FFT with FFTW Example

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FFTW is a collection of C routines for computing DFTs. It's very fast and freely available at http://www.fftw.org. Here's an example on how to compute the DFT of an array with FFTW and inverse the result back.

/*
 * fft.c: compute FFT and IFFT from an array
 */
#include <stdio.h>
#include <fftw3.h>

#define SIZE 4

int main( int argc, char** argv )
{
    double          input[SIZE] = { 1.0, 1.0, 1.0, 1.0 };
    fftw_complex    *data, *fft_result, *ifft_result;
    fftw_plan       plan_forward, plan_backward;
    int             i;
    
    data        = ( fftw_complex* ) fftw_malloc( sizeof( fftw_complex ) * SIZE );
    fft_result  = ( fftw_complex* ) fftw_malloc( sizeof( fftw_complex ) * SIZE );
    ifft_result = ( fftw_complex* ) fftw_malloc( sizeof( fftw_complex ) * SIZE );
    
    plan_forward  = fftw_plan_dft_1d( SIZE, data, fft_result, FFTW_FORWARD, FFTW_ESTIMATE );
    plan_backward = fftw_plan_dft_1d( SIZE, fft_result, ifft_result, FFTW_BACKWARD, FFTW_ESTIMATE );
    
    /* populate input data */
    for( i = 0 ; i < SIZE ; i++ ) {
        data[i][0] = input[i];
        data[i][1] = 0.0;
    }

    /* print initial data */
    for( i = 0 ; i < SIZE ; i++ ) {
        fprintf( stdout, "data[%d] = { %2.2f, %2.2f }\n",
                    i, data[i][0], data[i][1] );
    }
    
    fftw_execute( plan_forward );
    
    /* print fft result */
    for( i = 0 ; i < SIZE ; i++ ) {
        fprintf( stdout, "fft_result[%d] = { %2.2f, %2.2f }\n",
                    i, fft_result[i][0], fft_result[i][1] );
    }

    fftw_execute( plan_backward );

    /* print ifft result */
    for( i = 0 ; i < SIZE ; i++ ) {
        fprintf( stdout, "ifft_result[%d] = { %2.2f, %2.2f }\n",
                    i, ifft_result[i][0] / SIZE, ifft_result[i][1] / SIZE );
    }

    /* free memory */
    fftw_destroy_plan( plan_forward );
    fftw_destroy_plan( plan_backward );

    fftw_free( data );
    fftw_free( fft_result );
    fftw_free( ifft_result );
    
    return 0;
}

Keywords: using fftw, fftw examples, fft with fftw, fft in opencv

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