Databases
StackOverflow2013
Postsdbo
POFFTW: Inverse of forward fft not equal to original function
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POFFTW: Inverse of forward fft not equal to original function
    primarykey
    data
    text
    <p>I'm trying to use FFTW to compute fast summations, and I've run into an issue: </p> <pre><code>int numFreq3 = numFreq*numFreq*numFreq; FFTW_complex* dummy_sq_fft = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_complex* dummy_sq = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_complex* orig = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_plan dummyPlan = FFTW_plan_dft_3d( numFreq, numFreq, numFreq, orig, dummy_sq_fft, FFTW_FORWARD, FFTW_MEASURE ); FFTW_plan dummyInvPlan = FFTW_plan_dft_3d( numFreq, numFreq, numFreq, dummy_sq_fft, dummy_sq, FFTW_BACKWARD, FFTW_MEASURE ); for(int i= 0; i &lt; numFreq3; i++) { orig[ i ][ 0 ] = sparseProfile02[ 0 ][ i ][ 0 ]; //img. part == 0 orig[ i ][ 1 ] = sparseProfile02[ 0 ][ i ] [ 1 ]; } FFTW_execute(dummyPlan); FFTW_execute(dummyInvPlan); int count = 0; for(int i=0; i&lt;numFreq3; i++) { double factor = dummy_sq[ i ][ 0 ]/sparseProfile02[ 0 ][ i ][ 0 ]; if(factor &lt; 0) { count++; } } std::cout&lt;&lt;"Count "&lt;&lt;count&lt;&lt;"\n"; FFTW_free(dummy_sq_fft); FFTW_free(dummy_sq); FFTW_destroy_plan(dummyPlan); FFTW_destroy_plan(dummyInvPlan); </code></pre> <p>(Here sparseProfile02[0] is of type FFTW_complex*, and contains only positive real data.)</p> <p>Since we have dummy_sq = IFFT(FFT(sparseProfile02[ 0 ])), we must have dummy_sq = n^3*sparseProfile02. But this is true only some of the time; in fact, values on the dummy_sq grid are negative whenever corresponding values on the sparseProfile02 grid are zero (but not vice-versa). Does anyone know why this is happening?</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USdmckee
    1. USuser448788
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POFFTW: Inverse of forward fft not equal to original function
      1. USBruno
      1. VTFavorite
    2. VO
      singulars
      1. POFFTW: Inverse of forward fft not equal to original function
      1. USKizaru
      1. VTFavorite
    3. VO
      singulars
      1. POFFTW: Inverse of forward fft not equal to original function
      1. This table or related slice is empty.
      1. VTUpMod
    1. COTake note that there is a difference between quote formatting (using ">") and code formatting (using a four space indent). See also http://stackoverflow.com/editing-help
      singulars
      1. POFFTW: Inverse of forward fft not equal to original function
      1. USdmckee
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload