Hi 3image - can you please post the 3DM model file that you used to create your screenshot? That will help me to be more precise in answering your question, for example I'm not sure what size your cylinder is in this case, and it's possible that is having some effect. It is very helpful in general to always supply a 3DM model file along with your question, because there are often particular details that play some role in what's happening and often times those details cannot be determined by just looking at a screenshot.

But to give you an answer now - in general part of the job that Loft has to do is to make all the curves that are being lofted to be compatible with each other. Being compatible means the curves have the same structure, with equal control point counts, and the same "knot vector". There are different methods that can be used to make compatible curves, and the method that loft will use can be controlled using the "Profiles" option in the Loft command.

The "Profiles: Exact" mode will combine the knot vector of all curves together to make a single unified one, and then insert knots as needed on each original curve until they all share the same knot structure. This preserves the exact shape of each curve, but can be problematic if the curves were only slightly different from each other, when that happens there can be a lot of knots clustered slightly around each other.

The "Profile: Refit" option does a synchronized rebuild of each curve, the rebuilt curve will be fit to a tolerance and so won't have the exact same shape as the original but it will be within 0.001 units of it. This is kind of the most general purpose method, it tends to avoid having any tightly clustered together points but still gives a high amount of accuracy in the result. This is the method that is being used in your case there.

The "# Points" method samples a fixed number of points that you specify along each curve and then builds a curve passing through those points. It is the least accurate at preserving the shape but can also yield the most simple results. So for example in your case there if you set the Loft option for "Profiles: # Points" mode and then set the "Point count" option that then shows up to 10 , that will make a very simple surface with sparse control points but if you zoom in closely in the top you will see that it sags away from a true circle by some amount.

The default "Auto" mode chooses between either "Exact" or "Refit" modes depending on the structure of the curves. If the curves have simple knot structures it will use "Exact" mode, otherwise it will use "Refit" mode. The structure of an exact NURBS circle curve is actually somewhat complex because there are "fully multiple knots" in it, which are like corner points when you draw a curve. It's possible for surfaces or curves to have a sharp corner in these areas unless the control points on either side of it are specially positioned to be tangent to each other.

This may also be a little confusing because when you turn on control points for a circle, you don't see those corner points - that's because of a mechanism in MoI where if you turn on control points for a circle or surface of revolution it will actually show you the control points of a rebuilt version of the circle or surface, so if you edit the points the result will squish around in a smooth manner rather than getting sharp corners in it when the points are disturbed.

So anyway because the real underlying circle curves have fully multiple knots in them, the "Auto" mode will choose the Refit synchronization mechanism. If you switch that Profiles option to "Exact" it would work more like you were expecting.


But also if you care a lot about preserving the same control points, you could also use the Extrude command instead of Loft for a shape like this, Extrude doesn't have to do this same job of figuring out a combined compatible structure between multiple cross-sections and so the surface generated from Extrude will always inherit the exact same structure as the generating curve.

Also in general you may be worrying too much about the control point structure of the surface - the result that you're complaining about should be totally fine for regular use really. It's only if you're doing surface control point editing that you would really want to highly prioritize the resulting control point structure and if you are focused on that probably a polygon sub-d modeling program would be a better fit for that type of work, NURBS modeling works best when you are building surfaces from 2D curves, it's not as suitable for sculpting shapes by 3D surface control point editing.


Hope this helps, sorry the answer is so complex!

- Michael