Plotting a Gradient Vector Field in OpenCV

Question

I want to compute the gradient of a gray-scale image (smoothed_plane in the code) and plot it as a vector field in OpenCV, superposed to an existing image.

I tr

Answer 1

The following worked for me, after trying to use your code directly

Scharr(smoothed_plane,grad_x,ddepth,1,0,scale);
Scharr(smoothed_plane,grad_y,ddepth,0,1,scale);

for (int i = 0 ; i < image_height ; i ++){
    for (int j = 0 ; j < image_width ; j ++){
        Scalar xval = grad_x.at<uchar>(i,j); // Notice <uchar> not float
        Scalar yval = grad_y.at<uchar>(i,j);
        gradient_field.at<Point2f>(i,j) = Point2f(xval.val[0],yval.val[0]);
    }
}

The < float > suggestion gave me high and unusable values.

Answer 2

I solved my problem. The main mistake lied in the access method of the partial derivatives matrices, grad_x and grad_y. As described here, the method at.<>() returns a Scalar object, so to get access to the pixel intensity value one should use the .val[] field. This is how the code has to be changed:

Scharr(smoothed_plane,grad_x,ddepth,1,0,scale);
Scharr(smoothed_plane,grad_y,ddepth,0,1,scale);

for (int i = 0 ; i < image_height ; i ++){
    for (int j = 0 ; j < image_width ; j ++){
        Scalar xval = grad_x.at<float>(i,j);
        Scalar yval = grad_y.at<float>(i,j);
        gradient_field.at<Point2f>(i,j) = Point2f(xval.val[0],yval.val[0]);
    }
}

and this is the expected result: