[GRASS-dev] RFC: variations of statistics in r.neighbors (and the stats lib)

[Francesco Paolo Lovergine]

On Tue, Jan 31, 2023 at 07:47:49PM +0100, Markus Metz wrote:
>Hi Francesco,
>

Hi Markus,

>the proposed change to r.neighbors is interesting, but maybe too specific:
>you have introduced two new functions, and many more functions would be
>needed to e.g. get the filtered standard deviation or median.
>

Indeed, that was the reason of my doubt on that approach. The changes
introduced were simple enough and workerd properly for my specific case,
but it is neither elegant nor general at all.

>Therefore I suggest adding some filtering option to r.neighbors consisting
>of a filtering function and a comparison operator. The filtering function
>would be any of the currently supported neighborhood functions returning
>some value. The comparison operator would be one of gt, ge, le, lt (>, >=,
><=, <). r.neighbors would then first get the result value of the filtering
>function, then set all values in the neighborhood to NULL that do not
>fulfil the condition "value <comparison operator> <result value">, then
>call the actual neighborhood operation with the filtered values. This would
>be more flexible, because the user can freely combine a neighborhood filter
>function with a neighborhood operation.
>

This seems reasonable to me. The same approach could be used for at least
a pair of different population selection filter, i.e. by-quantile 
and by-quartile.

filter ::= by-quantile | by-quartile
filter-cmp-op ::= ge | le | gt | lt | range

with target value taken from quantile or quartile arguments. Eventually, one
could use a range too and specify a pair of new cmd line arguments:

quantile_low | quantile_high 
quartile_low | quartile_high

or change quantile and quartile into a list of 1..2 comma separated values.

Much better, isn't it?

>
>
>On Wed, Jan 25, 2023 at 10:55 AM Francesco P. Lovergine <frankie at debian.org>
>wrote:
>
>> Hi,
>>
>> for some specific needs of a research project, I had to make a little
>> change to r.neighbors (the target version was 7.8.5 but that's not
>> essential).
>>
>> Essentially, the idea behind is computing first order statistics on partial
>> populations as identified by selected quantiles (samples >= or <= of a
>> threshold value of quantile).
>>
>> For that, I introduced average_ge_quantile and average_le_quantile
>> operators modes.
>>
>>
>> https://github.com/fpl/grass/commit/6b83795b037c6645a32d6a525cfdee3cc65d521c
>>
>> (the html file is still not updated)
>>
>> I'm not persuaded this is the most elegant way of doing this kind of
>> things,
>> maybe it would be better using an option (as in the case of -w for weighted
>> operations) to compute average and possibly other statistics. Even, one
>> could
>> think in general to other multiple ways of selecting population on the
>> base of
>> quantiles/percentiles of population in a window.
>>
>> Any hint/opinion/alternative/critic about that?
>>
>> All this in the remote hypothesis that a pull request could have sense
>> for such a kind of features.
>>
>> Thanks
>>
>>
>> --
>> Francesco P. Lovergine
>> _______________________________________________
>> grass-dev mailing list
>> grass-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/grass-dev
>>

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-- 
Francesco P. Lovergine