[GRASS-dev] hints appreciated for work on a r.zonal.geometry module

[Moritz Lennert]

On 20/06/16 23:20, Markus Metz wrote:
> On Mon, Jun 20, 2016 at 6:40 PM, Moritz Lennert
> <mlennert at club.worldonline.be> wrote:
>> Hi,
>>
>> While work is ongoing concerning improving r.to.vect performance [1], I am
>> looking at the possibility of creating an r.zonal.geometry module which
>> would calculate a series of form statistics for raster zones (aka clumps). I
>> would begin simply by trying to replicate the v.to.db statistics, i.e. area
>> and perimeter, with compactness and fractal dimension derived from the two
>> former.
>>
>> Area calculation is quite simple (and already exists, for example, in
>> r.stats.zonal with the count method). However, perimeter is a bit more
>> complicated. My current idea is to 'simply' fetch raster info into a SEG
>> structure and to check for each pixel whether the upper, lower, left and
>> right neighbor has the same cat value as the current pixel. For each
>> neighbor where this is not the case, I add one pixel edge to the total zonal
>> perimeter.
>
> For raster-based perimeter calculation, you do not need a SEG
> structure. Instead, you could use a 2x2 moving window and compare top
> right - bottom right and bottom left - bottom right. Simply have two
> rows at a time accessible. Whenever a pair is different, increment the
> perimeter of each member of the comparison that is not NULL. You would
> need to pad the region with a NULL row on top and bottom and a NULL
> column to the left and right.

Ok, thanks ! That's pretty much exactly like clump.c in r.clump, or ?

> This works only with unique
> cluster/clump IDs and not with classes.

Yes. Not sure how usefule class-based geometries would be...

> Do you want to obtain any
> other parameters than area size and compactness? Another one might be
> smoothness = perimeter of the cluster / perimeter of the cluster's
> bounding box.

The idea is to extend gradually, so yes, bounding box based statistics 
would be nice. Probably also statistics based on the eigenvalues of the 
x,y coordinates matrix which allows approximating the form by an ellipse 
and calculating length/width ratio statistics.

But at this stage, I guess that for all these a two-row approach might 
be enough.

I don't know when I will have time for this, but I would love to be able 
to get this done before foss4g where I will present the GRASS OBIA 
toolchain...


Moritz