# How to spline together Bezier curves to form a smoth closed curve?

Given $k=m\cdot n$ points: $P_1,P_2,...,P_k$ (all points are two dimensional points), how can I spline together $m$ Bezier curves of $n$ degree to form a smooth closed curve?

Denote $B_{i,j}(t)$ to be the Bezier curve defined by points $P_i$ through $P_j$ (meaning $P_i$ and $P_j$ are the edge points). I want to spline together the curves: $B_{1,n+1}(t),B_{n+1,2n+1}(t),...,B_{(m-1)\cdot n+1,1}(t)$

For example, given the following: $P_1,P_2,...,P_8$

I want to find the Bezier curves $B_{1,3}(t),B_{3,5}(t),B_{5,7}(t),B_{7,1}(t)$ that will also create a nice smooth closed curve.

I know how to find Bezier curve of order $n$ given $n+1$ points, but the problem is how to spline them together, in a way that the edges (the points where two Bezier curves "meet") will be smooth.
In the example above, I want the curve to be smooth at points $P_1,P_3,P_5,P_7$.

When interpolating (say, natural) cubic spline, we solve the above problem (smoothness in the knots) by adding constraints that states the derivative of every cubic spline at point $x_i$ must be equal to the derivative of the next cubic spline in $x_i$ (namely $S_i'(x_i)=S_{i+1}'(x)$, $\forall i=1,2,...,n-1$).
Can I do something similar here?

• You should first define what you mean by smooth. It sounds like you want $C^1$. – AlexR May 20 '15 at 18:00
• By smooth I mean differentiable. The same demand we give when interpolating cubic spline. – so.very.tired May 20 '15 at 18:21
• In that case (this is $C^1$), I have provided the general derivation of an algorithm. Does this suffice or do you need further clarification? – AlexR May 20 '15 at 18:39

We can take a look at the problem for each dimension individually, since the cubic splines can be written as $$B_i : [t_i, t_{i+1}] \to \mathbb R^2; ~~t\mapsto \pmatrix{a_3 t^3 + a_2 t^2 + a_1 t + a_0 \\ b_3 t^3 + b_2 t^2 + b_1 t + b_0}$$

For the $1D$-case we call the points $x_i$. Furthermore we assume that the number of points is even and at least four - this will ensure that we can form a closed curve and have "junctions" at the indices $2k-1$ and "internal points" at indices $2k$.

The individual cubic segments $B_i$ will then follow the constraints \begin{align*} B_i(t_i) &= x_{2i-1} \\ B_i(\frac12 (t_i + t_{i+1})) &= x_{2i} \\ B_i(t_{i+1}) &= x_{2i+1} \\ B_i'(t_{i+1}) - B_{i+1}'(t_{i+1}) & = 0 \end{align*} Where the last two equations become \begin{align*} B_n(t_{n+1}) &= x_1 \\ B_n'(t_{n+1}) - B_1'(t_1) & = 0 \end{align*} Where there are $2n$ knots and $n$ segments. These ensure the closedness.
All in all this gives us a system $4n$ linear equations and $4n$ unknowns ($4$ for each cubic). We can chose the points $\{t_i\}_{i=1}^{n+1}$ to our liking, they only affect the "speed" in wich the parameterization traverses the curve.

Patching the result together to a function $$B : [t_1, t_{n+1}]\to\mathbb R$$ in a piecewise fashion ensures that every point $x_i$ is traversed at $t_i$ and $x_1$ is re-traversed at $t_{n+1}$, so $B$ is periodic. If we do this twice with the same choice of $\{t_i\}$ and the $y$-coordinates instead, we can join these two to a piecewise cubic $C^1$-curve in $\mathbb R^2$, each component given by the appropriate $B$.

• Thanks for your answer. Can I generalize it to Bezier curve of any degree? (not just cubic Beziers) – so.very.tired May 20 '15 at 18:48
• @so.very.tired Yes, you just need less junctions and a different multiple of points. Precisely, a degree-$n$-curve will need $n+1$ equations, one of wich will be the $C^1$-condition. The other $n$ must then be point-equations. Thus to increase the degree by one, you must add one interior point per segment. A degree-$4$ curve thus needs a multiple of $3$ points with points $3k-2, 3k+1$ as junctions and $3k-1, 3k$ as interior points. – AlexR May 20 '15 at 18:56
• What's $C^1$? I'm not familiar with this notation. I looked online and couldn't find it. – so.very.tired May 21 '15 at 9:06
• @so.very.tired Continuous with continuous first derivative, but possibly discontinuous higher-order derivatives. $C^0$ means continuous and $C^k$ means continuous with derivative in $C^{k-1}$. – AlexR May 21 '15 at 9:07
• @so.very.tired You may (this is colloquially called "curvature continuous"). Note that this will take care of one degree of freedom, so you must either sacrifice an interior point per segment for it, or increase the order of the splines. – AlexR May 21 '15 at 14:27