PBRBasicsAten-minuteGuideforQtdevelopers

DrSeanHarmerManagingDirector–UKTheKDABGroupApril2016

(ImagecourtesyofNVIDIA)

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Physicallybasedrendering(PBR)emulatestheinteractionbetweenlightandmaterialsandisatrendinreal-timerendering.Here’saten-minuteguidetotheessentials.

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PBRBasicsforQtDevelopers

Whatexactlyisphysicallybasedrendering(PBR)?Itisthelatestinanever-endingpursuitofmorerealisticcomputergeneratedimagery,asystemthatreplacescommonshortcutsforrenderingsurfaceswithmethodologiesrootedinthephysicalworld.Fastbecomingastandardingamedevelopmentcircles,PBRisageneralsetofprinciplesratherthanahard-and-faststandard,meaningthatyoucanfindmultipleinterpretationsthatallrightfullycallthemselvesPBR.TheprimarydefiningfeatureofanyPBRsystemthough,isthatitmoreaccuratelysimulatesthebehavioroflightthroughthemethodicalapplicationofreflection,scattering,andabsorptionphysics.

Otherthanenhancedrealism,oneofthebiggestbenefitsofaPBRapproachisthatitismuchmorefoolproof.PBRrendersmaterialsthatlookcorrectregardlessofthelightingenvironment.Artistsnolongerneedtotinkerwithesotericparametersoftheirobjectstogetthingsto“lookright”becausetheshaderproperlyrendersanobject’slookbasedonitspropertiesandthegivenlighting.What’smore,PBR’sinherentabilitytocreategreat-lookinggraphicswithconsistentlydefinedpropertiesmakesiteasiertocollaborateandshareobjectlibraries.

Thewords“physical”and“physics”bothderivefromtheLatinwordfornature,soit’snotacoincidencethatphysicalrenderingthatmimicsnaturerequiresphysics.Butaren’twealreadyemulatingphysicsinhowwemimicthepathoflightinexistingnon-PBRrenderingmodels?Yesandno.Let’stakealookatthemainpropertiesthatdistinguishPBRfromearliermodels.

Material

Representinganobject’stexturewithphysicallyderivedpropertiesisprobablythemostdistinguishingfeatureofaPBRsystem.Whiletheremaybedifferencesinthepropertiesofanygivenimplementation,thesethreefundamentalattributesarecommontonearlyall.

KDABdemoofDodgeViperusingPBR

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PBRdiffersfromolderrenderingmodelsinthattheGPUshaderscancreaterealisticrenderingwithoutneedingspecializedobjecthintsorlightingtricks.

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Albedo

Thealbedoofanobjectisthecolorofitsdiffusereflectedlight.Thinkofthisasthecoloroftheobjectunderperfectlyeven,omni-directional,balancedwhitelight.Youmightbethinkingalbedoseemslikejustamorescientificnamefor“diffusecolor”—what’sthedifference?PBRdiffersfromolderrenderingmodelsinthatshadercomputationstaketheplaceoftexturehints(inlegacyOpenGL)ornon-energyconservingshaderformulations.PBRdoesnotneedtotweakcolorsorincludedirectionallighthighlightstoensuretheobjectdisplaysproperlyunderthegivenlightingconditions.Younolongerneedextra“artistsupplied”informationtotricktherenderingengineintodisplayingasdesired.WithPBR,yourepresentanobjectwithrealphysicalcharacteristicsandaPBRtexturemapofanobjectcontainsitsflat,unalteredcolor.

Conductivity

We’renotusedtothinkingabouttheconductivityofasurfacetounderstandhowtorenderit.Butrememberingthatlightisactuallyanelectromagneticphenomenon,itmakessensethataPBRsystemneedstoknowanobject’sconductivitytopredicthowlightwillinteractwithitssurface.Ameasureofconductivity—oftensimplifiedas“metalness”—specifieshowstronglylightwillreflect,aswellaswhetherreflectedlightwillshifttowardsanewhue.Insulatingsurfaces(likerubber,paint,orwood)absorbmostlighthittingthesurface,whileconductingsurfaces(mostmetals)reflectmostlightandpotentiallycolorit.SomePBRsystemsareabletoalsospecifymaterialslikerubies,emeralds,orsapphiresassemi-conductors—somethingbetweenaconductorandaninsulator—allowingpartialreflection,absorption,andcolorationoflight.

Aconductivitymapallowsthe“metalness”propertytovaryacrossanobject.Conductivitytexturescanrealisticallydepictpaintedmetalbyindicatingwherethesurfaceispainted(anddiffuselyreflectinglight),andwheretheunderlyingmetalisvisiblethroughwearorscratches(andreflectinglightinaspecularmanner).

“Bymodelingphysicalphenomena

ratherthanapproximating

observation,wecanachievemore

mathematicallystableand

photorealisticvisualfidelity”

KorayHagen,Sony

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OnewayinwhichthemicrofacettheorycomesintoPBRsystemsisintheselectionoftheshaderalgorithmsusedtocalculatethespecularreflection.

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Microfacets

Understandingamaterial’sappearancerequiresanunderstandingofthemicroscopicpropertiesofthesurface.Ifthesurfaceisbrokenintomanytinyrandomlyorientedfacets,itwillspreadreflectedlightacrossawiderangeofangles,blurringreflectionsandspreadingouthighlights.Ifthesurfaceisverysmooth,reflectedlightwillbouncebackinapreciseanglefromthesurface,resultinginsharpreflectionsandnarrowhighlights.Ofcourseit’scompletelyimpracticaltospecifymicroscopicsurfacecharacteristicswithatremendousarrayofpolygons,sogenerallya“roughness”mapisusedtospecifythemicroscopicnatureofasurface.

Aroughnessmapmayalsobepairedwithacavitymap,whichcapturesfeaturesthatarelargerthanthesurfaceroughnessbutalsomuchsmallerthantheobject’spolygonmesh.Thosesmallsurfacefeatureshavethepropertyoftrappingandabsorbinglight.Acavitymapallowstherenderertomoreaccuratelyrepresentfeatureslikepits,nicks,groovesorotherfeaturesthatareimpracticaltorepresentintriangles.Inpractice,thisisoftenrepresentedbythecombinationofaheightmapandanormalmap,andusedinconjunctionwithoneofanumberofpossibleocclusionmappingalgorithms.

Inadditiontotheroughnessmap,theotherwayinwhichthemicrofacettheorycomesintoPBRsystemsisintheselectionoftheshaderalgorithmsusedtocalculatethespecularreflection.Manysuchalgorithmsexist;eachspecializedtobesthandlecertainlightingconditionsormaterials.Usingdifferentalgorithmsallowsthenumberandorientationofasurface’smicrofacetstoreactdifferentlytolightdependingonanobject’sdesiredappearance.Forexample,theGGXdistributiongivesawiderfalloffforthespecularhighlightsthanthetraditionalBlinn-Phongmodel,providingabetter-lookingappearanceforrealworldsurfaces.Therearealsoshaderstailoredforspecificusecases,suchashandlingroughmattesurfaceslikeclaypotsordealingwithtransparentmaterials.Generallyyou’llwanttominimizethetotalnumberofshadersemployed,aseachshaderchangerequiresacostlycontextshiftontheGPU.

Nowthatwe’redonetalkingaboutmaterials,whatotherfeaturesdistinguishaphysicallybasedrenderer?TherearetwomainbehaviorsoflightthatchangewithinaPBRsystem.

Microfacetscontrolreflectionsfromroughness

ofthecarpettosmoothnessofthechairs

(ImagecourtesyofNVIDIA)

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PBR’semulationoftheFresneleffectshowslightataveryshallowanglecreatingbrightreflectionsfor“shiny”objects…andforflatandmatteobjects.

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Conservationofenergy

Thisisn’tjustalawfromyourundergraduatephysicsclass.It’sanobservationthattheamountoflightanobjectreflectscanneverbemorethantheamountoflighthittingthatobject.Althoughthismightseemobvious,earliermodelstoemulatespecularhighlights(suchastheBlinn-Phongreflectionmodel)oftenresultedinproducingmorelightthanwhatactuallyhitthesurface.Whileitmostlyworks,italsodoesn’tlooknatural.

ObeyingconservationofenergyinPBRmeansthattheamountoflightreflectedbydiffuseandspecularsourcesshouldneverexceed100%.(Ofcoursethisruledoesn’tapplyiftheobjectrenderedisalightemitter,likeaneonsignorfluorescenttube.)Conservationofenergyandsurfaceroughnesscombinetocontrolanobject’shighlightsandoverallappearance.Anobjectwitharoughsurfacereflectsmorediffuselight,whileasmoothsurfacereflectsmorehighlights.Butduetoconservationofenergy,switchinganobject’stexturebetweenroughandsmoothdoesnotchangethetotalamountoflightreflected.

Fresneleffect

Haveyounoticedthatifyoulookstraightdownintoapoolyoucanseethebottombutifyoulookacrossitssurfaceyou’llseeareflectionofthesky?ThatisduetotheFresneleffect,whichinplainEnglishmeansthemoreglancinganangleoflightthathitsasurface,themorethatsurfacewillreflectthelight.ThecriticalthingthatmakesPBRdifferentfromotherrenderingmethodsisanacknowledgementthattheFresneleffectdoesn’tjustapplyto“shiny”objects(likethesurfaceofapool),itappliestoeveryobject.Yes,evenseeminglydrabsurfaceslikebricksandcardboardexhibittheFresneleffect.

SimulatingtheFresneleffectwithprecisionrequiresaconsiderableamountofcomplicatedmath,asignificantspeedconcerninareal-timerenderingsystem.InPBRsystemsthatneedrapidrendering,aroughandreadyapproximationfortheFresneleffectcomputesaninterpolatedreflectedcolorusingtheangleoflight,theconductorcolor,andthemetalnessmap.

“Physicallybasedrenderingisgreat

becauseitsimplifiestheassetcreation

processandmakesitdifficultfor

artiststocreateunrealisticresults”

MartinThomas,

Extremeistan

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ShinyslatetilesshowaninstanceoftheFresneleffect

(ImagecourtesyofNVIDIA)

GettinggraphicstodisplaywithaPBRrenderingmodelcomesdowntousingaPBR-awareengineandprovidingmaterialinformationinyourtextures.

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UsingPBR

GettingyourgraphicstodisplaywithaPBRrenderingmodelcomesdowntousingaPBR-awareengineandprovidingappropriatematerialinformationinyourtextures.Manyexistingrenderingengines(UnrealEngine4,Unity5,Frostbite)andtools(Marmoset,Sketchfab)havealreadymadethemovetoPBRforyou.However,ifyouaren’tusinganexistingengine,you’llneedtoimplementthenecessaryphysicsequationsinyourshader.

Ifyou’reusingQt,PBRcanbeimplementedwithcustomshadersinQt3D.Foranexampleofthisinaction,seeourDodgeViperdemo.ThisdemoshowsamixofstandardQtcontrolsmixedwithPBR-based3Drendering,andismadepossiblethroughanewPBRlayerthatKDABisdevelopingontopofQt3D.WeareupstreamingthisQtPBRcomponentintoQtAutomotiveasafirsttestdeploymentofthetechnology.TheeventualgoalistomakeitpartofafutureQt3Drelease,makingQtjustascapableandcurrentastherestofthegraphicsworld.

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AbouttheKDABGroup

TheKDABGroupistheworld'sleadingsoftwareconsultancyforarchitecture,developmentanddesignofQt,C++andOpenGLapplicationsacrossdesktop,embeddedandmobileplatforms.KDABisthebiggestindependentcontributortoQt.Ourexpertsbuildrun-times,mixnativeandwebtechnologies,solvehardwarestackperformanceissuesandportingproblemsforhundredsofcustomers,manyamongtheFortune500.KDAB’stoolsandextensiveexperienceincreating,debugging,profilingandportingcomplexapplicationshelpdevelopersworldwidetodeliversuccessfulprojects.KDAB’strainers,allfull-timedevelopers,providemarketleading,hands-on,trainingforQt,OpenGLandmodernC++inmultiplelanguages.Foundedin1999,KDABhasofficesthroughoutNorthAmericaandEurope.

www.kdab.com

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