[SciPy-User] fft convolutions

[Alex Flint]

I am trying to perform 2d convolutions between a large 2d array A and a
bunch of small 2d arrays B1...Bn. My approach is roughly:

a = fft(A,size)
for b in bs:
   ans = ifft(fft(b,size)*A)
   slow = convolve2d(A, b, 'same')

However, as implemented above, ans is offset an inconsistent amount from the
answer produced by convolve2d, presumably because convolve2d is treating b
as if the origin is in the center whereas fft treats b as if the origin is
at the top left (but it doesn't seem to be quite as simple as this). What am
I missing?

Also, I'm not using fftconvolve at the moment because I want to compute the
fft of A just once, then use it repeatedly for each b.

Cheers,
Alex
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