Changing the perspective using Opencv

edit

I'm working on a project, in which I use a chessboard, the problem that I'm facing, is when I recognize the board I want to crop the part of the frame that contains it and put it "straight", for that I'm using the cv::warpPerspective function, bellow is my code and the result that I get :

int main (){
    cv::Size board(6,4);
    cv::Mat src,result,quad,transformationMatrix;
    std::vector<cv::Point2f> imageCorners;
    std::vector<cv::Point2f> top, bot;



  std::vector<cv::Point2f> not_a_rect_shape;
    cv::VideoCapture cap(0);
    char fileName[20] = "MYROI";
    int index =0;
    int key = 0 ;   
    cap >> src;
    while ( key != 27){
        cap >> src;
        if(cv::findChessboardCorners(src,board,imageCorners,CV_CALIB_CB_FILTER_QUADS)){
            int xMin =imageCorners.at(0).x  ,xMax = imageCorners.at(0).x; 
            int yMin = imageCorners.at(0).y, yMax =imageCorners.at(0).y;
        for (int i  = (imageCorners.size()-1) ; i>0;i--){
                if(xMin > imageCorners.at(i-1).x)
                        xMin = imageCorners.at(i-1).x;
                if(xMax < imageCorners.at(i-1).x)
                    xMax = imageCorners.at(i-1).x;
                if(yMin > imageCorners.at(i-1).y)
                    yMin = imageCorners.at(i-1).y;
                if(yMax < imageCorners.at(i-1).y)
                    yMax = imageCorners.at(i-1).y;
            } 
        cv::Rect  myroi(xMin-5,yMin-5,(xMax-xMin)+5,(yMax-yMin)+5);
        if ( myroi.area() > 0){
            cv::imshow("ROI",(src)(myroi));  
        result = (src)(myroi);
        not_a_rect_shape.clear();
        not_a_rect_shape.push_back(imageCorners[0]);
        not_a_rect_shape.push_back(imageCorners[board.height-1]);
        not_a_rect_shape.push_back(imageCorners[board.area()-board.height-1]);
        not_a_rect_shape.push_back(imageCorners[board.area()-1]);
        std::vector<cv::Point2f> approx;
        cv::approxPolyDP(cv::Mat(not_a_rect_shape),approx,cv::arcLength(cv::Mat(not_a_rect_shape),true)*0.02,true);
        if (approx.size()!=4){
            std::cout << " Not quadrilateral!"<<std::endl;

            not_a_rect_shape.clear();
            approx.clear();

            not_a_rect_shape.push_back(imageCorners[0]);
            not_a_rect_shape.push_back(imageCorners[board.width-1]);
            not_a_rect_shape.push_back(imageCorners[board.area()-board.width-1]);
            not_a_rect_shape.push_back(imageCorners[board.area()-1]);
            cv::approxPolyDP(cv::Mat(not_a_rect_shape),approx,cv::arcLength(cv::Mat(not_a_rect_shape),true)*0.02,true); 
        }

        // center 
        cv::Point2f center(0,0);
        for (int i = 0 ; i <not_a_rect_shape.size(); i++)
                center+= not_a_rect_shape[i];
            center *=( 1./not_a_rect_shape.size()); // the center position 

            top.clear();
            bot.clear();
        // ordering  the  4 points 
        for (int i = 0; i < not_a_rect_shape.size(); i++){
            if (not_a_rect_shape[i].y < center.y)
                top.push_back(not_a_rect_shape[i]);
            else
                 bot.push_back(not_a_rect_shape[i]);
            }
        std::cout << center << std::endl;
        if(top.size()== 2 && bot.size()==2){ 
            cv::Point2f tl = top[0].x > top[1].x ? top[1] : top[0];
            cv::Point2f tr = top[0].x > top[1].x ? top[0] : top[1];
            cv::Point2f bl = bot[0].x > bot[1].x ? bot[1] : bot[0];
            cv::Point2f br = bot[0].x > bot[1].x ? bot[0] : bot[1];
            not_a_rect_shape.clear();
            not_a_rect_shape.push_back(tl);
            not_a_rect_shape.push_back(tr);
            not_a_rect_shape.push_back(br);
            not_a_rect_shape.push_back(bl);

            // Define the destination image
            quad = cv::Mat::zeros(300, 220, CV_8UC3);
            //quad = cv::Mat::zeros(result.rows,result.cols,CV_8UC3);
        // Corners of the destination image
            std::vector<cv::Point2f> quad_pts;
            quad_pts.push_back(cv::Point2f(0, 0));
            quad_pts.push_back(cv::Point2f(quad.cols, 0));
            quad_pts.push_back(cv::Point2f(quad.cols, quad.rows));
            quad_pts.push_back(cv::Point2f(0, quad.rows));
            transformationMatrix= cv ...