r.resamp.filtergrass - Online in the Cloud

PROGRAM:

NAME

r.resamp.filter - Resamples raster map layers using an analytic kernel.

KEYWORDS

raster, resample, kernel filter

SYNOPSIS

r.resamp.filter
r.resamp.filter --help
r.resamp.filter [-n] input=name output=name filter=string[,string,...]
[radius=float[,float,...]] [x_radius=float[,float,...]] [y_radius=float[,float,...]]
[--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:
-n
Propagate NULLs

--overwrite
Allow output files to overwrite existing files

--help
Print usage summary

--verbose
Verbose module output

--quiet
Quiet module output

--ui
Force launching GUI dialog

Parameters:
input=name [required]
Name of input raster map

output=name [required]
Name for output raster map

filter=string[,string,...] [required]
Filter kernel(s)
Options: box, bartlett, gauss, normal, hermite, sinc, lanczos1, lanczos2, lanczos3,
hann, hamming, blackman

radius=float[,float,...]
Filter radius

x_radius=float[,float,...]
Filter radius (horizontal)

y_radius=float[,float,...]
Filter radius (vertical)

DESCRIPTION

r.resamp.filter resamples an input raster, filtering the input with an analytic kernel.
Each output cell is typically calculated based upon a small subset of the input cells, not
the entire input. r.resamp.filter performs convolution (i.e. a weighted sum is calculated
for every raster cell).

The module maps the input range to the width of the window function, so wider windows will
be "sharper" (have a higher cut-off frequency), e.g. lanczos3 will be sharper than
lanczos2.

r.resamp.filter implements FIR (finite impulse response) filtering. All of the functions
are low-pass filters, as they are symmetric. See Wikipedia: Window function for examples
of common window functions and their frequency responses.

A piecewise-continuous function defined by sampled data can be considered a mixture (sum)
of the underlying signal and quantisation noise. The intent of a low pass filter is to
discard the quantisation noise while retaining the signal. The cut-off frequency is
normally chosen according to the sampling frequency, as the quantisation noise is
dominated by the sampling frequency and its harmonics. In general, the cut-off frequency
is inversely proportional to the width of the central "lobe" of the window function.

When using r.resamp.filter with a specific radius, a specific cut-off frequency regardless
of the method is chosen. So while lanczos3 uses 3 times as large a window as lanczos1, the
cut-off frequency remains the same. Effectively, the radius is "normalised".

All of the kernels specified by the filter parameter are multiplied together. Typical
usage will use either a single kernel or an infinite kernel along with a finite window.

NOTES

Resampling modules (r.resample, r.resamp.stats, r.resamp.interp, r.resamp.rst,
r.resamp.filter) resample the map to match the current region settings.

When using a kernel which can have negative values (sinc, Lanczos), the -n flag should be
used. Otherwise, extreme values can arise due to the total weight being close (or even
equal) to zero.

Kernels with infinite extent (Gauss, normal, sinc, Hann, Hamming, Blackman) must be used
in conjunction with a finite windowing function (box, Bartlett, Hermite, Lanczos).

The way that Lanczos filters are defined, the number of samples is supposed to be
proportional to the order ("a" parameter), so lanczos3 should use 3 times as many samples
(at the same sampling frequency, i.e. cover 3 times as large a time interval) as lanczos1
in order to get a similar frequency response (higher-order filters will fall off faster,
but the frequency at which the fall-off starts should be the same). See Wikipedia:
Lanczos-kernel.svg for an illustration. If both graphs were drawn on the same axes, they
would have roughly the same shape, but the a=3 window would have a longer tail. By scaling
the axes to the same width, the a=3 window has a narrower central lobe.

For longitude-latitude locations, the interpolation algorithm is based on degree
fractions, not on the absolute distances between cell centers. Any attempt to implement
the latter would violate the integrity of the interpolation method.

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