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Patrick Guio wrote:

I like your utilities idea. I also have some wrapper functions to read/write HDF4/5 files and use FFTW 1D transforms (in/out of place, real/complex). The Fits file format for multidimensional array might also be of interest to consider.

Might be somehow relevant - an example Blitz++/HDF5 combination code: https://github.com/igfuw/libmpdataxx/blob/master/libmpdata%2B%2B/output/hdf5.hpp

A relevant comment by "Shakes" (@shakes76) from SF (https://sourceforge.net/p/blitz/patches/2/):

The following is the code for getting FFT's on Blitz Arrays using the mature and highly recognised FFTW C Library.

template <typename type, int size>
void
FourierTransform<type,size>::FFTW(Array<complex<double>,2>
&field, Array<complex<double>,2> &fftOfField)
{
complex<double> *ptrField, *ptrFFT;
fftw_plan plan;

ptrField = field.data();
ptrFFT = fftOfField.data();
plan =
fftw_plan_dft_2d(field.rows(),field.cols(),reinterpret_cast<fftw_complex*>(ptrField),reinterpret_cast<fftw_complex*>(ptrFFT),FFTW_FORWARD,FFTW_ESTIMATE);

fftw_execute(plan);

fftw_destroy_plan(plan);
}

I have tested and its does work. Note that this works because as FFTW points out:

"C++ has its own complex<T> template class, defined in the standard header file. Reportedly, the C++ standards committee has recently agreed to mandate that the storage format used for this type be binary-compatible with the C99 type, i.e. an array T[2] with consecutive real [0] and imaginary [1] parts. (See report WG21/N1388.) Although not part of the official standard as of this writing, the proposal stated that: “This solution has been tested with all current major implementations of the standard library and shown to be working.”

To the extent that this is true, if you have a variable complex x, you can pass it directly to FFTW via reinterpret_cast<fftw_complex>(x)."

Also data reordering has to be done if DC is wanted in the centre (http://www.fftw.org/faq/section3.html#centerorigin). I have done it in the following:

void
FourierTransform<type,size>::centerDC(Array<complex<double>,2>
&field, Array<complex<double>,2> &result)
{
Array<complex<double>,2> tmp(field.rows(),field.cols());

for(int k = 0; k < field.cols(); k ++)
for(int j = 0; j < field.rows(); j ++)
tmp((j+field.rows()/2)%field.rows(),(k+field.cols()/2)%field.cols())
= field(j,k);

result = tmp;
}

Code Excerpt is taken from the FourierTransform Class in the up and coming Quantum Mechanics and Discrete Geometry Toolkit based on Blitz++ and Qt - qC++ (qcplusplus.sourceforge.net). It currently supports BEC simulations and the Fourier class can be found in the Subversion area.

Keep up the good work guys, more work on the tiny stuff would be appreciated :)

Cheers Shakes