[Ifeffit] FFT in Ifeffit

[Matt Newville]

Hi Scott, 

All Fourier transforms in Ifeffit use arrays of length 2048,
padding with zeros as needed.

In earlier versions of Feffit, doing repeated FTs in the fitting
loop was noticeably slow.  This is probably one of the reasons
fitting in R-space was uncommon.  Anyway, using arrays of length
N_fft = 1024, 512, or 256 definitely helped speed things up.

In principle, one could use N_fft ~= N_idp, but it turns out that
if you use too few FT points, you can easily get into a real
'false, local, minima'. So Feffit used 1024,512,or 256 for fits,
and then go back to 2048 to write out the data. Strictly speaking,
the FFT doesn't need to use an array size that is a power of 2,
but it turns out to make a very noticeable difference with the FFT
routines used.

With Ifeffit, several "anti-optimizations"  were made that make
the fits slightly slower but be more straightforward and simple
(relatively speaking, anyway!).  The use of double precision and
N_fft=2048 being the main examples.  Part of the reason for
sticking with 2048 is that you want to be able to view the data at
any point in the process so 'pretty output' is always needed.  
Also, I doubt many people will be trying to run artemis on a 386
or a microVAX.

--Matt

> As I was working on my talk for the EXAFS workshop, I came across a 
> rather technical question: how does Ifeffit choose the number of 
> points for the Fourier transforms? As I recall, FEFFIT used an 
> algorithm which required an integer power of 2, so its default 
> behavior was to use the smallest integer power of 2 greater than the 
> number of points in the chi(k) data, and then pad the rest with 0's. 
> This default behavior could also be over-ridden if desired.
> 
> It doesn't look to me like Ifeffit is handling that in the same 
> way--is it always using a very large number of points and padding 
> with 0's, or is it using an algorithm that doesn't require the 
> integer power of 2, or is it interpolating the chi(k) on to a grid of 
> the appropriate size first, or...?
> 
> I appreciate any info you can give me on this.
> 
> --Scott Calvin
> Sarah Lawrence College
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>