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;
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);
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 <complex> 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 with
consecutive real  and imaginary  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<double> *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)
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 :)
PS: Subversion migration? :P
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