[GRASS-dev] [GRASS GIS] #3545: i.superpixels.slic: behaviour of step parameter confusing

[GRASS GIS]

#3545: i.superpixels.slic: behaviour of step parameter confusing
-------------------------------------+-------------------------
 Reporter:  mlennert                 |      Owner:  grass-dev@…
     Type:  defect                   |     Status:  new
 Priority:  minor                    |  Milestone:
Component:  Addons                   |    Version:  unspecified
 Keywords:  i.superpixels.slic step  |        CPU:  Unspecified
 Platform:  Unspecified              |
-------------------------------------+-------------------------
 The behavior of the step parameter in i.superpixels.slic is a bit
 confusing:


 {{{
 GRASS 7.5.svn (Belgique31370):/data/home/mlennert > i.superpixels.slic
 orthos2016_p46_internal step=200 out=test200 perturb=10 compact=1 --o

 WARNING: Initialized 9 of 24 seeds


 GRASS 7.5.svn (Belgique31370):/data/home/mlennert > i.superpixels.slic
 orthos2016_p46_internal step=20 out=test20 perturb=10 compact=1 --o

 WARNING: Initialized 1169 of 2925 seeds

 GRASS 7.5.svn (Belgique31370):/data/home/mlennert > i.superpixels.slic
 orthos2016_p46_internal step=2 out=test2 perturb=10 compact=1 --o

 WARNING: Initialized 76 of 187 seeds
 }}}

 Normally, one would expect a larger number of seeds with step=2 than
 step=20. This behavior comes from lines 277ff of main.c, which read:


 {{{
 superpixelsize = step * step;
 if (step < 5) {
     superpixelsize = 0.5 + (double)nrows * ncols / n_super_pixels;

     step = sqrt((double)superpixelsize) + 0.5;
 }

 Why this limit at 5 ? If it is really necessary, it should at least be
 documented in the man page to avoid confusion.

 }}}

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/3545>
GRASS GIS <https://grass.osgeo.org>