Function edge_crossings 

pub fn edge_crossings(
    model: &Model,
    face_id: FaceId,
    coedge_id: CoedgeId,
    curve: &NurbCurve3D,
    d: &Vector3,
) -> GeopResult<Vec<Scalar>>

Where the edge curve, bounding the face face_id along the coedge coedge_id, crosses the face’s silhouette seen along d: the parameters where the face’s normal there turns perpendicular to d.

Projected, an edge touches the silhouette of a face it bounds tangentially where it crosses it — a contact no search on the paper can isolate, so it is found here, in space: g’s sign sampled along the edge, and each change bisected. Only isolated crossings: an edge along which the face is seen edge on throughout has none.