I am working in C++ and opencv

I am detecting the big contour in an image because I have a black area in it.![My image](https://i.stack.imgur.com/HM7wm.jpg)

In this case, the area is only horizontally, but it can be in any place.

```
Mat resultGray;
 cvtColor(result,resultGray, COLOR_BGR2GRAY);
 medianBlur(resultGray,resultGray,3);
 Mat resultTh;
 Mat canny_output;
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
 Canny( resultGray, canny_output, 100, 100*2, 3 );

    findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
    Vector<Point> best= contours[0];
    int max_area = -1;
    for( int i = 0; i < contours.size(); i++ ) {
            Scalar color = Scalar( 0, 0, 0 );

            if(contourArea(contours[i])> max_area)
            {
                max_area=contourArea(contours[i]);
                best=contours[i];
            }
    }
    Mat approxCurve;
    approxPolyDP(Mat(best),approxCurve,0.01*arcLength(Mat(best),true),true);

```

Wiht this, i have the big contour and it approximation (in approxCurve). Now, I want to obtain the corners of this approximation and get the image inside this contour, but I dont know how can I do it.

I am using this [How to remove black part from the image?](https://stackoverflow.com/questions/10632617/how-to-remove-black-part-from-the-image)
But the last part I dont understad very well.

Anyone knows how can I obtain the corners? It is another way more simple that this?

Thanks for your time,