I'm having trouble deciphering the output of the rlft3 function for a 2d image. I've read the book and I'm still stumped. I started with trying to just take the fft of a sin(x*y) but without knowing how the output array is set up makes it hard to know if its doing it right. I've tried the scaling bit mentioned in the book for the inverse normalization, still no luck. I thought I saw something about wave numbers so I inverted the output (right or wrong, I'm not sure), then it at least looked like a frequency spectrum, but not the correct one. Is it some sort of big secret or does nobody really know and we just tell people to read the book? A little help would be divine.

hi.

i'm but a humble artist, trying to do a simple 2D fft of an image to give me an 2D amplitude map (i don't need the real/imaginary parts).

but can i understand what the rlft3 is going on about? 1st, 2nd, 3rd frequency components? nyquist critical? speq? excuse me? can anyone unscramble this obviously erudite tangle?

...2D fft of an image to give me an 2D amplitude map...

Which version of Numerical Recipes? (Version 2? Version 3? What?)
Which language (C? C++? Fortran? What?)

Regards,

Dave