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

[Moritz Lennert]

[Sent this from a wrong address, so sending this again for completeness.]


-------- Forwarded Message --------
Subject: Re: [GRASS-dev] i.segment gives different results in G72 and G73
Date: Mon, 05 Dec 2016 09:09:48 +0100
To: Markus Metz <markus.metz.giswork at gmail.com>, Martin Landa 
<landa.martin at gmail.com>
CC: GRASS developers list <grass-dev at lists.osgeo.org>



Le 5 décembre 2016 08:54:01 GMT+01:00, Markus Metz 
<markus.metz.giswork at gmail.com> a écrit :
>On Sun, Dec 4, 2016 at 9:56 AM, Martin Landa <landa.martin at gmail.com>
>wrote:
>>
>> Running `i.segment` on the same data in G72 and G73 gives different
>> results. Both commands should use the same method - G72 supports only
>> 'region growing', G73 has this method as default one.
>
>Both versions use the same method by default. However, the G73 version
>of
>region growing in i.segment has been modified, it is now faster for
>larger
>objects (larger thresholds). A side effect is that objects might differ
>slightly between G72 and G73, but both results are correct in terms of
>the
>concept of region growing. The reason for the difference is when
>finding
>the most similar neighbor region, there can be several equally best
>neighbors. Which one is selected is more or less chance, to be precise,
>the
>order of visiting the neighbors, and this order of visiting changed.


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...

Moritz


>
>Markus M
>
>>
>> Region:
>>
>> {{{
>> north:      -1057730
>> south:      -1058250
>> west:       -713860
>> east:       -712960
>> nsres:      0.25
>> ewres:      0.25
>> rows:       2080
>> cols:       3600
>> cells:      7488000
>> }}}
>>
>> Command:
>>
>> {{{
>> i.segment group=seg output=seg1 threshold=0.01 --o
>> }}}
>>
>> G72:
>>
>> {{{
>> ...
>> Segmentation converged after 8 iterations
>> Writing out segment IDs...
>> i.segment complete. Number of segments created: 4367082
>> (Sun Dec  4 09:42:19 2016) Command finished (3 min 36 sec)
>> }}}
>>
>> G73:
>>
>> {{{
>> Segmentation converged after 14 iterations
>> Assigning region IDs to remaining single-cell regions...
>> Writing out segment IDs...
>> i.segment complete. Number of segments created: 4359666
>> }}}
>>
>> Segmentation performed in G73 gives slightly different results and
>> takes longer time to finish.
>>
>> Thanks for clarification in advance, Martin
>>
>> --
>> Martin Landa
>> http://geo.fsv.cvut.cz/gwiki/Landa
>> http://gismentors.cz/mentors/landa
>> _______________________________________________
>> grass-dev mailing list
>> grass-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-dev
>
>
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Université Libre de Bruxelles
Bureau: S.DB.6.138
CP 130/03
Av. F.D. Roosevelt 50
1050 Bruxelles
Belgique

tél. + 32 2 650.68.12 / 68.11 (secr.)
fax  + 32 2 650.68.30