June 10, 2004
Quote from the NIH Image manual p. 34:
FFT - Computes the Fourier transform and displays the power spectrum. Requires a square, power of two size image or selection. The transformed frequency domain image is stored in a 32-bit real buffer attached to the window displaying the power spectrum. The only commands in Image that currently recognize this real image are "Inverse FFT", "Redisplay Power Spectrum" and "Image Math". All other commands "see" only the 8-bit power spectrum.
A helpful trick to remember when attempting to make power of two sized selections is to hold down the shift key, which forces the selection to be square. Or, even easier, use the macros for creating power of two sized selections in the "FFT Macros" file distributed with Image. The "FFT Macros" file also contains macros for high and low pass filtering
These are instructions for making a 256 x 256 pixel image, suitable for use with the Process -> FFT menu.
(ImageJ - the square is yellow with small handles, rather than marching ants, but it behaves the same.)