Hello, I'm having some trouble to use FFTw library... I'm using two plans like this: fftw_plan fftw_plan_r2r_1d(SIZE_OF_IN, in, out, R2HC, FFTW_FORWARD); fftw_plan fftw_plan_r2r_1d(2*SIZE_OF_IN +2, out, originalInput, HC2R, FFTW_BACKWARD); If i'm not wrong, i should be getting, after executing those two plans, the same as 'in' array, in 'originalInput'. But all i get is noise. What's wrong ???. I know for sure it's something really easy... but i can't figure it out..... Thank you!!!
FFTw Forward/Backwards :s
Started by ●April 16, 2007
Reply by ●April 16, 20072007-04-16
On Apr 16, 9:26 am, "Jorge" <jleandrope...@gmail.com> wrote:> fftw_plan fftw_plan_r2r_1d(SIZE_OF_IN, in, out, R2HC, FFTW_FORWARD); > > fftw_plan fftw_plan_r2r_1d(2*SIZE_OF_IN +2, out, originalInput, HC2R, > FFTW_BACKWARD); > > If i'm not wrong, i should be getting, after executing those two > plans, the same as 'in' array, in 'originalInput'. But all i get is > noise.You should be passing the same size (SIZE_OF_IN) for creating both plans. This is the order of the transform to be computed, regardless of the format of the data. Note also that both in[] and out[] should be arrays of length SIZE_OF_IN. Regards, Steven G. Johnson PS. After you do that, the result should be the original data scaled by SIZE_OF_IN, since FFTW computes an unnormalized DFT.
Reply by ●April 16, 20072007-04-16
Jorge wrote:> I'm having some trouble to use FFTw library... > > fftw_plan fftw_plan_r2r_1d(SIZE_OF_IN, in, out, R2HC, FFTW_FORWARD); > > [...] all i get is noise.Yous should give the FFTW_ESTIMATE flag, otherwise your array contents may get overwritten by garbage during planning. Hendrik vdH
Reply by ●April 16, 20072007-04-16
Thanks you both... but i'm still having trouble. Basically, my code is like this (simplified,of course) #define SIZE_OF_IN 32 forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, FFTW_FORWARD | FFTW_ESTIMATE ); backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, FFTW_BACKWARD | FFTW_ESTIMATE ); fftwf_execute(forward); fftwf_execute(backwards); I get WAY too much noise. *ANY* idea will be very welcome. Thanks a lot!! regards On 16 abr, 15:23, Hendrik van der Heijden <h...@gmx.de> wrote:> Jorge wrote: > > I'm having some trouble to use FFTw library... > > > fftw_plan fftw_plan_r2r_1d(SIZE_OF_IN, in, out, R2HC, FFTW_FORWARD); > > > [...] all i get is noise. > > Yous should give the FFTW_ESTIMATE flag, otherwise your array > contents may get overwritten by garbage during planning. > > Hendrik vdH
Reply by ●April 16, 20072007-04-16
SOLVED !!!!! "Recall that these transforms are unnormalized, so r2hc followed by hc2r will result in the original data multiplied by n".... so... all i had to do is divide every element of 'out2' array (ouput of forward-backwards) by SIZE_OF_IN. Thanks! On 16 abr, 19:01, "Jorge" <jleandrope...@gmail.com> wrote:> Thanks you both... but i'm still having trouble. Basically, my code is > like this (simplified,of course) > > #define SIZE_OF_IN 32 > > forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, > FFTW_FORWARD | FFTW_ESTIMATE ); > backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, > FFTW_BACKWARD | FFTW_ESTIMATE ); > > fftwf_execute(forward); > fftwf_execute(backwards); > > I get WAY too much noise. *ANY* idea will be very welcome. > > Thanks a lot!! > regards > > On 16 abr, 15:23, Hendrik van der Heijden <h...@gmx.de> wrote: > > > > > Jorge wrote: > > > I'm having some trouble to use FFTw library... > > > > fftw_plan fftw_plan_r2r_1d(SIZE_OF_IN, in, out, R2HC, FFTW_FORWARD); > > > > [...] all i get is noise. > > > Yous should give the FFTW_ESTIMATE flag, otherwise your array > > contents may get overwritten by garbage during planning. > > > Hendrik vdH- Ocultar texto de la cita - > > - Mostrar texto de la cita -
Reply by ●April 16, 20072007-04-16
On Apr 16, 6:01 pm, "Jorge" <jleandrope...@gmail.com> wrote:> forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, > FFTW_FORWARD | FFTW_ESTIMATE ); > backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, > FFTW_BACKWARD | FFTW_ESTIMATE );You shouldn't be passing FFTW_FORWARD or FFTW_BACKWARD, either. These do not belong in the "flags" argument. The sign of the transform is specified by the 4th argument (FFTW_R2HC == forward, FFTW_HC2R == backward). i.e. do: forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, FFTW_ESTIMATE ); backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, FFTW_ESTIMATE ); Please also note the normalization that I mentioned before. Upon executing the two plans in sequence, out2[i] will be in[i] * SIZE_OF_IN, plus a small relative error on the order of the floating- point precision (1e-7 or so in single precision). Steven
Reply by ●April 16, 20072007-04-16
Thank you very much Steven!!! Regards On 16 abr, 19:25, "Steven G. Johnson" <stev...@alum.mit.edu> wrote:> On Apr 16, 6:01 pm, "Jorge" <jleandrope...@gmail.com> wrote: > > > forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, > > FFTW_FORWARD | FFTW_ESTIMATE ); > > backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, > > FFTW_BACKWARD | FFTW_ESTIMATE ); > > You shouldn't be passing FFTW_FORWARD or FFTW_BACKWARD, either. These > do not belong in the "flags" argument. The sign of the transform is > specified by the 4th argument (FFTW_R2HC == forward, FFTW_HC2R == > backward). i.e. do: > > forward = fftwf_plan_r2r_1d(SIZE_OF_IN, in, out, FFTW_R2HC, > FFTW_ESTIMATE ); > backwards = fftwf_plan_r2r_1d(SIZE_OF_IN, out, out2, FFTW_HC2R, > FFTW_ESTIMATE ); > > Please also note the normalization that I mentioned before. Upon > executing the two plans in sequence, out2[i] will be in[i] * > SIZE_OF_IN, plus a small relative error on the order of the floating- > point precision (1e-7 or so in single precision). > > Steven
