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May be something like this

int main(int argc, char* argv[])
{
    VideoCapture v(0);
    Mat im=imread("14596150505143125.jpg");
    Mat img,dst;
    double thresh=0;
    vector<vector <Point>> ctr;
    cvtColor(im,img,CV_BGR2GRAY);
    threshold(img,dst,thresh,255,THRESH_OTSU);
    findContours(dst,ctr,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

    int idx,maxCtr=-1;
    double pi = acos(-1.0);
    for (int i = 0; i<ctr.size();i++)
        if (saturate_cast<int>(ctr[i].size())>maxCtr)
        {
            maxCtr=ctr[i].size();
            idx=i;
        }
    RotatedRect r=minAreaRect(ctr[idx]);
    cout<<r.center<<"\t Size ="<<r.size<<"\tAngle="<<r.angle<<"\n";
    Mat rt1=(Mat_<float>(3,3)<< 1,0,-r.center.x,0,1,-r.center.y,0,0,1);
    double theta=(270-r.angle)/180*pi;
    Mat rr=(Mat_<float>(3,3)<< cos(theta),-sin(theta),0,sin(theta),cos(theta),0,0,0,1);
    Mat rt2=(Mat_<float>(3,3)<< 1,0,r.center.x,0,1,r.center.y,0,0,1);
    Mat rAffine;
    Mat rt=(rt2*rr*rt1);
    rt(Rect(0,0,3,2)).copyTo(rAffine);
    Mat imR;
    cout<<rAffine;
    warpAffine(im, imR,rAffine, im.size());
    imshow("result",imR);
    waitKey();
    return 0;
}

May be something like this

int main(int argc, char* argv[])
{
    VideoCapture v(0);
    Mat im=imread("14596150505143125.jpg");
    Mat img,dst;
    double thresh=0;
    vector<vector <Point>> ctr;
    cvtColor(im,img,CV_BGR2GRAY);
    threshold(img,dst,thresh,255,THRESH_OTSU);
    findContours(dst,ctr,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

    int idx,maxCtr=-1;
    double pi = acos(-1.0);
    for (int i = 0; i<ctr.size();i++)
        if (saturate_cast<int>(ctr[i].size())>maxCtr)
        {
            maxCtr=ctr[i].size();
            idx=i;
        }
    RotatedRect r=minAreaRect(ctr[idx]);
    cout<<r.center<<"\t Size ="<<r.size<<"\tAngle="<<r.angle<<"\n";
    Mat rt1=(Mat_<float>(3,3)<< 1,0,-r.center.x,0,1,-r.center.y,0,0,1);
    double theta=(270-r.angle)/180*pi;
    Mat rr=(Mat_<float>(3,3)<< cos(theta),-sin(theta),0,sin(theta),cos(theta),0,0,0,1);
    Mat rt2=(Mat_<float>(3,3)<< 1,0,r.center.x,0,1,r.center.y,0,0,1);
    Mat rAffine;
    Mat rt=(rt2*rr*rt1);
    rt(Rect(0,0,3,2)).copyTo(rAffine);
    Mat imR;
    cout<<rAffine;
    warpAffine(im, imR,rAffine, im.size());
    imshow("result",imR);
    waitKey();
    return 0;
}

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/videoio/videoio.hpp"
#include "opencv2/calib3d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/highgui.hpp"

#include <iostream>
#include <fstream>
#include <algorithm>
#include <ctype.h>

using namespace cv;
using namespace std;



int main(int argc, char* argv[])
{
    VideoCapture v(0);
    Mat im=imread("C:/Users/Laurent.PC-LAURENT-VISI/Downloads/14596150505143125.jpg");
    Mat img,dst;
    double thresh=0;
    vector<vector <Point>> ctr;
    cvtColor(im,img,CV_BGR2GRAY);
    threshold(img,dst,thresh,255,THRESH_OTSU);
    findContours(dst,ctr,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

    int idx,maxCtr=-1;
    double pi = acos(-1.0);
    for (int i = 0; i<ctr.size();i++)
        if (saturate_cast<int>(ctr[i].size())>maxCtr)
        {
            maxCtr=ctr[i].size();
            idx=i;
        }
    vector<Point> ctrApprox;
    double epsilon=2;
    do
    {
        approxPolyDP(ctr[idx], ctrApprox, epsilon, true);
        epsilon++;    
    }
    while (ctrApprox.size()>4);
    RotatedRect r=minAreaRect(ctr[idx]);
    vector <Point2f> p2(4);
    p2[0] = Point2f(r.center.x - r.size.width/2,r.center.x - r.size.height/2);
    p2[1] = Point2f(r.center.x - r.size.width/2,r.center.x + r.size.height/2);
    p2[2] = Point2f(r.center.x + r.size.width/2,r.center.x + r.size.height/2);
    p2[3] = Point2f(r.center.x + r.size.width/2,r.center.x - r.size.height/2);
    Mat rth=findHomography(ctrApprox,p2);
    warpPerspective(im, imR,rth, Size(im.size().width*2,im.size().height*2));
    imshow("result",imR);
    waitKey();
    return 0;
}