Draw largest/rect contour on this image

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sample code for finding and drawing biggest contour.

#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>

using namespace cv;
using namespace std;

int main( int argc, char** argv )
{
    Mat src = imread( argv[1] );

    int largest_area=0;
    int largest_contour_index=0;
    Rect bounding_rect;

    Mat thr;
    cvtColor( src, thr, COLOR_BGR2GRAY ); //Convert to gray
    threshold( thr, thr, 125, 255, THRESH_BINARY ); //Threshold the gray

    vector<vector<Point> > contours; // Vector for storing contours

    findContours( thr, contours, RETR_CCOMP, CHAIN_APPROX_SIMPLE ); // Find the contours in the image

    for( size_t i = 0; i< contours.size(); i++ ) // iterate through each contour.
    {
        double area = contourArea( contours[i] );  //  Find the area of contour

        if( area > largest_area )
        {
            largest_area = area;
            largest_contour_index = i;               //Store the index of largest contour
            bounding_rect = boundingRect( contours[i] ); // Find the bounding rectangle for biggest contour
        }
    }

    drawContours( src, contours,largest_contour_index, Scalar( 0, 255, 0 ), 2 ); // Draw the largest contour using previously stored index.

    imshow( "result", src );
    waitKey();
    return 0;
}

Try adding this at the very end of your code:

imshow("dst_image", dst);
imshow("src_image", src);
waitKey();