Whatdoes“linear”meaninalinearworkflow

Let’sstartwiththebasicswhentalkingaboutalinearworkflow.Weneedtodefinewhatthewordlinearactuallymeans.Thereislinear“light”andlinear“data”.Linearlightisthewaythatwewanttorepresentimagestooureyes.Lineardataishowwewanttostoretheimagedata.Westorethingswithlineardatainmindbecauseweneedtoaccountforgammaindisplaydevices.Mostcomputerdisplaydeviceshaveaninherentgammaofapproximately1/2.2=0.4545.Thisiswhat’scalledsRGBgamma.Videomonitorshaveagammaofrec709.Theyarecloseinvaluesbutnotexactlythesame.Havingsaidthat,agammaof0.4545isnotexactlysRGBorrec709butwe’regoingtocallthatcloseenoughforourpurposes.Becauseourmonitorsdarkenimages,weneedtocounteractthat.Hereisabitofagraphofwhatmonitorsdotoimagedata,thelinearimagedata,andthesRGBcurvethatweusetocorrectforthemonitor.

Themaththatcreatesthecurvesisapproximatelyasfollows:

MonitorEffect=pow(pixelvalue,2.2)sRGB–pow(pixelvalue,(1/2.2))

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Asyoucansee,yourmonitordoesaprettysignificantgammacorrectiontotheimagedatabeingdisplayedonit.Becausewecancorrectforthat,it’snotaproblem,aslongaswetakeafewthingsintoaccount.Let’stalkmoreaboutwhathappenstovaluesbecauseofthiscorrection.AsCGIartists,wehavebecomeaccustomedtoavalueof.5tobeexactly50%ofbrightness.However,that’snottechnicallycorrect,asyoucanseebythepreviouschart.

Lookingatthechartalittlemoreclosely,wecanseethatthehorizontalvalueof0.5isjustbelow0.25ontheverticalaxis.Technicallyit’s0.18.Doesthisseemfamiliar?Usingphotographicterms,that’s18percentgrey.Eighteenpercentgreyintermsoflineardatameans50%ofvisualbrightness.Whyisthisthecase?Well,oureyesareverysensitivetolowlightlevelsandweperceive0.18as0.5.Whenyougotobuya“greycard”ataphotographyshop,itwillbe18%greybutyoureyeswillseeitas50%.So,nowthatweknowthatoureyesturn0.18into0.5,andourmonitormakesthingsdarker,wecorrectforthatinsomethingcalledaLUT(lookuptable)thatmapsthevaluesweseeonthemonitortosomethingoureyesareusedto.ThatLUTgetsappliedinseveralplaces.Beforewegetintothethefullnittygrittyofwherealltheplacesshouldbe,let’slookatimagesthatcanhelptellthestoryalittlebetter.

Thefirstsetofimagesisaverysimpleillustrationoflightattenuation.ThefirstimageiswithoutanysRGBapplied.Thesetuphereisa100wattlightinaroomthatis12’x15’x9’,withGIturnedonandaninfinitenumberofGIbounces.Twoproblemsareevidenthere.A100‐wattbulbcaneasilylightthisroom,butthisfirstimagedoesnotshowthatishappening:

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Deleted: Usingthehorizontalaxisonthechart,wecanseethatavalueof0.5isvisuallyequaltoaround0.75.That’saprettysignificantdifference.

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Inadditiontothelightnotfillingtheroom,thisnonlinearrepresentationofthelightfalloffiswildlyinaccurate.It’sjustugly.However,inthissecondimage,wecanseethatthelightiscertainlylightinguptheroomquitenicely,andthelightattenuationissignificantlymorepleasing:

Thoseexamplesareverybasicbutgiveaquickideaofwhatistocomeintermsoflightingaccuracy.

Nextisarenderthatisnotcorrectedforthedarknessthatmonitorsapplytoimages.AspecialthankstothetalentedarchitectsatMarch(http://www.marchmade.com),whoprovideddesignedandmodeledthisenvironment.Thisimagewasrenderedwithasun/skysystem,andusesGItogetdiffusebounces.Thesunisaround20degreesoffthehorizonsothereisaprettygoodamountoflightbutcertainlynothighnoonstylelighting.

Thisimageseemsverydark.It’stoodarkhonestlybecauseifwethinklogicallyweknowthatataround4‐5pmyouwouldseemorebouncelightaroundtherestofthisstructure.Weknowthatbecausewe’reusedtoseeingthingsat4‐5pmallthetime.However,thecomputerandmonitorhavenoidea.Theyarebutdumbdisplaypresentationdevices.Let’sputansRGBgammacorrectioncurveontheimagetoshowwhathappenswhenwecorrectforwhatoureyessee.

Thisisofcoursemuchmorerepresentativeofwhatyouthinkyouseeataround4‐5pm.EventhoughthesRGBseemstoremovesignificantamountsofcontrast,thisimagecertainlyhasplentyofthat.However,it’simportanttonotethatthisimagewasgeneratedwithsRGB(linear)workflowinmind.Thatmeansthetextures,shadercolors,andvalueswereadjustedbasedonthatsRGBcurve.

LinearWorkflowina3DApplicationSo,whatdoesastandardworkflowlooklikeinthe3Dprocess?Let’sfirsttakealookatPhotoshop’scolorsettings.Sincenearlyeverytexturepaintedisfrom

Photoshop,weneedtomakesureoursettingsaresetofforalinearworkflow.Photoshopbakesinagammatoits8bitoutput,sowhenwesaveanimage,it’sbeengammaencoded.Thisisnotidealbutweneedtofindawayaroundit.Alternatively,wecanuseopenEXRimagesfortexturesasthereisnogammaencodingonthoseimagesoranyotherfloatingpointformat.

MakesuretousesRGBfortheRGBpartoftheworkingspacesarea.Nowthatthepaintingtoolissetup,weneedtotakealookatthe3Dapp.However,let’slookattheidealpaththatimagedatatravelsthroughthe3Dapplicationbeforeanythingelse.

Weneedtoassumethatthedatagoingintotherenderislinear.Thismeansweneedtoremoveanygammaencodingthatimageshave,iftheyare8bitdata.However,32or16bitfloatingpointimagesareassumedtoalreadybelinear,it’simportantthatwedonotcorrectthem.Intheend,ourrenderergenerateslinearfloatingpointdatathatweneedtopreviewinanimageviewing.Sinceit’sdatalinear,weneedtoaddthesRGBLUT,whichwillmakeitvisuallylinear.

Let’stakealookatafewexamplesofwhathappenstothetexturedatawhenlinearworkflowisproperlyappliedandwhenit’snot.HerewehaveasimplegradientcreatedinNuke,asafloatingpointopenEXR.Thetopimageislinear,andthebottomimageisanon‐linearimage.Inthisinstance,thenon‐linearmeansansRGBlut/gammaisbakedintotheimage.Wewillcallthisagamma‐encodedimage.

Noticethedifferencewithrespecttothemidtonesandtheblacksoftheimages.Sincethetopimageislinear(nosRGBlutbakedin)thisshouldgiveuscorrectresultsintherenderer.

Now,let’sseewhathappenswhenthelinearandthenon‐linearimagegetsusedasareflectionmap.Thisrenderwassetupwithawhitebackgroundsoanyreflectionwouldbepure1.0.Thegroundplaneisblackandthereflectiontexturechanneloftheplanewassetwiththeaboveimages.Theimageshavebeentaggedwithluminancevaluestoshowwhere50%brightnessis.

Hereisthelinearresult:

Hereisthenon‐linear(gammaencoded)result:

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Deleted: (akait’sencodedwith

Deleted: sRGBencoded

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Usingtheframebuffer’sabilitytosample,wegetavalueofaround.7onthe50%markwhenweexaminethenon‐linearimage.However,whenwesamplethelinearresult,thevalueis.5.Thisisdata‐linearcorrect.

Now,let’stakethatonestepfurthertodisplacementmaps.Thetopimageisgeneratedfromalinearfile.Thebottomimageisusingagammaencodedfile.

Hereitshouldbeevenmoreclearthatmakingsurethatweputlineardataintotherenderer.Wecanseewhatresultsfromnottakingtheextracaretoensureaccuracy

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Deleted: Wecanevenseethatthe3Dappisremovingpixeldata,andinthiscasevaluesbelow.01!

Deleted: notgammaencoded

Deleted: gammaencodedtobelinear

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inthesesteps.Thisisthestartofit,sincetexturesmakeour3Dworldgoround.Thereismoregroundtocoverthatwillhelpshedtheproverbiallightonthesubject.

Whyisworkinginlinearspaceimportant?Wehavealreadyseensomeearlyexampleofwhyusingalinear,orgammacorrectedworkflowisimportant.Wecanseethatrendersthatareproperlytransformedandwhenourlinearlightisaccountedfor,thatourrenderslookmoreaccurate.Achievingthislightinglevelwithoutusingagammacorrectedworkflowispossible.However,weneedtopushwaymorelightenergyintotherenderssothattheGIcanbouncetheraysaroundmore.Whenthathappenswehavelongerrendertimes.Generallyspeaking,rendertimesare3‐4xlongerwhenmakingadjustmentstolightingscenariosinordertoaccountforwhatwe“want”toseewhennotworkinginalinear,orgammacorrectedmanner.Goingevenfurtherdowntheoutputpipeline,compositingandcolorcorrectingisaffectedbyusingalinearworkflow.Thisisbecauseifwedon’tworklinearly,wecolorcorrectourdatainanon‐linearmanner.Thiswillresultinwildlyinaccuratevaluesforsomethinglikebrightness(orgain/multiplyincompositingterms).Hereisaflowchartofwhattheprocessshouldlooklike.

Manypiecesofsoftwaremakeafewassumptionsbasedonthefileformatandthebitdepthoftheimage.AcompositingapplikeNukeassumesthatifyouhavesavedyourdataasaformatlikeopenEXR,thatyouareworkingwithlineardataandthatanygammaisonlybeingcorrectedbythedisplay,whichNukedoesinitsframebuffer.Italsoassumesthatifweworkwith8bitdata(jpg,png,tga,etc)thatwehave(ortheapplicationhas,rather)bakedinthesRGBgammaandneedstotakeoutthegammatomaketheimagelinearagain.

Hereistheresultoftakingourcorrectimageandturningupthebrightness(gain)by3x.

Wecanseethatthebrightsarereallybrightandthemidtonesaremuchbrighteralso.Thatimagehadnogammaappliedtoitwhenitwassavedoutonlytheviewingsystemappliedthegamma.Now,let’ssaythattheimagedidhavegammabakedintoit.Thesame3xbrightnesswouldlooksomethinglikethis:

Visually,wecantellthatthisimageisnotright.Everythingismuchtoobrightandthevalueswehadbeforearenot3xbrighter.Theyaremuchbrighterbecausewe’renowdoinganon‐linearcolorcorrection.Wealwayswanttodolinearcolorcorrectionbecauseit’smuchmoreaccuratetowhathappenstovaluesintherealworld(linearlight,remember?)versuswhathappenstodataintheCGIworld(nonlinear).So,nowthepointcomeswhereweneedtosaveoutourimagery.Hereisthecompletedflowchartofhowa3Drendergetshandledbycompositingsoftwarethatisawareandmanagesgammaandlinearworkflow:

Hereit’sevidentthatwritingoutto,andreadingfromanonfloatingpointformatisthebigdifferentiatortotheworkflow.However,whilethesekindsoffilesaregamma(sRGB)encoded,thesoftwaremakesaccountsforthemontheinputandtheoutput.Evenontheweb,thebrowsersassumethatimageshavebakedinsRGBcurvestothem.It’sfunnytothinkofwebbrowsersbeingcompliantwithcolorstandards,buttheyare.

Howtoworklinearlyin3DSMaxusingbuilt‐inrenderersHerewehavethestandard3DSMaxpreferencespanelattheirdefaultsettings:

Weneedtomakeafewchangestoittoworklinearly.Remember,weneedtohaveaviewingLUTandalsocorrectinputdataforthesRGBgamma.Hereisthemodifiedpreferencespanel:

Firstthing’sfirst,weneedtoenablethegammacorrectionandmakesureourgammaissetto2.2.ThisnumberiscloseenoughtosRGBthatwe’rejustgoingtogowithit.NextofcoursewewanttoseethatthegammachangeshaveeffectintheMaterialEditor(fortheshaderballswatches)aswellasthecolorselectors.Thiswaywecanvisuallyseehowourchoicesaffecttheresultingcolors,becauseasweknow,avalueof0.18isactually0.5.Weneedtobeabletoseethatorelsesettingupvalueswillbeverydifficult.Finally,weneedtosettheinputgammaofbitmapsto2.2.Why?Well,rememberweareassumingthat8bitdatahasabakedingammaof2.2.Bysettingtheinputgammato2.2andtheoutputgammato1.0,welinearizetheimagesothatdiffusecolorsgettransformedproperlysotheylookhowweexpectthemtofromoursessioninPhotoshop.

Howtoworklinearlyin3DSMaxusingVRayWe’regoingtoswitchovertoVRaybecauseitoffersreallygreatsupportforlinearworkflows.HereisthebasicVRaycolormappingrollout:

You’llnoticesomegammasetting,andafewcheckboxes.Hereistheresultingpanelafterit’sallsetup.

What’simportanttonoteisofcoursethegammasettingof2.2butalsothecheckboxfor“adaptationonly”.ThisisbecauseofcoursewewanttolettherendererworklinearlybutitbecameapparentthatifwewerenotsamplingatsRGBgammathatalotofnoisewouldgounnoticedbytherenderer.Why?Well,it’stimetobreakoutthecalculatorsagain.Let’ssayweputanoisethresholdof0.005.Thatisaprettyacceptablenoisefloortolookfor.However,theproblemisthatbecausewhilevisuallywe’relookingfor0.005therendererseeslineardata.Thisisonceagainduetothe0.5reallybeing0.18duetothelineargamma.Whenwerun0.005throughthepow(0.005,2.2)wegetavalueof:8.664310539439e‐06.Thisvalueisalotlowerthan.005,sothesamplerwon’tcatchit.Weneedtofigureoutwhatnumber0.005isinthelineargammaworld.Byusingthepowerfunctionpow(0.005,(1/2.2))tolookuppixelvaluesfordecidingonwhentosample,weknowwe’relookingfor0.009.Ofcoursethisisallforeducation’ssake.SinceVRaytakesitallintoaccountwiththesimple“adaptationonly”checkbox,wecansimplyignorethemathifwewantto.

ConclusionsLinearworkflowandunderstandinggammaisaverytrickyproposition.Itactuallyhelpstosometimesgoonfaiththatthenumbersworkaslongaswefollowtheformula.Theshorthandoftheformulaisasfollows:

• Transformalldatagoingintotherenderersothatwehanditlineardata• RenderlinearlyandgammacorrecttheviewingsystemwithaLUT• Outputtofloatingpointfiles(openEXR)forcompositing• Compositingsoftwareshouldaccountforlinearworkflowwithaviewing

LUTthatmatchestheoneintherenderer

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Deleted: checkboxlabeled“Linearworkflow”.

Deleted: Seemseasyright?Yep!

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• Outputfromcompositingif8bitshouldhavesRGBgammabakedin.Iftherenderisfloatingpoint(openEXR)thenit’simportantnottobakethegammain.

Oncealinearworkflowisstarted,artistsnoticethattheimagerylooksmorephotographicandit’seasiertogettheresultsdesired.Ultimately,that’sthegoalisn’tit?