[petsc-users] Efficient FFTShift-implementation for vectors/matrices

[Barry Smith]

  I found a variety of things on the web, below. I don't understand this but for the even case it seems one simply modifies the input matrix before the FFT http://www.fftw.org/faq/section3.html#centerorigin

  https://stackoverflow.com/questions/5915125/fftshift-ifftshift-c-c-source-code <https://stackoverflow.com/questions/5915125/fftshift-ifftshift-c-c-source-code>

  https://www.dsprelated.com/showthread/comp.dsp/20790-1.php <https://www.dsprelated.com/showthread/comp.dsp/20790-1.php>



> On May 18, 2021, at 9:48 AM, Roland Richter <roland.richter at ntnu.no> wrote:
> 
> Dear all,
> 
> I tried to implement the function fftshift from numpy (i.e. swap the
> half-spaces of all axis) for row vectors in a matrix by using the
> following code
> 
> void fft_shift(Mat &fft_matrix) {
>         PetscScalar *mat_ptr;
>         MatDenseGetArray (fft_matrix, &mat_ptr);
>         PetscInt r_0, r_1;
>         MatGetOwnershipRange(fft_matrix, &r_0, &r_1);
>         PetscInt local_row_num = r_1 - r_0;
>         arma::cx_mat temp_mat(local_row_num, Ntime, arma::fill::zeros);
>         for(int i = 0; i < Ntime; ++i) {
>             const PetscInt row_shift = i * local_row_num;
>             for(int j = 0; j < local_row_num; ++j) {
>                 const PetscInt cur_pos = j + row_shift;
>                 if(i < (int)(Ntime / 2))
>                     temp_mat(j, i + int(Ntime / 2)) = *(mat_ptr + cur_pos);
>                 else
>                     temp_mat(j, i - int(Ntime / 2)) = *(mat_ptr + cur_pos);
>             }
>         }
>         for(int i = 0; i < Ntime; ++i) {
>             const PetscInt row_shift = i * local_row_num;
>             for(int j = 0; j < local_row_num; ++j) {
>                 const PetscInt cur_pos = j + row_shift;
>                 *(mat_ptr + cur_pos) = temp_mat(j, i);
>             }
>         }
>         MatDenseRestoreArray (fft_matrix, &mat_ptr);
> }
> 
> but I do not like the approach of having a second matrix as temporary
> storage space. Are there more efficient approaches possible using
> PETSc-functions?
> 
> Thanks!
> 
> Regards,
> 
> Roland Richter
> 

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