[GRASS-user] Question to the input seed grid of i.segment

[Moritz Lennert]

On 24/01/17 20:40, Raphael Knevels wrote:
> Thanks for the quick response :-)
>
> The extent of my region is also the extent of my imagery-group. For i.segment I used as memory 10240 MB with a threshold of 0.25.

That's a pretty large threshold which generally leads to 
under-segmentation. You might want to start with a value closer to 0.01.

>
> In SAGA I used at first Seed Generation (Band Width of 18, but I
> also
> varied this parameter) for producing the Seed Output. The Seed Output is
> a raster with floating point values. Moreover, the Seed Output contains
> single pixels distributed over the hole area. The "space" between those
> pixels is "no data". The segmentation is then computed by Seeded Region
> Growing with the seed grid as input.
>
> "Seeds in i.segment have to be polygons not points. These polygons
> are
> represented by identical positive integer values (= IDs) in adjacent
> pixels, and they have to cover the entire region. When used as seeds for
> a segmentation, these polygons are the further merged."

Sorry, but this was actually wrong information from my part: i.segment 
also supports seed pixels. From the man page:

"The seeds map can be used to provide either seed pixels (random or 
selected points from which to start the segmentation process) or seed 
segments. If the seeds are the results of a previous segmentation with 
lower threshold, hierarchical segmentation can be performed. The 
different approaches are automatically detected by the program: any 
pixels that have identical seed values and are contiguous will be 
assigned a unique segment ID. "


However, the seed map has to have integer values. To get that from you 
SAGA output (floating-point) seed map, just run r.clump.

> -> alright. That means, I definitely must convert the no data values in the SAGA Seed Output to zero or any other integer number.

No, I actually don't think this would help in any way as this would 
create one big region out of all the no data pixels...

> Meanwhile, I also tried the segmentation result of SAGA Seeded Region Growing as Seeds for GRASS - this works...

And how long did this take ?

> " You could try using the brand new i.superpixels.slic add-on to create superpixels which you can then use as seeds."
> -> I could successfully install and open it ("g.extension i.superpixels.slic", GRASS 7.2.0).
> However, when I run the tool with default settings, GRASS gives a problem message and finishs the process.

Ok, this sounds like bug #3247 [1] for which the author hasn't applied 
the fix, yet. Rashad, will you have the opportunity to apply just the 
fix for the actual bug (not necessarily the other proposals in the 
patch), soon, or should I do it ?

Moritz