[GRASS-dev] A few additional questions about i.segment

[Markus Metz]

On Thu, Jul 4, 2013 at 11:13 AM, Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> As you might have gathered from my other messages on the list, we are
> currently running tests of i.segment in our department. A few questions came
> up (with my attempts at answers).
>
>
> ***************************
>>
>> I had a look at the results, they are interesting. Maybe you could also
>> try
>> a combination of panchromatic, red and near-infrared bands and leave the
>> other bands out. If it is possible to give different weights to the bands,
>> then maybe 2 for the pan, 1 for red and 1 for NIR.
>
>
> I think that is possible, but I have to check how. I imagine you would
> work at the resolution of the pan for this...

You could first resample the pan band to the resolution of the other
bands, then r.mapcalc - multiply the pan band with 2 and use i.segment
-w.

>
>>
>> Regarding the size of the segments, we generally combine different
>> segmentations, fine and coarse, depending on the type of objects that we
>> have to classify. We also combine different segmentations because then we
>> can use the classification of a fine segmentation as attribute for
>> classifying a coarse segmentation (e.g. counting trees or houses in a
>> large
>> segment). Here we have very small segments and also very large ones
>> together
>> in the results, it is a bit different.
>
>
> So to be able to do what you do now, you would need homogenous segment
> sizes, i.e. not only a minsize (which I set fairly low in this first
> trial: 2 pixels), but also the equivalent of a maxsize. Then, if you set the
> two fairly close to each other (i.e. minsize=900 and maxsize=1100 to get
> segments of +/- 1000 pixels).

This is currently controlled with the threshold value. I am not sure
if a max size makes sense. There might be homogenous objects with
identical pixel values (difference = 0) that are larger than a given
max size. Splitting such objects to respect a max size would be
arbitrary. I would rather try to find a good combination of threshold
and minsize, and do hierachical segmentation using the output of the
previous run as input for the next run with increasing threshold.
Technically, a maxsize option is possible for i.segment.

Markus M

>
> I'll see how that can be achieved with the currently implemented module.
> *******************************
>
> Any hints on possible answers to these issues ?
>
> Moritz