create box of car plates - c++

Question

I have a program that creates boxes of car plates.
Input1:
Output1:
Input2:
Output2:
Code:
#include <cv.h> // open cv general include file
#include <highgui.h> // open cv GUI include file
#include <iostream> // standard C++ I/O
#include <vector>
#include <list>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <regex>
#include <string>
#include <fstream>
using namespace cv;
using namespace std;
int main( int argc, char** argv )
{
Mat img;
int largest_area=0;
int largest_contour_index=0;
Rect bounding_rect;
img = imread( argv[1], 1 );
Mat tmp = img;
cvtColor( img, img, CV_BGR2GRAY );
GaussianBlur(img, img, Size(5,5), 0);
threshold(img, img, 0, 255, CV_THRESH_OTSU);
Mat temp(img.rows,img.cols,CV_8UC1);
Mat dst(img.rows,img.cols,CV_8UC1,Scalar::all(0));
img.copyTo(temp);
vector<vector<Point>> contours; // storing contour
vector<Vec4i> hierarchy;
findContours( temp, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE );
for( int i = 0; i< contours.size(); i++ ) // iterate
{
double a=contourArea( contours[i],false); //Find the largest area of contour
if(a>largest_area)
{
largest_area=a;
largest_contour_index=i;
bounding_rect=boundingRect(contours[largest_contour_index]); // Find the bounding rectangle for biggest contour
}
}
drawContours( dst, contours,largest_contour_index, Scalar(255), CV_FILLED, 8, hierarchy );
rectangle(tmp, bounding_rect, Scalar(255,255,255),1, 8,0);
Mat roi = tmp(bounding_rect);
imshow("Final", roi);
waitKey();
return(0);
}
Do you know how to compare the license plate to be straight? Or another approach to create these boxes?
Here's my test set: http://www.filedropper.com/carboxtar