[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.gisw...@gmail.com>, Martin Landa
<landa.mar...@gmail.com>
CC: GRASS developers list <grass-dev@lists.osgeo.org>
Le 5 décembre 2016 08:54:01 GMT+01:00, Markus Metz
<markus.metz.gisw...@gmail.com> a écrit :
On Sun, Dec 4, 2016 at 9:56 AM, Martin Landa <landa.mar...@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
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Belgique
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