B-spline Basis Functions: Important Properties
Let us recall the definition of the B-spline basis functions as follows:
This set of basis functions has the following properties, many of which
resemble those of Bézier basis functions.
- Ni,p(u) is a degree
p polynomial in u
- Nonnegativity -- For all i, p and u,
Ni,p(u) is non-negative
- Local Support --
Ni,p(u) is a non-zero
This has been discussed on previous page.
- On any span [ui,
p+1 degree p
basis functions are non-zero, namely:
Ni-p+2,p(u), ..., and
- Partition of Unity -- The sum of all non-zero degree p
basis functions on span [ui,
ui+1) is 1:
The previous property shows that
Ni-p+2,p(u), ..., and
Ni,p(u) are non-zero on
This one states that the sum of these p+1 basis functions
- If the number of knots is m+1, the degree of the basis
functions is p, and the number of degree p basis
functions is n+1, then m = n + p + 1
This is not difficult to see.
Let Nn,p(u) be the last degree
p basis function. It is non-zero on
un+p+1). Since it is the last
basis function, un+p+1 must be the
last knot um. Therefore, we have
un+p+1 = um
and n + p + 1 = m. In summary, given m and
p, let n = m - p - 1 and the degree p
basis functions are
N2,p(u), ..., and
- Basis function Ni,p(u)
is a composite curve of degree p polynomials with
joining points at knots in
The example shown on the previous page illustrates this property
well. For example, N0,2(u),
which is non-zero on [0,3), is constructed from three parabolas
defined on [0,1), [1,2) and [2,3). They are connected together
at the knots 2 and 3.
- At a knot of multiplicity k, basis function
Therefore, increasing multiplicity decreases the level of
continuity, and increasing degree increases continuity.
The above mentioned degree two basis function
N0,2(u) is C1 continuous
at knots 2 and 3, since they are simple knots (k = 1).
The Impact of Multiple Knots
Multiple knots do have significant impact on the computation of basis
functions and some "counting" properties. We shall look at two of them and a
calculation example will be provided on the next page.
- Each knot of multiplicity k reduces at most k-1
basis functions' non-zero domain.
Consider Ni,p(u) and
Ni+1,p(u). The former is
on [ui, ui+p+1)
while the latter is non-zero on
If we move ui+p+2 to
ui+p+1 so that they become a double knot.
Then, Ni,p(u) still has p+1
knot spans on which it is non-zero; but, the number of knot spans
on which Ni+1,p(u) is
non-zero is reduced by one because the span
This observation can be generalized easily. In fact, ignoring
the change of knot span endpoints,
to create a knot of multiplicity k,
k-1 basis functions will be affected. One of them loses
one knot span, a second of them loses two, a third of them loses
three and so on.
The figures below show the basis functions of degree 5 where the
left end and right end knots have multiplicity 6, while all knots in
between are simple (Figure (a))). Figure (b) is
the result of moving u5 to u6.
Those basis functions ended at u6 have fewer
knot spans on which they are non-zero. Then,
u4 and then u3 are moved to
u6, making u6 a knot of
multiplicity 4 (Figure (c) and (d). Figure (e) shows the result after
moving u2 to u6, creating
a knot of multiplicity 5.
At each internal knot of multiplicity k, the number of
non-zero basis functions is at most p - k + 1, where
p is the degree of the basis functions.
Since moving ui-1 to
ui will pull a basis function whose
non-zero ends at ui-1 to end at
ui, this reduces the number of non-zero
basis function at ui by one.
More precisely, increasing ui's
multiplicity by one will reduce the number of non-zero basis
functions by one. Since there are at most p+1 basis
functions can be non-zero at ui, the
number of non-zero basis functions at a knot of multiplicity k
is at most (p + 1) - k = p - k + 1.
In the above figures, since the multiplicity of knot
u6 are 1 (simple),
2, 3, 4 and 5, the numbers of non-zero basis function at
u6 are 5, 4, 3, 2 and 1.