FFTW R2C two-dimensional size parameters

Question

I cannot get what size parameters are for fftwf_plan_dft_r2c_2d

• Input: a N rows by M cols matrix
• Output: a N rows by floor(M/2) + 1 cols matrix ?

Are parameters input or output size?

Tried to give input size. This is what GDB sais

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1676.0x768]
0x637eed72 in n1fv_8 () from C:\devfiles\bin\libfftw3f-3.dll
(gdb) backtrace
#0  0x637eed72 in n1fv_8 () from C:\devfiles\bin\libfftw3f-3.dll
#1  0x7c91a000 in ntdll!RtlpUnWaitCriticalSection ()
   from C:\WINDOWS\system32\ntdll.dll

No other thread uses fftw at the time.

The context:

Herbs::MatrixStorage<float> spectrum_in(frame_a.nRowsGet(),frame_a.nColsGet());
Herbs::MatrixStorage<std::complex<float>> 
    spectrum_out(frame_a.nRowsGet(),frame_a.nColsGet()/2+1);

Check for heap corruption issued by possible alllocation mistakes in MatrixStorage class . The rows in the matrix is one large block. rowGet returns the pointer to the given row.

memset(spectrum_in.rowGet(0),0
    ,sizeof(float)*spectrum_in.nRowsGet()*spectrum_in.nColsGet());
memset(spectrum_out.rowGet(0),0
    ,sizeof(float)*spectrum_out.nRowsGet()*spectrum_out.nColsGet());
heapdump(); //Heap seams to be fine after these

Causes sigsevg

FFT::PlanFloat_2dR2C plan(spectrum_in,spectrum_out);

The plan constructor does the following

plan=FFT::PlanFloat_2dR2C::PlanFloat_2dR2C(Herbs::MatrixStorage<InputType>& buffer_in
,Herbs::MatrixStorage<OutputType>& buffer_out)
{
plan=fftwf_plan_dft_r2c_2d
    (
     buffer_in.nRowsGet()
    ,buffer_in.nColsGet()
    ,buffer_in.rowGet(0)
    ,(fftwf_complex*)buffer_out.rowGet(0)
    ,FFTW_MEASURE
    );
}

EDIT:

I used a precompiled DLL instead. GCC may produce bad code on 32-bit Windows (From release notes):

Removed an archaic stack-alignment hack that was failing with gcc-4.7/i386. Added stack-alignment hack necessary for gcc on Windows/i386. We will regret this in ten years (see previous change).

EDIT 2:

The dll became bad due to wrong options given to the configure script. The documentation is now updated.

Answer 1

You are correct. For an NxM float or double input you should allocate Nx(M/2+1) fftwf_complex or fftw_complex output.

The parameters are the input size. For example,

#include "fftw3.h"

#define Width 1024
#define Height 768

int main(){
        float input[Width*Height];
        fftwf_complex *fft = new fftwf_complex[((Width/2)+1)*Height];
        fftwf_plan fplan = fftwf_plan_dft_r2c_2d(Height, Width,
                                 (float*)input, fft, FFTW_ESTIMATE);
        fftwf_execute(fplan);
        fftwf_destroy_plan(fplan);
}

See Chapter 4.3 Basic Interface of the FFTW User Manual