Introduction - If you have any usage issues, please Google them yourself
This "slow" but simple One- and Two-dimensional FFT Demo can be downloaded from here (source included).
The Demo generates a 1D or 2D input signal (with size 256 or 256x256) which is displayed in the left bitmap (8 bit gray palette).
The signal contains a centered rectangular pulse (pulse size from 2 to 256). Since the input is real, the FFT routine ftu() produces biased spectrum. If "Reorder" is checked, the zero frequency is shifted to the center of the spectrum by swapping data from the respective quadrants. The complex result is displayed in the right bitmap in one of four possible modes: Module, Phase, Real part or Imaginary part. If "InvFFT" is checked, an inverse FFT is applied on the spectrum, the result being the recovered input signal. However, to recover the input signal correctly "Reorder" has to be unchecked. In order to keep up with tradition, the 2D signal may be fed from Lenna.bmp file which comes with the Demo. (See about Lenna below).