Using Numpy stride_tricks to get non-overlapping array blocks

Question

I\'m trying to using numpy.lib.stride_tricks.as_strided to iterate over non-overlapping blocks of an array, but I\'m having trouble finding documentation of the parameters,

Answer 1

import numpy as np
n=4
m=5
a = np.arange(1,n*m+1).reshape(n,m)
print(a)
# [[ 1  2  3  4  5]
#  [ 6  7  8  9 10]
#  [11 12 13 14 15]
#  [16 17 18 19 20]]
sz = a.itemsize
h,w = a.shape
bh,bw = 2,2
shape = (h/bh, w/bw, bh, bw)
print(shape)
# (2, 2, 2, 2)

strides = sz*np.array([w*bh,bw,w,1])
print(strides)
# [40  8 20  4]

blocks=np.lib.stride_tricks.as_strided(a, shape=shape, strides=strides)
print(blocks)
# [[[[ 1  2]
#    [ 6  7]]
#   [[ 3  4]
#    [ 8  9]]]
#  [[[11 12]
#    [16 17]]
#   [[13 14]
#    [18 19]]]]

Starting at the 1 in a (i.e. blocks[0,0,0,0]), to get to the 2 (i.e. blocks[0,0,0,1]) is one item away. Since (on my machine) the a.itemsize is 4 bytes, the stride is 1*4 = 4. This gives us the last value in strides = (10,2,5,1)*a.itemsize = (40,8,20,4).

Starting at the 1 again, to get to the 6 (i.e. blocks[0,0,1,0]), is 5 (i.e. w) items away, so the stride is 5*4 = 20. This accounts for the second to last value in strides.

Starting at the 1 yet again, to get to the 3 (i.e. blocks[0,1,0,0]), is 2 (i.e. bw) items away, so the stride is 2*4 = 8. This accounts for the second value in strides.

Finally, starting at the 1, to get to 11 (i.e. blocks[1,0,0,0]), is 10 (i.e. w*bh) items away, so the stride is 10*4 = 40. So strides = (40,8,20,4).

Answer 2

scikit-image has a function named view_as_blocks() that does almost what you need. The only problem is that it has an extra assert that forbids your use case, since your blocks don't divide evenly into your array shape. But in your case, the assert isn't necessary, so you can copy the function source code and safely remove the pesky assert yourself.

Answer 3

Using @unutbu's answer as an example, I wrote a function that implements this tiling trick for any ND array. See below for link to source.

>>> a = numpy.arange(1,21).reshape(4,5)

>>> print a
[[ 1  2  3  4  5]
 [ 6  7  8  9 10]
 [11 12 13 14 15]
 [16 17 18 19 20]]

>>> blocks = blockwise_view(a, blockshape=(2,2), require_aligned_blocks=False)

>>> print blocks
[[[[ 1 2]
   [ 6 7]]

  [[ 3 4]
   [ 8 9]]]


 [[[11 12]
   [16 17]]

  [[13 14]
   [18 19]]]]

[blockwise_view.py] [test_blockwise_view.py]