The straight forward way / concept to create a parametric curve is to define a polyline with a width. Express your x and y of the sine S-bend as a function of t, i.e. x(t) and y(t). Create a polyline out of the points it traces and convert it into a polygon by adding a width. There is a polyline2polygon function for that. Make sure you discretize your polyline in t not too little (optically bad) and not too much (gds file gets bigger). If you search for ‘pcurve’ in the Nazca code or on the Forum you will find some more background information on parametric curves. A tutorial on pcurves has been made. It will be posted with the Nazca 0.4.4 release.