I would think you have small "loops" in either the top or bottom curve. Maybe it's too small to see on an antialiased curve, but the shaded mesh is revealing the problem?

jonah

Hi andras, that likely means that your surface has little bumps in it just like it looks.

Could you please post the 3DM model file with those curves in it and tell me which command you used to create the surface? I'll take a look and see what is going on.

There is a curve refitting process that happens on curve inputs for several surfacing commands, sometimes it is possible for that process to introduce wiggles similar to that.

If you can send me a 3DM file where I can reproduce the construction steps I may be able to do something about it.

- Michael