[Numpy-discussion] neighborhood iterator speed

[Nadav Horesh]

I am trying to replace an old code (biliteral filter) that rely on ndimage.generic_filter with the neighborhood iterator. In the old code, the generic_filter generates a contiguous copy of the neighborhood, thus the (cython) code could use C loop to iterate over the neighbourhood copy. In the new code version the  PyArrayNeighborhoodIter_Next must be called to retrieve every neighbourhood item. The results of rough benchmarking to compare bilateral filtering on a 1000x1000 array:
Old code (ndimage.generic_filter):  16.5 sec
New code (neighborhood iteration):  60.5 sec
New code with PyArrayNeighborhoodIter_Next  omitted: 1.5 sec

* The last benchmark is not "real" since the omitted call is a must. It just demonstrates the iterator overhead.
* I assune the main overhead in the old code is the python function callback process. There are instructions in the manual how to wrap a C code for a faster callback, but I rather use the neighbourhood iterator as I consider it as more generic.

If the PyArrayNeighborhoodIter_Reset could (optionally) copy the relevant data (as the generic_filter does) it would provide a major speed up in many cases.

  Nadav