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

[Markus Metz]

On Wed, Dec 7, 2016 at 11:27 AM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:
>
> 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.

OK. Figuring out a reasonable threshold based on small sample regions is
IMHO of general interest for i.segment, because you would expect similar
results when you increase the region. Therefore I think the behaviour in
G73 is an improvement.

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