[GRASS-dev] PCA (i.pca) in G7: filtering and rescaling

[Markus Metz]

On Wed, Dec 4, 2013 at 3:42 PM, Nikos Alexandris
<nik at nikosalexandris.net> wrote:
> Nikos Alexandris wrote:
>> > > > 1) I can't see any differences in the derived Principal Components
> ..
>> > okay, to clarify: I mean the resulting images which, initially are
>> > Principal Components (synthetic images) and, after applying filtering &
>> > backward PCA, the resulting images approach the original data -- still
>> > they are modified.
>
>> Example (using i.pca in G7, though r.info below executed from G64):
>
>>..
>>PC1      2.78 ( 0.4947, 0.5922, 0.5743, 0.2735) [69.53%]
>>..
>
> Ignore please. I picked the wrong example. It works with percentage = 99 for
> example. Because 70 + 20 = 90 was still inside the 1st component (=96.52).
>
> Sorry for the noise. Then I need to see why the rescaling is not getting at
> [0,255].

Rescaling applies to PC scores. With filtering, the output is not PC
scores, but a filtered version of the input. If you want the filtered
output to be rescaled, please use r.recode afterwards.