how to find the coordinate of top/bottom/left and right points

edit

You can follow below method to achieve this.

• Segment the four dotted region. Use inRange to segment the dots, better to convert to HSV color space before inRange.
• Use OpenCV Find contour get the contour in image.

• Use OpenCV moments to find the centre point of your contour

  Code:-

  #include <iostream>
  #include "opencv2/opencv.hpp"
  #include <stdio.h>     
  using namespace std;
  using namespace cv;
  
  int main( int argc, char** argv )
  {
      Mat src=imread("img.png",1);
      Mat hsv,thr;
      cvtColor(src,hsv,CV_BGR2HSV);
     // inRange(hsv,Scalar(160,0,120), Scalar(179,255,200), thr);
      inRange(src,Scalar(20,0,120), Scalar(25,0,200), thr);        
     vector<vector<Point> > contours;
     vector<Vec4i> hierarchy;
     findContours( thr, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
  
    /// Get the moments
    vector<Moments> mu(contours.size() );
    for( int i = 0; i < contours.size(); i++ )
       { mu[i] = moments( contours[i], false ); }
  
    ///  Get the mass centers:
    vector<Point2f> mc( contours.size() );
    for( int i = 0; i < contours.size(); i++ )
       { mc[i] = Point2f( mu[i].m10/mu[i].m00 , mu[i].m01/mu[i].m00 ); }
  
    /// Draw contours
    for( int i = 0; i< contours.size(); i++ )
       {
         circle( src, mc[i], 10, Scalar(0,0,255), 1,CV_AA, 0 );
       }
  
  imshow("src",src);
  waitKey();
  
  
  waitKey();
  
  }
  

Result: