#3545: i.superpixels.slic: behaviour of step parameter confusing
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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 mmetz):
Replying to [comment:2 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.
OK, I changed the condition to `step < 2` and updated the option
description in r72634.
--
Ticket URL: <https://trac.osgeo.org/grass/ticket/3545#comment:4>
GRASS GIS <https://grass.osgeo.org>
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