On Thu, Oct 3, 2013 at 4:05 PM, Moroney, Catherine M (398D) <
[email protected]> wrote:
> I know I have a lot yet to learn about array striding tricks, so please
> pardon the triviality of this question.
>
> Here is the problem both in words and "dumb" python:
>
> I have a large NxM array that I want to break down into smaller nxn chunks
> where n divides evenly into both N and M. Then I want to calculate the
> fraction of pixels in each nxn chunk that meets a certain criteria: say (x
> > 1) & (x < 2).
>
If n divides both N and M, you can simply reshape your array to 4D, then
reduce it back to 2D:
import numpy as np
N, M = 1000, 2000
n = 100
a = np.random.rand(N,M) * 5
a_view = a.reshape(a.shape[0] // n, n, a.shape[1] // n, n)
a_fractions = np.sum((a_view > 1) & (a_view < 2), axis=(1, 3)) / (n * n)
>>> a_fractions.shape
(10L, 20L)
>>> a_fractions
array([[ 0.1965, 0.1964, 0.202 , 0.1997, 0.1976, 0.1997, 0.2026,
0.1951, 0.2051, 0.1995, 0.1926, 0.2006, 0.1973, 0.1964,
0.2046, 0.1977, 0.2066, 0.2009, 0.2003, 0.2013],
...
[ 0.2028, 0.1943, 0.2036, 0.1992, 0.2 , 0.2009, 0.1971,
0.1996, 0.196 , 0.196 , 0.1983, 0.2021, 0.2031, 0.1955,
0.1916, 0.1939, 0.202 , 0.2064, 0.2021, 0.1954]])
Jaime
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