How to find the distance(physical in m and pixels) between two object(points) in image in OpenCV and C+++?

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Hi Im using OpenCV and C++. I have already done the code where can fine the center of the object. Also I have the prior information about the distance between the camera and object(s) in the image. So know all this just need to find the distance (real physical one in m or cm, also also the one in pixels ) between two objects in the image (or between two centers of the mass of those objects) . Here is the code to find the center moment of rectangle object. Similar approach would be to find the center of other shape (object).

int main(int argc,char** argv)
{

           Mat image = imread("000167.png");
           Mat gray,bw,dil,erd, dst_final;
       Mat new_src=image.clone();

     for(int y = 0; y < image.rows; y++ )
    {
       for(int x = 0; x < image.cols; x++ )
       {
              for(int c = 0; c < 3; c++ )
              {
                     new_src.at<Vec3b>(y,x)[c]= saturate_cast<uchar>( 1.5*(image.at<Vec3b>(y,x)[c] ));

              }
       }
     }


      cv::GaussianBlur(new_src, src_gray, cv::Size(3,3),1,1,BORDER_DEFAULT); //original
      medianBlur(new_src, src_gray, 11);
      blur( new_src, src_gray, Size(3,3) );
      cvtColor(src_gray,gray,CV_BGR2GRAY);
      Canny(gray,bw,600,1200,5,true);
                    Mat grad,bw1;
                    Mat morphKernel = getStructuringElement(MORPH_ELLIPSE, Size(20,10));
                    morphologyEx(bw, grad, MORPH_GRADIENT, morphKernel);        
                    threshold(grad, bw1, 255.0, 255.0, THRESH_BINARY | THRESH_OTSU);                

          vector<vector<Point> > contours;
      vector<vector<Point> > rough;
      vector<vector<Point> >rough_color;
      vector<vector<Point> >precise;
          vector<Vec4i> hierarchy;
      Mat dst = image.clone();
          findContours(bw1.clone(), contours, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

      vector<Point> approx;

      for( int i = 0; i< contours.size(); i++ )
      {

           approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true) * 0.01, true);
           if (fabs(contourArea(approx)) <2000 || fabs(contourArea(approx))>50000)                                     
               continue;

                  Rect r = boundingRect(contours[i]);
                  if( (float) (r.height/r.width) > 1.5 && (float) (r.height/r.width) <3)
                  {
                   rough.push_back(approx);
                   }
      }

      for(int i = 0; i < rough.size(); i++)
      {
          Rect bound=boundingRect(rough[i]);
          Rect x, y, w, h = boundingRect(rough[i]);
          rectangle(dst,Point(bound.br().x,bound.br().y), Point(bound.tl().x,bound.tl().y),Scalar(255, 0, 0),2);
          RotatedRect rectPoint = minAreaRect(rough[i]);

              Point2f fourPoint2f_rough[4];
              rectPoint.points(fourPoint2f_rough);
          vector<Point> fourPoint_rough;
          for(int i = 0; i <4; i++)
         {
             fourPoint_rough.push_back(fourPoint2f_rough[i]);
         }


        {
            line(dst, fourPoint2f_rough[i], fourPoint2f_rough[i + 1], Scalar(255,0,0), 3);
        }
        line(dst, fourPoint2f_rough[0], fourPoint2f_rough[3], Scalar(255,0,0), 3); */
      }

      if(rough.size() !=0 )
      {
      for( int i = 0; i< rough.size(); i++ )
      {
          vector<Moments> mu(1);
          vector<Point2f> mc(1);
          int gray_level;
          // compute the central momment
             mu[0] = moments( rough[i], false );
             mc[0] = Point2f( mu[0].m10/mu[0].m00 , mu[0].m01/mu[0].m00 );
          circle( dst, mc[0], 4, Scalar(0,0,255), -1, 8, 0 );
          gray_level=gray.at<uchar>(mc[0]);
          if(gray_level<200 && gray_level>20)
          {rough_color.push_back(rough[i]);}
      }


      for(int i = 0; i < rough_color.size(); i++)
      {
          Rect bound=boundingRect(rough_color[i]);
          Rect x, y, w, h = boundingRect(rough_color[i]);
          RotatedRect rectPoint = minAreaRect(rough_color[i]);
              Point2f fourPoint2f_color[4];
              rectPoint.points(fourPoint2f_color);
         vector ...