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Maybe you could use Sobel derivatives

 /// Gradient X
 //Scharr( src_gray, grad_x, ddepth, 1, 0, scale, delta, BORDER_DEFAULT );
 Sobel( src_gray, grad_x, ddepth, 1, 0, 3, scale, delta, BORDER_DEFAULT );
 convertScaleAbs( grad_x, abs_grad_x );

 /// Gradient Y
 //Scharr( src_gray, grad_y, ddepth, 0, 1, scale, delta, BORDER_DEFAULT );
 Sobel( src_gray, grad_y, ddepth, 0, 1, 3, scale, delta, BORDER_DEFAULT );
 convertScaleAbs( grad_y, abs_grad_y );