Fix bug in Objective C OpenCV based Sobel edge detector code

hi I have a openCV based Objective C project for video which works correctly by itself, where I am trying to calculate intensity gradient using Sobel operator using the stack overflow example ([login to view URL]) but it is giving me error. I need to calculate average gradient and resulting direction of image from phone. Can you please fix the error. STACK OVERFLOW CODE Mat img = imread("[login to view URL]", IMREAD_GRAYSCALE); // compute the image derivatives for both the x and y direction Mat dx, dy; Sobel(img, dx, CV_32F, 1, 0); Sobel(img, dy, CV_32F, 0, 1); Scalar average_dx = mean(dx); Scalar average_dy = mean(dy); double average_gradient = atan2(-average_dy[0], average_dx[0]); cout << "average_gradient = " << average_gradient << endl; And to display the resulting direction Point center = Point([login to view URL], [login to view URL]); Point direction = Point(cos(average_gradient) * 100, -sin(average_gradient) * 100); Mat img_rgb = imread("[login to view URL]"); // read the image in colour line(img_rgb, center, center + direction, Scalar(0,0,255)); imshow("image", img_rgb); waitKey();