);

        waitKey(0);
    }