[GRASS-dev] i.segment gives different results in G72 and G73

[Moritz Lennert]

On 05/12/16 21:06, Moritz Lennert wrote:
> On 05/12/16 14:52, Markus Metz wrote:
>>
>>
>> On Mon, Dec 5, 2016 at 1:46 PM, Martin Landa <landa.martin at gmail.com
>> <mailto:landa.martin at gmail.com>> wrote:
>>>
>>> Hi,
>>>
>>> 2016-12-05 9:09 GMT+01:00 Moritz Lennert <moritzlennert at posteo.net
>> <mailto:moritzlennert at posteo.net>>:
>>> > What colleagues here have also noticed is that between the versions
>> (but I can't remember if it was 7.0 vs 7.3 or 7.2 vs 7.3) quite
>> different thresholds were needed to reach similar segmentation results.
>> In 7.3 reasonable thresholds are often an order of magnitude lower, i.e.
>> 0.001 instead of 0.01...
>>>
>>> this wasn't my case.
>>
>> I am also getting very similar results with the same threshold. Can your
>> colleagues provide an example where such different thresholds need to be
>> used to obtain comparable results?

Taïs has prepared an example and can send it by private mail, but he 
reminded me of the cause of the difference he observed: in G73 the 
threshold is calculated based on the entire image, whereas in G72 the 
threshold is calculated based on only the current computational region. 
We had asked for this for i.segment.uspo (parameter optimization) as 
this module works on small sample subregions, but if the threshold is 
calculated based on the region, then the "optimal" threshold values 
coming out of the uspo procedure are not comparable between sample regions.

See some discussion here: 
https://lists.osgeo.org/pipermail/grass-dev/2016-June/080592.html.

Moritz