Polyline inside rect intersection = Empty?

[DasUberbadger]

Hi, I've been using Clipper2 to clip OpenStreetMap vector data to a tile bounds and am finding it regularly returns an empty solution for polylines which are completely enclosed by a tile's bounds.

Here's a real-world-data example, (all int64) with:
An open line added as subject with points (3172, 2361) to (3168, 2381)
A closed rect added as clip with points (0,0), (4096, 0), (4096, 4096), (0,4096)

Clip called via Execute with clip type Intersection and fill rule NonZero gives me a successful clip, but an empty polytree.

This isn't consistent, for the most part the returned polytrees are correct for both enclosed and overlapping polylines of various lengths, but every now and again, such as the above example, I get nothing.

Any ideas? Has anyone encountered anything like this? Or - and this is quite likely - am I doing something dumb/missing something in the setup?

Test code is as follows:

Clipper2Lib::Paths64 subjects;
Clipper2Lib::Paths64 clip;
Clipper2Lib::Paths64 solution;

Clipper2Lib::Path64 Path;

Path.push_back(Clipper2Lib::Point64(0,0));
Path.push_back(Clipper2Lib::Point64(4096, 0));
Path.push_back(Clipper2Lib::Point64(4096, 4096));
Path.push_back(Clipper2Lib::Point64(0, 4096));

clip.push_back(Path);

Clipper2Lib::Path64 part;
part.push_back(Clipper2Lib::Point64(3172, 2361));
part.push_back(Clipper2Lib::Point64(3168, 2381));

subjects.push_back(Part);

Clipper2Lib::Clipper64 Clipper;

Clipper.AddOpenSubject(subjects);
Clipper.AddClip(clip);

bool bSucceeded = false;

Clipper2Lib::PolyTree64 LineSolution;
bSucceeded = Clipper.Execute(Clipper2Lib::ClipType::Intersection, Clipper2Lib::FillRule::NonZero, LineSolution);

and LineSolution comes up as size 0.

Oh - of course it's pertinent that this that this is part of the Unreal 5 codebase so could be missing a more recent fix or something. Going to go and have a compare with latest Clipper2 in github.

[DasUberbadger]

There are some differences, maybe hold off this Q until I've merged in latest and checked if it's still happening.

[DasUberbadger]

OK, updated to latest Clipper2 from the repo - same result though, empty polytree :(

[AngusJohnson]

  Paths64 subjects, clip, lineSolution;
  clip.push_back(MakePath({ 0, 0, 4096, 0, 4096, 4096, 0, 4096 }));
  subjects.push_back(MakePath({ 3172, 2361, 3168, 2381 }));

  Clipper2Lib::Clipper64 Clipper;
  Clipper.AddOpenSubject(subjects);
  Clipper.AddClip(clip);

  Clipper2Lib::PolyTree64 PolygonSolution; //only gets **closed** paths
  bool bSucceeded = Clipper.Execute(Clipper2Lib::ClipType::Intersection, 
    Clipper2Lib::FillRule::NonZero, PolygonSolution, lineSolution);

  if (lineSolution.size())
    std::cout << lineSolution[0].size();

[DasUberbadger]

Oh I knew I'd have done something dumb... many thanks.