[GRASS-dev] [GRASS GIS] #3892: i.superpixels.slic: if kmeans converges, result should be one superpixel, not NULL

[GRASS GIS]

#3892: i.superpixels.slic: if kmeans converges, result should be one superpixel,
not NULL
-------------------------+-------------------------------------------------
  Reporter:  mlennert    |      Owner:  grass-dev@…
      Type:  defect      |     Status:  new
  Priority:  normal      |  Milestone:
 Component:  Addons      |    Version:  unspecified
Resolution:              |   Keywords:  i.superpixels.slic convergence
       CPU:              |  color
  Unspecified            |   Platform:  Unspecified
-------------------------+-------------------------------------------------

Comment (by mmetz):

 Replying to [ticket:3892 mlennert]:
 > In an automated script in which I run i.superpixels.slic, I sometimes
 have the situation that the zone I run the latter on is very small and
 very homogeneous leading to a convergence within the very first iteration
 of the kmeans in i.superpixels.slic (given the automatically determined
 parameter settings). I am ok with the entire area being considered as one
 single superpixel and would thus expect i.superpixels to label all pixels
 with 1. However, it labels all pixels with NULL. Is there any reason not
 to label a single superpixel ?

 Homogeneous input has been fixed in a01f12c5, including your fix for color
 definitions.

 Note that in case of homogeneous input, i.superpixels.slic will produce as
 many superpixels as there are seeds, not only one superpixel, because the
 distance to a superpixel center is considered together with spectral
 similarity.
 >
 > Somewhat as a side note, i.superpixels.slic crashes in that situation
 because of the color definition call in main.c (lines 707+708):
 >
 > {{{
 >             Rast_make_random_colors(&colors, 1, numlabels);
 >             Rast_write_colors(outname, G_mapset(), &colors);
 > }}}
 >
 > This can be easily solved by wrapping these calls into a check:
 >
 > {{{
 >     if (numlabels > 1) {
 >             Rast_make_random_colors(&colors, 1, numlabels);
 >             Rast_write_colors(outname, G_mapset(), &colors);
 >     }
 > }}}.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3892#comment:1>
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