Using KissFFT to get frequency domain image and back

Question

we've compiled the KissFFT library and it appears to be running fine, but we're not sure how we use it to get the frequency domain of a 2d image. Any help would be fantastic.

PS. We're running it in the After Effects SDK.

Answer 1

I haven't used KissFFT with images (only with audio) but here's my attempt based on what I've read in the docs. You should create 3 arrays for the individual colour components (i.e. create img1/img2/img3).

 //(I can't test this code now because I don't have kiss_fft)

 kiss_fft_scalar img1[M][N]; //array containing red pixels 
 kiss_fft_scalar img2[M][N]; //array containing green pixels
 kiss_fft_scalar img3[M][N];//array containing blue pixels


int mDim[2] = { N, M }; 
const int numDim = 2; //number of dimensions

kiss_fft_cpx *cinRed =  new kiss_fft_cpx[N * M]; 
kiss_fft_cpx *coutRed = new kiss_fft_cpx[N * M];
kiss_fft_cpx *cinGreen =  new kiss_fft_cpx[N * M]; 
kiss_fft_cpx *coutGreen = new kiss_fft_cpx[N * M];
kiss_fft_cpx *cinBlue =  new kiss_fft_cpx[N * M]; 
kiss_fft_cpx *coutBlue = new kiss_fft_cpx[N * M];

kiss_fftnd_cfg state = kiss_fftnd_alloc (mDim, numDim, 0, 0, 0);

int k=0;
for (int i=0;i<M;++i) {
    for(int j=0;j<N;j++){
        cinRed[k].r = img1[i][j];
        cinRed[k].i = 0; 
        cinGreen[k].r = img2[i][j];
        cinGreen[k].i = 0; 
        cinBlue[k].r = img3[i][j];
        cinBlue[k].i = 0; 

        k++;
    }
}

kiss_fftnd(state,cinRed,coutRed);//coutRed contains 2D fft results for img1 (red channel)
kiss_fftnd(state,cinGreen,coutGreen);
kiss_fftnd(state,cinBlue,coutBlue);

//for inverse 2D FFT just use
state = kiss_fftnd_alloc (mDim, numDim, 1, 0, 0);//1 sets inverse to true

kiss_fftnd(state,coutRed,cinRed); //note that cin and cout have switched positions
kiss_fftnd(state,coutGreen,cinGreen);
kiss_fftnd(state,coutRed,cinGreen);

Thank you Cris for letting me know that I should use kiss_fftnd (I've deleted the old code and procedure)