Reply by divy...@gmail.com●March 29, 20102010-03-29

On Mar 29, 2:50�pm, Kamran <kam...@uio.no> wrote:

> Hi,
> Sorry if this is not the right forum but, I didn't know who else to turn to.
> I have a quite simple process. I use FFTW library to take a forward fft
> of a one dimensional signal and then do some multiplication in freq.
> domain and then a backward (inverse) fft. �(called in a C++ code).
>
>
> Kamran

Great that you could figure that out.
Just to help others who might be having issues, here's link to my
CFFTWWrapper class (includes usage code as well) that uses FFTW. Made
it as a plugin for Image Apprentice couple of years ago.
http://code.google.com/p/cfftwwrapper/
best regards,
Divya Rathore
www.adislindia.com

Reply by Kamran●March 29, 20102010-03-29

Sorry guys, disregard the whole thing. Id forgotten to normalize after
taking inverse. stupid.
Kamran wrote:

>
> Hi,
> Sorry if this is not the right forum but, I didn't know who else to turn
> to.
> I have a quite simple process. I use FFTW library to take a forward fft
> of a one dimensional signal and then do some multiplication in freq.
> domain and then a backward (inverse) fft. (called in a C++ code).
> I check the result with matlab (as I understand matlab uses the same
> library). The forward result is identical in both matlab and my C++
> code, but the backward fft is completely off mark, this is the relevant
> code: (I use the number of samples in the signal for number of frequency
> samples, nfft)
>
> fftwf_complex* m_array;
>
> for (int i= 0; i < ns; i++)
> m_array[i][0] = some_values;
> m_array[i][1] = some_other_values;
> }
>
> // take forward fft
> fftwf_plan p = 0;
> p = fftwf_plan_dft_1d(size, (fftwf_complex*)
> &m_array[0][0],(fftwf_complex*) &m_array[0][0],FFTW_FORWARD,
> FFTW_ESTIMATE);
> fftwf_execute(p);
> fftwf_destroy_plan(p);
>
>
> // m_array now contains correct coefficients
>
> // take backward fft, this should get me back to my original data
> // but it doesn't
>
> fftwf_plan p = 0;
> p = fftwf_plan_dft_1d(size, (fftwf_complex*)
> &m_array[0][0],(fftwf_complex*) &m_array[0][0],FFTW_BACKWARD,
> FFTW_ESTIMATE);
> fftwf_execute(p);
> fftwf_destroy_plan(p);
>
> // m_array is now filled with rubbish
>
> appreciate any help,
>
> Kamran

Reply by Kamran●March 29, 20102010-03-29

Hi,
Sorry if this is not the right forum but, I didn't know who else to turn to.
I have a quite simple process. I use FFTW library to take a forward fft
of a one dimensional signal and then do some multiplication in freq.
domain and then a backward (inverse) fft. (called in a C++ code).
I check the result with matlab (as I understand matlab uses the same
library). The forward result is identical in both matlab and my C++
code, but the backward fft is completely off mark, this is the relevant
code: (I use the number of samples in the signal for number of frequency
samples, nfft)
fftwf_complex* m_array;
for (int i= 0; i < ns; i++)
m_array[i][0] = some_values;
m_array[i][1] = some_other_values;
}
// take forward fft
fftwf_plan p = 0;
p = fftwf_plan_dft_1d(size, (fftwf_complex*)
&m_array[0][0],(fftwf_complex*) &m_array[0][0],FFTW_FORWARD, FFTW_ESTIMATE);
fftwf_execute(p);
fftwf_destroy_plan(p);
// m_array now contains correct coefficients
// take backward fft, this should get me back to my original data
// but it doesn't
fftwf_plan p = 0;
p = fftwf_plan_dft_1d(size, (fftwf_complex*)
&m_array[0][0],(fftwf_complex*) &m_array[0][0],FFTW_BACKWARD,
FFTW_ESTIMATE);
fftwf_execute(p);
fftwf_destroy_plan(p);
// m_array is now filled with rubbish
appreciate any help,
Kamran