[GRASS-dev] [GRASS-user] Gauss filter in r.neighbors

[Markus Metz]

Glynn Clements wrote:
>
> Can someone determine whether there's a problem with
> "r.neighbors gauss=..." in 6.x?

There is no problem with the Gaussian filter, but a problem with the
output map type of r.neighbours in G64: the output type is in G64 the
same like the input type, whereas in G7 the output type depends on the
chosen function. That means, if you use r.neighbours method=average in
G64 with a CELL map, the output is also CELL.

Markus M

>
> Martin Album Ytre-Eide wrote:
>
>> I followed you examples and it did not work in grass 6.4.4 so I
>> tried them in grass7.1 and they worked for me as well.
>>
>> Tried my map - and it worked. I result looks good :)
>>
>> Thanks!
>>
>> (maybe there is a problem in r.neighbors for grass 6.4.4?)
>>
>> Martin
>>
>> ________________________________________
>> Fra: Glynn Clements [glynn at gclements.plus.com]
>> Sendt: 2. oktober 2014 14:27
>> Til: Martin Album Ytre-Eide
>> Kopi: grass-user at lists.osgeo.org
>> Emne: Re: [GRASS-user] Gauss filter in r.neighbors
>>
>> Martin Album Ytre-Eide wrote:
>>
>> > I am using the the r.neighbors function and I have an idea about
>> > what the 'Gauss' parameter will do for me, but I can't find any
>> > documentation for it. Experimenting with it, left me clueless.
>>
>> Using gauss= is equivalent to using weight= with a weights file
>> corresponding to a Gaussian filter.
>>
>> The exact weights which are used can be found in the
>> gaussian_weights() function in raster/r.neighbors/readweights.c. They
>> are effectively:
>>
>>         ncb.weights[i][j] = exp(-(x*x+y*y)/(2*sigma2))/(2*M_PI*sigma2);
>>
>> where x, y is the offset (in cells) from the centre of the
>> neighbourhood and sigma2 is the square of the value passed to the
>> gauss= option.
>>
>> > What I won't to do:
>>
>> > Apply a Gaussian weighting of cell neighbors and sum the values of
>> > the neighbors. If all cells=1, I would get only 1's back.
>>
>> You need to use method=average to make the effective weights sum to
>> one (when weights are used, method=average divides the weighted sum of
>> the cell values by the sum of the weights).
>>
>> IIRC, the weights are such that they would sum to one if you had an
>> infinitely-large neighbourhood, but the fact that you're cropping the
>> curve to a finite neighbourhood means that the sum will always be less
>> than one.
>>
>> The weights are determined by sigma alone, and are unaffected by the
>> neighbourhood size.
>>
>> > I have a map of zeros and ones. I tried this:
>> >
>> > r.neighbors -c input='input_map' output='output_map' method=sum gauss=10 size=33
>> >
>> > The result is a map with only zeros.
>>
>> It works for me; e.g.
>>
>>         $ r.mapcalc --o 'foo = rand(0,2)' seed=1
>>         $ r.neighbors --o -c in=foo out=bar method=average size=33 gauss=10
>>         $ r.info -r bar
>>         min=0.442173333781042
>>         max=0.568925311919939
>>         $ r.neighbors --o -c in=foo out=bar method=sum size=33 gauss=10
>>         $ r.info -r bar
>>         min=0.0985901293262625
>>         max=0.449425795562072
>>
>>         $ r.mapcalc --o 'foo = 1'
>>         $ r.neighbors --o -c in=foo out=bar method=average size=33 gauss=10
>>         $ r.info -r bar
>>         min=1
>>         max=1
>>         $ r.neighbors --o -c in=foo out=bar method=sum size=33 gauss=10
>>         $ r.info -r bar
>>         min=0.221413499330273
>>         max=0.812157275619236
>
> --
> Glynn Clements <glynn at gclements.plus.com>
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