Add 2D spectrum output

[otto-link]

libfftw3-dev

#include <fftw3.h>
#include <vector>
#include <cmath>
#include <iostream>

int main() {
    int N = 256;  // Size of the 2D array

    // Input array (real data)
    std::vector<double> in(N * N);  
    
    // Output array for complex numbers (N * (N/2+1) because of the symmetry in FFTW's output)
    std::vector<fftw_complex> out(N * (N / 2 + 1));  

    // Fill the input array with some example data (e.g., a simple gradient)
    for (int i = 0; i < N * N; ++i) {
        in[i] = static_cast<double>(i);
    }

    // Create the FFTW plan for real-to-complex transformation
    fftw_plan plan = fftw_plan_dft_r2c_2d(N, N, in.data(), out.data(), FFTW_ESTIMATE);

    // Execute the FFT
    fftw_execute(plan);

    // Compute and print the modulus of the first few output values
    std::cout << "Modulus of FFT output:\n";
    for (int i = 0; i < 5; ++i) {
        double real_part = out[i][0];
        double imag_part = out[i][1];
        double modulus = std::sqrt(real_part * real_part + imag_part * imag_part);
        std::cout << "modulus[" << i << "] = " << modulus << "\n";
    }

    // Clean up the FFTW resources
    fftw_destroy_plan(plan);
    fftw_cleanup();

    return 0;
}