[SciPy-Dev] Re FFTs in SciPy (and NumPy)

Charles R Harris charlesr.harris@gmail....
Tue Jul 13 07:54:00 CDT 2010

On Mon, Jul 12, 2010 at 11:45 PM, Sturla Molden <sturla@molden.no> wrote:

> There has been some discussion on FFTPACK lately. Problems with FFTPACK
> seems to be:
> - Written in old Fortran 77.
> - Unprecise for single precision.
> - Can sometimes be very slow, depending on input size.
> - Can only handle a few small prime factors {2,3,4,5} efficiently.
> - How to control integer size portably for 64-bit and f2py?
> - Only 1D transforms.
> There is another FFT library in cwplib from Colorado Mining School. It
> has some very nice properties:
> - Written in C, not Fortran, and the FFT-part of the library is just one
> single C file.
> - License is BSD it seems.
> - Quite fast (see benchmarks in http://www.fftw.org/fftw-paper.pdf and
> http://www.fftw.org/speed/)
> - Can handle larger set of prime factors than FFTPACK
> {2,3,4,5,7,8,9,11,13,16}.
> - 1D and 2D inplace transforms.
> The problem with cwplib is that it does not allow arbitrary sized FFTs.
> But til will be fast whenever FFTPACK is fast (it seems FFTPACK is only
> competitive for N=2**k case). It will also be fast for other
> factorizations where FFTPACK will fallback to O(N**2).
> How to handle arbitrary sized arrays with cwplib? One possibility is
> using Bluestein's FFT algorithm, which just requires a few lines og
> Python. This was even suggested to avoid O(N**2) fallback in FFTPACK
> here a while ago. If we are going to do that for FFTPACK, the cwplib's
> FFT will actually be more flexible with respect to array size.
> All in all I think it is worth looking at.
> The C code is in the "Seismic U*nx" download from CWP, which is huge,
> but contains a lot of nice C library functions for scientific computing
> apart from FFTs as well (all of which are BSD licensed it seems).
> http://www.cwp.mines.edu/cwpcodes/index.html
Looks interesting.

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