python - Scipy ndimage median_filter origin -

i have binary array, say, a = np.random.binomial(n=1, p=1/2, size=(9, 9)). perform median filtering on using 3 x 3 kernel on it, say, b = nd.median_filter(a, 3). expect should perform median filter based on pixel , 8 neighbours. however, not sure placement of kernel. documentation says,

origin : scalar, optional.

the origin parameter controls placement of filter. default 0.0. 

if default zero, should taking current pixel , 3 x 3 grid right , bottom, no? shouldn't default center of footprint? in our 3 x 3 example correspond (1, 1) opposed (0, 0)?

thanks.

origin says accepts scalar, me accepts array-like input case scipy.ndimage.filters.convolve function. passing 0 indeed center of footprint. origin's value relative center. 3x3 footprint, can specify values -1.0 1.0. here examples. notice in example origin not specified filter centered expected.

import numpy np import scipy.ndimage  a= np.zeros((5, 5)) a[1:4, 1:4] = np.arange(3*3).reshape((3, 3))  default_out = scipy.ndimage.median_filter(a, size=(3, 3)) shift_pos_x = scipy.ndimage.median_filter(a, size=(3, 3), origin=(0, 1)) shift_neg_x = scipy.ndimage.median_filter(a, size=(3, 3), origin=(0, -1))  print(a) print(default_out) print(shift_pos_x) print(shift_neg_x) 

output:

input array:

[[ 0.  0.  0.  0.  0.]  [ 0.  0.  1.  2.  0.]  [ 0.  3.  4.  5.  0.]  [ 0.  6.  7.  8.  0.]  [ 0.  0.  0.  0.  0.]] 

centered output:

[[ 0.  0.  0.  0.  0.]  [ 0.  0.  1.  0.  0.]  [ 0.  1.  4.  2.  0.]  [ 0.  0.  4.  0.  0.]  [ 0.  0.  0.  0.  0.]] 

shifted right output:

[[ 0.  0.  0.  0.  0.]  [ 0.  0.  0.  1.  0.]  [ 0.  0.  1.  4.  2.]  [ 0.  0.  0.  4.  0.]  [ 0.  0.  0.  0.  0.]] 

shifted left output:

[[ 0.  0.  0.  0.  0.]  [ 0.  1.  0.  0.  0.]  [ 1.  4.  2.  0.  0.]  [ 0.  4.  0.  0.  0.]  [ 0.  0.  0.  0.  0.]]