Guilloshes

Guilloche is an ornament (widely used on banknotes as a protection against forgery) in the form of two or more bands or strings twisted over each other in a continued series, leaving circular openings which are filled with round ornaments.
It can be implemented as a set of phase shifted sinusoids, with amplitudes modulated by inner and outer outlines. The tricky part is that the axis of sinusoids is an average between inner and outer lines, and the sinusoids incline towards the perpendicular of the axis.