advanced modeling and surfacing techniques with autodesk alias studio tools

7/30/2019 Advanced Modeling and Surfacing Techniques with Autodesk Alias studio tools

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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


23/33

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.


24/33

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.


25/33

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


26/33

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.


27/33

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.


28/33

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.


29/33

Fillettobedone

Theresultingfilletoftenhasproblemsregardingtheentrylinesorthehighlightalongthefilletknee.

Thisfilletisaconstantfillet.Theradiusentrylinesshowan

unevenandbadbehavior.

Thisfilletisachordalfillet.Thethicknessofthehighlightat

thekneevariesunevenly.

Thereasonforthisbadfilletbehavioristheunevenchangeoftheanglebetweenbothsurfaces.Inthiscasetheanglebetween

thetwosurfacesbecomessmallerandattheenditbecomesbiggeragain.Thetangentcheckershowsaninflectionoftheangle

change(in

the

middle).

Thissimpleexampleillustrateshowarolling

ballwouldmarkunevenradiusentrylineon

bothreferencesurfaceswhentheirangleto

eachotherisnotcontrolled.


30/33

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


31/33

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


32/33

A

C

B

Hereisanexamplefromindustrialdesignwherethetheoreticallineisnotcrispandcausesirritationwiththefinaldesign where

filletsareincorporated.

flat Cur ved(convex) Curved

(concave)

Thiseffectisabasicruleintechnicalsurfacingtomakemajorfeaturesappearcrispandstrongfromalldifferentviews.Software

productsoffertoolsthatcanplanarizeacurveorasurfaceedgetogetthiseffect.Theplanethatisthebaseforsuchcurves

rarelyappearsparalleltothethreemainviews (side,frontandtop).Soitisneededtobeabletomodifyacurvesetwhilethe

curvestillmaintainsitsplanarcharacterin3D.


33/33

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

advanced modeling and surfacing techniques with autodesk alias studio tools

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