__all__ = ["snd_deriv"]

import numpy as np


def snd_deriv(image):
	m = image.shape[0]
	n = image.shape[1]

	out = np.zeros((m, n))

	out = -4 * image
	out[1:, :] += image[: m - 1, :]
	out[: m - 1, :] += image[1:, :]
	out[:, 1:] += image[:, : n - 1]
	out[:, : n - 1] += image[:, 1:]
	return out
