Hi Petr,

> Does it mean MoI replaces an original surface by a
> "simplified" version when some conditions are met?

This replacement does happen during point editing but only for surfaces of revolution.

In the case that you are showing here, it is sweep that is doing the simplification of the profile curves - this is controlled by the Profiles: option inside of Sweep.

With Profiles: Auto (default mode), it will do a refit of the profile curves if the merged profiles have a lot of points in them, or if the profiles have any fully multiple interior knots - in this case it is the interior fully multiple knots condition that makes it switch to refitting.

If you switch that option to Profiles: Exact, that will override this, and then there will not be any refitting and you will get a surface with fully multiple knots in it. This one will get a kink in it in MoI if you drag its points around. The other simplification mechanism that kicks in with spheres will not kick in for this case because this is not a surface of revolution.

You can get the same smoothly deformable result in Rhino if you do the sweep over there by clicking the "refit within xx units" radio button in the sweep dialog.

- Michael