botcs botcs - 1 year ago 173
Python Question

scipy convolve2d outputs wrong values

Here is my code which I used for checking the correctness of convolve2d

import numpy as np
from scipy.signal import convolve2d

X = np.random.randint(5, size=(10,10))
K = np.random.randint(5, size=(3,3))
print "Input's top-left corner:"
print X[:3,:3]
print 'Kernel:'
print K

print 'Hardcording the calculation of a valid convolution (top-left)'
print (X[:3,:3]*K)
print 'Sums to'
print (X[:3,:3]*K).sum()
print 'However the top-left value of the convolve2d result'
Y = convolve2d(X, K, 'valid')
print Y[0,0]

On my computer this results in the following:

Input's top-left (3x3) corner:
[[0 0 0]
[1 1 2]
[1 3 0]]
[[4 1 1]
[0 3 3]
[2 1 2]]
Hardcording the calculation of a valid convolution (top-left)
[[0 0 0]
[0 3 6]
[2 3 0]]
Sums to
However the top-left value of the convolve2d result

Background story: I've been debugging a convnet library, and somehow the gradients were always wrong. After a few weeks I concluded that everything should be working fine, so I checked the convolve2d function by bare hand.

Answer Source

The expression (X[:3,:3]*K).sum() is not correct. For convolution, you have to reverse the kernel, e.g. (X[:3,:3]*K[::-1,::-1]).sum()

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