:orphan:
.. _ahelp_3dDFT:
*****
3dDFT
*****
.. contents:: :local:

.. codeblock:: none
Usage: 3dDFT [options] dataset
where 'dataset' is complex or floatvalued.
* Carries out the DFT along the time axis.
* To do the DFT along the spatial axes, use program 3dFFT.
* The input dataset can be complexvalued or floatvalued.
If it is any other data type, it will be converted to floats
before processing.
* [June 2018] The FFT length used is NOT rounded up to a convenient
FFT radix; instead, the FFT size is actual value supplied in option
'nfft' or the number of time points (if 'nfft' isn't used).
* However, if the FFT length has large prime factors (say > 97), the
Fast Fourier Transform algorithm will be relatively slow. This slowdown
is probably only noticeable for very long files, since reading and
writing datasets seems to take most of the elapsed time in 'normal' cases.
OPTIONS:

prefix PP == use 'PP' as the prefix of the output file
abs == output float dataset = abs(DFT)
* Otherwise, the output file is complexvalued.
You can then use 3dcalc to extract the real part, the
imaginary part, the phase, etc.; see its 'cx2r' option:
3dcalc cx2r REAL a cxset+origexpr a prefix rset+orig
* Please note that if you view a complex dataset in AFNI,
the default operation is that you are looking at the
absolute value of the dataset.
++ You can control the way a complex IMAGE appears via
the 'Disp' control panel (ABS, PHASE, REAL, IMAGE).
++ You can control the way a complex TIME SERIES graph appears
via environment variable AFNI_GRAPH_CX2R (in 'EditEnv').
nfft N == use 'N' for DFT length (must be >= #time points)
detrend == leastsquares remove linear drift before DFT
[for more intricate detrending, use 3dDetrend first]
taper f == taper 'f' fraction of data at ends (0 <= f <= 1).
[Hamming 'raised cosine' taper of f/2 of the ]
[data length at each end; default is no taper]
[cf. 3dPeriodogam help for tapering details!]
inverse == Do the inverse DFT:
SUM{ data[j] * exp(+2*PI*i*j/nfft) } * 1/nfft
instead of the forward transform
SUM{ data[j] * exp(2*PI*i*j/nfft) }
++ Compile date = Jun 14 2021 {AFNI_21.1.12:linux_ubuntu_16_64}