@ Brian - re:

> Use Construct > Curve > Project to project your curve to the plane, as a line.

The tricky part is that although it will have the shape of a line, it is going to be a closed curve that's all squashed down and running back over itself. So it's not going to have 2 endpoints at the extrema like you would need. If I have understood.

@ Peer - thanks for the further examples. I don't think there is any way to automate this currently but I'll see about adding a method on curves to collect all the locations where there is a common tangent to a point or direction. Then to weed out the internal ones you could make a line and intersect it with the curve, if there is an intersection other than at the tangent point then you would skip it. And you'd do a similar line intersection test for the endpoints of an open curve as well.

- Michael