外文翻譯文獻Constructing Rules and Scheduling Technology for 3D Building Models

ConstructingRulesandSchedulingTechnologyfor3DBuildingModels

ZhengweiSUI,LunWU,JingnongWENG,XingLIN,XiaoluJI

Abstract

3Dmodelshavebecomeimportantformofgeographicdatabeyondconventional2Dgeospatialdata.Buildingsareimportantmarksforhumantoidentifytheirenvironments,becausetheyareclosewithhumanlife,particularlyintheurbanareas.Geographicinformationcanbeexpressedinamoreintuitiveandeffectivemannerwitharchitecturalmodelsbeingmodeledandvisualizedinavirtual3Denvironment.Architecturalmodeldatafeatureswithhugedatavolume,highcomplexity,non-uniformrulesandsoon.Hence,thecostofconstructinglarge-scalescenesishigh.Meanwhile,computersarelackofprocessingcapacityuponalargenumberofmodeldata.Therefore,resolvingtheconflictsbetweenlimitedprocessingcapacityofcomputerandmassivedataofmodelisvaluable.Byinvestigatingthecharacteristicsofbuildingsandtheregularchangesofviewpointinvirtual3Denvironment,thisarticleintroducesseveralconstructingrulesandschedulingtechniquesfor3Dconstructingofbuildings,aimingatthereductionofdatavolumeandcomplexityofmodelandthusimprovingcomputers’efficiencyatschedulinglargeamountofarchitecturalmodels.Inordertoevaluatetheefficiencyofproposedconstructingrulesandschedulingtechnologylistedintheabovetext,theauthorscarryoutacasestudyby3DconstructingthecampusofPekingUniversityusingtheproposedmethodandthetraditionalmethod.Thetworesultsarethenexaminedandcomparedfromaspectsofmodeldatavolume,modelfactuality,speedofmodelloading,averagerespondingtimeduringvisualization,compatibilityandreusabilityin3Dgeo-visualizationplatforms:ChinaStar,oneChina’sownplatformfor3DglobalGISmanufacturedbytheauthorsofthispaper.Theresultofcomparisonrevealsthatmodelsbuiltbytheproposedmethodsaremuchbetterthanthosebuiltusingtraditionalmethods.Fortheconstructingofbuildingobjectsinlarge-scalescenes,theproposedmethodscannotonlyreducethecomplexityandamountofmodeldataremarkably,butcanalsoimprovingcomputers’efficiency.

Keywords:Constructingrules,Modelscheduling,3Dbuildings

I.INTRODUCTION

Inrecentyears,withthedevelopmentof3DGIS(GeographicalInformationSystem)softwarelikeGoogleEarth,Skyline,NASAWorldWind,large-scale3Dbuildingmodelswithregionalcharacteristicshavebecomeimportantformofgeographicdatabeyondconventional2Dgeospatialdata,likemulti-resolutionremotesensingimagesandvectordata[1].Comparedtotraditional2Drepresentation,geographicinformationcanbeexpressedinamoreintuitiveandeffectivemannerwitharchitecturalmodelsbeingmodeledandvisualizedinavirtual3Denvironment.3Drepresentationandvisualizationprovidesbettervisualeffectandvividurbangeographicinformation,andthusplaysanimportantroleinpeople'sperceptionsoftheirenvironment.Meanwhile,the3Dbuildingdataisalsoofgreatsignificancefortheconstructionofdigitalcities.

Buthowtoefficientlyvisualizethousandsof3Dbuildingmodelsinavirtual3Denvironmentisnotatrivialquestion.Themostdifficultpartofthequestionistheconflictsbetweenlimitedprocessingcapacityofcomputerandmassivevolumeofmodeldata,particularlyintheprocedureofmodelrendering.Takingthe3Dmodelingofacityfortheexampleusingtraditional3Dmodelingmethod,supposethereare100000buildingstomodelintheurbanareaandtheaveragesizeofmodeldataforeachbuildingisroughly10M.SothetotaldatavolumeofbuildingmodelsinthecitycouldreachaTBlevel.However,thecapacityofordinarycomputermemoryisonlyintheGBscale.Basedonthisconcern,theauthorsproposedtheschedulingtechnologyforlarge-scale3Dbuildingsmodelsinaspectsofmodelloadingandrendering.Duetothelackofbuildingconstructingrulesandstandard,modelsofbuildingsvaryinaspectsofconstructingmethods,texturescollectionandmodeldatavolume,especiallyinaspectsofmodelreusabilityandfactuality.Suchalargeamountofdatawithoutuniformconstructingrulesbecomesahugechallengefordatastorage,processingandvisualizationincomputers.Italsobringstheproblemofincompatibilityamongdifferent3DGISsystems.

AfteryearsofresearchinGIS(GeographicInformationSystem),peoplehaveaccumulatedanumberofwaystosolvetheaboveproblems[3].Howeverinvirtual3Denvironment,becauseofthedifferenceindataorganizationandmannersofhumancomputerinteraction(HCI),weneedtoapplyanewstandardizedmethodofmodelingandschedulingfor3Dmodels.Atpresent,thereisnosuchauniformmethodastheconstructingspecificationorstandardforthemodelingof3Dbuildings.Existingapproachesareinsufficientandinefficientintheschedulingoflarge-scalebuildingmodels,resultinginpoorperformanceorlargememoryoccupancy.Inresponsetosuchquestions,theauthorsproposedanewmethodfortheconstructionof3Dbuildingmodels.Modelsbuiltusingtheproposedmethodscouldbemuchbetterthanthosebuiltusingtraditionalmethods.Forthe3Dmodelingofbuildingobjectsinscenesoflargescale,theproposedmethodscannotonlyremarkablyreducethecomplexityandamountofmodeldata,butcanalsoimprovingthereusabilityandfactualityofmodels.Concerningtheschedulingoflarge-scalebuildingmodels,theModelLoadingJudgmentAlgorithm(MLJA)proposedinthispapercouldsolvetheoptimaljudgmentproblemofmodelloadingin3Dvisioncone,particularlyincircumstancewithuncertainuserinteractions.

Thispaperfirstexaminesandanalyzesexistingproblemsinconstructingandschedulingstepsof3Dbuildingmodels.Thentheauthorsproposeasetofconstructingrulesfor3Dbuildingmodelstogetherwithmethodsofmodeloptimization.Besides,specialschedulingtechnologyandoptimizationmethodformodelrenderingisalsoappliedinthispaperforlarge-scale3Dbuildingmodels.Inordertoevaluatetheefficiencyofproposedrulesandmethods,acasestudyisundertakenbyconstructinga3DmodelforthemaincampusofPekingUniversityandShenzhenusingboththeproposedmethodandthetraditionalmethodrespectively.Thetworesulting3DmodelsofPekingUniversitycampusandShenzhenarethenexaminedandcomparedwithoneotherinaspectsofmodeldatavolume,modelfactuality,speedofmodelloading,averagerespondingtimeduringvisualization,compatibilityandreusabilityinvarious3Dgeo-visualizationplatformslikeChinaStar(oneChina’sownplatformfor3DglobalGISmanufacturedbytheauthors),Skyline,etc.Resultofcomparisontellsthatprovidedsimilarfactualityofmodels,usingtheproposedmethodofus,thedatavolumeofmodelswasreducedby86%;thespeedofmodelloadingwasincreasedby70%;theaveragerespondingtimeofmodelduringvisualizationandinteractionspeedwasreducedby83%.Meanwhile,thecompatibilityandreusabilityof3Dmodeldataarealsoimprovediftheyareconstructedusingourapproach.

II.MODELINGRULESOF3DBUILDINGS

3Dsceneisthebestformofvisualizationfordigitalcitysystems.Whileconstructing3Dmodelsforbuildingsobjects,propermethodsandrulesshouldbeused,whicharemadewithfullconcernsofthecharacteristicsof3Dbuildingmodels[2].Theresultingmodelsshouldberobust,reusableandsuitableenoughfortransmissionovercomputernetwork,andshouldatthesametimebeautomaticallyadaptedtosystemcapability.

Generallyspeaking,methodsofconstructing3Dbuildingmodelscanbeclassifiedintothreetypes:wireframemodeling,surfacemodelingandsolidmodeling.Innormalcircumstances,tomodelbuildingsin3Dformat,theframeworkofbuildingshouldbeconstructedfirstaccordingtothecontourfeatures,numberoffloors,floorheight,aerialphotographandliveactionphotosofbuildings.Then,gatherthecharacteristicsofscenethatthebuildingstomodelarerepresenting.Importantcharacteristicsincludebuildingsaerialphotographorliveactionshootingphotos.Finally,mapthegatheredtexturetomodelframework,optimizethemodelandcreatedatabaseofthe3Dbuildingmodels.

Althoughtherehavealreadybeenmanyapproachesfortheconstructionof3Dbuildingmodels,aunifiedmodelingmethodandrulesarestillneededtoimprovetheefficiency,quality,facilitatechecking,reusabilityandarchivingofconstructedmodels.Byinvestigatingthecharacteristicsofbuildings,wefoundthatbuildingshaveregulargeometricsolidformodeling,similartextureonthesurfacesofdifferentdirections,highsimilarityinsmall-scalemodelsofbuildings,etc.Accordingtothese,thisarticlegivesadiscussiononthemodelingrulesfromthreeaspects,includingconstructingrulesofthe3Dbuildingmodels,texturemappingrulesof3Dbuildingmodelsandoptimizationmethodforconstructedmodelsbasedonmentionedconstructingrules.

A.Constructingrulesofthe3Dbuildingmodels

The3Dbuildingmodelingreferstotheprocedureofrepresentingtruebuildingsfromtherealworldintocomputerintheformof3Dobjects[4].Humanbeings,asthecreatorandatthesametimepotentialusersofmodels,playakeyroleinthisprocedure.Peoplearedifferentfromeachotherintheunderstandingofthebuildingobjects,methodsofmodelingandthesoftwaretoolstheyuseformodeling.Suchdifferencesamongpeoplewhocarryoutmodelingworkatthesametimeleadtothe3Dmodelsofdiversequalityandlowefficiency.Sothe3Dbuildingconstructingrulesproposedinthisarticlebecomenecessaryandhelpfultosolvetheaboveproblems.

1)Combinesimilarfloorsasawholeandkeeptheroofindependent

2)Sharesimilarmodelsandprocessthedetailsespecially

3)Constructingintheunitofmeters

4)Definecentralpointofthemodel

5)Unifiedmodelcodes

6)Reducenumberofsurfacesinasinglemodel

7)Reducecombinationofthemodels

8)Rationalsplitofmodels

B.Texturemappingrulesof3Dbuildings

Basedontheframeworkof3Dmodels,weneedtoattachthesemodelswithpropertexturestocreateabettervisualizationeffectfor3Dbuildings.Thequalityoftexturemappinghasadirectimpactonthevisualeffectofthescenewhilingbeingrendered[5].Sincethegraphicscardofcomputerwillloadallthetexturestogetherwhenrenderingamodel,texturemappingrulesandthequalityofthetexturemappingcandirectlyinfluencetheefficiencyofrenderingaswell.

C.Optimizationofmodelsbasedonconstructingrules

Basedonconstructingrulesandthecharacteristicsof3Dbuildingmodels,theauthorsdevelopasoftwaretooltooptimizethe3Dbuildingmodelsautomatically.Theoptimizationsimplementedinthesoftwaretoolcontainthedeletionofmodels’internaltextures,mergingadjacentvertices/lines/surfaces,removingun-mappedframeworkandsoon.Besides,thesoftwarecanenhancetheshapeofthewholemodel,texturepositionandmodelfacticityintheprocedureofmodeloptimization.

III.SCHEDULINGTECHNOLOGYOFLARGE-SCALE3DBUILDINGMODELS

Forthe3Dvisualizationoflarge-scalearchitecturalmodels,aseriesofmeasurescouldbeappliedtoensuretheefficientrenderingofmodels.Importantmeasuresincludesthesceneorganization,visionconecutting,eliminationoftexturesonthebacksideofmodels,Shaderoptimization,LODAlgorithm,mathlibraryoptimization,memoryallocationoptimization,etc..

Howtodisplaythousandsof3Dcitybuildings’modelsinavirtual3Denvironmentisnottrivial.Themainproblemistheschedulingofmodels[7].Itdetermineswhenandwhichmodelstobeloaded.Thisproblemcanbedividedintotwosmallerproblems:Findvisiblespatialregionofmodelsin3Denvironment,andoptimizationmethodofmodelrenderingefficiency.

A.Findvisiblespatialregionofmodelsin3Denvironment

Accordingtooperatingmechanismofcomputersduring3Dvisualizationandthecharacteristicsoflarge-scale3Dscene,weneedtodeterminethepositionofcurrentviewpointfirstbeforeloadingsignalmodelsorurban-unitmodels.Theninresponsetotheregularchangesofviewpointinvirtual3Denvironment,thesystemwillpreloadthe3Dmodeldataintomemoryautomatically.Inthisway,frequentIOoperationscanbereducedandthusoverallefficiencyofsystemgetsimproved.AnewalgorithmnamedMLJA(ModelLoadingJudgmentAlgorithm)isproposedinthispaperinordertofindoutvisibleregionofmodelsinthe3Denvironment.Thealgorithmintegratesthegraticulesandelevationinformationtodeterminethecurrentviewpointofusersinthe3Dspace.Andwiththemovementofviewpoint,thealgorithmschedulestheloadingofmodelcorrespondinglyandefficiently.

B.Optimizationmethodofmodelrenderingefficiency

Theschedulingmethodoflarge-scale3Dbuildingmodelsproposedaboveisaneffectivewaytosolvetheproblemcausedthecontradictionbetweenlargemodeldatavolumeandlimitedcapacityofcomputers.Accordingtothealgorithm,wecanavoidloadingthewholelarge-scale3Dbuildingmodelsatonetimeforthesakeoflimitedcomputermemory,andthenimprovesystemefficiencyintheprocedureofmodelloadingandabandoning.DuetothelimitedcapacityofGPUandlocalvideomemory,weneedafurtherresearchonhowtodisplaytheloadedmodeldatainmoreefficientmanner.Intheremainingpartofthispaper,theauthorswillcontinuetointroduceseveralmethodsontheoptimizationofmodelrenderinginthevisioncone.

1)Eliminationoftexturesonthebacksideofmodels

Thebacksideofthe3Dmodelisinvisibletotheusers.Ifweomitthetexturemappingforthe3Dmodelonthebackside,theprocessingloadofgraphiccardwillbereducedasmuchasatleast50%.Besides,accordingtoaninvestigationonprocedureofactualmodelrendering,theauthorsfoundthatonthebacksideofthe3Dmodel,theinvisibletextureisrenderedinacounter-clockwisemanneragainstthedirectionofeyesight,whilethevisibletexturemappingisrenderedinclockwisemanner.Sowecanomittherenderingofmodelswhichisintendedtoberenderedincounterclockwisemanner.Therefore,thetextureswon’texistonthebackof3Dmodels.Thegraphiccardcouldthenworkmorerapidlyandefficiently.

2)Eliminatetheshieldedmodel

Bycalculatingthegeometricrelationshipbetween3Dmodelsinthescene,theshieldedmodelscanbeomittedwhiledisplayingthescenewithappropriateshieldingpatches.Throughthisway,wecaneffectivelyreducetheusageofgraphicscardmemory,andthusachievehigherrenderingefficiencyandfaster3Dvirtualsystem.

Inthevirtual3Dgeographicinformationsystem,weoftenobserve3Dmodelsfromahighaltitude.Itisespeciallytrueforlarge-scaleoutdoor3Dmodels.Theusualarrangementof3Dbuildingmodelsarealwayssparse,howevertherealblockisverysmall.Therefore,establishinganindexforvisualcontrol,whichissimilartotheBSPtree,doesn’tamounttomuch.ThroughcarefullystudyingDirectX,wefoundthatwecantakeadvantageofthelatestZ-bufferingtechnologyofDirectXtoimplementtheshieldingcontrolofmodels.

3)OptimizationmethodoftheShaderinstructions

Inshader3.0technology,SM(ShaderModel)isamodelwhichcanoptimizetherenderingengine.A3Dsceneusuallycontainsseveralshaders.Amongtheseshaders,somedealwiththesurfacesandskeletonsofbuildings,andothersdealwiththetextureof3Dbuildingmodels.

Geometrycanbehandledquicklybyshaderbatchprocess.Theshadercancombinesimilarculminationin3Dbuildingmodels,dealwiththecorrelationoperationofasinglevertex,determinethephysicalshapeofthemodel,linkthepoint,line,triangleandotherpolygonsforarapidprocessingwhilecreatenewpolygons,etc.WecanassignthecomputingtasktoshaderandlocalvideomemorydirectlyinaveryshorttimewithoutbotheringtheCPU.Inthiscase,visualeffectsofsmoke,explosionsandotherspecialeffectsandcomplexgraphicsarenolongernecessarytobeprocessedbytheCPUofcomputer.SuchfeaturesofshadercanspeedupboththeCPUandgraphiccardinprocessinghugeamountof3Dmodels.

4)LODalgorithmoflarge-scale3Dscene

LOD(LevelofDetail)isacommonandeffectivesolutiontoresolvetheconflictsbetweenrealtimevisualizationandtheauthenticityofmodels[8].ByinvestigatingthemainfeaturesandtypicalalgorithmsofLODtechnology,theauthorsproposedanewstructurefordynamicmulti-leveldisplay.Thisstructuren