Hi Schbeurd, as you suspected this is not a bug but is a side effect of the "flat" style.

The "flat" style rotates the object around the world Z axis, but swings it around to point as much as possible in the tangent direction of the curve. The spots where you are seeing the switch over there are places where the curve tangent has passed to the "other side" of the world z axis direction.

The "flat" style is really intended to be used on curves that do not have any part in them that approaches the upwards/z-axis direction too closely. For curves that do not point too close to upwards, it lets the object rotate relative to the curve but stay stabilized in the upwards/z-axis direction.

See here: http://moi3d.com/forum/index.php?webtag=MOI&msg=263.10 for another discussion on it.

Here is another example - given this input:

Using freeform will generate a rotation around the curve that gradually twists around the curve tangent. This will produce only gradual changes from one object to the next, but is not "stabilized" with regard to any particular direction, so it is even possible on longer loopy curves for the object to eventually flip up-side down.

Freeform example:


Using flat will rotate the object around the world z axis, and try to point it as much as possible in the direction of the curve tangent. This essentially stabilizes the object with regard to the z-axis direction, but it behaves poorly when the curve tangent approaches the vertical/z-axis direction, rotations will become sort of wild in those points and the orientation at a point exactly along the z-axis is not well defined since the rotation axis and "stabilizing direction" are pointing in the same direction there.

Flat example:



"None" just copies the object to that point without rotating it in any fashion:

None example:


Please let me know if you need any additional explanation.

- Michael