There are

Equation 87 defines a piecewise continuous
function. A *knot vector*,
,
must
be specified. This determines the values of *t* at which the pieces of
curve join, like knots joining bits of string. It is necessary that:

The *N*_{i,k} depend *only* on the value of *k* and the values in
the knot vector. *N* is defined recursively as:

This is essentially a modified version of the idea of taking linear interpolations of linear interpolations of linear interpolationsn

At this point it would be instructive for you to work out *N*_{1,1},
*N*_{2,1}, *N*_{3,1}, *N*_{1,2}, *N*_{2,2}, *N*_{1,3} for the knot
vector (0,2,3,6). It helps if you draw the graphs for these
functions.

There are several things that you should note about these
equations. Each
*N*_{i,k}(*t*) depends only on the *k*+1 knot values
from *t*_{i} to *t*_{i+k}.
*N*_{i,k}(*t*)=0 for *t* < *t*_{i} or
so
only influences the curve for
.
Formally,
is a polynomial of order *k* (degree
*k*-1) on each interval
.
Across the knots
is *C*^{k-2}-continuous.
is, of course,
continuous in all its derivatives between the knots.
is
validly defined for
where
and
.
Even more properties of B-splines are
described in Rogers and Adams pp. 306-7.

The above explanation shows that the knot vector is very important. The knot vector can, by its definition, be any sequence of numbers provided that each one is greater than or equal to the preceding one. Some types of knot vector are more useful than others. Knot vectors are generally placed into one of three categories: uniform, open uniform, and non-uniform.

**Uniform.**- These are knot vectors for which

(90)

For example:

**Open Uniform.**- These are uniform knot vectors which have
*k*equal knot values at each end:

(91)

For example:

**Non-uniform.**- This is the general case, the only constraint
being the standard
(Equations 88). For example:

The shapes of the *N*_{i,k} basis functions are determined entirely by
the *relative* spacing between the knots. Scaling
(
)
or translating (
)
the knot vector has no effect on the shapes of the
*N*_{i,k}.

The above gives a description of the various types of knot vector but it doesn't really give you any insight into how the knot vector determines the shape of the curve. The following subsections look at the different types of knot vector in more detail. However, the best way to get to feel for these is to derive and draw the basis functions yourself.

For simplicity, let *t*_{i} = *i* (this is allowable given that the
scaling or translating the knot vector has no effect on the shapes of
the *N*_{i,k}). The knot vector thus becomes
and
Equation 89 simplifies to:

N_{i,1}(t) |
= | ||

N_{i,k}(t) |
= | (92) |

You should be easily able to graph the first few of these for yourself. The principle thing to note about the uniform basis functions is that, for a given order

**Moving control points.**- Moving the control points obviously changes the shape of the curve.
**Multiple control points.**- Sticking two adjacent control points on top of one another causes the curve to pass closer to that point. Stick enough adjacent control points on top of one another and you can make the curve pass through that point.
**Order.**- Increasing the order
*k*increases the continuity of the curve at the knots, increases the smoothness of the curve, and tends to move the curve farther from its defining polygon. **Joining the ends.**- You can join the ends of the curve to make a
closed loop. Say you have
*M*points, . You want a closed B-spline defined by these points. For a given order,*k*, you will need*M*+(*k*-1) control points (repeating the first*k*-1 points): . Your knot vector will thus have*M*+2*k*-1 uniformly spaced knots.

If you wish your B-spline to start and end at your first and last
control points then you need an open uniform knot vector. The only
difference between this and the uniform knot vector being that the
open uniform version has *k* equal knots at each end.

An order *k* open uniform B-spline with *n*+1=*k* points is the Bezier
curve of order *k*. It would be a useful exercise for you to prove
this for *k*=3. For ease of calculation take the knot vector to be
[0,0,0,1,1,1].

Any B-spline whose knot vector is neither uniform nor open uniform is non-uniform. Non-uniform knot vectors allow any spacing of the knots, including multiple knots (adjacent knots with the same value). We need to know how this non-uniform spacing affects the basis functions in order to understand where non-uniform knot vectors could be useful. It transpires that there are only three cases of any interest: (1) multiple knots (adjacent knots equal); (2) adjacent knots more closely spaced than the next knot in the vector; and (3) adjacent knots less closely spaced than the next knot in the vector. Obviously, case (3) is simply case (2) turned the other way round.

**Multiple knots.**- A multiple knot reduces the degree of
continuity at that knot value. Across a normal knot the continuity is
*C*^{k-2}. Each extra knot with the same value reduces continuity at that value by one. This is the only way to reduce the continuity of the curve at the knot values. If there are*k*-1 (or more) equal knots then you get a discontinuity in the curve. **Close knots.**- As two knots' values get closer together, relative to the spacing of the other knots, the curve moves closer to the related control point.
**Distant knots.**- As two knots' values get further apart, relative to the spacing of the other knots, the curve moves further away from the related control point.

However, non-uniform B-splines are the general form of the B-spline
because they incorporate open uniform and uniform B-splines as special
cases. Thus we will talk about *non-uniform B-splines* when we
mean the general case, incorporating both uniform and open uniform.

- Move the control points.
- Add or remove control points.
- Use multiple control points.
- Change the order,
*k*. - Change the type of knot vector.
- Change the relative spacing of the knots.
- Use multiple knot values in the knot vector.

*k*=4 (cubic);- no multiple control points;
- uniform (for a closed curve) or open uniform (for an open curve) knot vector.

where it is usual for the patch to have the same order (i.e.