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

[Markus Metz]

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.

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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