Create mask to select the black area

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I have a black area around my image and i want to create a mask using OpenCv C++ that select just this black area so that i can paint it later. how Can i do that with out affect the image it self.

I try to converted to grayscale image and then using threshold to converted it to binary, but its affect my image since its contain black pixels.

This is the code i used to find the biggest contoure

#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp""
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/photo/photo.hpp"
#include <iostream>
using namespace cv;
using namespace std;
int main( )
{
    Mat src;
    src = imread("pano1.jpg", CV_LOAD_IMAGE_COLOR);
    Mat gray;
    cvtColor(src, gray, CV_BGR2GRAY);
    threshold(gray, gray,30, 255,THRESH_BINARY_INV); //Threshold the gray
    imshow("gray",gray);
    int largest_area=0;
    int largest_contour_index=0;
    Rect bounding_rect;
    vector<vector<Point>> contours; // Vector for storing contour
    vector<Vec4i> hierarchy;
    findContours( gray, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE );
    // iterate through each contour.
    for( int i = 0; i< contours.size(); i++ )
    {

    //  Find the area of contour
    double a=contourArea( contours[i],false); 
    if(a>largest_area){
        largest_area=a;cout<<i<<" area  "<<a<<endl;
        // Store the index of largest contour
        largest_contour_index=i;               
        // Find the bounding rectangle for biggest contour
        bounding_rect=boundingRect(contours[i]);
    }
}

Scalar color(255,255,255);  // color of the contour in the
//Draw the contour and rectangle
drawContours( src, contours,largest_contour_index, color, CV_FILLED,8,hierarchy);
rectangle(src, bounding_rect,  Scalar(0,255,0),2, 8,0);

Mat inpainted;
inpaint(src, gray, inpainted, 3, INPAINT_NS);
imshow("inpainted image", inpainted);
namedWindow( "Display window", CV_WINDOW_AUTOSIZE );
imshow( "Display window", src );    
waitKey(0);                                         
return 0;

}

This is the result of the code:

Another question: if i want to crop the image instead of paint it, how can i do it??

Thanks in advance,