[Scilab-users] Re : fftshift and ifftshift are way too different

[sgougeon at free.fr]

Hello Antoine,

fttshift() switches all halves along all dimensions. This is mandatory.
So for instance in 2D, opposite quadrants (wrt the center) are switched.
In 3D, opposite cubes are switched. etc.

To me, the current implementation is right:

--> m=grand(3,5,"uin",0,9)
 m  = 
   2.   5.   9.   3.   0.
   2.   4.   5.   5.   6.
   4.   1.   8.   5.   9.

--> fftshift(m)
 ans  =
   5.   9.   4.   1.   8.
   3.   0.   2.   5.   9.
   5.   6.   2.   4.   5.

--> ifftshift(fftshift(m))
 ans  =
   2.   5.   9.   3.   0.
   2.   4.   5.   5.   6.
   4.   1.   8.   5.   9.

--> and(ifftshift(fftshift(m))==m)
 ans  =
  T

BR
Samuel

----- Mail d'origine -----
De: antoine monmayrant+scilab <antoine.monmayrant+scilab at laas.fr>
À: International users mailing list for Scilab. <users at lists.scilab.org>, List dedicated to development questions <dev at lists.scilab.org>
Envoyé: Tue, 09 Oct 2018 11:02:20 +0200 (CEST)
Objet: [Scilab-users] fftshift and ifftshift are way too different

Hello all,

 From what I understand of fast Fourier transforms, fftshift and 
ifftshift should be almost identical.
The only difference is when there are an odd number of elements in the 
dimension along which the shift is performed (ie 
fftshift([1:4])==ifftshift([1:4]) but fftshift([1:5])!=ifftshift([1:5])).
However, it seems that in scilab6.x fftshift and ifftshift are based on 
completely different codes and do not accept the same arguments.
In particular, with fftshift, one can specify the dimension along which 
to perform the shift, while it is not the case for ifftshift.
I think some update of ifftshift would be welcome.
I propose to use the code below (myifftshift), where I just changed 
"ceil" by "floor" in the definition if fftshift.
What do you think, does it look right to you?

Cheers,

Antoine

function x = myifftshift(x,job)
     if argn(2)<2 then job="all",end
     deff("sel=fun(sk)","c=floor(sk/2);sel=[c+1:sk,1:c]")
     if job=="r" then job=1,elseif job=="c" then job=2,end
     ind=list()
     if job=="all" then
         for sk=size(x),ind($+1)=fun(sk),end
     else
         for sk=size(x),ind($+1)=:,end;
         ind(job)=fun(size(x,job))
     end
     x=x(ind(:))
endfunction