Count the pixel (black or white) in real time.

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I need to count the black and white pixels of the object in REAL TIME. What is the best method to use for this ? In my program, First I get the biggest contour .The next process is to count the pixels(black) of the object that have no mask but I don't know how to do it. Thanks in advance.

 #include <opencv2/imgproc/imgproc.hpp>
 #include <opencv2/highgui/highgui.hpp>

 using namespace cv;
 int main(int argc, char** argv)
 {
 vector<Vec4i> hierarchy;
     CvCapture* capture = cvCaptureFromCAM(0);
 cvSetCaptureProperty(capture, CV_CAP_PROP_FRAME_WIDTH, 270);
 cvSetCaptureProperty(capture, CV_CAP_PROP_FRAME_HEIGHT, 190);

    cv::Mat src;

    while(true) {
        src = cvQueryFrame( capture );

    Mat gray;
    cvtColor(~src, gray, CV_BGR2GRAY);
    //medianBlur(gray,gray,27);
    threshold(gray, gray, 220, 255, THRESH_BINARY);
    vector<vector<Point> > contours;
    findContours( gray.clone(), contours, hierarchy,CV_RETR_EXTERNAL,
    CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );


    for (int i = 0; i < contours.size(); i++)
    {
        double area =contourArea(contours[i]);
        Rect rect = boundingRect(contours[i]);

        if (area >= 30&& abs(1 - ((double)rect.width / (double)rect.height)) <=
            0.1)   
        {

            Mat mask = Mat::zeros(gray.rows, gray.cols, CV_8UC1);
            drawContours( mask, contours, i, CV_RGB(255,255,255), -1, 8,
            vector<Vec4i>(), 0,Point() ); //masking//
            Mat crop(src.rows, src.cols, CV_8UC3);
            crop.setTo(Scalar(0,0,255));
            src.copyTo(crop, mask);


            Mat masked;
            gray.copyTo(masked, mask);
            int count_black = cv::countNonZero(masked == 255);
            printf("count_black = %d\n",count_black);
            printf("ContourArea = %.0f\n\n",
                  ((contourArea(contours[i]))+( arcLength( contours[i],
                    true )/2)));


           imshow("cropped", crop);
           imshow("source", src);
            waitKey(33);
        }


    }



}
}