[R-sig-Geo] How to upscale (reduce the spatial resolution) an image using a Gaussian filter in R?

[Nikolaos Tziokas]

I managed to solve the issue using the OpenImageR package and the function
down_sample_image.

library(raster)
library(OpenImageR)

r = raster("path/tirs.tif")

m = as.matrix(r)

psf = down_sample_image(m,
                        factor = 4.6, # zoom factor
                        gaussian_blur = T,
                        gauss_sigma = 0.5 * 100) # sigma * pixel size

e <- extent(r)

m2r <- raster(psf)
extent(m2r) <- e
raster::crs(m2r) <- "EPSG:7767"
res(m2r)

writeRaster(m2r, "path/tirs460.tif")

The function seems to do exactly what the creator of the area-to-point
regression Kriging, Qunming Wang, did with in his Matlab code (
https://github.com/qunmingwang/Code-for-S2-fusion/blob/master/PSF_template.m).
That is, he multiplied the width of the PSF with scale factor.


Στις Τετ, 7 Σεπ 2022, 15:46 ο χρήστης Greg Snow <538280 using gmail.com> έγραψε:

> The `image_resize` function in the `magick` package may be a simple
> option.  A gaussian filter is one of the options.  This uses the
> ImageMagick program behind the scenes, but is a quick and easy
> interface within R for images (raster data).
>
> On Tue, Sep 6, 2022 at 10:46 AM Nikolaos Tziokas
> <nikos.tziokas using gmail.com> wrote:
> >
> > Hi,
> >
> > I want to resample a raster from 15m to 460m (pixel size) using a
> Gaussian
> > filter.
> >
> > *The goal*
> >
> > I am having a coarse image which I want to downscale. I also have a fine
> > resolution band to assist the downscaling. The downscaling method I am
> > using is called geographically weighted area-to-point regression Kriging
> > (GWATPRK). The method consists of two steps:
> >
> >    1. GWR
> >    2. area-to-point Kriging on the GWR's residuals
> >
> > In order to perform GWR using raster data, those needs to have the same
> > pixel size. This means that, my fine resolution image needs to be
> upscaled
> > to match the spatial resolution of the coarse band. This upscaling of the
> > fine band needs to be done using a Gaussian kernel with σ=0.5(i.e., the
> > PSF).
> >
> > How can I upscale (reduce the spatial resolution) a satellite image
> using a
> > Gaussian kernel (i.e., point spread function)?
> >
> > For reference, I am following the paper The effect of point spread
> function
> > on downscaling continua
> > <https://www.sciencedirect.com/science/article/pii/S092427162030229X>
> where
> > the authors at p.253 in Eq (9) mention:
> >
> > *the coarse image produced by upscaling the corresponding fine band k
> using
> > a PSF.*
> >
> > I googled how I can achieve that but unfortunately I couldn't find any
> > solution. So to do this, how can I use this Gaussian filter to change the
> > resolution (pixel size) of my image?
> >
> > Here is the image
> > <
> https://drive.google.com/drive/folders/18_1Kshb8WbT04gwOw4d_xhfQenULDXdB?usp=sharing
> >
> > I
> > am trying to convolve.
> >
> >
> > Many thanks
> >
> >
> > --
> > Tziokas Nikolaos
> > Cartographer
> >
> > Tel:(+44)07561120302
> > LinkedIn <http://linkedin.com/in/nikolaos-tziokas-896081130>
> >
> >         [[alternative HTML version deleted]]
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo using r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
>
>
> --
> Gregory (Greg) L. Snow Ph.D.
> 538280 using gmail.com
>

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