proximate contour to rectagle - c++

Question

I'm trying to detect and extract playing cards from an image. The plan is to detect the contours of the cards and then extract them using the area of the contours. (Is there a more efficient way of doing it?)
The problem is that I was having trouble with non-closed contours:
With this contours I'm not able to calculate the area of the rectangles. Hence, I performed morphological transformations to close the contours, producing this:
And after edge extracting:
Leaving me with these "rectangles" with twisted corners in the edges. How can I approximate these pseudo-rectangles to perfect geometric rectangles?
Is there a more efficient way of doing this?
Here is my code so far:
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <stdlib.h>
#include <stdio.h>
using namespace cv;
using namespace std;
#define BKG_THRESH 60 // preProcessImg
Mat src;
void preProcessImg(Mat* _img){
Mat aux_gray;
cvtColor(*_img, aux_gray, CV_BGR2GRAY );
GaussianBlur(aux_gray, *_img, Size(5,5), 0);
}
int main( int argc, char** argv ){
vector<vector<Point>> contours;
/// Load an image
src = imread("img.jpg");
preProcessImg(&src);
Canny(src, src, 30, 200);
//Mostrar imagem
namedWindow( "canny_output", CV_WINDOW_NORMAL); // Create a window
imshow( "canny_output", src);
waitKey(0);
Mat structuringElement = getStructuringElement(MORPH_ELLIPSE, Size(7, 7));
morphologyEx(src, src, MORPH_CLOSE, structuringElement);
//Mostrar imagem
namedWindow( "morph_transf", CV_WINDOW_NORMAL); // Create a window
imshow( "morph_transf", src);
waitKey(0);
findContours(src, contours, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE);
Mat drawing = Mat::zeros( src.size(), CV_8UC3 );
for( int i = 0; i< contours.size(); i++){
Scalar color( rand()&255, rand()&255, rand()&255 );
drawContours( drawing, contours, i, color );
}
//Mostrar imagem
namedWindow( "contours", CV_WINDOW_NORMAL); // Create a window
imshow( "contours", drawing);
waitKey(0);
return 0;
}

Answer 1

The more robust way is to find lines (Hough lines) afer Canny, intersect they and find rectangles. Contours are not robust for noise.