Function intersection_polylines 

pub fn intersection_polylines(
    mesh_a: &Mesh,
    bvh_a: &Bvh,
    mesh_b: &Mesh,
    bvh_b: &Bvh,
    xf: &Rigid,
    out: &mut Vec<Polyline>,
) -> Result<usize>

Appends to out the polylines where mesh_a and mesh_b (placed in A’s space by xf) intersect, and returns how many were appended. Elements already in out are kept and stay in front.

A pair of triangles that only touch at a point contributes no segment; coplanar pairs contribute only area overlaps, as their boundary (interior edges shared by two pairs cancel; a touch along an edge or at a point contributes nothing); a degenerate triangle contributes the overlap of its longest edge. Segments are welded by exact f32 equality: endpoints computed from the same edge and plane are bit-identical, and endpoints on a shared vertex snap to it exactly, so loops close.

The order is deterministic: open polylines first, each from its smaller end, then closed loops, each from its smallest point; each group is sorted by first point.

Errors

QueryError::InvalidInput for an invalid xf (see Rigid); then QueryError::MeshMismatch or QueryError::StaleBvh when a BVH does not match its mesh; and QueryError::OutOfMemory when out or a temporary cannot grow. On any error out keeps its original length.