[GRASS-dev] [GRASS GIS] #2856: i.segment: warning "Region consists of only one cell, nothing to update" slows down module

[GRASS GIS]

#2856: i.segment: warning "Region consists of only one cell, nothing to update"
slows down module
--------------------------+------------------------------
  Reporter:  mlennert     |      Owner:  grass-dev@…
      Type:  defect       |     Status:  new
  Priority:  normal       |  Milestone:  7.0.4
 Component:  Imagery      |    Version:  svn-trunk
Resolution:               |   Keywords:  i.segment region
       CPU:  Unspecified  |   Platform:  Unspecified
--------------------------+------------------------------

Comment (by mmetz):

 Replying to [ticket:2856 mlennert]:
 > In the NC dataset:
 >
 >
 > {{{
 > g.region n=221318 s=220073 w=637746 e=639646 res=1
 > i.group ortho in=ortho_2001_t792_1m
 > i.segment group=ortho at user1 output=ortho_seg_001 threshold=0.01
 memory=2500
 > }}}
 >
 > Then, when I try to use the output of this as seeds for the next level:
 >
 >
 > {{{
 > i.segment group=ortho at user1 output=ortho_seg_002_seeded threshold=0.02
 seed=ortho_seg_001 memory=2500
 > }}}
 >
 > I get tons of
 >
 > {{{
 > WARNING: Region consists of only one cell, nothing to update
 > }}}
 >
 > The module run takes +/- 1m25.
 >
 > When I comment
 [https://trac.osgeo.org/grass/browser/grass/trunk/imagery/i.segment/create_isegs.c#L1086
 line 1086]  in imagery/i.segment/create_isegs.c so that the warning is not
 issued, module run only takes about 20s.
 >
 > Is this warning really necessary ? Or is it a bug that it appears in
 this context ?

 It is a bug that appears in this context. The warning is justified because
 it appears only when regions are merged. A merged region must always
 consist of more than one cell (merge of two original regions each
 consisting of at least one cell). The bug seems to be an inappropriate
 call to merge_regions().

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2856#comment:2>
GRASS GIS <https://grass.osgeo.org>