FFTW Real to Complex of Multidimensional data

[Raivyn]

I am trying to utilize the multidimensional FFT fftwf_plan_dft_r2c_2d from a single array of data. I have a M data points, N number of times:

float *input = (float*)malloc( M * N * sizeof( float ) );
// load M*N data points data
fftwf_complex *outputFFT = (fftwf_complex*)fftwf_malloc( N * ((M/2) + 1) * sizeof( fftwf_complex ) );
fftwf_plan forwardFFTPlan = fftwf_plan_dft_r2c_2d( N, M, input, outputFFT, FFTW_ESTIMATE );

fftwf_execute( forwardFFTPlan );
fftwf_destroy_plan( forwardFFTPlan );

// Plot M data points, ignore the rest. Plotting magnitude of the data sqrtf( ([REAL] * [REAL]) + ([IMAG] * [IMAG]) ) 

The FFT results are not correct, but if I just take the FFT of M data points

for( int i = 0; i < N; i++ )
{
   float *input = (float*)malloc( M * sizeof( float ) );
   // load M data points
   fftwf_complex *outputFFT = (fftwf_complex*)fftwf_malloc( ((M/2) + 1) * sizeof( fftwf_complex ) );
   fftwf_plan forwardFFTPlan = fftwf_plan_dft_r2c_1d( M, input, outputFFT, FFTW_ESTIMATE );

   fftwf_execute( forwardFFTPlan );
   fftwf_destroy_plan( forwardFFTPlan );
}
// Plot M data points, ignore the rest. Plotting magnitude of the data sqrtf( ([REAL] * [REAL]) + ([IMAG] * [IMAG]) ) 

the result is correct. The data being loaded into the array is the same. What am I not understanding about multi-dimensional FFTs? I read through the help pages and I "thought" I was doing it right, but obviously I am missing something...

[Raivyn]

I was using the wrong plan: needed to use fftwf_plan_many_dft_r2c instead of a 2d. I missread the documentation that I was doing a 2 dimensional FFT (FFT rows then columns) instead of a one dimensional FFT across multiple "rows" of data.

[qZanity]

Out of curiosity which FFT library are you using? As I have a audio matching program I must develop and I think FFT would be the way to go as there only small audio files

[Raivyn]

FFTW 3.3 (www.fftw.org)