Function flatten_point 

pub fn flatten_point(
    cp: &Vector4,
    origin: &Vector3,
    e1: &Vector3,
    e2: &Vector3,
) -> Vector3

The homogeneous 3-D point cp = (w p, w) in the plane through origin along the orthonormal e1 and e2, homogeneous: (w x, w y, w) for x = (p - origin) . e1, y = (p - origin) . e2 — the inverse of embed_point for a point of the plane, and like it without division.