//returns the interpolated value of the first derivative
func (s *Spline) AtD1(time Double) *Vec3 {
a := s.NodeBefore(time)
b := s.NodeAfter(time)
dt := b.T - a.T
t := (time - a.T) / dt
return glmath.InterpolateHermiteD1Vec3(a.P, a.V, b.P, b.V, t)
}
func (n *Node) InterpolateV(o *Node, t Double) *Node {
p := glmath.InterpolateHermiteVec3(n.P, n.Tv, o.P, o.Tv, t)
tu := n.Tu.Interpolate(o.Tu, t)
tv := glmath.InterpolateHermiteD1Vec3(n.P, n.Tv, o.P, o.Tv, t)
tc := n.Tc.Interpolate(o.Tc, t)
u, v := n.U+(o.U-n.U)*t, n.V+(o.V-n.V)*t
return NewNode(p, tu, tv, tc, u, v)
}