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英文原文ExtendingBlender:DevelopmentofaHepaticAuthoringToolAbstract-Inthispaper,wepresentourworktoextendawellknown3Dgraphicmodeler-Blender-tosupporthepaticmodelingandrendering.TheextensiontoolisnamedHAMLAT(hepaticApplicationMarkupLanguageAuthoringTool).WedescribethemodificationsandadditionstotheBlendersourcecodewhichhavebeenusedtocreateHAMLATFurthermore,wepresentanddiscussthedesigndecisionsusedwhendevelopingHAMLAT,andalsoanimplementationroadmapwhichdescribesthechangestotheBlendersourcecode.Finally,weconcludewithdiscussionofourfuturedevelopmentandresearchavenues.KeywordsHepaticsHAMLGraphicModelersBlenderVirtualEnvironments.I.IntroductionA.MotivationTheincreasingadoptionofhepaticmodalityinhuman-computerinteractionparadigmshasledtoahugedemandfornewtoolsthathelpnoviceuserstoauthorandedithepaticapplications.Currently,thehepaticapplicationdevelopmentprocessisatimeconsumingexperiencethatrequiresprogrammingexpertise.Thecomplexityofhepaticapplicationsdevelopmentrisesfromthefactthatthehepaticapplicationcomponents(suchasthehepaticAPI,thedevice,thehepaticrenderingalgorithms,etc.)needtointeractwiththegraphiccomponentsinordertoachievesynchronicity.Additionally,thereisalackofapplicationportabilityastheapplicationistightlycoupledtoaspecificdevicethatnecessitatestheuseofitscorrespondingAPI.Therefore,deviceandAPIheterogeneityleadtothefragmentationanddisorientationofbothresearchersanddevelopers.Inviewofalltheseconsiderations,thereisaclearneedforanauthoringtoolthatcanbuildhepaticapplicationswhilehidingprogrammingdetailsfromtheapplicationmodeler(suchasAPI,device,orvirtualmodel).ThispaperdescribesthetechnicaldevelopmentofthehepaticApplicationMarkupLanguageAuthoringTool(HAMLAT).ItisintendedtoexplainthedesigndecisionsusedfordevelopingHAMLATandalsoprovidesanimplementationroadmap,describingthesourcecodeoftheproject.B.BlenderHAMLATisbasedontheblender1softwaresuite,whichisanopen-source3Dmodelingpackagewitharichfeatureset.Ithasasophisticateduserinterfacewhichisnotedforitsefficiencyandflexibility,aswellasitssupportsformultiplefileformats,physicsengine,modemcomputergraphicrenderingandmanyotherfeatures.Becauseofblendersopenarchitectureandsupportivecommunitybase,itwasselectedastheplatformofchoicefordevelopmentofHAMLAT.Theopen-sourcenatureofBlendermeansHAMLATcaneasilyleverageitsexistingfunctionalityandfocusonintegratinghepaticfeatureswhichmakeitacompletehapto-visualmodelingtool,sincedevelopinga3Dmodelingplatformfromscratchrequiresconsiderabledevelopmenttimeandexpertiseinordertoreachtheleveloffunctionalityofblender.Also,wecantakeadvantageoffutureimprovementstoblenderbymergingchangesfromitssourcecodeintotheHAMLATsourcetree.HAMLATbuildsonexistingBlendercomponents,suchastheuser-interfaceandeditingtools,byaddingnewcomponentswhichfocusontherepresentation,modification,andrenderingofhepaticpropertiesofobjectsina3Dscene.ByusingBlenderasthebasisforHAMLAT,wehopetodevelopa3DhepaticmodelingtoolwhichhasthematurityandfeaturesofBlendercombinedwiththenoveltyofhepaticrendering.C.ProjectGoalsAspreviouslystated,theoverallgoalfortheHAMLATprojectistoproduceapolishedsoftwareapplicationwhichcombinesthefeaturesofamodemgraphicmodelingtoolwithhepaticrenderingtechniques.HAMLAThasthelookandfeelofa3Dgraphicalmodelingpackage,butwiththeadditionoffeaturessuchashepaticrenderingandhepaticpropertydescriptors.Thisallowsartists,modelers,anddeveloperstogeneraterealistic3Dhippo-visualvirtualenvironments.Ahigh-levelblockdiagramofHAMLATisshowninFigure1.Itillustratestheflowofdatainthehepaticmodeling.HAMLATassiststhemodeler,orapplicationdeveloper,inbuildinghapto-visualapplicationswhichmaybestoredinadatabaseforlaterretrievalbyanotherhepaticapplication.Byhippo-visualapplicationwerefertoanysoftwarewhichdisplaysa3Dscenebothvisuallyandhecticallytoauserinavirtualsetting.AnXMLfileformat,calledHAML2,isusedtodescribethe3Dscenesandstorethehippo-visualenvironmentsbuiltbyamodelerforlaterplaybacktoanenduser.Traditionally,buildinghippo-visualenvironmentshasrequiredastrongtechnicalandprogrammingbackground.Thetaskofhecticallyrenderinga3Dsceneistedioussincehepaticpropertiesmustbeassignedtoindividualobjectsinthesceneandcurrentlytherearefewhigh-leveltoolsforaccomplishingthistask.HAMLATbridgesthisgapbyintegratingintotheHAMLframeworkanddeliveringacompletesolutionfordevelopmentofhippovisualapplicationsrequiringnoprogrammingknowledge.Theremainderofthepaperisorganizedasfollows:inSection2,wepresenttheproposedarchitectureextensionsanddiscussdesignconstraints.Section3describestheimplementationdetailsandhowhepaticpropertiesareaddedandrenderedwithintheblenderframework.Insection4wediscussrelatedissuesandfutureworkavenues.II.SYSTEMOVERVIEWANDARCHITECTURETheblenderdesignphilosophyisbasedonthreemaintasks:datastorage,editing,andvisualization.Accordingtothelegacydocumentation3,itfollowsadatavisualize-editdevelopmentcycleforthe3DmodelingA3DsceneisrepresentedusingdatastructureswithintheBlenderarchitecture.Themodelerviewsthescene,makeschangesusingtheeditinginterfacewhichdirectlymodifiestheunderlyingdatastructures,andthenthecyclerepeats.Tobetterunderstandthisdevelopmentcycle,considertherepresentationofa3DobjectinBlender.A3Dobjectmayberepresentedbyanarrayofverticeswhichhavebeenorganizedasapolygonalmesh.Usersmaychoosetooperateonanysubsetofthisdataset.Editingtasksmayincludeoperationstorotate,scale,andtranslatethevertices,orperhapsare-meshingalgorithmtocleanupredundantverticesandtransformfromaquadtoatriangletopology.Thedataisvisualizedusingagraphical3Drenderedwhichiscapableofdisplayingtheobjectasawireframeorasashaded,solidsurface.Thevisualizationisnecessaryinordertoseetheeffectsofeditingonthedata.Inanutshell,thisexampledefinesthedesignphilosophybehindblendersarchitecture.Inblender,dataisorganizedasaseriesoflistsandbasedatatypesarecombinedwithlinksbetweenitemsineachlist,creatingcomplexscenesfromsimplestructures.Thisallowsdataelementsineachlisttobereused,thusreducingtheoverallstoragerequirements.Forexample,ameshmaybelinkedbymultiplesceneobjects,butthepositionandorientationmaychangeforeachobjectandthetopologyofthemeshremainsthesame.AdiagramillustratingtheorganizationofdatastructuresandreuseofsceneelementsisshowninFigure2.Asceneobjectlinkstothreeobjects,eachofwhichlinktotwopolygonalmeshes.Themeshesalsoshareacommonmaterialproperty.Theentiresceneisrenderedononeofseveralscreens,whichvisualizesthescene.WeadopttheBlenderdesignapproachforourauthoringtool.Thedatastructureswhichareusedtorepresentobjectsina3Dscenehavebeenaugmentedtoincludefieldsforhepaticproperties(e.g.,stiffness,damping)；userinterfacecomponents(e.g.,buttonpanels)whichallowthemodelertochangeobjectpropertieshavealsobeenupdatedtoincludesupportformodifyingthehepaticpropertiesofanobject.Additionally,aninteractivehippo-visualrenderedhasbeenimplementedtodisplaythe3Dscenegraphicallyandhectically,providingthemodelerorartistwithimmediatefeedbackaboutthechangestheymaketothescene.AclassdiagramoutliningthechangestotheblenderframeworkisshowninFigure3.ComponentswhicharepertinenttoHAMLATareshadedingray.HAMLATbuildsonexistingblendersub-systemsbyextendingthemforhepaticmodelingpurposes.Thetactileandkinestheticcues,whicharedisplayedduetointeractionwithvirtualobjects,hisdatatype.Otherscenecomponentssuchaslamps,cameras,orlinesarenotintuitivelyrenderedusingforcefeedbackhepaticdevicesandarethereforenotofcurrentinterestofchaoticrendering.AnaugmentedversionofthemeshdatastructureisshowninFigure4.Itcontainsfieldsforvertexandfacedata,plussomespecialcustomdatafieldswhichallowdatatobestoredto/retrievedfromdiskandmemory.Wehavemodifiedthisdatatypetoincludeapointertoahepaticsdatastructure,whichstoreshepaticpropertiessuchasstiffness,damping,andfrictionforthemeshelements(Figure5).Similarlytothebuilt-ingraphicalrendered,HAMLATusesacustomrenderedfordisplaying3Dsscenesgraphicalandhapcall,andisinendoftheconnectivityisgiveninthenextsection.IMPLEMENTATIONDataStructureBlenderusesmanydifferentdatastructurestorepresentthevarioustypesofobjectsina3Dsceneapolygonalmeshcontainsthelocationandtopologyofvertices；alampcontainscolorandintensityvalues；andacameraobjectcontainsintrinsicviewingparameters.TheMeshdatastructureisusedbytheblenderBlenderrendered.Thiscomponentisdevelopedindependentlysincehereticalandgraphicalrenderingmustbeperformedsimultaneouslyandsynchronously.Asimulationloopisusedtoupdatehepaticrenderingforcesataratewhichmaintainsstabilityandquality.Wheretheforcefeedbackvectorwhichisdisplayedtotheuseriscomputedusingthestiffnesscoefficient(variablenamestiffness)fortheobjectandxthe|penetrationdepth(displacement)ofthehepaticproxyintoanobject.Thestiffnesscoefficienthasarangeof0,1,where0representsnoresistancetodeformationand1representsthemaximumstiffnesswhichmayberenderedbythehepaticdevice.ThedampingofanobjectdefinesitsresistancetotherateofdeformationduetosomeappliedstrutHepaticesforce.Itistypicallyrenderedusingtheforceequation:HepaticPropertiesInthissectionwellbrieflydiscussthehepaticpropertieswhichmaycurrentlybemodeledusingHAMLAT.Itisimportantforthemodelertounderstandthesepropertiesandtheirbasisforuseinhepaticrendering.Thestiffnessofanobjectdefineshowresistantitistodeformationbysomeappliedforce.Hardobjects,suchasarockortable,haveveryhighstiffness；softobjects,suchasrubberball,havelowstiffness.Thehardness-softnessofanobjectistypicallyrenderedusingthespring-forceequation:rangeof0,1andmaybeusedtomodelviscousbehaviorofamaterial.Italsoincreasesthestabilityofthehepaticrenderingloopfordstiffmaterials.Thestaticfriction(variablenamestriation)anddynamicfriction(variablenamedirection)coefficientareusedtomodelthefrictionalforcesexpe