c++ - fftw shift zero-frequency to image center -


i struggling on result of fftw applied on image.

i can fourier transform , inverse fourier transform on image, sure 2 functions working. trying analyze result of fourier transform , try apply filter on it, etc.

currently confusing on ft result. know the zero-frequency should on top-left corner. should shift image center if want check frequency friendly. there @ least 2 way have tested. 1 method described on fftw faq webpage, pixels in image multiplied (-1)^(i + j) before fourier transform, i, , j pixel's index.

image multiplied (-1)^(i+j), magnitude is

umg

but ,it should (which did in imagej) fourier transform in imagej:

img

i tried using code switch them after calculating magnitude, got similar wrong result.

void ftshift2d(double * d, int w, int h) {      int nw = w/2;     int nh = h/2;     if ( w%2 != 0) {nw++;}     if ( h%2 != 0) {nh++;}          printf("%d, %d\n", nw, nh);         (int = 0; < nh ; i++)         {                 for(int j = 0; j < nw; j++)                 {                         int t1 = * w + j;                         int t2 = (i + nh + 1) * w + j + nw;                         swapxy<double>(d[t1], d[t2]);                 }                  (int k = nw; k < w; k++)                 {                         int t1 =  * w + k;                         int t2 = (i + nh　＋ 1 ) * w + k - nw;                         swapxy<double>(d[t1], d[t2]);                 }         } }

i understood layout of ft result in fftw, described in website. checked output, layout can confirmed, top half have data, rest 0. therefore, seems explain why 2 methods used above cannot work. trying design new method shift zero-frequency image center. give me better way it?

i insist on this, aim use filter, easier understand. if zero-frequency did not shifted center, guys give me suggestion can, example, apply high gaussian filter on in frequency domain.

thanks lot.

the input image is:

img

the ft , ift code are

    void imgr2c(double * d, fftw_complex * df, int w, int h) {         fftw_plan p = fftw_plan_dft_r2c_2d(h, w, d, df, fftw_estimate);         fftw_execute(p);          fftw_destroy_plan(p);  }   void imgc2r(fftw_complex * in, double * ftreverse, int w, int h) {         fftw_plan p = fftw_plan_dft_c2r_2d(h, w, in, ftreverse, fftw_estimate);         fftw_execute(p);          int l = w * h;         (int = 0; < l; i++)         {                 ftreverse[i] /= l;         }          fftw_destroy_plan(p); }

[edit spektre]

this how should more or less like:

img

you may want check out how did here: https://github.com/kvahed/codeare/blob/master/src/matrix/ft/dft.hpp functions doing job @ bottom. if there issues, feel free contact me personally.


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