“…Akin to our framework of linear evolutions on orientation scores, cf. [13,17], this means that we enforce horizontal diffusion and convection, i.e. transport and diffusion only takes place along so-called horizontal curves in H r which are curves t → (p(t), q(t), s(t)) ∈ H r , with s(t) ∈ (0, 1), along which connection between (p(0), q(0), s(0)) and (p(t), q(t), s(t)) and the actual horizontal…”