Well, it is a different case from your initial post.
When i read it i asked myself
- what if there are 2+ curves,
- what if they are not orthogonal,
- what if there are plenty of them in flattened list…
As you see, so many ifs…
So i simplified and guessed you might need very solution for that and only that problem posted…
In case 1) extension approach as @ [Seghier khaled] would be the best. The question is how to chose proper extension distance so it dont mess all possible cases, like…
In case 2) @ Laurent Delrieu Clipper based solution suits well.
In case 3) preproccessing the curves by grouping them on proximity is neccessary…
By my humble opinion, when the curves are grouped properly, general solution is to find the closest control points ( similiar as in my definition), extend the proper segments by measured distance + small number so it intercests the polyline and then do some “region bounding”.
How to choose proper segment to extend because there are 2 on every control or disontinuity point?
Test which one has larger angle on closest polyline…for example