Hi Pilou,

re:
> What is the use to Flat the "moved lines" and not just vertical extrude
> the lines bottom not moved?

Flattening it has the added benefit of making the extruded faces able to be handled as analytic plane surfaces which can then get special treatment in things like intersection calculations.

It would also work to extrude the edges directly, it would result though in surfaces that have slanted control points with a parallelogram type shape. It's not like that's horrible but it means they will be generic surfaces and not able to be handled as an analytic plane surface.

So flattening it out just helps to make the end surfaces as simplified as possible.

- Michael