Group Title: Department of Computer and Information Science and Engineering Technical Reports
Title: Adjustable speed subdivision surfaces
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Title: Adjustable speed subdivision surfaces
Alternate Title: Department of Computer and Information Science and Engineering Technical Report
Physical Description: Book
Language: English
Creator: Peters, J.
Karciauskas, K.
Publisher: Department of Computer and Information Science and Engineering, University of Florida
Place of Publication: Gainesville, Fla.
Copyright Date: 2007
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Bibliographic ID: UF00095660
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Adjustable Speed Subdivision Surfaces

K. Kar6iauskas and J. Peters

Abstract-We introduce a non-uniform subdivision algorithm
that partitions the neighborhood of an extraordinary point in the
ratio a : 1 a, where a G (0, 1). We call a the speed of the non-
uniform subdivision and verify C1 continuity of the limit surface.
For a 1/2, the Catmull-Clark algorithm is recovered. Other
speeds are useful to vary the contraction near extraordinary

With few exceptions, polynomial subdivision algorithms
generalize uniform (box-)spline subdivision [BHR93]. In the
univariate case, standard B-spline knot insertion yields a
stable, local non-uniform refinement of the control polygon,
one knot at a time. Cashman et al. [CDS07], [CDS08] recently
proposed factoring non-uniform B-spline subdivision into
small stencils suitable for simultaneous knot insertion. Earlier,
Goldman and Warren [GW93] extended uniform subdivision
of curves to knot intervals in globally geometric progression.
When the univariate refinement is tensored and applied to
tensor-product splines, the knot intervals must be carried over
to parallel iso-curves. In practice, the least superset of all
iso-knot intervals is enforced by inserting knots. By con-
trast, Non-uniform Recursive Subdivision Surfaces, proposed
by Sederberg et al. [SZSS98], allow freely assigning knot
intervals to every edge of the control net. Subdivision in the
regular part surrounding an extraordinary node, i.e. a control
point associating more or fewer than four direct neighbors, is
constructed by halving each knot interval.

izations for high-quality surface constructions. Our approach
is philosophically different. We take the point of view that
subdivision surfaces are splines with singularities and that
subdivision is a process for generating a sequence of nested
surface rings. Once the Bernstein-B6zier (BB-) form of a
surface ring is well-defined, e.g. as in Appendix V-C, we
do not refine the control net further, but work with the
polynomial representation. In this scenario, the non-uniformity
is motivated by the the desire to adjust, possibly repeatedly,
the relative width of polynomial surface rings. Figure 1 gives
an example of such an adjustment in practice. The resulting
family of algorithms has properties akin to and specializes to
Catmull-Clark subdivision. In Section II, we derive the rules
and in Section III, we verify the properties. Besides adjustable
control net refinement for computer graphics and adjustable-
width surface rings in the framework of guided subdivision for
high-quality surfaces [KP07], Figure 1 and 9 present a third
application, namely the bi-3 (bicubic) completion of a surface
after one or two subdivision steps.


(c a a (c
= 2



blend with hole

Fig. 1. Shape completion. (top) Regular speed a 1/2. (bottom) High
speed a 3/4. (left) Blend with hole. (middle) Two steps of subdivision.
(right) Highlight lines when filling the remaining hole according to [SS05].
Note the strong fault in the highlight line in the center area of the zoomed in
detail (top, right).

We came across a different generalization, not covered by
[GW93], [SZSS98], [CDS07], when looking for reparameter-

Fig. 2. Univariate (left) uniform, (right) adjustable speed subdivision of speed
2/3. (top two) control point refinement, eon = extraordinary node; (bottom)
knot sequences.

_2 3
(T 0 7 (T ,

Fig. 3. (left) Knot sequences oftensored adjustable speed subdivision. (right)
The adjustable knot spacings around an extraordinary node are consistent and
define a sequence of C2 connected bi-3 surface rings in n = 5 sectors.

Figure 2 illustrates adjustable speed subdivision in one
variable. Instead of subdividing uniformly, we subdivide in



[1] a) [2 () [3 (c) [4]

(d) ^

Fig. 4. Derivation of subdivision rules converting one sector of the input
Extraordinary rule.

a ratio a : a where cr (0, 1) and a := 1 o. We call
a the speed of the non-uniform subdivision. We subdivide
only at the extraordinary node since the remainder defines
polynomial pieces. So, while the knot intervals for fixed a
show a geometric progression, this is different from the global
recursive splitting with geometrically scaled knots considered
in [GW93]. If we recursively applied the adjustable speed
subdivision, we would have knot ratios aT2, , a, 2 in the
second step. Tensoring this subdivision, as shown in Figure 3,
we obtain an adjustable speed subdivision that is well-defined
except near nodes with n / 4 neighbors. Adding rules for
this extraordinary case, we obtain a family of subdivision
algorithms, depending on the parameter a E (0,1) and
generating a sequence of bi-3 polynomial surface rings. Since
our knot spacing will be non-uniform, we cannot hope to build
on inherently uniform box-spline subdivision. Instead, we use
a less orthodox route and derive the missing rules by surface
ring extension in the BB form, as illustrated in Figure 4 and
detailed next.
Input: A net p (Figure 4,left) representing coefficients of a
bi-3 spline.
Output: The refined net p (Figure 4,right) representing coef-
ficients of a bi-3 spline.
Step [1] Convert p to bi-3 BB form (a).
Step [2] Extend (a) C2 inwards to yield coefficients (b).
Step [3] Subdivide (b) with ratio c : a to yield coefficients
Step [4] Revert (c) to p with the extraordinary node defined
by the stencil of Figure 4 right, and the choice

18rar(or2 + a) 92or2
3: l ( :) .(1
S9 +(n -4)(a +1)2 : 9+(n 4)(a + 1)2 (

Combinatorially, this refinement step mimics Catmull-Clark
subdivision. The weights 3 and 7 are chosen within a range
that yields C1 surfaces. The specific choice of 3 and 7 places
the central limit point at the central limit point of Catmull-
Clark subdivision (see Section III).
With the labels ij of the input control points p' shown
in Figure 5, left, and the labels k of the refined points ph in
Figure 5, right, steps [1]-[4] yield the following subdivision

Pk w 'pi (2)

n n

n n

S: 1 3 /-

p to the output p via the Bernstein-Bezier (BB-)representation. (right)

04 14 24 34

33 1 123

32 12 32

01 11 1 31

00 0 20 30

Fig. 5. (left) Indexing of the net p and (right) indexing of the refined p.
The indexing (right) is also used for the analysis.

where the non-zero w" are given by el := c2 + a, e :
S2 + 3a and
k\ij 11 12 13 21 22 23 31 32 33
22 ~ 2
22 0-1 2 -
3 c-2 c7- c, c(2
w : 4 4 2 3 2 (3)
_4 32
7 6 32 32 -22

Due to the symmetry across the sector bisectrix, w" = wi,
(and w, = k 3, 7), and, due to the symmetry across the
sector p., oii,.'i,, ray, = ,-, 'k 2,5,8. Only one of
the n sectors (see Figure 7) needs to be considered as we
construct a rotationally symmetric scheme. As in Catmull-
Clark subdivision, the 'outer' refined control points with
refinement rules w3 = wh k 1, 2,

19 (T 19 12 19 11
" 2 1 (T, W
, .1 = e1, o = owlo
i= oC, W = 1,

need not be considered for the next subdivision step. We
used two different indexing strategies for input and output,
since one, the indexing of the input (Figure 5,left), is natural
for representing patches of the surface ring, while the output
indexing (Figure 5,right) is commonly used in the analysis of
The BB coefficients of Figure 4 (c) defined by p'i, together
with the outer layer of BB coefficients defined by the refined
net p1 form a bi-3 surface ring in BB form. One sector of
this ring is shown in Figure 4 (d) with the contribution of p'i
drawn in black, and the contribution of pk in grey. Explicit
formulas for the BB coefficients are given in Appendix V-C.

7n x 7n-net

Fig. 6. Two views of the control net refined by adjustable speed subdivision.
(left) defines a complete bi-3 surface ring, (right) suffices to determine the
subdivision matrix A.

Indexing also the input mesh according to Figure 5, right,
we analyze the refinement

P7i+k P7i+k := Ap7yi+, i = 0, . ., n- 1, k = 1, . .,7.
The indices of one sector are shown in Figure 5, right.
Replacing the extraordinary node by n copies as in [PR98],
we obtain the 7n x 7n subdivision matrix with the familiar
uniform block-circulant structure

Ao Al ... A, -
A_ 1 Ao ... An-2
A : ..
A, ... An, 1 Ao!

Ak e RD7x7

The discrete Fourier Transform diagonalizes A into blocks
n-l 1
S= A, 0,...,n-, : exp(27i ) -
k 0

The eigenvalues of the 0-th Fourier block Ao are:
1, a3, r4, r5, C6 and the roots of quadratic equation

A2+(3+7- 2+2(a2 +1))A+a2(a2 ( +)a+au) 0.
The eigenvalues of the k-th Fourier block, Ak, k 1 0, are
0, a3, r4, 5, a6 and the roots of quadratic equation

A2_ a((1 + cn,k)+ 2o)A+ + 0r 0, Cn,k : cos( ).
We determine the central limit or extraordinary point by calcu-
lating the left eigenvector to the eigenvalue 1 [HKD93]. The
stencil for computing the extraordinary point is structurally
equivalent to that of Figure 5, right. We only need to replace
a, 3, 7 by a := 1 3 , 3, 7, where

S2a(3 + 7) + a3
(a2 + 3a)3 + (2g2 + g + 1)y + a(a2 + 2+ ) '
a2 (/ + 2y) + a-y
(a2 + 3a)3 + (2a2 + a + 1)y + a(a2 + 2a) "

Converting a Catmull-Clark-net {cij} to {pk} by (11), we
verify that extraordinary point of Catmull-Clark subdivision
coincides with that of our adjustable speed subdivision if 3
and 7 are defined by the formulas (1); and applying (11), then
(3) and reversing (11) proves that a 1/2 yields a subdivision
scheme coinciding with Catmull-Clark subdivision.

Fig. 7. Construction of one sector of the bi-3 characteristic ring. (left) The
tensor-border {tij }. (middle) The tensor-border is subdivided with ratio a : 0.
(right) The bi-3 sector construction is completed by A{tio} and A{toj}, the
outermost layers of the tensor-border scaled by A.

By inspection, and with c := cos(27/n), we verify that the
subdominant eigenvalue is the root

A ((1 + c)a2 + 2a + 0 (1 + c)((l + c)a2 + 4a)), (8)
of the quadratic equation (7) for k = 1. The subsubdominant
eigenvalue p is also a root of (7) but for k = 2. As with
Catmull-Clark subdivision, the limit curvature is generically
unbounded since y > A2, and the limit surface is generically
hyperbolic at the extraordinary point. The characteristic ring
is a normalized bi-3 symmetric scalable C2 map for any speed
oa (0, 1). Due to the rotational and bisectrix symmetry only
one half of one sector needs to be considered. Figure 7, left,
identifies all but one of the BB control points tij of a bi-
3 patch. We call this collection a tensor-border of depth 2.
With the normalization too : (1,0), tensor-borders from
adjacent sectors are C2 connected if, with c := cos(7/n),
s := sin(7/n),

tio = (2zs + z2c, -2zic + z2s), t2o = (zoc + zis, zoS zic),

t30 = (zoc, zos), tll (24,0), t21 (z3 + z4S, Z3S

t31 = (zc, Z3S), t22 = (zs Z4 C, 0), (9)
1 + c cc 1 + c cc
t32 (c(z3 Z4 ), SZ3 24 ))
2 2s 2 2s
Choosing the scalars zi according to (12), the ring and the
ring scaled by A are C2 connected. Regularity and injectivity
of characteristic ring have been checked numerically for
n 3, 5, 6,..., 20 and ar [1/10, 9/10], following [RP05]:
using the BB representation of Appendix V-B, we formed
for two of the three BB patches of a sector the differences
ti+,j tij, resp. t ij+ tj and verified that the coordinates
are of one sign. This implies regularity. Injectivity follows by
monotonicity of the boundary coefficients of the patch.
Since our main interest is in the characteristic ring, and
the characteristic ring is independent of 3 and 7, we did not
analyze different choices of 3 and 7.

At any subdivision step, a can be changed. Figure 8
illustrates how different choices of a affect a sector of the
characteristic ring and Figure 9 shows corresponding varia-
tions of the width of surface rings. Although it is possible

ring net


n= _6

transformation between the Catmull-Clark net {c' } and the
adjustable speed subdivision net {pk} (see Figure 5 for the
Pk e ciw (10)

where nonzero entries w@j are given below unless they follow
from bisectrix symmetry or sector symmetry:

fl:= a, f2 := + 1, f3:= 22- 1, f4: 3 + 2-r,
i33 1 ,i3 f2 2 ,.g fl
S 3a 3 '



Fig. 8. A sector of the characteristic ring for different valences and speeds.



W3 9,2 3
*21 f2 @31
S18,4 4
.32 f1f4 .23
w4 ga4 W4
4 9o4 4
19 1 3
6G3 '
< 1 21

!I. 7

flf2 .33 A2
9a2 3 92 '
fl .22 f2f4
18a4 9a4 '
1ff2 3 33 12f3
18a4 4 18a4'
f4 .23 ff13
3a3 603 '

1.-.-f 37 36a6 '

13.4 33 6a6
1. -.' 36a6

Fig. 9. (left) Input with gap (scaled to 2/5th size). (right three) (top) One
and (bottom) two adjustable speed subdivision steps.

to apply the adjustable speed subdivision to Catmull-Clark-
type control nets using high speeds, the surface shape for
higher speeds is increasingly poor when a > 3/4. This should
not surprise since in the limit a -- 1, we obtain the well-
known construction of a finite bi-3 cap consisting of the
bi-3 tensor-border plus the central limit point of Catmull-
Clark-subdivision that has also been patented as [SS05]. The
primary function of the construction is to our mind within the
framework of guided subdivision [KP07] and finite surface
constructions as illustrated in Figure 1 and 9. In a similar
fashion, an adjustable-speed variant of Doo-Sabin subdivision
has been derived.

The complete conversion formulas are presented here. Only
the formulas of Appendix V-C are needed to output nested
surface rings in BB form. Appendix V-B provides the constants
used in the BB representation of the characteristic ring (9)
in terms of a. Appendix V-A gives and optional an initial
conversion from a Catmull-Clark net to an adjustable speed
subdivision net that is used to prove that adjustable speed
subdivision subsumes Catmull-Clark subdivision.

A. Transformation from a Catmull-Clark net to an adjustable
speed subdivision net
By matching the BB representation of a Catmull-Clark net
with the BB representation in Figure 4 (b), we derive the

B. Definition of the characteristic ring
Here, we show all constants involved in the definition of the
characteristic ring (9). Representing single set of formulas for
a whole family of algorithms in terms of the speed a must be
more complex than those of a single instance such as Catmull-
Clark. It should therefore not surprise but rather delight that
the arrays DEN, NUM[r],r 0... 4, have been generated
and written by the symbolic tool Maple in the Maple form
Then for r 0... 4,

jh, E NUM,|| 1 li i]cj 1+AEj NUMII I| -'||]cJ
S ij DEN[, ][1[.i]cj1 + AEij DEN- [-1 [,.:5j-1

ho h2 h8
zo := U ,Z := his ,z2 := 3 := h3 z4 : 4 .
2c 2c 2c


2044, 19011,-5302, 66], [16
72353,8639,4963,93], [6981,3E
258,-995,-3167,-63], [-2133,
4007,-1807,1387,21], [438,50
748,1621,-405,-3], [-54,-31,2
612,-756,72], [3,-16,-36,-6]]
9,220,-6], [0,3,3]],

1 Y

1' -- ^ ^ 1

1'5 1 -

pS -tij

Fig. 10. Indices for converting the (left) adjustable speed subdivision net to
(right) tensor-border BB form.

[ [3612,-2407,-403] [3,-42,-30,3] ],
[ [609,499,94] [0, 3, 3] ],
[ [63,-65,-14], [0]] ,
[[3,4,1], [0]]]:

subdivision net p'" as

w n(1o+er22t" a i
where nonzero entries are given in (13) unless they followTS

where nonzero entries .' are given in (13) unless they follow

from bisectrix symmetry .,

,, or sector symmetry

dl := 1 + d2 : 6- 40- +2,

WOo a 6d d Ta4 d1d2 oo .' a3 d

w22 -2a2d2 2 a3a d2 33 w 04
-21 -4 7 -3 di 0_T -2 7
=24dl ,1 = oa3d1 .;,, 2aa d2 ,

9-1 a4 i 1 -3 i T-2 a d2 ,

29 adT 7 a a ( T 2
i. ,-i =IIa d -2 .1 I o a

91 41 .1 3 d
2 ,

1-o2d2 ,
2aa0 d2,

22 =aad, 2aa ^ , *, a2,

21 21 W21 a
22 2 32 2 23 33

W31 o W31 w31 -- W31

22 -2 2 233 1
W22 w22 w22 1

22 1 2 32
W32 0 32

[[127732,88788,315,266,-9], [22792,-7920,1006,10,-20]],
[[-161010,-110639,-1358,-332,9], [18334,5283,82,-324,48,-3]],
[[154696,107286,3216,243, -3, [-10818,-2669,-834,440,-45]],
[[ 114902, 82039, 4477, 108], [4654,1041,846, 311,18]],
[[66172,49570,4181,27], [-1422,-317, 475,129,-3]],
[[-29338,-23483,-2772,-3],[292,74,164, 30]]
[ [9832,8562,1326] [-36, 12,-33,3] ],
[ [2408,-2324,-452] [2,1,3] ],
[[406,443,105], [0]],
[ [42, 53, -15], [0]],
[[2,3,1], [0]]]:
[ [480,80] [480,-80]] ,
[ [3960,-390,10], [2520,-66,-18]],
[[15400,790,-10], [-6400,842, 94]],
[[ 37490,-914,-81,3], [10250,-1815,219,16]],
[[63866,1003,248,9], [11396,1918,-92,-14]],
[[80505,-1850,-326,9 ], [9167, 936, 234,48, 3]],
[[77348,3432,243,3], [5409,-170,404, 45]],
[[57451, 4531, 108], [2327,582, 305,18]],
[[33086,4187,27], [711,415,129,-3]],
[ [14669,2772,3], [146,158, 30]],
[[4916,1326], [-18,-33,3]],
[ [1204, 452] [1,3]],
[[203,105], [0]],
[ [21, 15], [0]],
[[1,1], [0]11 :

C. C-,i.., i,,i. an adjustable speed subdivision net to a BB

With the index conventions of Figure 10, the tensor-border
BB coefficients tij are obtained from the adjustable speed

-23 1 33
0 W32 0 W32


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05,479,116,-9], [911
8,2210,248,9], [54
51, 4051,219, 3], [:
6,3926,-105], [711,
69,-2556,27], [146,

8,11747,2185, 6],
2, 3304, 764] [3,
658,185], [0,3, 3]

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