How to use the given FFT function in C?

Question

Sorry this is the first time for me to ask question here, I've given a FFT function

void fft_1d(int n, float xr[256], float xi[256], int ntype)
{
  /* compute the FFT of a complex signal
     xr and xi are the real and imaginary parts respectively
     xr and xi contain the signal as input and the FT as output
     n is the number of points, it should be a power of 2
     ntype is set to 1 for forward transform and -1 for inverse transform
  */

And I have been told to extend this function to do the 2D (DFT) of the given image.

My problem is:

1. I can get every itensity value of the given image but how can I deal with the complex components (real part /imaginary part) of the fft_1d? In other words, what should I put in the parameters float xr[256] and float xi[256] in order to do the 2D (DFT) of the image?

Thanks for your attention!

Answer 1

An image is, generally, a real-only function. Therefore, you can set the real inputs to be the image values, and the imaginary inputs to zero when calling the FFT function.

However, in order to perform a 2D FFT, you must first call the 1D FFT on each row, and then call the 1D FFT on each column of the results. These intermediate results will be complex, and must be passed as such to the second set of FFTs.