Hi, I am trying to convert a FFT and IFFT Matlab code to C using the fftw libraries. Hoiwever, I am stuck at a particular step: The Matlab command is: G = ifftn(P21,'symmetric'); ow do I convert this statement to C using the fftw library. I have tried the following steps: 1.I used fftw_plan_dft_2d to compute the FFT for a double array of size nx by ny. the FFT values match the Matlab values. I also used fftw_plan_dft_2d(..,..,..,..,fftw_backward, fftw_estimate) to compute the IFFT. This also did not provide the desired results. 2. I then thought of using r2c and a c2r IFFT. This also gives me wrong results. Can somebody suggest an alternative?? Thanks in advance, Vivek

# IFFT conversion using FFTW libraries

Started by ●November 9, 2007

Reply by ●November 9, 20072007-11-09

On Nov 9, 7:48 am, "vivekv80" <vivek_...@yahoo.com> wrote:> 1.I used fftw_plan_dft_2d to compute the FFT for a double array of size nx > by ny. the FFT values match the Matlab values. I also used > fftw_plan_dft_2d(..,..,..,..,fftw_backward, fftw_estimate) to compute the > IFFT. This also did not provide the desired results.Matlab's ifft differs from FFTW's fftw_backward by a scale factor of 1/ N (N = # points = nx * ny). Regards, Steven G. Johnson

Reply by ●November 9, 20072007-11-09

>On Nov 9, 7:48 am, "vivekv80" <vivek_...@yahoo.com> wrote: >> 1.I used fftw_plan_dft_2d to compute the FFT for a double array of sizenx>> by ny. the FFT values match the Matlab values. I also used >> fftw_plan_dft_2d(..,..,..,..,fftw_backward, fftw_estimate) to computethe>> IFFT. This also did not provide the desired results. > >Matlab's ifft differs from FFTW's fftw_backward by a scale factor of 1/ >N (N = # points = nx * ny). > >Regards, >Steven G. Johnson > >Well as per Matlab's reference manual: I think I need to convert my FFT coeffs into conjugate symmetric before I can pass them to the FFTW library. Can someone provide pointers??

Reply by ●November 11, 20072007-11-11

vivekv80 wrote:>> On Nov 9, 7:48 am, "vivekv80" <vivek_...@yahoo.com> wrote: >>> 1.I used fftw_plan_dft_2d to compute the FFT for a double array of size > nx >>> by ny. the FFT values match the Matlab values. I also used >>> fftw_plan_dft_2d(..,..,..,..,fftw_backward, fftw_estimate) to compute > the >>> IFFT. This also did not provide the desired results. >> Matlab's ifft differs from FFTW's fftw_backward by a scale factor of 1/ >> N (N = # points = nx * ny). >> >> Regards, >> Steven G. Johnson >> >> > Well as per Matlab's reference manual: I think I need to convert my FFT > coeffs into conjugate symmetric before I can pass them to the FFTW > library. Can someone provide pointers??Yes. The FFTW manual: http://www.fftw.org/fftw3_doc/Multi_002dDimensional-DFTs-of-Real-Data.html I wrote a C++ wrapper for FFTW that understands various 2d (and 1d & 3d) matrix types. The idea was that this frontend would make programming a lot easier. There is a bit of example code there that demostrates the layout as well. http://sip.unige.ch/cvmlcpp/source/doc/Fourier.html http://sip.unige.ch/cvmlcpp/ Best, Fokko Beekhof

Reply by ●November 14, 20072007-11-14

>vivekv80 wrote: >>> On Nov 9, 7:48 am, "vivekv80" <vivek_...@yahoo.com> wrote: >>>> 1.I used fftw_plan_dft_2d to compute the FFT for a double array ofsize>> nx >>>> by ny. the FFT values match the Matlab values. I also used >>>> fftw_plan_dft_2d(..,..,..,..,fftw_backward, fftw_estimate) tocompute>> the >>>> IFFT. This also did not provide the desired results. >>> Matlab's ifft differs from FFTW's fftw_backward by a scale factor of1/>>> N (N = # points = nx * ny). >>> >>> Regards, >>> Steven G. Johnson >>> >>> >> Well as per Matlab's reference manual: I think I need to convert myFFT>> coeffs into conjugate symmetric before I can pass them to the FFTW >> library. Can someone provide pointers?? > >Yes. The FFTW manual: >http://www.fftw.org/fftw3_doc/Multi_002dDimensional-DFTs-of-Real-Data.html > >I wrote a C++ wrapper for FFTW that understands various 2d (and 1d & 3d) >matrix types. The idea was that this frontend would make programming a >lot easier. There is a bit of example code there that demostrates the >layout as well. >http://sip.unige.ch/cvmlcpp/source/doc/Fourier.html >http://sip.unige.ch/cvmlcpp/ > >Best, >Fokko Beekhof >Has someone worked on a C implementation of the conjugate symmetric conversion of the coefficients?? This way the coefficients match the values obtained by doing a IFFT(..., Symmetric) in Matlab Vivek