Intensity value based tracking

edit

I want to draw a rectangle in opencv according to the pixel intensity i.e. i don't want to give a static values to cvRectangle like cv::point1 or cv::point2. I want that whenever specific intensity value is encountered it draws a rectangle around that object. I got a sample like this but it's not what i want how should i modify this

pMOG->operator()(frame, fgMaskMOG);

while(frame.data!=NULL)

 {
     for(int i=0;i<frame.row;i++)
     {
         for(int j=0;j<frame.col;j++)
         {
     if(frame.at<uchar>(i,j)=='0')
         //std::vector<Rect> frame;
         cv::rectangle(frame,                    // the dest image 
          cvPoint(100, 100),      // top left point 
          cvPoint(300, 30),       // bottom right point 
          cvScalar(255, 0, 0, 0), // the color; blue 
          3, 8, 0);               // thickness, line type, shift 

 }
     }