def _grad_y(arr):
    """ Convolution with vertical derivative kernel
    H = [[-1,-2,-1],
         [ 0, 0, 0],
         [ 1, 2, 1]]
    """
    dy = -(arr[-1,-1] + 2*arr[-1,0] + arr[-1,1]) + \
           arr[ 1,-1] + 2*arr[ 1,0] + arr[ 1,1]
    return dy