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Advanced Modeling and Surfacing Techniques with AutodeskAliasStudio (Part I)Uwe Rossbacher - Autodesk
MA115-2P Like the real world, the virtual world too provides different ways to model form. This session takesclassic geometry representation, NURBS (Non-Uniform Rational B-Splines) and looks at it from theperspective of a sculptural material. As a technology, there are strengths and weaknesses, but as amaterial, the properties of the math become a part of the allure of the medium.
Take a fascinating and intimate look at surface modeling and learn why this material has become an
essential part of shape definition for all the automotive companies and the top product design firms inthe world.
About the Speaker:Uwe Rossbacher brings 10 years of experience in industrial design and 3D surface modeling in the
automotive industry to his role as Marketing Manager for Modeling/Technical Surfacing at Autodesk,including more than six years with the Alias Studio Tools product development team.
Uwe has a deep knowledge of automotive concept design and technical surfacing and currentlyfocuses on creating and delivering demonstrations, technical marketing materials and field education.Uwe also supports Autodesk product management efforts by identifying and defining market and userrequirements.
During his time with Alias, Uwe focused his efforts on refining the technical surfacing productspecifications according to the needs of customers worldwide, and was a key resource in thedevelopment of many new surfacing tools within AliasStudio. Before joining Alias, Uwe held marketingand technical positions at ICEM. Most recently Uwe worked at the VW concept studio in Potsdam,Germany.
Uwe received a Masters Degree in Engineering and Technical design from the University ofTechnology in Dresden, Germany.
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Like the real world, the virtual world too provides different ways to model form. This session takes a classic
geometry representation, NURBS (NonUniform Rational BSplines) and looks at it from the perspective of a
sculptural material. As a technology, there are strengths and weaknesses, but as a material, the properties of
the math become a part of the allure of the medium.
Take a fascinating and intimate look at surface modeling and learn why this material has become an essential
part of shape definition for all the automotive companies and the top product design firms in the world.
Beforecomputercameintothegame
Back in the days before computers, architects, engineers, and artists would draw their designs for buildings, roads,
machine parts by using pencil, paper, and various drafting tools. These tools included rulers and Tsquares for drawing
straight lines, compasses for drawing circles and triangles and protractors for making precise angles. Of course, a lot of
interestingshaped objects couldn't be drawn with just these simple tools, because they had curved parts that weren't just
circles or ellipses. Often, a curve was needed that went smoothly through a number of predetermined points.
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Thisproblemwasparticularlyacuteinshipbuilding:althoughaskilledartistordraftsmancouldreliablyhanddrawsuchcurves
on
a
drafting
table,
shipbuilders
often
needed
to
make
life
size
(or
nearly
life
size)
drawings,
where
the
sheer
size
of
the
requiredcurvesmadehanddrawingimpossible.
Becauseoftheirgreatsize,suchdrawingswereoftendoneintheloft
areaofalargebuilding,byaspecialistknownasaloftsman.Theterm
loftingoriginallycamefromtheshipbuilding industrywhereloftsmen
workedon
"barn
loft"
type
structures
to
create
the
keel
and
bulkhead
formsoutofwood.Thiswasthenpassedontotheaircraftthen
automotiveindustrieswhoalsorequiredstreamlineshapes.
Theresultingcurvesweresmooth,andvariedincurvaturedependingonthepositionoftheducks.
Toaidinthetask,theloftsman wouldemploylong,thin,flexiblestripsofwood,plastic,ormetal,calledsplines.Thesplineswere
heldin
place
with
lead
weights,
called
ducks
because
of
their
resemblance
to
the
feathered
creature
of
the
same
name.
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Ascomputerswereintroducedintothedesignprocess,thephysicalpropertiesofsuchsplineswereinvestigatedsothatthey
could
be
modeled
mathematically
on
the
computer.
TheDuckswerereplacedwithsocalledControl
Vertices(CV).
Atfirst
NURBS
were
only
used
in
the
proprietary
CAD
packages
of
car
companies.
Later
they
became
part
of
standard
computer
graphicspackages.
Realtime,interactiverenderingofNURBScurvesandsurfaceswerefirstmadeavailableonSiliconGraphicsworkstationsin
1989.In1993,thefirstinteractiveNURBSmodeller forPCs,calledNRBS,wasdevelopedbyCASBerlin,asmallstartupcompany
cooperatingwiththeTechnicalUniversityofBerlin.
DevelopmentofNURBS(NonUniformRationalBasis,orBzier Spline)beganinthe1950byengineerswhowereinneedofa
mathematicallyprecise
representation
of
freeform
surfaces
like
those
used
for
car
bodies
and
ship
hulls,
which
could
be
exactly
reproducedwhenevertechnicallyneeded.
ThepioneersofthisdevelopmentwerePierreBzier whoworkedasanengineerat
Renault,andPauldeCasteljau whoworkedatCitron,bothinFrance.Bzier worked
nearlyparalleltodeCasteljau,neitherknowingabouttheworkoftheother.But
becauseBzier publishedtheresultsofhiswork,theaveragecomputergraphics
usertodayrecognizessplines(whicharerepresentedwithcontrolpointslyingoff
thecurveitself)asBzier splineswhiledeCasteljaus nameisonlyknownandused
forthealgorithmshedevelopedtoevaluateparametricsurfaces.Inthe1960sit
becameclearthatnonuniform,rationalBsplinesareageneralizationofBzier
splines.
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Thelayoutofthepatchesisthebasicknowledge
Often surface modeling is described as a magic thing. The modeler community mostly keep the secret of how to model the
highest quality level, called Class A. There is a lot of myths about it around. Following are described some techniques to
find and train patch layout. This knowledge is independent from the software that will be used and fits most of the time
even traditional clay modeling techniques.
1Howwouldyoumakeitinfoam?
Tofindthefirststepsinbuildingasurfacemodelithelpstoimaginehowthisprocesswouldbe,whenusingablockoffoam and
asawtoworkouttheshape.
Similartothemostpreferredsketchingview,theNURBSmodelerstartstodevelopthecenterline.Thisisthesectionofthe
modelthatlaysinthemiddleofitdefiningthesideview.Buildingthecurvesofthecenterlinethemodelergetsanideaofhow
manycurvesheneedstodescribetheshapeoftheobject.
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Centerlinesurfaces
1Allsurfacescrossingthecenterlineshouldbebuildasone
Beziersurface.Thishelpstoavoiddiscontinuitiesofthe
surfacesat
the
centerline.
Lots
of
designs
require
astrict
radiusbehaviorofthesurfacesinacertainareaclosetothe
centerline.Theonesurfaceacrossthecenterline
approachhasthedisadvantage thatwhenCVmodification
onthesurfaceisrequired,alwaysthecorrespondingCVhas
tobepickedaswell,tomaintainthesymmetry.Thiscosta
bitmoretime,butthepriceis,thatthesurfaceisalways
100%smoothacrossthecenterline.
Widthofthecar
2Differenttoaclassicalapproach(atfirstlayoutsomecurves
todescribethesurfacesbuildingthewidthofthecar)often
theuse
of
simple
surface
planes,
their
positioning
and
some
directmodelingisthefastermethodthatfulfillstheideaof
modeling.Inclaythedesigneralwaysseesthematerial,the
volume.Itcanbesaidhatthereisalwaysphysicalproperty.
Thesamewouldcomewithusingshadeable propertyin
softwareprocess. Thatswhyitisgoodtoshowinvirtual
worldassoonaspossiblesurfacesthatcanbeshaded.
Amodelingworkflowbasedonsimpleplanesrequiresafastreparameterizationofthoseplanes.Whentheformer
planesalreadyareshapedandwithcertaindegree,itisoftentootimeconsumingtochangethepositionofeachCV
individually.Areapproximationtechniqueismuchfaster.Thesurfacewillbetrimmedandthenregardingthistrimedge
recalculated.Theresultisacompletelyreshapedsurfacedonebytheinternalreapproximationalgorithm.
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Marrytheparts
3Withthisstepitbecomesclearifthepartsdefiningthewidth
ofthemodelfittogether.Forcarsitisimportant,thatthe
crowningof
the
side
window
and
the
body
side
fit
together.
Thiscanbeverifiedbuildingtheshouldersurfaceswith
correctangleconditionstobothsidesurfaces.
SeetangentangleexamplethatwillcomeinPart2!
RockerandFillets
5
connectionandmuscles
4
2Detectthefeaturelinesandbuildtheslabsfirst
Sometimesthe
modeler
just
gets
ascan
of
aphysical
model.
For
further
design
iterations
this
scan
has
to
re
modeled
in
NURBS
(ReverseEngineering).UsingGaussianShaderthemoldercandetecttheareasofcurvaturechangeonthemodel.Thisgivesa
goodhintregardingthelayoutofthesurfaceboundaries.
Scandata(polygons)ofaphysical model
Thecolorizedareasgivesaprettygoodhintaboutthemajorslabsurfacesthathavetobebuildfirst.Itsimportanttosaythat
thosephysicalmodelsusuallyarefilletedandwithalltransitionsurfaces.Thereforeslabswillnotappearanymorewiththeirreal
sizebutpartialhiddenortrimmed.Themodelermustextendthecolorizedareastocreatetheslabsintheiroriginalsize.
Itsagoodmethodtofittheslabsfirstonthesmallerportion(liketheyappearonthefilletedphysicalmodel)toextendthem
afterfitting.Whenthefitiswithsmoothandlowdegreesurfaces,theextendwillprovidereasonableresults.
InAliasSudio Blendcurvescanbecreateddirectlyon
topofthepolygons(Mesh).
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Stilltherearecaseswherejustasectionmodelis
available.Herethemodelerfindstheslabsby
interpretingsectionlines.
Oneapproachistoisolatesmainsectionlines.Then
theywillberebuildwithNURBScurves.Thiscanbe
donewithallthemainsectionlinesandisasave
methodtofindtheareaswhereaSlabcharacterturns
intoaTransition character.
?
?
Thesectionlinestillisonecontinuesline.
Thebluesectionintheimagebesideis
withthreeparts,twomainpiecesand
onetransitionpiece(fillet).
Themodeler
needs
to
find
the
points
wherethefilletgoesintothemain
curves.
Sectiondata(curves,rawdata)ofaphysicalmodel
Thenthemodelerbuildthetransition
curve.Itishelpful,whenthecurvehas
somekindofhistorysothatthemodeler
canmovetheendpointsofthetransition
curvealongthemaincurvetofindtheright
point,wherethetransitionstarts.In
AliasStudiotheBlendcurvetypeisidealfor
thisjob.
Thecurvature
plots
calculated
on
the
createdcurvesalsocangiveagoodhint
aboutthetransitionpoint.
Firstthemodelercreatesthetwomain
curves.Heoverbuildsthesectionline.Itis
withthe
modelers
skill
to
interpret,
where
themaincurvesleavesthesectionline.
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Withthismethodallpoints,whereaslab
partgoesintoatransitionpart,canbe
marked,like
in
the
image
beside.
This
knowledgeisthebaseforbuildingmain
surfaces(slabs)andtransitionsurfaces.
Forexampleasimpledraftsurfacewillbe
created.Thisfirstsurfacedoesnthaveto
fullyfitthesectionsyet.
Interpretingeveryorlotsofscanlinesseemsabittimeconsuming.Thatswhythereisasecondapproachusingsurfacesto
interpretmanysectionlinesinonestep.Forthat,theuserworkswithsimplesurfacesandvisualsections,cutthroughthese
surfaces.Thesurfaceshastostayrelativelysimple(movinghullsinsteadofmovingeverysingleCV)andthevisualsections givea
permanentfeedbackabouttheleveloffitting.
Basedonthefirstdraftsurfacesanotherdraftwillbecreated.Itisveryhelpfulwhenthisseconddrafthasahistorical
connectiontothefirstdraft.Inthiscasethesecondsurfaceshouldjustfitthesectionlinesintermsoftherightangle(being
parallel).
Fittingthefirstdrafttothesections,theseconddraftalwayswillfollow.Theseconddraftwillbefitmodifyingthefirstone
only.Withthismethodthetheoreticallinecanbedefinedveryfast.
Calculatevisualsections!
Adjustthedraftangletofollow
thesectionlines!
FitthissurfacebymovingCVs!
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Finishingasurfacemodeloftenitcomestoareasthataredifficulttoproceedwith.Typicallytheseareareaswheremorethan
tworadiicometogether,orwheretransitionsurfacesarewashingout.
3Closetheentiremodelfirstintheory
Agoodmethodforthosecasesistoclosethemodelfirstjustusingsharpedges.Allfilletsshouldbeavoided.Oftensurfaces
mustbebuildthatwontevenappearafterfilletingandthatusuallywontbeconsideredfindingtherightpatchlayout.
Thenextstepistobuildthefilletsaslongastheyreachwithoutgettingcomplicatedorwashedout.Thisstepisrelativelyeasyto
do.Justareaswithaclearlydefinedfilletsituationwillbeworkedon.
Theleftoverregionsarewashoutsormagiccornerswheremanyradiicometogetherandhavetobeblended.Tofilltheseblend
regionsfirsttheentrylinesfortheblendshavetobemarked.Asageneraladvicethenaturalflowofthefilletedgesshouldbe
continued.Itsobviousthatthesurfacesbetweenthesecontinuedfilletedgesactasablendandisnotafilletanymore.Withthis
methodmostofthosesituationscanbemanaged.
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1
2
3
4
Thenaturalflowofthefilletedgeswillbemaintainedand
blendedintothenaturalflowofthewheel.
But!
Thismethodhastheadvantagethatthesoftwarecontrolstheinternalcontinuitybyitselfandthedigitalsculptorhasthe
freedomtoconcentratejustontheformheisworkingon.Itseemsthattheabilityofmodifyingtheshapeofthoseonesurface
objectsareendlessbutwhenitcomestoconcreteengineeringorpackagerequirementsoftenthismethodhaslimitations.The
modelerchecksthetechnicalrequirementsusinganalysestoolsandadaptsthesurfacemodelviamovingCVs.Hecannotuse
thetechnicalinputdirectlyasabaseforhismodellikehecoulddoitusingatraditionalpatchlayout.
Sometimesestablishingaclassicalpatchlayoutcanbecometootimeconsuming.Thereareshapeswhereitisbettertojust
sculpttheentiremodeloutofonesurfacebyinsertingsegments (raisingitsmathematicalcomplexity).Itiscomparableto
craftsmanshipwhere
the
sculptor
cuts
the
form
out
of
one
block
of
foam.
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Thedesignoftheshowerheadwasdefinedbasedonthesplittinglinealongtheentiremodel.
Theengineerhaddifficultiestoestablishthepatchlayoutbasedonthisline.Themostcriticalareaisonthetipoftheshower
headwheretheclassicallayoutjustoffersaretrianglepatch.Ontheotherhand,theengineerfoundtoomanysurfaces.That can
bedifficultbecausethehighernumberofsurfacesmakesitmoredifficulttocontrolthehighlightflowthroughtheentiresurface
set.
Adigitalsculptor usedtheonesurface
approach.Healmostmatchedthesplitline
thatwasgivenasatechnicalrequirement.
Literallyhecamecloserandcloserby
adjustingtheCVs.Savingpatcheshecould
easilycontrolthehighlightofhispatch.
Thismethodisnotasaccurateasbuilding
surfacesbasedonthesplitlinebutithelps
tosculptorganicsurfaces.
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SurfaceEvaluation
Software renders surfaces using shader, environments and shadow calculation to become close to the reality. But i n surface
modeling the stakeholders just need to evaluate the s hape of a sculpture. Colors and si milar effects are trouble. Therefore in themodel shop models will be coated just in white (reduced to the necessary) to get the pure s hape, avoiding any distraction by
visualization gadgets. Only shadows play a role to evaluate form.
NURBS technology offers another method of surface evaluation. Skilled software modelers are able to judge surfaces in accordance
the CV spacing and their flow. In competition to shaded evaluation techniques the CV picture becomes an reasonable instrument
for surface evaluation.
1Shading
Toevaluate
Surfaces
in
Clay,
the
molder
works
with
foil,
called
Dynoc.
A
little
bit
of
water
on
top
of
the
smooth
clay
surface
sucks
theDynocontoit.Nowtheenvironmentreflectsonthefoilandanyinconsistenciesofthesurfaceshapewillbevisible.Ifthe
shapeoftheclayshouldbeimproved,theDynochastoberemoved.
UsingDynocinclayprocessthemodelercannotwatchthereflectionswhileheisscrapingtheClay.Insoftwareprocessthe
modelercanmodifythesurfaceswhilethereflections(diagnostics)areon.Thisisahugeadvantagetoreachsurfacequality much
faster.
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Itisimportanttoknowhowtheevaluationtool
calculatesthereflectionlines.Additionaltothatitis
goodwhenthelightsourcecreatingthereflectionlines
canbemovedabovethemodel.
InAliasStudio the
Iso Angle
Tool
is
the
most
accurate
evaluationregardinghighlightsonthesurfaceset,even
whenthesehighlightsarenotcomparablewith
reflectionsintherealworld.
MostsoftwareforNURBSmodelingoffershadingtoolsthatvisualizetheselectedsurfaces.Butoftentheusercalculatezebra
stripes,whichareoftensphericalenvironment.Thisevaluationisnotgoodenoughtojudgehighendsurfacequality.
ReasoningbehindIsoangleanalyses
Isoangleliterallymeans"alineofconstantangle"ona
surface.Hereisananalogy:onamapoftheworld,
sometimesyoucanseewhatarecalled"isotherms"
thesearelinesofconstant temperature.Similarly,"iso
bars"arelinesofconstantbarometricpressure.These
lineareusedtoindicatetemperatureandpressure
trendsontheglobe.
Isoangle,thetool, bydefault,showsyouthelineof
zeroanglebetweenthesurfacenormals ofthesurface
andaninfinite"lightsourcedirection".Infact,you
shouldstopthinkingofthisasalightingkindof
calculation,butasananalysisofthesurfaceproperties
purely.Thepointedarrowintheinterfaceisthereto
justindicatethedirectionwithwhichthezeroangleis
beingevaluated.
Isoanglethereforeprovidesaverytightandaccuratewayofevaluatingthepropertiesofasurface.Itwillindicatesurface
properties(like
trends,
continuity
across
boundaries
etc.)
way
more
accurately
than
spherical
maps
of
zebra
stripes.
Whenusedinthemultipleband(thecoloredbands)mode,eachcoloredbandisindicatingalineofconstantanglewiththe
directionofthevector forexample, thewhitebandcouldbethezeroangle,theredonenexttoitcouldbethelineofconstant
5degreeangle,
thegreenonenexttoitthe10degreeangleetc.Thiskindofevaluationiscalledisophotes actually,andisusedinmathematical
analysisofsurfaces.
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2CVpicture
Theanalogybetweenducksusedbyloftsman andCVsinNURBStechnologyisaperfectwaytounderstandtheimportanceof
agoodCVflowtogetgoodsurfaces.
NURBSmodelingsoftwareoftenoffersvariousUserInterface
capabilitiesforsurfacescreationandmanipulation.Butallthisis
justamask.ThecommongroundisalwaysthepositionoftheCVs.
Foraloftsman theduckplacementwastheonlypossibilitytogeta
nicecurvewithaestheticaccelerationandsoistheCVplacement
foraNURBSmodeler.Sothesecretofwellshapedsurfacemodelsis
withtheirCVlayout.ChoosingaNURBSmodelingsoftwareit
shouldbetakencare,thatCVscanbevisualizedandmodeled
directly.
CommonrulesaboutCVspacing
Asafirstruleitcanbesaid,thattheCVshastolookniceandtheyshouldflowbeautifully(thereareseveralexceptions whenit
comestotheCVflow).ExperiencedNURBSmodelerscanreadtheshapeofaNURBSmodelbyinvestigatingtheCVsonly.
Butwasisniceandwhatisbeautiful?Somepeoplesay:Beautyisthephenomenonoftheexperienceofpleasure,throughthe
perceptionofbalanceandproportionofstimulus.Itinvolvesthecognitionofabalancedformandstructurethatelicitsattraction
andappealtowardsaperson,animal,inanimateobject,scene,music,idea,etc.
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EvenwhenthejudgmentofCVstructureisrelativetherearesomerulesintroducingtwoparameters.
A lengthofthelinethatconnectstwoCVswitheachother CVspacing
B lengthoftheNormalthatisbetweenaCVandtheunderlyingcurve/surface
A
B
Judgingbeautyisalwaysdependedontheaudienceandrelative.ToillustratethisthereisanicequotefromFrancisBacon.
Thereisnoexcellentbeautythathasnotsomestrangenessintheproportion.Technical surfacingexistsbetweencreativityofdesignandrequirementsofEngineering.AnydefinitionofBeautyseemstobe
relativeanddoesntfitinstrictengineeringrequirements.ButtheSurfacemodelerneedstoputagoodunderstandingofa
beautifulsenseinhisworktocreategoodsurfacesetsandthereforealotofinspirationisavailable.
2.ConcentrateCVswherethegeometryshowsmorecurvatureinitsshape.Agoodanalogyistoimaginethespeedofthehand
whendrawingacurvewithapencil.Wherethespeedisslow,theartisttriestocreatecurvedshape,theCVsshouldbe
concentrated.Where
the
speed
gets
faster,
less
CVs
are
needed
to
describe
this
portion.
1.Both Parameters shouldchangesmoothlyandwithinsymmetryifappropriate.DrawingthechangeofAandBinagraph,the
resultingcurves,mustbenice,smoothandnotwavy.Thereisnoengineeringruleaboutthesecurves.Theusermustconsider
hisown
sense
of
beauty,
symmetry
and
proportion.
!
!A/B
CV#1 CV#2 CV#3#ofCV
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PositionG0
3SurfaceContinuity
Thiscontinuityspecifiesiftheedgeoftwosurfaces(naturaledgeortrimmededge)fitwithinagiventolerance.Ifthegeometry
edgeorendpointfitswithinatolerancetheresultisgoodintermsofG0continuity.Thismeasurementcanbequalifiedasa
digitalcheck.
Whendefiningaform,animportantfactoristhecontinuitybetweensurfaces howsmoothlytheyconnecttooneanother.
Oneexampleofwheresurfacingexcelsisautomotivebodypanels.Iftwocurvedareasofthepanelhavedifferentradiiof
curvatureand
are
blended
together,
maintaining
tangential
continuity
(meaning
that
the
blended
surface
doesn't
change
directionsuddenly,butsmoothly)won'tbeenough.Theyneedtohaveacontinuousrateofcurvaturechangebetweenthetwo
sections,orelsetheirreflectionswillappeardisconnected.
Thecontinuityisdefinedusingtheterms G0 position(touching)
G1 tangent(angle)
G2 curvature(radius)
G3 acceleration(rateofchangeofcurvature)
Tangency G1
Thiscontinuityspecifiestheanglebetweencorrespondingnormal'sthatstandsontheedgesofthereferenceandthegeometry.
IfthisangleiswithinatolerancethemeasurementisgoodintermsofG1continuity. Thismeasurementcanbequalifiedasa
digitalcheck.
Additionallytothemeasurementofthesurfacenormal'sthereisacheckintermsofcolinearityofthetangents(linesconnecting
theCVs).Thiscriterionisinterestingjustforcontinuitymeasurementsbetweentwonaturaledges.NoteveryNURBSmodeling
systemoffersthischeck(AliasStudio)butitshouldbeinvestigatedbyeveryuser.
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CurvatureG2(radius) AccelerationG3(rateofchangeofcurvature)
Thecontinuitymeasurementintermsofcurvatureshouldnotbejudgedasadigitalcheckanymore.Inhereitismoreimportant
toevaluatetheflowandthesmoothnessofthecurvatureacceleration.Aperfectcurvatureresultjustcanbeevaluatedusing
curvatureplots.
CurvatureG2/G3
4Curvatureplots
WhenitcomestocurvaturemeasurementlotsofsystemsofferlocatorsthatmeasureG2continuityonthepatchbordersand
thesetypicalzebrastripestoevaluatehighlights.
Thesetwosurfacesobviouslylooksfinetogether,butthisisnottrue.The
evaluationtoolsarejustnottherightones.
TheG2locatorsimplycomparesthevalueofcurvatureonbothsurface
edgeswithinacertaintolerance.Thisinformationisnotenoughintermsof
judginghighlightquality.Eventhezebrastripesoftenarejustaspherical
maponthesurfacesetanddoesntgiveausefulfeedbackaboutthe
smoothnessoftheentiresurfaceset.Theydontshowaccelerationof
curvaturewithintheshape.
ThatswhyitmakesnosensetojudgethehighlightqualityofasurfacesetjustusingG2checker.Thesurfacesetmustbecutand
curvaturecombsmustbecalculatedonthesections.
C
Justacurvatureplotonasectioncutthroughthesurfacesetshowsthatthe
accelerationofthecurvatureofbothsurfacesdoesntmatch.SureG2
continuityisachieved,butitsisnotagoodresultregardingtheflowofthe
highlights.
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Thiscurvaturecombshowawell
balancedG3result. Thisjudgmentis
withthe
skill
level
of
the
operator
and
canbequalifiedasanalog,because
thereisnotacertaintolerancethat
wouldmakesenseallthetime.
Thisresultiswellbalancedtoo.The
smallstepinthecurvatureplot(G2
break)doesnthaveanoticeable
impactonthefinalhighlightpicture.In
fact,thissurfacesethasonlyG1
continuitybutitisstillgood.
Itisveryhelpfultoseethetwosurfacesbesidemilledout.OftenitbecomesclearthatastrictG2achievementisnotnecessaryto
reachhighendsurfacequalityintermsofvisualhighlights.
ClassA
For reaching the top there must be always a bar, a measurement, a level. In freeform surfacing such a top level can be found
in the automotive industry. Here is enough potential to build surfaces as good as possible and to accumulate knowledge
and skills to create those high end surfaces called Class A.
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ClassAsurfacesisatermusedinautomotivedesigntodescribeasetoffreeformsurfacesofhighquality. Although,strictly, itis
nothingmorethansayingthesurfaceshavecurvatureandtangencyalignmenttonearperfectaestheticalreflectionquality.
ThepointwhenClassAqualityforthefirsttimeisrequiredisnotfixed.Asmoreasthedesignbecomesfinal,asmore
engineeringandfeasibility requirementshavetoberecognized.Acleanandlightsurfacestructureisthebasetoincorporate
theserequirementsinadigitalmodel.ThefinalresultisasurfacesetthatwillbegiventotoolingortofurtherCADdepartments
anditsfinalqualityleveliscalledClassA.
Inautomotivedesign,thefirstvirtualsurfacemodelsappearattheconcept stagealready.Afastvisualizationofshapeand
proportionisvalid.Basedonsketches,packagesandscannedquarterscalemodels,thedigitalsculptorcreatesthefirstsurface
model.Atthisstagetechnicalrequirementsdoesntplaysucharole.Theinterpretationofthestylingideaisthemainapproach.
ItscalledConceptModeling.
1WhatisClassA?
2TwodifferentaspectsofClassA
Curvaturecombs
G3
Accelerationofhighlights
kneeratio
Tolerances
Splitlines
Firstflange
Panelgap
Fullyfilleted
ThechallengeforClassAmodelingistocombinetheaestheticsuperioritywithhardrequirementsfromengineering.Asurface
setthatappearsjustbeautifulbutwithoutmeetingengineeringrequirementswillnotbeconsideredasClassA.
AsthedigitaldesignprocessgetsmaturelessDataControlModelsgetmilledoutbutthequalitylevelremains.TheStudiostry to
pushthepointofdesignfreezemoreandmorebackwardstogaintimetoimprovethestyling.ForClassAmodelingthismeans,
thatithastobeestablishsometimesalongtheprocesstobereadywhentheprojectisdone.
HighlightQuality Engineeringrequirements
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ClassAmodelingmeansalotofinvestmentintime.Duringextensivedesigniterationsoftenitmakesnosensetorequirethe
highestqualitylevelofsurfaces.InthepastClassAqualityoftenwasrequiredjustafterthedesignfreeze.Thispointwas
reachedwith
aso
called
Data
Control
Model
done
and
available
only
in
clay.
This
model
had
the
final
design
features
included. It
wasscannedandthenthemodelhadtoberecapturedasNURBSinClassAqualitytoservethefollowingCADprocesses.Sothe
ClassAmodelingwasmainlyusingscandataasinputandacertaindeviationtolerancewasgiventothemolder.Itwascalled as
reverseengineering.
Thoserequirementswereidealforoutsourcing
thework.ThatiswhytodaymostoftheClassA
modelingworkisdonewithTearOneSuppliers
aroundthebigautomotivecompaniesandso
mostoftheknowledgeandskillsaboutit.Itsan
interestingphenomenonthatthebestClassA
modelersareworkingascontractorsforthose
suppliersandcannotbefoundasemployees.
3ReverseEngineering
Tomakethedesignprocessmoreeffective,thestudiosengageengineersearlyonlookatengineeringrequirements
to
get
guidance
from
it.
Another
point
to
make
the
process
more
effective
is
to
improve
the
surface
quality
of
the
firstmodelstomakethemreuseableforthenextdesignsteps.Someautomotivecompaniesevendontspeak
aboutconceptualdesignanymore.Theyestablished aprocesswherethedesignmodelingflowsmeaninglessinto
engineeringandtooling.
4AndwherecouldClassAgointhefuture?
AcommunityofmodelershasestablishedgeneralcriteriafordatatobeconsideredclassA. Priortocheckingedgecontinuity,curvature,highlights,orshapesitisthesurfacestructurethathastobewellestablished. InpresentationsregardingclassAsurfacingItrytomakeitclear:ThestructureofthesurfacedataisthebasisofClassAsurfacing.DigitalDesigner BarryKimball NissanDesignAmerica,FarmingtonHills,Michigan
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SurfaceModelingParadigms
In comparison with traditional modeling virtual modeling is still very young. It heavily depends on the mathematical
progress. The tremendous development speed of technology every year opens new kind of workflows. Especially modeling
techniques from the Entertainment industry are influencing workflows in the Automotive industry. Polygon modeling and
Subdivision Surfaces are just a few topic coming from there. Regarding NURBS modeling three paradigms will be touched
on the following pages.
Theimageshowstwocurveshavingthesameshape.
Theuppercurvehas6CVsand3segments(4editpoints).ItsaNURBScurve
Thecurvebelowhas6CVsandjust1segment(2editpoints).ItsaBeziercurve.
1NURBSversusBeziermathematic
MostsurfacemodelingsystemshaveacommonmathematicalfoundationcalledBSplines.SomeofthemcreatesNURBS
geometryandsomecreatesBeziergeometry.MathematicallyasinglesegmentNURBSsurfaceisequivalenttoasingleBezier
surfacepatch.
Mathematically
Bezier
surfaces
are
asubset
of
NURBS.
AliasStudio is
capable
of
creating
NURBS
and
Bezier
surfaces.
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WhenmovingthemarkedCVsitbecomesobviousthataBezierCVinfluencestheentirecurveandaNURBSCVjusthas
influencewithinitssegment. Thispartialinfluenceincreasestheriskofwavycurvatureplotandwiththistheriskofabad
highlightpicturewhenitcomestosurfaces.Thatswhyoftenitisrecommendedtoworkwithsinglesegmentedgeometryknow
asBEZIERmathematic.Ithasthedisadvantage ofmoreworktakingcareforthesurfaceconnections,butalmostguarantees good
highlights.
Usingmultisegmentgeometrycanbeanadvantageinseveralsurfacemodelingsituations.ButsoftwareisunabletoprovideG3
continuitybetweenthesegmentswhenthereisnotenoughmathematicalfreedom.
ToavoidinternalG3breaks(likeshownbelow)thegeometryshouldalwayshaveaminimumdegreeof5.Thedegreevalidation
usedinhereisforAliasStudio.Othersoftwarehavedifferentrulestospecifythedegreeofasurface.Basicallythegeometry
musthavesixControlVerticestoavoidinternalG3breaks.
Asolutionwherethedegreeofthegeometrydoesntcountiswhenusingstrictlysinglesegmentation approach.Thisisknown
asusing BezierSurfaces.
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CADsoftwarepackagesusetwobasicmethodsforthecreationofsurfaces.
Thefirst
begins
with
construction
curves
(splines)
from
which
the
3D
surface
is
then
swept
(section
along
guide
rail)
or
meshed
(lofted)through.
Thesecondmethodisdirectcreationofthesurfacewithmanipulationofthesurfacepoles/controlpoints.
Bothmethodshavetheirstrength.Theirusedependsonthemodelingsituation.
2Modelingmethod
Curvenetworkapproach:Fitsdesignerswhothinkinfeaturelines.
Givesafast3Dinterpretationoftheintendedshape.
traditionalpatchlayoutbasedoncurves:LacksincontrollingtheinnerCVs
Requiresahugeeffortofplanningtokeephistoryof
edgecurvesandsurface.
Thesurfaceisaresultofcurvemodificationandcannot
beshapeddirectly.
Surfacesruledbyparameters:Additionallytocurvessurfacescanbedefinedby
parameterslike
radii,
vectors,
length.
Servesengineeringapproach
Proceduralmodeling
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Withthistechnique,NURBSbecomesthefirstlegitvirtualmaterial.
Directmodeling
Directmodelinglivesofof CVmanipulation.Todoso,itisneededtoholdtheoverallnumberofCVsdown.Toaligntwosurface
witheachother,aspecialalignormatchingcommandisneeded.Thisimpliesmassagingeverysurfacematchindividually.
Currentlyitisthebestwaytoensureatophighlightqualityoftheentiresurfaceset,butitrequiresagentlesurfacelayoutthatis
basedonprimarysurfacesandsecondarysurfacesthataredependentonthis.
ItmaysoundthreateningtomanipulateallCVsindividually.Butaslongaspossible,thedigitalsculptormanipulatesentireCV
rowsorusestoolsthatallowamodificationofmanyCVsbasedonarulehowtheinfluenceoneachCVgraduallybecomeless.
PracticalModelingTechniques
One helpful approach to understand and master digital surface modeling is to base design workflows on proven methods from the
traditional craftsmanship and translate them into the world of computers, software and NURBS.
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1Surfacesmightbebiggerthantheyappearonafinalmodel
Thebasicsurfaces (oftentheyarecalledslabs)appearonthefinalmodelalwaystrimmed.Agoodanalogyisthecraftsmanshipofclay
modelingwherethemodelermovesafixedtemplatealongaguide.Thetemplatecutsclaywhereittouchesthevolumeanditcutair
to.Theentiresurfacethetemplateiscuttingisexactlythesurfacethathastobemodeledinsoftware,evenwhenthetemplatecuts
airandthecorresponding digitalsurfacestandsout.Thedigitalsurfacewillbetrimmedlater,butitsbasicshapemustbemodeledfirst.
Thisdependsonthestageof
thedesignprocess.Production
carsthat
has
been
modeled
afterdesignfreezehavea
differentlayoutthatnot
necessarilyfollowingthisrule.
Thecolorizedwheellipshould
getacertainhighlightflow.To
achievethis,theunderlying
trimmedsurfacesmustbe
shapedtosupportthefinal
sculpturingofthewheellip
surface.
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Thebluefilletsurfacesetmusthaveanice
highlightflow.Butbecauseoftheholeinthe
bumperitlookslikethereisnosurfacethe
twomiddlefilletpiecescanbealignedto.
Thesurfacesaroundtheholeinthe
bumpershowsareactuallybiggerandthey
areconnectedestablishinganicehighlight
flow,evenwhentheyaretrimmedlateron
describingthe
hole.
Butonlythiscurvatureconnectionallowsto
alignthebluefilletsallwaylong,eventhere
wheretheyappearashangingintheair
(whenthemodelfinallyistrimmed).
Thisisanexamplehowsurfacesthatevenwontbeseeninthefinalmodelholdafeature(bluefillet)together.
2Angleconditionsalongsurfaceboundaries
Finalsurfacesetsarealwaysfilleted.Thosefiletsoftenplayasignificantroleexpressingfeatures.Inthekneeofafillettheremight
beanultrashinyhighlightgloss.Thetangentlinesofafillet(thelineswhereafilletstarts)marktheregionwhereaflatandslow
highlightfrom
the
big
slab
surfaces
starts
to
speed
up
into
the
much
more
curvy
fillet.
Soitisclearthattheshapeofsuchfilletsareimportantforthedesignexpressionintwoways:
NiceanddeterminedRadiusentrylines Constant orsmoothchangeofkneeradiitoensureanicehighlight.
Theshouldersurfaceusuallywillbebuildwith
positioncontinuity(FIXED).
Buildingthefilletjustwiththissimplepositioncontinuityisthereasonoftheproblemwiththefilletdescribedfollowing.
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Fillettobedone
Theresultingfilletoftenhasproblemsregardingtheentrylinesorthehighlightalongthefilletknee.
Thisfilletisaconstantfillet.Theradiusentrylinesshowan
unevenandbadbehavior.
Thisfilletisachordalfillet.Thethicknessofthehighlightat
thekneevariesunevenly.
Thereasonforthisbadfilletbehavioristheunevenchangeoftheanglebetweenbothsurfaces.Inthiscasetheanglebetween
thetwosurfacesbecomessmallerandattheenditbecomesbiggeragain.Thetangentcheckershowsaninflectionoftheangle
change(in
the
middle).
Thissimpleexampleillustrateshowarolling
ballwouldmarkunevenradiusentrylineon
bothreferencesurfaceswhentheirangleto
eachotherisnotcontrolled.
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Controllingtheanglechangingfromoneendtotheothergivesamuchsmootherandcontrolledresultforthequalityofthefinal
fillet.ThetangentcheckershowsjustthemaximumandtheMinimumoftheangleexactlyatStartandEndofthefillet.
InAlias
this
can
be
controlled
using
the
TANGENT
ANGLE
option
in
the
surface
creation
tools.
Thisfilletisaconstantfillet.Theradiusentrylinesareeven
(linearinterpolationbetweenthetwoends).
Thisfilletisachordalfillet.Thethicknessofthehighlightat
thekneevariesevenly(inaccordancealinearinterpolation).
1
2
BothSlabsurfaces willbecutfirst,usingalong
blade.Thecuttingedgeofthebladecouldbe
imitatedin
Software
by
aprofile
curve.
ThemodelerhastobalancethecrownofbothSlabs
toeachother.Hecandothatbyexaminationof
bothsurfacesfromthesideandthetopview.
Thefinalindication isthesharpedgebetweenboth
Slabs.Itsshapeshowsifbothcrownsfittoeach
other.
Todefinethetransition(oftenitsaradius)the
modelertapesthetwoentrylines(Tangentoffset)
onthesmoothslabs.Themodelerexamsbothlines
bycheckingthemfromtheappropriateview.
3TheoryModeling
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4Usingarake,themodelercutstheclaybetweenthe
tapes.Afterheroughlyachievedtheshapeofthe
transition,heusesatemplatebladewithacutting
edgethatwasmadeaftertheprofilethetransitional
surfaceshouldhaveorafterthespecificradius.
Themodelerslidesthebladealongbothtapesto
nottouchingthealreadyfinishedslabs.
Thelaststepistoremovethetapesandtocarefully
smooth
the
entire
surface
set.
Whenthetransitionshouldbearadius,themodeler
usesadrawingcircletogettherightdistancefor
eachtape
from
the
sharp
edge.
3
Afinishedsurfacemodelalmosthasnosharplines.Everythingiswithfilletsandtransitionsurfaces.Butrecognizingmodeling step1
theremusthavebeenasharptheoretical linebeneaththosefillets.Regardingthisitisaninterestingexperimenttoinspect thehood
ofacar
like
shown
below.
The
inspector
moves
along
the
front
of
acar
and
notices
there
three
vital
points
(A
B
C)
where
the
feature
of
thehoodappearsinthreedifferentways.
C
B
A
Thereisoneposition(B),wherethevirtualtheoretical
line,and
with
this
even
the
fillet
that
is
build
on
top,
appearsstraight.OnPositionAitappearsconcave,on
positionCitappearsconvex.SoinBthetheoretical
lineMUSTbestraight,meansplanar.
AB
C
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A
C
B
Hereisanexamplefromindustrialdesignwherethetheoreticallineisnotcrispandcausesirritationwiththefinaldesign where
filletsareincorporated.
flat Cur ved(convex) Curved
(concave)
Thiseffectisabasicruleintechnicalsurfacingtomakemajorfeaturesappearcrispandstrongfromalldifferentviews.Software
productsoffertoolsthatcanplanarizeacurveorasurfaceedgetogetthiseffect.Theplanethatisthebaseforsuchcurves
rarelyappearsparalleltothethreemainviews (side,frontandtop).Soitisneededtobeabletomodifyacurvesetwhilethe
curvestillmaintainsitsplanarcharacterin3D.
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NURBS modeling is a fascinating modeling technique only existing in the virtual world. When we understand it
as tool among others used in the design process, interacting with the real world via milling and scanning, it can
become a powerful next generation element in Design and Styling.
Advanced modeling and surfacing techniques
with Autodesk Alias Studio