All works fine when i take an Half-Half circle but fails when i take an Half circle!
(points are added to the surface BEFORE use of the Blend Function!
The object on the background if an half circle in the two cases!


here with a simple arc Half Arc circle! That fails!
Seems the surface is divided by Half for any reason! :)



If you choose points on the left surface will be divided! :(
https://moiscript.weebly.com/uploads/3/9/3/8/3938813/the_2cases.3dm

Hi Pilou, exact NURBS circles and arcs are made up of quadrants like that which will get creased if you move their control points around. MoI tries to replace those with an automatically rebuilt curve on surfaces of revolution but your extruded arc is not detected as a surface of revolution and so doesn't get the automatic rebuilt points with out the corner points at the quadrants.

Try using Rebuild on the arc before you extrude it, or move one control point of the arc before extruding it so that you won't have the surface that has corner points in its interior anymore.

- Michael

Wonderful : the trick of the rebuild works like a charm!
Excellent! :)

The "move point" method works also but is less practical if you want not a deformation before the extrusion...
The move and take back of a control point is some acrobatic :)
Draw a point or a line as a mark , move the control point and take back, kill the marks...
Extrusion
Not a big deal...but ...or maybe there is more simple...easy if your control point is on the grid... :)