Out of curiosity, I did some testing myself, using the ortho photo
available in the GRASS North Carolina demo data set [1]. Several
conclusions (based on default parameter settings in SAGA and GRASS):
- Actually the seed map coming out of SAGA is not floating point. The
created tiff is Float32, but in fact pixel values are integer. So, in
GRASS you can just run r.mapcalc "int_map = int(map)" and then use it as
seed input to i.segment.
- The SAGA seeded region algorithm as AFAIU is more similar to the
i.superpixels.SLIC algorithm than to our region-growing algorithm. That
explains the different execution times. See [2] (grass segmentation with
threshold 0.1), [3] (saga segmentation), [4] (grass superpixels).
- Providing the seeds coming from SAGA as input to i.segment does not
change neither the execution time, nor the result.
So: I would suggest you try segmenting with a lower threshold. In order
to not loose time by segmenting the whole image at each try, you can
either create a few small named regions and use the i.segment.uspo addon
to try to automagically determine the best threshold, or you just define
a small computational region and test manually before applying the
result to the entire image.
Moritz
[1] http://tomahawk.ulb.ac.be/moritz/ortho_photo.png
[2]
http://tomahawk.ulb.ac.be/moritz/segmentation_grass_noseeds_thresh_01.png
[3] http://tomahawk.ulb.ac.be/moritz/segmentation_saga.png
[4] http://tomahawk.ulb.ac.be/moritz/grass_superpixels.png
On 25/01/17 11:22, Moritz Lennert wrote:
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
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