[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
Resolution:               |   Keywords:  i.superpixels.slic step
       CPU:  Unspecified  |   Platform:  Unspecified
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Comment (by mlennert):

 Replying to [comment:1 mmetz]:
 > Replying to [ticket:3545 mlennert]:
 > > The behavior of the step parameter in i.superpixels.slic is a bit
 confusing:
 > >
 > > [...]
 > >
 > > 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.
 > >
 > > }}}
 > >
 > The reason is that with small step sizes, you would get mini-
 superpixels, mostly squares. The SLIC algorithm likes a somewhat larger
 distance between superpixel centers in order to produce reasonable
 results. You can modify the condition to `step < 2` and see what happens.

 Doing this and asking for a step of 2 both on my VISNIR aerial photos and
 on the BW NC orthophoto, I get tiny superpixels of divers shapes. I don't
 find them very useful for my particular application, but this should be up
 to the user to decide, not imposed by the module.

 If there are other reasons why step has to be above a certain threshold I
 would prefer the code to just raise the value to that threshold and tell
 the user about it, rather than creating a step value which is not very
 understandable for users. Something like this:


 {{{
 if (step < 5) {
     G_warning("Setting step to the minimum value of 5.");
     step = 5;
 }
 }}}

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3545#comment:2>
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