将Armadillo与FFTW连接 [英] Interfacing Armadillo with FFTW

问题描述

我正在尝试在Armadillo软件包的矩阵中使用FFTW.我需要在2D矩阵上执行N个独立的FFT.遵循FFTW和其他在线资源的手册，我有以下代码:

I am trying to use FFTW in the matrix of Armadillo package. I need to perform N independent FFT on a 2D matrix. Following the manual of FFTW and other resource online, I have the following code:

#include <armadillo>
#include <iostream>
#include "fftw3.h"

using namespace std;
using namespace arma;

fftw_plan fplan;

int main(int argc, char *argv[])
{
  cx_mat psi;
  int NCOL=2, NROW=4;
  int frank=1;
  int howmany=NCOL;
  int n[]={NROW};         
  int idist=NROW, odist=NROW; 
  int istride=1, ostride=1;
  int *inembed=n, *onembed=n;

  psi.resize(NROW, NCOL);
  psi(0,0)=cx_double(1,1); psi(0,1)=cx_double(1,1);
  psi(1,0)=cx_double(2,1); psi(1,1)=cx_double(1,2);
  psi(2,0)=cx_double(3,1); psi(2,1)=cx_double(1,3);
  psi(3,0)=cx_double(4,1); psi(3,1)=cx_double(1,4);
  cout << psi << endl; // the output is correct at here

  fftw_complex* in = reinterpret_cast<fftw_complex*>(psi.memptr());

  fplan = fftw_plan_many_dft(frank, n, howmany,
                             in, inembed, istride, idist,
                             in, onembed, ostride, odist,
                             FFTW_FORWARD, FFTW_MEASURE);

  cout << endl << "after fftw plan: " << endl << psi << endl; // all zeros

  // fftw_execute(fplan); // output zeros again if execute
  cx_double* A_mem=psi.memptr();
  cout << endl << "by reference: " << endl << *A_mem << " " << *(A_mem+1) << endl;

  return 0;
}

代码编译没有任何错误.但是，如果我运行代码，则在fftw_plan_many_dft之后，它将输出全零.如果删除fftw_plan_many_dft，则输出正确.我什至没有执行fftw计划，那么为什么只通过设置计划来清理我的数据呢?

The code compiles without any error. But if I run the code, after fftw_plan_many_dft, it outputs all zeroes instead. If I remove fftw_plan_many_dft, the output is correct. I don't even execute the fftw plan so why it clean my data by just setting up the plan?

推荐答案

通过 FFTW_MEASURE 标志，缓冲区实际上用于调整FFT算法.您应该首先创建计划，然后初始化输入数据.

With the FFTW_MEASURE flag the buffers are actually used to tune the FFT algorithm. You should create the plan first, and then initialise the input data.

请注意， FFTW_ESTIMATE 不会没有表现出这种行为，因此您也可以在当前代码中更改此标志以解决此问题，但是您的代码可能会运行慢一点.

Note that FFTW_ESTIMATE does not exhibit this behaviour, so you could also just change this flag in your current code to fix the problem, but then your code would probably run slower.

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