Working on some new way to fill, it is classical way, some parallel filling but it is still a single path.
The idea is to fill with closed shapes (rectangle cutted by interiors and exterior)
Red curves are the borders and black are path. I choose here to have nice border.
Then link/bridge the shapes, in order to have a single path
No smoothing
Some smoothing
Others patterns
So it is now possible to alternate direction on each layer.
Blue dot is an indication of the starting/ending point. So if there are 2 dots it means 2 curves, good if 2 layers, bad if just one layer !