[GRASS-dev] Implementation of the High Pass Filter Additive Fusion technique (i.fusion.hpf)

[Nikos Alexandris]

Nikos Alexandris wrote:

> > together with Nikos Ves, we share the "i.fusion.hpf" idea/proof of
> > concept. At the moment, we have a custom shell script named
> > `i.fusion.hpf` (an attempt for a proper GRASS add-on), which implements
> > the High Pass Filter Additive (HPFA) Fusion Technique for Pan-Sharpening
> > [*]. Nikos V started already porting to Python. How can we proceed in
> > sharing it?
> 
> [...]
> 
> > Two questions
> > 
> > ? Can someone confirm that the part of the existing "i.pansharpen" code
> > that performs histogram matching (code lines 348 - 431), do so as
> > "linearly stretching an image to match another image's Mean and StdDev"?

Moritz Lennert:

> AFAICT, it applies the method described in [1].

Is that a reference also indicated in i.pansharpen's manual?

> I don't know (and don't have the time to think about) what this method does
> in terms of mean and stddev.

My guess was/is that it is not the same, i.e. it does not match Mean and StdDev. As a quick 
test, I tried the identical (me thinks) tool in WhiteboxGIS "Histogram Matching (Two Images)", 
does not give identical Means and StdDevs after the operation -- which is the case with 
i.pansharpen too if I am not wrong.

> > ? Would it be desired to get the HPFA algorithm integrated in
> > i.pansharpen?
> 
> Yes. I think that if we have a generic module such as i.pansharpen, it
> would be preferable to have all pansharpening methods in that one module.

There is one "difference" in that HPFA treats all bands to be sharpened separately. And, in 
this manner, it can be (mis-)used to sharpen any low-res band. For example, WorldView-2 
products have 8 multi-spectral bands. Hence the "not red= green= blue=" design so far from 
my side.

@Michael, is this of interest for you?

Thank you Moritz, Nikos
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