B-Splines

This page describes how to use B-splines in FreeCAD. It gives also background information what B-splines are and for what applications they are useful.

Motivation
If you know already about B-splines and their application, you can directly continue with section B-splines in FreeCAD.

Let's assume you want to design a part that should be produced with a 3D printer. The part must have an edge this way:



You have to print the part in direction of the sketch's bottom to the top. Outer support structures might not be an option. Therefore you need to add a support directly to your part. What options do you have?


 * Option 1: you could add a line from point (20, 0) to point (80, 40):



However this solution needs a lot of volume, thus weight and material.


 * Option 2: you can connect the two points with an arc of a circle. To save volume, the arc should end tangentially in point (80,40). Then your solution looks like this:



OK. But at the bottom you don't need immediate support.


 * Option 3: you could save some more volume if the connection between the 2 points is a curve that begins tangentially at (0, 20) and ends tangentially at (80, 40):



So a curve with which you can connect two points tangentially to a reference point can be very useful for constructions. Bézier curves provide this feature.

Derivation
Bézier curves are polynomials to describe the connection between 2 points. The simplest polynomial connecting 2 points is a line ($$A*x^1+B$$) thus also linear Bézier curves are linear:



However a polynomial becomes first be useful when we can control it. So there should be a point between the 2 endpoints that allows us to define how the endpoints are connected. Like in the above example option 3 the curve is helpful when it starts and ends tangentially to lines crossing the endpoints. And this is a main feature of Bézier curves. So let's add a control point between the 2 endpoints. The curve will start tangentially towards this control point, meaning it is tangential to the line that we can draw between the startpoint and the control point. Going backwards from the endpoint the curve will also be tangential to the line we can draw between the control point and the end point. Animation 2 shows how such a curve looks.



The animation makes clear what the curve basically is - a transition from P0 to P2 by rotating the line P0-P1 to become the line P1-P2. Therefore we get the nice tangential start/end feature.

Such a curve can only be described by a quadratic polynomial. (The number of left-hand/right-hand turns + 1 is the necessary polynomial order. A quadratic polynomial is a single turn, a cubic polynomial is a combination of a left-hand and right-hand turn, and so on.) Therefore a Bézier curve with one control point is a quadratic (second order) Bézier curve.

Having only control point is often not sufficient. Take the above motivation example. There in option 3 we end the curve tangentially in x-direction. But how can you connect the points (20, 0) and (80, 40) so that the curve ends tangentially in y-direction? To achieve this you need first a right-hand and then a left-hand turn, so a cubic (third order) polynomial. And that means for a Bézier curve that we need (or you can say we gain) a second control point. Animation 3 shows a cubic Bézier curve.



To answer the question, the solution with the tangential y-direction ending for the example is this one:



Math
If you are interested to understand the background math, here are the basics.

A Bézier curve is calculated using this formula:

$$\quad \textrm{Bezier}(n,t)=\sum_{i=0}^{n}\underbrace{\binom{n}{i}}_{\text{polynomial term}}\underbrace{\left(1-t\right)^{n-i}t^{i}}_{\text{polynomial term}}\; \underbrace{P_{i}}_{\text{point coordinate}} $$

n is hereby the degree of the curve. So a Bézier curve of degree n is a polygon with order n. The factors $$P_{i}$$ are hereby in fact the coordinates of the Bézier curves' control points. For a visualization see this page.

If you are further interested, have a look at this website with a nicely animated derivation of the math of Bézier curves.

Rules
In the above derivation you might already noticed some "rules" for Bézier curves:
 * The polynomial degree is also the degree of the curves.
 * If you need $$n$$ turns, you need at least a $$n+1$$ degree Bézier curve.
 * A Bézier curve always begins tangentially to the line between the startpoint and the first control point (and ends tangentially to the line between the last control point and the endpoint).

Basics
This video lists at the beginning the practical problems with Bézier curves. For example that adding or changing a control point changes the whole curve. These problems can be resolved by joining several Bézier curves. The result is a so-called spline, in particular a B-spline (basis spline). The video also explains that a union of quadratic Bézier curves form a uniform quadratic B-spline and that a union of cubic Bézier curves form a uniform cubic B-spline.

From the videos we can collect useful "rules" for B-splines:
 * The first and last control point is the end/start point of the spline.
 * Like for Bézier curves, splines always begin tangentially to the line between the startpoint and the first control point (and end tangentially to the line between the last control point and the endpoint).
 * A union of $$S$$ Bézier curves with the degree $$D$$ has $$S+D$$ control points.
 * Since one is in most cases working with cubic B-splines we can then state that $$N$$ control points lead to $$N-3$$ Bézier segments and in turn $$N-4$$ segment junction points.
 * A B-spline with the degree $$D$$ offers at every point a continuous $$D-1$$ order derivative.
 * For a cubic B-spline this means that the curvature (second order derivative) does not change when traveling from one segment to the next one. This is a very useful feature as we will later see.

If you are interested in more details about B-Spline properties, have a look at this video.

Basis
The name B-spline stands for Basis spline. Instead of forming the spline as a combination of Bézier curves, the approach is to to model the same spline a different way. The idea is hereby to use another set of polynomials as basis. A linear combination of these basis polynomials $$B_D(t)$$ with the order $$D$$ form the B-spline. This video explains the transition from the Bézier control points to the polynomial basis functions to describe the spline. Mathematically we can describe a B-spline with this formula:

$$\quad c(t)=\sum_{k=0}^{N}p_{k}B_{k, D}(t) $$

Hereby $$p_k$$ is the $$k$$-th control point of the B-spline and also a factor for the $$k$$-th base polynomial $$B_{k, D}(t)$$. Every basis polynomial describe the spline in a certain region and therefore moving a control point does not affect the whole spline. Tho understand this, it is highly recommended to have a look at this video starting at minute 2:23.

As explained in the video the basis polynomials are Bernstein polynomials. The set of basis polynomials for a certain B-spline can be visualized this way:



At every spline position $$t$$ the sum of polynomials is 1 (indicated by the orange line). At the start only the red polynomial has an influence since all other polynomials are there 0. At greater $$t$$ the spline is described by a linear combination of different basis polynomials. In the image above, every polynomial is greater than 1 for the whole range $$0 < t < 1$$. This is not necessarily the case. As shown in the video, the basis polynomials are basically only greater than 0 for a certain spline position range. The interval at which a basis polynomial is greater than 0 is described by the knot vector. If you are interested in learning about the knot vector, have a look at this video.

Non-uniform B-splines
A property of the Bernstein polynomials is that when looking at the different S-spline Bézier parts, the path length of every part is the same. (The path length is often called the travel time). As you can imagine, it can be useful to have B-splines whose Bézier parts have different path lengths. This can be achieved by weighting the different polynomials:

$$\quad c(t)=\sum_{k=0}^{N}d_{k}B_{k, D}(t)w_k $$

$$w_k$$ is hereby the weight of the $$k$$-th control point. When the weights are not equal, the B-spline is called non-uniform.

Especially when B-splines should be used for 3D modelling, normalized, non-uniform B-splines are necessary. The normalization is done by a division by the weighted basis functions. Thus when all $$w_k$$ are equal, we get a uniform B-spline, independent on the weight itself:

$$\quad c(t)=\cfrac{\sum_{k=0}^{N}d_{k}B_{k, D}(t)w_k}{\sum_{k=0}^{N}B_{k, D}(t)w_k} $$

These non-uniform and rational (because of the division) B-splines are often called NURBS. Looking at their formula, we see that they are in fact a B-spline with a weighted basis $$R_{k, D}(t)$$:

$$\quad c(t)=\sum_{k=0}^{N}d_{k}R_{k, D}(t) $$

whereas

$$\quad R_{k, D}=\cfrac{B_{k,D}(u)w_k}{\sum_{l=1}^N B_{l,D}(t)w_l} $$

B-splines in FreeCAD
FreeCAD offers to create uniform or non-uniform B-splines of any degree in 2D via the Sketcher workbench.

Creation
To create B-splines, go into a sketch and use the toolbar button. Then left-click to set a control point, move the mouse left-click to set the next control point and so on. Finally right-click to finish the definition and create the B-spline.

By default uniform cubic splines are created, except there are not enough control points to do this. So when you create a B-spline with only 2 control points, you get of course a spline that is single linear Bézier curve, for 3 control points you get a quadratic Bézier curve, first with 5 control points you get a cubic B-spline consisting of 2 Bézier segments.

To create periodic B-splines (B-splines that form a closed curve), use the toolbar button. It is not necessary to set the last control point onto the first one because the B-spline will automatically be closed:



B-splines can also be generated out of existing sketch segments. To do this, select the elements and press the the toolbar button.

Changing the Degree
Tho change the degree, select the B-spline and use either the toolbar button or.

Note: Decreasing the degree cannot revert a prior increase of the degree, see the Wiki page Decrease B-spline degree for an explanation.

Changing the Knot Multiplicity
The points where two Bézier curves are connected to form the B-spline are called knots. The knot multiplicity determines how the Bézier parts are connected, see the Wiki page Increase knot multiplicity for details.

To change the knot multiplicity, use the toolbar buttons or.

Note: Creating two B-Splines that are connected to each other will not unite to a single new B-spline. So their connection point is not a knot. The only way to get a new knot in an existing B-spline is to decrease the degree. However, you may get many new knots. Thus the better choice is to redraw the B-spline with more control points.

Changing the Weight
Around every control point you see a dark yellow circle. Its radius sets the weight for the corresponding control point. By default all circles have the radius 1. This is indicated with a radius constraint for the first control point circle.

To create a non-uniform B-spline the weights have to be non-uniform. To achieve that you can either change the radius constraint of the first control point circle:



or you delete the constraint that all circles are equal and then set different radius constraints for the circles.

If no radius constraint is set, you can also change the radius by dragging:



In the dragging example you see that a high weight attracts the curve to the control point while a very low weight changes the curve so as if the control point does almost not exist.

When you look at the creation function for non-uniform rational B-splines you see that a weight of zero would lead to a division by zero. Therefore you can only specify weights greater than zero.

Display Information
Since the form of a B-spline does not tell much about its properties, FreeCAD offers different tools to display the properties:

Limitations
At the moment (FreeCAD 0.19) there are some limitations when using splines you should know:
 * 1) You cannot set tangential constraints. In this example Sketcher_spline-limit-tangential.png you want to assure that the spline touches the blue curve 2 times tangentially. This would be useful because the blue line could for example be the spatial border for your design.
 * 2) You cannot insert a new control point between two selected existing control points. There is no other way than to redraw the spline.
 * 3) You cannot delete a control point. Also in this case you must redraw the spline
 * 4) You cannot create an offset curve for a B-spline using the tool Draft Offset.

Typical Use Cases
According to the properties of B-splines, there are 3 main use cases:
 * 1) Curves that start/end tangentially to a certain direction. An example for this is the motivation example above.
 * 2) Curves describing larger designs and providing the freedom of local changes. See this example below.
 * 3) Curves providing certain continuous order derivative at knots. See this example below.

Designing
Take for example the case that you design a housing of a kitchen mixer. Its desired shape should look like this one:



To define the outer form it is advantageous to use a B-spline because when you change a control point to change the curvature at the bottom, the curvature at the side and top will not be changed: