Hi Pilou - I guess what I was talking about was not clear.

The point is that it does not matter to sweep which side is the natural start or end, because sweep will automatically flip any curves as needed. It is not needed to do any manual "unrotate" step.

Flipping curves before you do the sweep should not have any effect, since sweep handles the orientations automatically.

Here's another example - note here I have several profiles set up:



Also note that the profiles have different random orientations, on some the natural direction is left to right, on others right to left (use MarkOpenCurveStart to see this):



However, you can just select all these profiles and sweep them *** without doing any manual flipping *** - the sweep command automatically handles any necessary flipping, to make this result:




So notice there how I did the sweep without any extra flipping and there is no twisting in the surface that was created?

You can try this yourself with the attached 3DM file.

If you have an example that behaves differently, *** Please post the 3DM file for it!!! *** - I'm not sure if the image you posted previously is drawn in a bitmap program or is an actual MoI screenshot? If it is a MoI model that you're seeing behave like that, please post the actual model file so I can look at it.

Under normal circumstances it should not ever be needed to worry about which side of a sweep profile curve is the natural start and end because of this automatic twist minimization that handles it for you automatically. Automatically means - you do not need to flip it yourself!!! :)

- Michael