center

The midpoint of the endpoints, carrying the dipole's size — a RoundPoint.

The center's aw sits radius2/2 above the null lift, so center.radius2 equals this dipole's radius2 — exactly what lets start and end fall out componentwise as center ∓ radius·attitude. Finite dipoles yield a unitized center (az = 1; read the coordinates off ax/ay); at infinity the center degenerates.