How to get the pixel coordinates of 4 corners points of (quasi) rectangle in image?

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I would like to find the pixel coordinates of the 4 corner points of the detected rectangle (its a airplane window that I assume is rectangle). Im able to detect the window (the rectangle) as shown in the image but then do not know how to get the pixel coordinates of the 4 rectangle corner points. I think they can be detected by harris corner , right? Then can store coordinates of corner points in matrix S?

here the code

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include<dirent.h>
#include<string.h>

 using namespace cv;
 using namespace std;

 Mat src; Mat src_gray;
 int thresh = 5;
 int max_thresh = 60000;
 RNG rng(12345);

 const char* source_window = "Source image";
 const char* corners_window = "Corners detected";

 //int thresh = 1;
 //int max_thresh = 300;

 //void thresh_callback(int, void* );

 static double angle(Point pt1, Point pt2, Point pt0)
 {
     double dx1 = pt1.x - pt0.x;
     double dy1 = pt1.y - pt0.y;
     double dx2 = pt2.x - pt0.x;
     double dy2 = pt2.y - pt0.y;
     return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
 }

 void setLabel(cv::Mat& im, const std::string label, std::vector<cv::Point>& contour)
 {
    int fontface = cv::FONT_HERSHEY_SIMPLEX;
    double scale = 0.4;
    int thickness = 1;
    int baseline = 0;

    cv::Size text = cv::getTextSize(label, fontface, scale, thickness, &baseline);
    cv::Rect r = cv::boundingRect(contour);

    cv::Point pt(r.x + ((r.width - text.width) / 2), r.y + ((r.height + text.height) / 2));
    cv::rectangle(im, pt + cv::Point(0, baseline), pt + cv::Point(text.width, -text.height), CV_RGB(255,255,255), CV_FILLED);
    cv::putText(im, label, pt, fontface, scale, CV_RGB(0,0,0), thickness, 8);
 }



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

         IplImage *desimg,*srcimg;
         string dirName = "C:/B787/" "/";

         DIR *dir;
         dir = opendir(dirName.c_str());
         string imgName;
         struct dirent *ent;

         if (dir != NULL) {
                 while ((ent = readdir (dir)) != NULL) {
                      imgName= ent->d_name;
                      if(imgName.compare(".")!= 0 && imgName.compare("..")!= 0)
                                {
                                  string aux;
                                  aux.append(dirName);
                                  aux.append(imgName);
                                  cout << aux << endl;
                                  Mat image= imread(aux);
                                  //imshow(aux,image);
                                  waitKey(0);

     //resize(image, image, Size(640,480), 0, 0, INTER_CUBIC);
     //char* source_window = "Source";
     //namedWindow( source_window, CV_WINDOW_AUTOSIZE );
     //imshow( source_window, image );
    //================================
    Mat gray,bw,dil,erd, dst_final;

    //cv::GaussianBlur(image, src_gray, cv::Size(9, 9), 2, 2); //original
    //cv::GaussianBlur(image, src_gray, cv::Size(3,3), 1,1,BORDER_DEFAULT); //original
    cv::GaussianBlur(image, src_gray, cv::Size(3,3),1,1,BORDER_DEFAULT); //original
    medianBlur(image, src_gray, 11);
    blur( image, src_gray, Size(3,3) );
    cvtColor(src_gray,gray,CV_BGR2GRAY);
     //cvtColor( src, src_gray, COLOR_BGR2HSV );
     //blur( src_gray, gray, Size(1,3) );
     //medianBlur(src, src_gray,, 5);

    Canny(gray,bw,600,1200,5,true);
    //Canny(gray,bw,1200,600,5,true);
    //Canny(gray,bw,thresh, thresh*1, 5,true);
    //Canny( src_gray, canny_output, thresh, thresh*1, 3 );
    //dilate(bw,dil,Mat());
    //erode(dil,erd,Mat());
    //Mat tmp=bw.clone();

     //cvtColor( src, src_gray, COLOR_BGR2GRAY);
     //blur( src_gray, src_gray, Size(1,3) );
    // medianBlur(src_gray, src_gray, 3);
     //cv::GaussianBlur(src_gray, src_gray, cv::Size(9, 9), 2, 2);

    /////////Mat gray ...