┌─────────────────┐
                           │      2         2
    d = \| tangent(t)\| =  │B' (t)  + B' (t)  
                          ⟍│  x         y

                           tangent (t)     B' (t)
^                                 x          x    
x(t) = \| tangent (t)\| =─────────────── = ──────
                 x       \| tangent(t)\|     d

                            tangent (t)     B' (t)
^                                  y          y
y(t) = \| tangent (t)\| = ─────────────── = ──────
                 y        \| tangent(t)\|     d