[SciPy-User] Why does generic_filter coerce input to float64?

[Juan Nunez-Iglesias]

Hi Eric,




Thanks for the response!




Changing it would be a very bad thing and probably break a lot of code out there. However, a kwarg would be most useful to me! Would a PR to add this be welcome?




At scikit-image, we converged on "preserve_dtype" as a slightly easier-to-parse name for such a kwarg. Thoughts?




Juan.

On Tue, Jan 13, 2015 at 12:26 AM, Moore, Eric (NIH/NIDDK) [F]
<eric.moore2 at nih.gov> wrote:

>> From: Juan Nunez-Iglesias [mailto:jni.soma at gmail.com] 
>> Sent: Sunday, January 11, 2015 6:21 PM
>> To: scipy-user at scipy.org
>> Subject: [SciPy-User] Why does generic_filter coerce input to float64?
>>
>> Hi all,
>>
>> I'm wondering, why does ndimage.generic_filter cast the input array as a float
>> before passing it to the user-supplied function? I have at least one application
>> that requires integer input. For small integers, this is not a problem (other
>> than resulting in at least two unnecessary casts), but for big ones this will
>> cause a loss of precision.
>>
>> I'd love to contribute a patch but unfortunately I couldn't find the place in
>> the C-code where this happens.
>>
>> Thanks!
>> 
>> Juan.
>>
> The cast happens in the expansion of the CASE_FILTER_POINT macro[1], which is used
> just below that in NI_GenericFilter.
> This looks like a deliberate choice though, so I don't think that we can just change
> it.  One option might be to add this as a keep_dtype argument, which would default 
> to false.
> Eric
> 1. https://github.com/scipy/scipy/blob/master/scipy/ndimage/src/ni_filters.c#L869
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