I am wondering what (p4-p2)/2 exactly represents. Numerically, it's the averaged difference between p3 and p4, but it shouldn't be a derivative like the one in the previous example.
@SainanChen: I believe the first derivative of a 3D curve is a tangent vector. It might be better to interpret if you think about each element of the vector in 2D.
When trying to interpolate points into a line, the order matters right? Is the main difference between this example and the previous one that the produced line does not only go in one direction?