The Fourier Duck

Behold the duck.
It does not cluck.
A cluck it lacks.
It quacks.
It is specially fond of a puddle or pond.
When it dines or sups, it bottoms ups.
The Fourier Duck originated in a book of optical transforms (Taylor, C. A. & Lipson, H., Optical Transforms, Bell, London 1964). An optical transform is a Fourier transform performed using a simple optical apperatus.
Back to the Fourier Duck.