Absolute frequency units `f_a`
`f_a=0` corresponds to the zero Fourier term (zero frequency), while `f_a=0.5` corresponds to Nyquist frequency `f_N`.
Nyquist frequency is the maximum spatial frequency in an image scanned with a pixel size p [Å] and in pixels `f_N=1/2`, while in Å `f_N=1/(2p) [1/Å]`.
For the pixel size p [Å], spatial frequency `f_s` [1/Å] is related to absolute frequency `f_a` by:
For an image size n pixels sampled at p Å, k'th Fourier pixel (0<=k<=n/2) is related to frequency by:
Resolution r [Å] (defined as inverse of spatial frequency) is:
For examples of code in which Fourier pixels are handled check sparx/filter.cpp.