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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