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how do I correct the perspective and keeping the aspect ratio of a car plate? I search for solutions but all solutions, I need to know cv::Point of corners of input image.

how do I correct the perspective and keeping the aspect ratio of a car plate? this is the code:

Mat src = imread(path);
Mat thr;
cvtColor(src, thr, CV_BGR2GRAY);
threshold(thr, thr, 70, 255, CV_THRESH_BINARY);

vector< vector <Point> > contours; // Vector for storing contour
vector< Vec4i > hierarchy;
int largest_contour_index = 0;
int largest_area = 0;

Mat dst(src.rows, src.cols, CV_8UC1, Scalar::all(0)); //create destination image
findContours(thr.clone(), contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE); // Find the contours in the image
for (int i = 0; i< contours.size(); i++){
    double a = contourArea(contours[i], false);  //  Find the area of contour
    if (a>largest_area){
        largest_area = a;
        largest_contour_index = i;                //Store the index of largest contour
    }
}

drawContours(dst, contours, largest_contour_index, Scalar(255, 255, 255), CV_FILLED, 8, hierarchy);
vector<vector<Point> > contours_poly(1);
approxPolyDP(Mat(contours[largest_contour_index]), contours_poly[0], 5, true);
Rect boundRect = boundingRect(contours[largest_contour_index]);

if (contours_poly[0].size() == 4){
    std::vector<Point2f> quad_pts;
    std::vector<Point2f> squre_pts;
    quad_pts.push_back(Point2f(contours_poly[0][0].x, contours_poly[0][0].y));
    quad_pts.push_back(Point2f(contours_poly[0][1].x, contours_poly[0][1].y));
    quad_pts.push_back(Point2f(contours_poly[0][3].x, contours_poly[0][3].y));
    quad_pts.push_back(Point2f(contours_poly[0][2].x, contours_poly[0][2].y));

    squre_pts.push_back(Point2f(boundRect.x, boundRect.y));
    squre_pts.push_back(Point2f(boundRect.x, boundRect.y + boundRect.height));
    squre_pts.push_back(Point2f(boundRect.x + boundRect.width, boundRect.y));
    squre_pts.push_back(Point2f(boundRect.x + boundRect.width, boundRect.y + boundRect.height));

    Mat transmtx = getPerspectiveTransform(quad_pts, squre_pts);
    Mat transformed = Mat::zeros(src.rows, src.cols, CV_8UC3);
    warpPerspective(src, transformed, transmtx, src.size());
    Point P1 = contours_poly[0][0];
    Point P2 = contours_poly[0][1];
    Point P3 = contours_poly[0][2];
    Point P4 = contours_poly[0][3];


    line(src, P1, P2, Scalar(0, 0, 255), 1, CV_AA, 0);
    line(src, P2, P3, Scalar(0, 0, 255), 1, CV_AA, 0);
    line(src, P3, P4, Scalar(0, 0, 255), 1, CV_AA, 0);
    line(src, P4, P1, Scalar(0, 0, 255), 1, CV_AA, 0);
    rectangle(src, boundRect, Scalar(0, 255, 0), 1, 8, 0);
    rectangle(transformed, boundRect, Scalar(0, 255, 0), 1, 8, 0);

    imshow("quadrilateral", transformed);
    imshow("thr", thr);
    imshow("dst", dst);
    imshow("src", src);
    waitKey();
}
else
    cout << "Make sure that your are getting 4 corner using approxPolyDP..." << endl;

these are the images original and result

How I search for solutions but all solutions, I need to know cv::Point of corners of input image. keep the aspect ratio?