def harris_corner_detector(gray_im):
    """
    :param gray_im: grayscale image to find key points inside
    :return: An array with shape (N,2) of [x,y] key point locations in gray_im
    """

    # Remember that differentiation is done left to right!
    der_xx = utils.blur_spatial(derivative_x(derivative_x(gray_im)), 3)
    der_yy = utils.blur_spatial(derivative_y(derivative_y(gray_im)), 3)
    der_xy = utils.blur_spatial(derivative_y(derivative_x(gray_im)), 3)
    der_yx = utils.blur_spatial(derivative_x(derivative_y(gray_im)), 3)

    det = der_xx * der_yy - der_xy * der_yx
    trace = der_xx + der_yy

    R = det - HARRIS_CORNER_K * trace
    # Binary array containing only local maxima
    s_R = sol4_add.non_maximum_suppression(R).T
    return np.argwhere(s_R > 0)