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A topological comparison of surface extraction algorithms

Abstract

In many application areas, it is useful to convert the discrete information stored in the nodes of a regular grid into a continuous boundary model. Isosurface extraction algorithms di er on how the discrete information in the grid is generated, on what information does the grid store and on the properties of the output surface.

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A topological comparison of surface extraction algorithms

Author: Andújar Gran, Carlos Antonio,Brunet Crosa, Pere,Fairén González, Marta,Navazo Álvaro, Isabel,Vinacua Pla, Álvaro
Year: 2006
Source: https://upcommons.upc.edu/bitstream/2117/10821/1/CAGD_05.pdf
A Topological Compa ison o Su ace
Ex ac ion Algo i hms
C. And´uja , P. B une , M. Fai ´en, I. Na azo, A. Vinacua
Modeling, Visualiza ion, In e ac ion and Vi ual Reali y G oup
So wa e Depa men
Uni e si a Poli `ecnica de Ca alunya, Ba celona, Spain.
{anduja , pe e, m ai en, isabel, al a }@lsi.upc.edu 1
Abs ac
In many applica ion a eas, i is use ul o con e he disc e e in o ma ion s o ed in
he nodes o a egula g id in o a con inuous bounda y model. Isosu ace ex ac ion
algo i hms di e on how he disc e e in o ma ion in he g id is gene a ed, on wha
in o ma ion does he g id s o e and on he p ope ies o he ou pu su ace. Recen
algo i hms o e di e en solu ions o he disambigua ion p oblem and o con ol-
ling he inal opology. Based on a numbe o p ope ies o he g id’s g ey cells and
o he econs uc ion algo i hms, a cha ac e iza ion o se e al su ace ex ac ion
s a egies is p oposed. The classi ica ion p esen ed shows he inhe en limi a ions
o he di e en algo i hms conce ning global opology con ol and econs uc ion
o local ea u es like hin po ions o he olume and almos non-mani old egions.
These limi a ions can be obse ed and a e illus a ed wi h some p ac ical examples.
We e iew in ligh o his classi ica ion some o he ele an pape s in he li e a u e,
and see ha hey clus e in some a eas o he p oposed hie a chy, making a case o
whe e i migh be mo e in e es ing o ocus in u u e esea ch.
Key wo ds: Su ace ex ac ion, Ma ching Cubes algo i hm, opologically
consis en isosu aces, ec angula g ids, disc e e olume models, opology
op imiza ion.
1 In oduc ion
In many applica ions olumes a e ep esen ed by a disc e e olume model,
some imes because ha is he way in which da a ha e been ga he ed (medical
1Wo k pa ially suppo ed by he TIN2004-08065-C02-01 o he spanish go e n-
men (MEC) and FEDER unding.
P ep in submi ed o Else ie Science 5 Decembe 2005
applica ions, nume ical simula ion esul s), some imes as an in e media e ep-
esen a ion o help in achie ing some objec i e (model epa a ion, cha ac e
sculp ing, simpli ica ion). In his con ex , i is o en necessa y o eco e om
his olume in o ma ion a su ace ep esen ing he bounda y o (a po ion
o ) he olume. I is his su ace-ex ac ion p oblem ha we ocus on in his
pape .
When he olume model is buil om a solid model, i may con ain bina y
in o ma ion (an in-ou classi ica ion o e e y e ex in a g id), o i may con-
sis o a sampling o a scala ield (a he same e ices), o example a signed
dis ance ield. Mo e in o ma ion can be s o ed, like He mi e da a, exac in e -
sec ion poin s, o he ac ha he olume en e s ( h ough a ace, o example)
in o a cell, bu hese a e seldom e ec i ely used in he li e a u e o eco e
he bounda y o he olume.
The ex ac ed su ace is gene ally modeled wi h a iangle mesh M. The clas-
sical algo i hm o achie e his — he o iginal Ma ching Cubes— da es back o
[1]. This o iginal algo i hm was soon seen o p esen inconsis encies in some
con igu a ions (see [2]), and much o he wo k done on his algo i hm he e-
a e has cen e ed on a oiding such inconsis encies. Di e en solu ions o his
p oblem, howe e , na u ally yield di e en opologies o he esul ing su ace.
These di e ences a ise in a eas whe e a dimension o he olume is compa able
o he scale o he disc e iza ion, ei he hin shee s o olume o hin ubula
po ions.
In his pape we p opose a way o classi y hese algo i hms depending on he
na u e o he cells hey examine, and some p ope ies o he su aces hey
p oduce. We e iew in ligh o his classi ica ion some o he ele an pape s
in he li e a u e, and see ha hey clus e in some a eas o he p oposed
hie a chy, making a case o whe e i migh be mo e in e es ing o ocus in
u u e esea ch.
To his end, we will s a by e iewing some o he ele an p e ious wo k in
sec ion 2, o help mo i a e he de ini ions on which we base ou classi ica ion,
in oduced in sec ion 3. In sec ion 4 we discuss b ie ly how di e en algo i hms
beha e ega ding he opology o he esul ing su ace, and hen show in sec-
ion 5 how di e en algo i hms beha e in econs uc ing some sui ably chosen
es models, in acco dance wi h he discussion in he p e ious sec ions.
2 Su ace ex ac ion algo i hms
Su ace ex ac ion algo i hms di e on how he g id in o ma ion is ob ained,
on how i is ep esen ed in he disc e e model, and on how he su ace is econ-
2
s uc ed. Mos algo i hms a e only using in o ma ion on g id e ices. Bu e -
ex alues can be ob ained by a simple sampling o he o iginal model/ ield [3]
o by analyzing he solid olume in he e ex neighbo hood [4,5].
Mul iple a ia ions o he o iginal Ma ching Cubes algo i hm [1] gi e di e -
en solu ions o he su ace ex ac ion p oblem. Howe e , mos echniques a e
based on local c i e ia and canno o e a di ec con ol o opological p ope -
ies o he ex ac ed mesh. As i s no ed by Du s [2], he o iginal Ma ching
Cubes algo i hm [1] may p oduce su aces wi h holes due o opologically
inconsis en decisions on he econs uc ion o ambiguous aces, whe e inde-
penden local decisions in he wo adjacen cubes may lead o wo di e en
su ace econs uc ions in hei common ace.
Disambigua ion echniques epo ed so a ha e ocused on wo majo con-
ce ns: opological consis ency, i.e. p oducing closed su aces by p ope cube
polygonaliza ion, and opological co ec ness, i.e. ex ac ing a su ace ai h ul
o he geome y o he eal su ace.
Inconsis ency o ambiguous aces (see igu e 1) appea s when adjacen cubes
ha sha e such a ace ake di e en local decisions (cases c and d in Figu e 1)
on he econs uc ion o he su ace wi hin he ambiguous ace. Consis ency
can be sol ed by jus conside ing he inside/ou side node classi ica ion, e-
ga dless o he ac ual da a alues. A i s solu ion o he consis ency p oblem
was gi en by p e e ed pola i y me hods. These algo i hms decide how o slash
ambiguous aces o a cell using a uni o m c i e ion: always join black nodes
o always join whi e nodes. This decision can be implemen ed ei he algo i h-
mically [6] o by using a lookup able [7]. All hese echniques a e gene ally
simple o implemen al hough hey gene a e a bi a y opologies.
   
Fig. 1. Unambiguous aces (a, b) gene a ing a single edge o he iangula mesh
and he wo possible choices o edges in an ambiguous ace (c, d).
Techniques add essing he opological co ec ness p oblem in e he p ope
polygonaliza ion o an ambiguous cube by analyzing i s ac ual da a alues.
These me hods a e equi ed o p o ide di e en polygonaliza ion schemes o
each ambiguous cube, [8,9]. Ini ial me hods only a emp ed o ensu e he
co ec ness o he e u ned su ace on he bounda y o ambiguous aces. The
analysis was based on ace cen e esampling [10,11], bilinea in e pola ion [12]
o g adien disambigua ion [13].
3
Ini ial disambigua ion echniques [14,12] we e ying only o disambigua e
wi hin g id aces. Nielson and Hamann [12] p oposed a s a egy based on he
saddle poin alue o he bilinea in e polan o dic a e he edge connec ions on
an ambiguous ace: he connec ion is made so as o sepa a e he saddle poin
om e ices o opposi e sign. This gi es a solu ion which is opologically co -
ec in e ms o he bilinea in e polan on he ace. In a simila way, Pasko e
al. [14] we e also aking he disambigua ion decision om he posi ion o he
cen e o he hype bolic ace o he ilinea in e polan on he ambiguous
ace. Mon ani e al. [15] used a simple-en y lookup able o sol ing he con-
sis ency p oblem, including an ex a con igu a ion o he complemen a y case
in se e al con igu a ions (3, 6, 7) o he classical Ma ching Cubes algo i hm.
This lookup able was also used in [16] o g ids wi h boolean in o ma ion. In
his case, e ices a he middle o he g id edges we e conside ed and a inal
ace me ging pos -p ocess was p oposed o educing he ace complexi y o
he ou pu model.
Te ahed a decomposi ion echniques [3,17] spli each cube in o i e o six
e ahed a, which always exhibi an unambiguous polygonaliza ion. Gueziec
e al. [18] di ide each cubic cell in o i e e ahed a. The i s ou e ahed a
a e cen e ed on ou al e na ing e ices o he cell and spli hei neighbo
aces h ough hei diagonal. The las e ahed on ills he cen al hole in
he cubic cell. Consis ency h ough g id aces is ensu ed by using al e na ing
subdi isions: he subdi ision in a pa icula cell is always symme ical o he
subdi ision in any o i s neighbo cells. The esul ing opology is o ced by
his al e na ing subdi ision and depends on he subdi ision choice a he i s
cell o he g id. Pascucci [19] p oposes an algo i hm o isosu acing a scala
ield de ined in a e ahed al g id. His implemen a ion is howe e es ic ed o
he case o egula g ids (no necessa ily ec ilinea ). In ec ilinea g ids, he
scheme leads o he same e ahed al decomposi ion as in [18].
Some me hods a emp o es ima e opology also inside he ambiguous cubes
ei he by using c i ical poin analysis [20] o ilinea in e pola ion. Isosu ace
ex ac ion algo i hms based on ilinea in e pola ion add ess he opological
co ec ness by imposing he local opology o he local ilinea in e polan
inside e e y g id cubic cell. Na a ajan [21] and Che nyae [22] independen ly
ecognized ha , besides he ace ambigui ies, he e a e addi ional ambigui ies
in he ep esen a ion o he ilinea in e polan in he in e io o he cube. In
he case o a cell wi h wo diagonally-opposi e black e ices, he su ace may
be in wo sepa a e pieces o he e may be a single unnel piece, opologically
equi alen o a cylinde (see igu e 2). Na a ajan used he alue o he body
saddle poin (whe e all h ee i s pa ial de i a i es o he ilinea in e polan
a e ze o) o disc imina e he wo cases. Ma eye [23] also discussed he in-
e io ambigui y p oblem, esol ing he ambiguous cases by conside ing he
beha io o he ilinea unc ion along he cell diagonals. Cignoni e al. [24]
designed a new mul i-en y lookup able (exhaus i e lookup able, ELUT) us-
4
ing he Na a ajan app oach, which p o ides, o each cell con igu a ion and
o each combina ion o he alues o he saddle poin s o he ambiguous aces
and/o o he body saddle poin , he co ec isosu ace pa ch con ained in
he cell. They include a inal mesh e inemen in o de o ensu e ha he i-
angula ed su ace will ha e he same opology as he ilinea in e polan in
he cell. An e icien and obus implemen a ion o he Che nyae me hod is
p esen ed in [25]. This algo i hm gua an ees a inal wo mani old su ace using
a ba e y o op imized es s and an ex ended Che nyae lookup able. This
sequence o pape s ha a emp o gua an ee econs uc ions wi h he local
opology o he ilinea in e polan inside e e y g id cell culmina es wi h he
wo pape s om Nielson [26] and Lopes and B odlie [27]. These wo pa allel
pape s p esen well- ounded and obus econs uc ion algo i hms based on
he analysis o ace and body saddle poin s and gi e exhaus i e cha ac e iza-
ions o he possible cases.









Fig. 2. The wo possible opologies a ising om an X-cube.
Among he algo i hms ha use ex a in o ma ion apa om alues a he
g id e ices we can ci e he Ex ended Ma ching Cubes scheme om Kobbel
e al. [28]. Kobbel ep esen s a ec o dis ance ield by s o ing h ee scala
alues a each g id poin . In addi ion, a local es ima ion o he su ace no -
mal ec o is s o ed a he g id e ices. The ec o dis ance ield is in ac
ep esen ing he exac loca ion o he su ace in e sec ions wi h he g id edges
(wi h a maximum o one in e sec ion pe edge). Edge and e ex ea u es can
be de ec ed and econs uc ed by de ec ing ea u e cells and inse ing an ap-
p op ia e poin in hem. The Dual Con ou ing algo i hm [29] econs uc s he
su ace om He mi e da a: he inpu alues a e he in e sec ion poin s and
no mal ec o s o he in e sec ions su ace wi h he g id edges (maximum o
one in e sec ion). The algo i hm uses a quad a ic e o me ic o compu e
a new poin inside each o he ou cells a ound each black-whi e g id edge
and gene a es a quad connec ing hese ou new poin s. The algo i hm is able
o eco e sha p ea u es o he ini ial solid. Va adhan e al. [30] e ec i ely
compu e up o wo in e sec ions on each edge o he g id. They hen p oceed
wi h an adap a ion o he Dual Con ou ing algo i hm, esul ing in a scheme
ha is able o e ec i ely eco e some hin ea u es o he o iginal. In [31]
hey complemen his p oposal wi h a cha ac e iza ion o cells whe e he algo-
5

i hm is able o econs uc he exac opology o he o iginal, and p opose an
adap i e subdi ision scheme ha e ines he g id locally un il all cells sa is y
ha c i e ion. The app oach in [32] is a a ia ion o Ma ching Cubes and Dual
Con ou ing, also equi ing He mi e da a. A Quad ic E o Me ics econs uc-
ion is pe o med a each ambiguous ace o a pa icula g id cell, compu ing
a new isosu ace poin in he ace. In he las s ep, he su ace inside he cell
is econs uc ed.
The olume-based app oach om Anduja e al. [33,34] uses he ini ial geom-
e y inside he cell o de ec g ey cells con aining pa s o he ini ial objec
su ace. The algo i hm pe o ms an addi ional subdi ision o he cells and
compu es i s e ex signs using he 26-adjacen cells. Resul ing subcell con ig-
u a ions a e gua an eed o be unambiguous. This app oach allows he econ-
s uc ion o hin pa s o he objec and ensu es a olume-Hausdo dis ance
condi ion be ween he ini ial and he econs uc ed objec .
A di e en app oach o sol ing he opological co ec ness p oblem is p e-
sen ed in [35]. In his case, he op imiza ion o he o e all opological p op-
e ies is achie ed h ough a global app oach. The con ol o he opological
beha io will be discussed in Sec ion 4.
3 Disc e e olume ep esen a ions: de ini ions and p ope ies
Le us assume ha Vis a bounded closed olume in
R
3and ha Sis he
bounda y o V. Le us also assume ha we ha e a ec angula uni o m g id R
wi h cubical cells cwi h sides o leng h `. The elemen s o Ra e i s cells c(R),
he g id e ices (R) and he cell aces and edges, (R) and e(R). We shall
always conside hese componen s as closed se s; hus, he edges include bo h
endpoin s, he aces include hei edges, and he cells include all six aces.
The e o e, o example, he cells c(R) do no cons i u e a pa i ion o R.
De ini ion 1 Whi e and black cells
A cell co Rwill be called whi e i he se in e sec ion be ween cand Vis
he oid se . A cell co Rwill be called black i he se in e sec ion be ween c
and Vis he same cell c.
Ve ices o Rcan also be classi ied as whi e o black in a simila way: The
classi ica ion o a whi e e ex is ou V , while black e ices can be inV o
onV . Edges eand aces o Rcan be classi ied in a simila way as cells.
Cells o Rno being whi e o black will be called g ey cells. G ey cells ha e a
non-null in e sec ion wi h S. G ey cells can be o ou di e en ypes:
6
De ini ion 2 G ey-0 cells (G0cells)
A g ey cell co Ris in G0i he numbe o i s whi e e ices is g ea e han
ze o and less han eigh . In o he wo ds, G0cells ha e a non-uni o m se o
e ices: some o hei eigh e ices a e whi e while some o he e ices a e
black.
De ini ion 3 G ey-1 cells (G1cells)
A g ey cell co Ris in G1i he e exis s some edge eo csuch ha he
in e sec ion be ween eand Sis no null.
De ini ion 4 G ey-2 cells (G2cells)
A g ey cell co Ris in G2i he e exis s some ace o csuch ha he
in e sec ion be ween and Sis no null.
De ini ion 5 G ey-3 cells (G3cells)
A g ey cell co Ris in G3i he in e sec ion be ween cand Sis no null.
No ice ha e e y cell in c(R) is ei he whi e, black, o belongs o G3. This
again is no a pa i ion, as a black cell whose bounda y has a non- oid in e -
sec ion wi h S=∂V is also in G3.
Abusing language we speak o Gkcells e e ing o cells in Gk. We will u he
abuse language by saying ha a su ace ex ac ion algo i hm is a Gkalgo i hm
i i ope a es by compu ing Gkcells. In all cases i will be clea by he con ex
o which o hese we e e . No ice ha when applied o algo i hms, his clas-
si ica ion e e s o he inpu , o a leas he pa o he inpu he algo i hm is
in e es ed in. Conce ning he ou pu , we also de ine:
De ini ion 6 k− econs uc ion algo i hms
We will say ha a su ace econs uc ion algo i hm is a k- econs uc ion i
he ou pu su ace is comple ely con ained in ∪c∈Gkcand he ou pu olume
in e sec s all he cells in Gk.
3.1 P ope ies
F om hese de ini ions alone, a se ies o use ul p ope ies can be shown. We will
lis hem he e, and la e apply hem in he analysis o he di e se algo i hms
ha ha e been p oposed in he li e a u e.
Fi s , i is i ially seen ha he Gk o m a hie a chy:
7
P ope y 1 Fo each k∈ {0,1,2},Gk⊂Gk+1
I a cell is in G0, i has an edge wi h di e ing classi ica ions a he e ices.
Because o Bolzano’s heo em, he bounda y mus in e sec ha edge a leas
once, hence he cell is in G1. The es o he inclusions ollow om he ac ha
we conside all elemen s closed, hence in e sec ing an edge implies in e sec ing
he aces ha sha e i , and in e sec ing a ace implies in e sec ing he cells
ha sha e i .
I a cell is in G3 G2, hen i mus con ain an isola ed po ion o Vin i s
in e io . The e o e G3can only di e om G2i Vcon ains connec ed com-
ponen s o olume less han `3. The con e se o his is ob iously no ue, as
an a bi a yly small connec ed componen may con ain a e ex o R, bu he
p e ious ob ious ema k implies ha
P ope y 2 Fo any olume V, he e is an ε∈
R
such ha i `<ε=⇒
G3(V) = G2(V)
since Vis bounded and closed, and hence compac .
Ob iously, Gkalgo i hms wi h k < 3 will miss hese small po ions, and will
no be able o o e a comple e econs uc ion unless `is chosen su icien ly
small. Howe e G3may also miss some o hese small po ions i hey do no
gua an ee a 3- econs uc ion.
The di e ences be ween o he se s in he g ey hie a chy canno be so easily
cha ac e ized. Cells in G2 G1a e cells ha con ain a po ion o he olume
ha en e s and exi s h ough aces o cells, wi hou dis u bing any edges.
The e o e hey a e hin po ions o V. Tipically hese oxels a e popula ed
by pipe-like po ions o he olume, bu his need no be so. Gi en a ce ain
ec angula g id R, conside he olume ob ained o illing i up comple ely
( aking he union o all he cells in R), and hen sub ac ing a bi a ily small
cylinde s wi h he edges o axes. The esul is a solid Vwhose olume is
a bi a ily close o ha o R, ye all he cells o Ra e in G2bu no in G1.
A simila case is G1 G0, whe e we can conside V=∪c∈c(R)c ∪ ∈ (R)B ,
whe e B deno es a small open ball cen e ed a he e ex .
These a e ex eme bu un eal examples in p ac ice. We will show howe e
wi h examples o which ex en he p oblem is no so dis an om p ac ical
econs uc ion p oblems.
Finally, no ice ha 3- econs uc ions gua an ee a bound on he Hausdo
dis ance be ween S=∂V and he esul o he econs uc ion:
P ope y 3 Gi en a olume V, and a 3- econs uc ion V0using a g id wi h
8
cell-size `, hen
Haus(V, V 0)≤q(3)`
No ice ha V0in e sec s all he cells in G3, and he e is a po ion o S, and
hence o V, in each cell o G3. Fu he mo e each poin on Vis ei he in a
black cell o in some cell in G3, and a 3- econs uc ion mus p oduce a olume
ha in e sec s ha cell. Since wo poin s in a cell canno be u he han he
s a ed bound, he p ope y ollows immedia ely.
This desi able p ope y, howe e , is no gua an eed o any o he econs uc-
ion le el, as we ha e seen ha in gene al G3 Gk6=∅ o k < 3, and po ions
on cells in G3 Gkmay lie a bi a ily a om he algo i hm’s ou pu su ace.
4 Topological p ope ies o isosu aces
The objec i e o his sec ion is o cha ac e ize he su ace ex ac ion algo-
i hms e iewed in Sec ion 2 by using he p ope ies de ined in he p e ious
Sec ion. A special ema k will be done on discussing he opological p ope ies
o he su aces ha hey econs uc , wi hou conside ing he inal applica ion
desc ibed in he pape s whe e hey a e p esen ed.
4.1 0- econs uc ion algo i hms
Mos o he e iewed algo i hms [6,7,15,17–19,28,32,35] a e a ia ions o he
o iginal Ma ching Cubes algo i hm [1]. They ha e a se o common cha ac-
e is ics: The inpu in o ma ion is he classi ica ion o he g id e ices wi h
espec o he olume ( hey only de ec G0cells), he ou pu su ace is a i-
angle mesh econs uc ed locally o each G0cell. Each iangle o he inal
mesh belongs o a unique cell, and he ou pu su ace S0in e sec s only once
each black-whi e edge o he g id. These p ope ies gua an ee ha he ou pu
su ace s abs all G0cells, so all hese algo i hms a e 0− econs uc ions.
The main di e ence among he o me algo i hms lies in he me hod hey use
o pe o m he local econs uc ion o he iangle mesh M. The e a e wo
decisions o conside : he selec ion o he local opology o Min a cell and
he iangula ion o he esul ing connec ed componen s (shee s) in such a
way ha i has no impac on he selec ed opology. [35] shows ha he only
MC con igu a ions gi ing choices o con ol he local opology a e hose ha
co espond o cells ha ing ambigui y aces (also called X- aces, see Figu e 1),
9
las wo igu es a e gene a ed by [35] using wo di e en op imiza ion s a e-
gies. S a egy used o Figu e 5(e) minimizes he numbe o solid componen s,
while Figu e 5( ) shows he esul o adop ing he oposi e s a egy and i is
he only one eco e ing he ini ial wo shells.
The disc e iza ion o he model in Figu e 6(a) p esen s abundan ambigu-
ous aces and cells. This example shows clea ly how he al e na ing me hods
(Figu e 6(b)) a bi a ily join o sepa a e po ions o he model. The ilinea
app oaches (Figu e 6(c)) ailed o econs uc diagonals. Figu e 6(d) clea ly
shows how he dual con ou ing algo i hm c ea es non-mani old e ices and
edges. Using he same wo s a egies as in Figu e 5, [35] yields he solu ions in
Figu es 6(e) and 6( ). No e ha he opology o Figu e 6(e) exac ly ma ches
he o iginal model.
6 Conclusions
Isosu ace ex ac ion algo i hms di e on how he disc e e in o ma ion in he
g id is gene a ed, on wha in o ma ion does he g id s o e and on he p ope -
ies o he ou pu su ace. Recen algo i hms o e di e en solu ions o he
disambigua ion p oblem and o con olling he inal opology.
Based on a numbe o p ope ies o he g id g ey cells and o he econs uc ion
algo i hms, a cha ac e iza ion o se e al su ace ex ac ion algo i hms has been
p oposed. The classi ica ion in Table 1 p esen s he inhe en limi a ions o
he di e en algo i hms conce ning global opology con ol and econs uc ion
o local ea u es like hin po ions o he olume and almos non-mani old
egions. These limi a ions ha e been obse ed and discussed in some p ac ical
examples.
Mos o he algo i hms discussed a e o ype G0and only p esen local opol-
ogy con ol. Non-global opology con ol usually leads o some a bi a y inal
opology which will no ul ill he use equi emen s and/o a ge s. Gkalgo-
i hms wi h k > 0 a e sca ce. In some cases, e en i He mi e da a a e s o ed in
g id edges, he algo i hms emain G0because hey ope a e by compu ing G0
cells and do no conside g id edges wi h mul iple su ace in e sec ions (wi h
he sole excep ion o [30]). As obse ed in P ope y 2, Gkalgo i hms wi h
k < 3 will miss small po ions o he olume unless he g id edge size is chosen
su icien ly small.
A po en ial line o u u e wo k is he de elopmen o new wo-mani old su ace
ex ac ion algo i hms wi h he 3- econs uc ion p ope y and/o global opol-
ogy con ol o he ou pu su ace. As s a ed in P ope y 3, 3- econs uc ion
schemes exhibi a bounded Hausdo dis ance p ope y ha gua an ee a sim-
16

(a) O iginal model (b) Al e na ing e ahediza ion
(c) T ilinea disambigua ion (d) Dual Con ou ing
(e) Minimizing componen s ( ) Maximizing componen s
Fig. 5. Di e en esul s om di e en G0algo i hms. The model has been gene a ed
wi h i e oc ee subdi ision le els.
ila i y be ween he ini ial olume and he econs uc ed one.
17
(a) O iginal model (b) Al e na ing e ahed iza ion
(c) T ilinea disambigua ion (d) Dual Con ou ing
(e) Minimizing componen s ( ) Maximizing componen s
Fig. 6. Di e en esul s om di e en G0algo i hms. The model has been gene a ed
wi h ou oc ee subdi ision le els.
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