Hi David, Danny has a great video there to demonstrate the situation where you would want to use the "set flat direction".

> 1.- I think that I had an accidental non-desire situation.
> If I draw any closed curve, and a path touching the curve,
> and after that I apply a 1 rail sweep-->Flat-->Set flat direction
> and I choose a diagonal direction I get a weird result.

Yeah that is not how it would be used - the flat direction for a planar path like that would be perpendicular to the path's plane, not in some diagonal direction inside the same plane.

It's not really an option that is needed for planar paths like that at all - planar paths will work the same with either Twist option. It's used for non-planar paths that curve around in many directions in 3D, that's where there is different behavior possible for how the profiles are rotated as they travel along the 3D path.

When traveling along a 3D path, the Twist: Freeform option will gradually bank the object to the left or right as it moves along the path, to make for a kind of minimal but constant rotation of the shape. This is the default because it is more general purpose for any kind of crazy looping path since it is not weighted towards any one particular fixed direction.

But if you do not wish for the profile to bank in that manner, you can set the Twist:Flat option which will only rotate the profile around the "flat direction" (default is z axis but now you can set it with the new option). This kind of rotation will only work well if your path does not have any tangent pointing towards that same direction though, because the profile placement is not well defined when the path tangent and "flat direction" are identical.

You may also want to look at the example in Array curve to see another comparison of Twist: Freeform and Twist: Flat
http://moi3d.com/1.0/docs/moi_command_reference8.htm#arraycurve


Twist:Flat is often used with a helix path to make the profile only be rotated around the helix axis direction and not tilted to the left or right as it travels along the path.


But again the key thing is that the Twist option is used for controlling behavior on non-planar paths, your examples all have planar paths. For planar paths it is easiest to leave it to Twist: Freeform which will produce the same result but without needing to specify the flat direction (which needs to be perpendicular to the path's plane to get the proper result).


- Michael