[GRASS-dev] Testing i.superpixels.slic

[Sophie Crommelinck]

Hello,

thanks a lot for spending time and work implementing *i.superpixels.slic*.
It was awesome to see how fast the ticket was addressed, in which I asked
if someone would be interested in implementing this addon.

I did some initial tests on a UAV orthoimage (Extent: 1000x1000m, GSD: 5cm)
and unfortunately it delivered quite different results than a comparable
Matlab implementation.

What I did:

   - i.superpixels.slic on parts of the RGB image with *num_pixels* = 2500
   and *compactness* = 1
   - i.superpixels.slic on parts of the RGB image with *num_pixels* = 2500
   and *compactness* = 20
   - i.superpixels.slic on parts of the RGB image with *num_pixels* = 2500
   and *compactness* = 1 and *-n*
   - I ran the matlab implementation with the same settings

What I observed:

   - i.superpixels.slic outlines rarely align with the road outlines in the
   image.
   - Numerous very small superpixels are created (especially for
   *compactness* = 1 and *compactness* = 1 + *-n*). The total number of
   superpixel is much larger than the one defined with *num_pixels*. The
   output looks rather like that of a classical edge detector in textured
   regions (see screenshots attached).

My questions:

   - Why does the number of superpixels not correspond to the one defined
   in *num_pixels?*
   - How can two implementations both based on Achanta et al. provide such
   different results (screenshots attached)?
- The manual says: '*Contrary to the Achanta et al. version of SLIC0,
   i.superpixels.slic takes into account the compactness value chosen by the
   user even when the -n flag is used.*' I understand SLICO as a version of
   SLIC, for which the compactness is adaptively refined for each
   superpixel. To me, the purpose of SLICO seems to be that the compactness
   does not need to be defined and that the superpixels outlines are more
   regular-shaped. However, in the manual it sounds as if -n would rather
   intend to favor irregular-shaped superpixels. It is not clear to me, how
   the GRASS version relates to the usual SLICO version.

Screenshots:

   - https://share4land.itc.utwente.nl:5566/sharing/fS3tbiWuW


References:

   - https://nl.mathworks.com/help/images/ref/superpixels.html
   - https://grass.osgeo.org/grass72/manuals/addons/i.superpixels.slic.html


Best,

Sophie Crommelinck
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