[SciPy-user] Efficient "windowing"
Robert Kern
robert.kern@gmail....
Wed Dec 5 14:11:44 CST 2007
Jose Luis Gomez Dans wrote:
> Hi,
> I am trying to apply a simple algorithm to a 2D matrix (an image). What I want to do is, for each pixel, choose the highest (... lowest) value in its 8- or 4-connected neighbours. I have done this using weave.inline, and using a couple of loops, but I was curious if there's some way of doing this using numpy slice syntax? My (allegedly, unelegant) attempts have been versions of the following:
>
> b[1:-1,1:-1] = scipy.array([a[0:-2,1:-1] , a[2:,1:-1] , a[1:-1,0:-2] ,\
> a[1:-1,2:],a[0:-2,0:-2], a[0:-2,2:], a[2:,0:-2], a[2:,2:]],'f').max()
>
> They don't work, because the max() call at the end refers to the whole array, so you are given a constant value array, equal to the max. value of a. Using for loops is very slow when dealing with large arrays.
.max() takes an `axis` argument. It also takes an `out` argument that will help
you save you from making some large temporaries.
In [1]: from numpy import *
In [2]: n = 256
In [5]: a = random.random((n,n)).astype(float32)
In [7]: b = zeros([n,n], dtype=float32)
In [8]: neighbors = array([a[0:-2,1:-1], a[2:,1:-1], a[1:-1,0:-2], a[1:-1,2:],
a[0:-2,0:-2], a[0:-2,2:], a[2:,0:-2], a[2:,2:]])
In [9]: neighbors.shape
Out[9]: (8, 254, 254)
In [10]: neighbors.max(axis=0, out=b[1:-1,1:-1])
Out[10]:
array([[ 0.94350582, 0.94350582, 0.94350582, ..., 0.98800218,
0.97231197, 0.61088812],
[ 0.94350582, 0.82977545, 0.94350582, ..., 0.61088812,
0.97231197, 0.97231197],
[ 0.94350582, 0.94350582, 0.94350582, ..., 0.95455241,
0.95455241, 0.95455241],
...,
[ 0.92812324, 0.92812324, 0.95367759, ..., 0.92305821,
0.92305821, 0.92305821],
[ 0.95591497, 0.95591497, 0.95367759, ..., 0.96583498,
0.85454881, 0.92305821],
[ 0.92812324, 0.95591497, 0.92812324, ..., 0.96583498,
0.92305821, 0.92305821]], dtype=float32)
In [11]: b
Out[11]:
array([[ 0. , 0. , 0. , ..., 0. ,
0. , 0. ],
[ 0. , 0.94350582, 0.94350582, ..., 0.97231197,
0.61088812, 0. ],
[ 0. , 0.94350582, 0.82977545, ..., 0.97231197,
0.97231197, 0. ],
...,
[ 0. , 0.95591497, 0.95591497, ..., 0.85454881,
0.92305821, 0. ],
[ 0. , 0.92812324, 0.95591497, ..., 0.92305821,
0.92305821, 0. ],
[ 0. , 0. , 0. , ..., 0. ,
0. , 0. ]], dtype=float32)
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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