import matplotlib.pyplot as plt
import numpy as np
from scipy.misc import face

def convolve(image, mask):
    ret = np.zeros_like(image).astype("f4")
    for y in range(1, image.shape[0]-1):
        for x in range(1, image.shape[1]-1):
            ret[y, x] = (image[y-1:y+2, x-1:x+2] * mask).sum()
    return ret[1:-1, 1:-1]

image = face(gray=True)
mask = [[-1, -1, -1], [2, 2, 2], [-1, -1, -1]]
mask = [[-1, 2, -1], [-1, 2, -1], [-1, 2, -1]]
#mask = np.ones((3, 3)) / 9
# mask = np.zeros((3, 3))

result = convolve(image, mask)

plt.subplot(121)
plt.imshow(image)
plt.subplot(122)
plt.imshow(result)
plt.show()