While testing the Euler characteristic computation function, nipy.algorithms.statistics.intvol.EC2d, I found some unexpected behavior that could be reduced to the following :

In [2]: import numpy as np

In [3]: from nipy.algorithms.statistics.intvol import EC2d

In [4]: a = np.ones(shape=(3, 3), dtype='int'); print a, EC2d(a)
[[1 1 1]
 [1 1 1]
 [1 1 1]] -1

In [5]: a[0, 0] = 0; print a, EC2d(a)
[[0 1 1]
 [1 1 1]
 [1 1 1]] 1

In [6]: a[0, 0] = 1; a[-1, 0] = 0; print a, EC2d(a)
[[1 1 1]
 [1 1 1]
 [0 1 1]] -1

Is that a expected consequence of those functions being approximate? Thanks for your help and sorry if I am misunderstanding the intended purpose of the functions.