Inverse complex FFTW transform

Question

I've been scanning all of the FFTW documentation, trying to figure out how to inverse FFT my FFT spectrum.

The documentation only mentions how to inverse FFT real-to-complex transformations, using the opposite c2r transform.

There are no FFTW_INVERSE flags anywhere but I suspect that the FFTW_FORWARD/BACKWARD transform might be what I need.

i.e. FORWARD for FFT and BACKWARD for IFFT. The documentation speaks about it flipping the sign; I remember something similar happening with the IFFT.

Is the backwards transform just an IFFT (for forward transform)? If not, how do you achieve an IFFT?

Answer 1

Is the backwards transform just an IFFT (for forward transform)?

Yes.

Compare the "what FFTW actually computes", here

Answer 2

FFTW is a nice library.

Real forward, real backward, complex forward, complex backward.

Refill the input arrays when executing a new plan. Works for me, see documentation via the link graciously supplied by MM for more details.

Extracted from working code:

//--- Have FFTW do the DFT via a FFTW Plan

        fftw_plan thePlan;

/*

//--- Real DFT

        thePlan = fftw_plan_dft_r2c_1d( theSignalN,        
                                        theSignalArray, 
                                        theDftArray, 
                                        FFTW_ESTIMATE );   

//--- Real Inverse DFT

        thePlan = fftw_plan_dft_c2r_1d( theSignalN, 
                                        theDftArray, 
                                        theSignalArray, 
                                        FFTW_ESTIMATE );

//--- Complex DFT
                                        
        thePlan = fftw_plan_dft_1d( theSignalN, 
                                    theSignalArray, 
                                    theDftArray, 
                                    FFTW_FORWARD, 
                                    FFTW_ESTIMATE );
                                    
*/
//--- Comples Inverse DFT

        thePlan = fftw_plan_dft_1d( theSignalN, 
                                    theDftArray, 
                                    theSignalArray, 
                                    FFTW_BACKWARD, 
                                    FFTW_ESTIMATE );

        fftw_execute( thePlan );
        fftw_destroy_plan( thePlan );