[GRASS-dev] r.series threshold patch

[Markus Neteler]

Glynn Clements wrote:
> 
> Markus Neteler wrote:
>> >> to easier operate on incomplete time series from MODIS (and
>> >> others), we would like to suggest attached patch. It
>> >> adds a threshold to filter out incomplete pixel series
>> >> before calling the aggregation function which saves us
>> >> to perform extra runs on counting valid pixels and to
>> >> post-filter the aggregated results.
>> >
>> > While I don't doubt that this is a useful optimisation for your
>> > particular case, I'm generally opposed to adding such optimisations
>> > for specific cases.
>> >
>> > A more general optimisation would be to extend the method= and output=
>> > options to accept multiple values, so that you can compute multiple
>> > aggregates in a single run. You would still need to combine the two
>> > outputs with e.g. r.mapcalc, but you would only need one run of
>> > r.series.
>> >   
>> The optimization you propose is of course far more general than what
>> we did, and could be extremely valuable.
>> 
>> Nonetheless, we think that introducing the threshold parameter is not
>> really a special case hack: all it really does is a straightforward
>> generalization of the current -n flag, transforming it from a ON/OFF
>> switch to an integer value.
>> 
>> The threshold parameter indicates the minimum number of non NULL
>> inputs required for passing over the inputs to the aggregation
>> function.
>> 
>> It varies in the range [1,num_inputs]; thresh=num_inputs is equivalent
>> to -n (return NULL unless the inputs are all non NULL), while thresh=1
>> is the standard behaviour (compute the aggregation if there is at
>> least 1 non NULL input).
> 
> Actually, thresh=0 would give the existing behaviour. If -n isn't
> used, the values are always passed to the aggregate function. If all
> of the values are null, most aggregates will return null, but the
> "count" aggregate will return 0.
> 

You are right.
Do you still vote against the patch in general (along with better
documentation)?

Markus
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