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();

Objective C OpenCV

ID Proiect: #18711611

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