[gmx-developers] gmx_fft_2d_real() output size

[Erik Lindahl]

Hi Peter,

All our FFTs employ the same standard output formats as used by e.g. FFTW
and and MKL, so for the full documentation it's probably easiest to read
there.

Short story: All values are stored as proper real+complex parts (i.e., 2
real values), so depending on input size the output array might be larger.
For in-place R2C transforms that usually means padding (but not
out-of-place ones).

Cheers,

Erik

On Sat, Aug 10, 2019 at 10:36 AM Peter Kasson <kasson at virginia.edu> wrote:

> Hi developers,
>   The doxygen docs are a little ambiguous on gmx_fft_2d_real(), and the
> source doesn't offer much clarification because we're wrapping external
> libraries.  Given a real-to-complex 2D FFT on a grid of size x by y, is the
> output a complex grid of size x by y also?  There are comments about
> in-place FFT (which I'm not doing) taking a 5*4 real array and writing a
> 5*2 complex array.
>
> (The context of this is trying to compute power spectra for an analysis
> routine.)
>
> Thanks,
> --Peter
>
> ----------------------------------------------------------------------
> Peter Kasson, MD, PhD
> Associate Professor
> Departments of Molecular Physiology and Biological Physics
> and of Biomedical Engineering
> University of Virginia
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-- 
Erik Lindahl <erik.lindahl at dbb.su.se>
Professor of Biophysics, Dept. Biochemistry & Biophysics, Stockholm
University
Science for Life Laboratory, Box 1031, 17121 Solna, Sweden
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