H16 Mantra User Guide THE LIMITS TAB - Houdini · H16 Mantra User Guide THE LIMITS TAB The Limits...

H16MantraUserGuide

THELIMITSTABTheLimitsTabcanbefoundundertheRenderingTabontheMantraNode.

Theparametersfoundonthistabcontroltheamountoftimesarayassociatedwithaspecificcomponentisallowedtopropagatethroughascene.Settingtheselimitshasinfluenceoverthefinallookofyoursceneaswellastheamountoftimeitwilltaketherenderyourimagewithoutnoise.

ReflectLimit

Thisparametercontrolsthenumberoftimesaraycanbereflectedinyourscene.

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Theaboveexampleshowsaclassic“HallofMirrors”scenariowiththesubjectplacedbetweentwomirrors.Thiseffectivelycreatesaninfiniteseriesofreflections.

Fromthiscameraanglethereflectionlimitsareobviousandhavealargeimpactontheaccuracyofthefinalimage.However,inmostcasesthereflectionlimitwillbesubtler,allowingyoutoreducethenumberofreflectionsinyoursceneandoptimizethetimeittakestorenderthem.

Rememberthatthefirsttimealightsourceisreflectedinanobject,itisconsideredadirectreflection.Therefore,evenwithReflectLimitsetto0,youwillstillseespecularreflectionsoflightsources.

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Tocontrolwhathappenswhenthemaximumnumberofreflectionsisexceeded,seetheAtRayLimitparameterontheLimitstab.

RefractLimit

Thisparametercontrolthenumberoftimesarayberefractedinyourscene.

Theaboveexampleshowsasimplescenewithtengridsallinarow.Byapplyingarefractiveshader,wewillbeableseethroughthegridstoanimageofasunsetinthebackground.

H16MantraUserGuide

Fromthiscameraangle,fortheimagetobeaccurate,therefractionlimitmustmatchthenumberofgridsthatthatareinthescene.However,mostsceneswillnothavethisnumberofrefractiveobjectsallinarowandsoitispossibletoreducetherefractlimitwithoutaffectingthefinalimagewhilealsoreducingthetimeittakestorenderthem.

KeepinmindthatthisRefractLimitreferstothenumberofsurfacesthattheraymusttravelthrough,notthenumberofobjects.

Rememberthatthefirsttimealightsourceisrefractedthroughasurface,itisconsideredadirectrefraction.Therefore,evenwithRefractLimitsetto0,youwillseerefractionsofLightSources.However,sincemostobjectsinyourscenewillhaveatleasttwosurfacesbetweenitandthelightsource,directrefractionsareoftennotevidentinyourfinalrender.

H16MantraUserGuide

Tocontrolwhathappenswhenthemaximumnumberofrefractionisexceeded,seetheAtRayLimitparametersontheLimitstab.

DiffuseLimit

Thisparametercontrolsthenumberoftimesdiffuserayscanpropagatethroughyourscene.

UnliketheReflectandRefractLimits,thisparameterwillincreasetheoverallamountoflightinyoursceneandcontributetomostglobalillumination.Withthisparametersetabovezerodiffusesurfaceswillaccumulatelightfromotherobjectsinadditiontodirectlightsources.

Inthisexample,increasingtheDiffuseLimithasadramaticeffectontheappearanceofthefinalimage.Toreplicaterealisticlightingconditions,itisoftennecessarytoincreasetheDiffuseLimit.However,sincetheamountoflightcontributionusuallydecreaseswitheachdiffusebounce,increasingtheDiffuseLimitbeyond4doeslittletoimprovethevisualfidelityofascene.Additionally,increasingtheDiffuseLimitcandramaticallyincreasenoiselevelsandrendertimes.

H16MantraUserGuide

SSSLimit

ThisparametercontrolsthenumberoftimeslightgeneratedfrommaterialswithSub-SurfaceScatteringwillbeincludedintheevaluationofindirectlight.ItisintrinsicallylinkedtotheDiffuseLimit,sincethepropagationofindirectdiffuseraysiswhatallowstheevaluationofnewSSSsamples.

Inthisexample,increasingtheSSSlimitallowsthegreySSSmaterialtoreceiveindirectilluminationfromtheorangeSSSmaterial.YoumayalsonoticearelationshipbetweenDiffuseLimitandSSSLimit–essentially,tomatchthecontributionofindirectlight,youwillusuallyneedoneextraSSSSample.

KeepinmindthatmaterialswithSSSenabledabsorbandscatterlight,sothelightcontributiontootherSSSobjectswilloftenbequitesmall.IncreasingSSSlimitsbeyond2willdolittletoimprovetherealismofafinalrenderbutmayrequiredramaticallymoreSSSsamples.Infact,inmanycasesevenanSSSLimitof1(essentiallylimitingthecontributiontotheobjectitself)willbesufficienttocreatehighlyrealisticrenders.

H16MantraUserGuide

VolumeLimit

Thisparametercontrolsthenumberoftimesavolumetricraycanpropagatethroughascene.ItfunctionsinasimilarfashiontotheDiffuseLimitparameter.

IncreasingtheVolumeLimitparameterwillresultinmuchmorerealisticvolumetriceffects.Thisisespeciallynoticeableinsituationswhereonlypartofavolumeisreceivingdirectlighting.Also,inorderforavolumetricobjecttoreceiveindirectlightfromotherobjects,theVolumeLimitparametermustbesetabove0.

WiththeVolumeLimitsettovaluesabovezero,thefogvolumetakesonthecharacteristiclightscatteringyouwouldexpectfromlighttravellingthroughavolume.However,aswiththeDiffuseLimit,thelightcontributiongenerallydecreaseswitheachbouncedrayandthereforeusingvaluesabove4doesnotnecessarilyresultinanoticeablymorerealisticimage.

Also,increasingthevalueofthisparametercandramaticallyincreasetheamountoftimespentrenderingvolumetricimages.

OpacityLimit

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Asaraytravelsthroughmanytransparentsurfaces,orthroughavolume,itwillcalculatethecumulativeamountofOpacity.WhenthisvalueexceedstheOpacityLimitmantrawillassumeallsurfacesbeyondthispointareopaque.

ThisparameterbehavesinasimilarfashiontoboththeReflectandRefractLimitbutoperatesonaccumulatedvaluesratherthansimplythenumberofsurfacestherayhaspassedthrough.

Intheaboveexample,eachgridhasashaderattachedwithanopacityvalueof0.1.Itisimportanttorememberthatinthiscase“transparent”referstoobjectswhoseopacityislessthan100%anddoesnotincluderefractiveobjectswhichcanappeartransparent.

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Intheaboveexample,thesphereofthelefthasanopacityof0.5,withnorefraction.ThesphereontherighthasanOpacityof1withrefractionenabled.YoucanseethattheOpacityLimithasnoeffectontheamountofrefraction,onlyaffectingobjectswhoseopacityvalueislessthan1.

WhilereducingtheOpacityLimitmaysaveasmallamountofrendertime(1–5%)usinglowvaluesmayresultinbandingandotherartifactswhenyourcameraismovingorananimationisevolving.Thiscanbeespeciallynoticeableinsmokesimulationswhereopacityvaluesareconstantlychanging.

ThedefaultvalueforOpacityLimitisquiteaggressive,changingthisvalueshouldbedonecarefullyandtheresultsinspectedacrossarangeofframesinananimatedsequence.

ColorLimit

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Thisparametercontrolsthemaximumvalueashadingsampleisallowedtoreturnfromindirectsources.

PhysicallyBasedRenderingcancause“spikes”incolorvalueswhenextremelybrightindirectlightsourcesareundersampled.Thisresultsin“fireflies”inthefinalrenderedimagewhichcanbeverydifficulttoremovewithoutveryhighsamplingrates.

Youcanseeintheexampleabove,thatevenat12x12pixelsamples,the“fireflies”stillremain.AdjustingMinandMaxindirectrayssamplesettingscouldremovethisnoise,butatthecostoflongerrendertimes.

DecreasingtheColorLimitparameterclampsthecolorvaluesintheseindirectsamplesandcanhelptoavoidthese“spikes”.

ReducingthecolorLimitcanbeaneffectivewayofremoving“fireflies”withoutincreasingsamplingrates.However,clampingthevaluesinindirectlightingcanresultinanoverall

H16MantraUserGuide

reductionintheamountoflightinyourscene.Thisisespeciallyevidentinsceneswhicharemostlyilluminatedbyindirectlight.

ColorLimitDecay

ThisparametercausestheColorLimittodecayasrayspropagatethroughthescene.

SincetheColorLimitparameteractsasclampontheindirectvaluesinyourscene,itcanoccasionallycauseindirectreflectionstoappeartoodim.Todisguisethiseffect,theColorLimitDecaycandecreasethecolorlimitaftereachbounce.Thiswaythedecreaseinlightonlybecomesapparentafterseveralbounceswheretheeffectislessnoticeable.

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Intheaboveexample(colorcorrectedwithabrightnessvalueof0.1)youcanseethatbyreducingtheColorLimitvalue,allsecondaryvaluesareclampedtothesameamount.However,bysettingthecolorlimitdecaywithoutadjustingthecolorlimit,thebrightnessofeachreflectionisreducedaftereachbounceproducingamoresubtlereductioninintensity.

SettingtheColorLimitDecayvalueto0.9willcausethecolorlimittobe90%ofitsoriginalvalueafteronebounce,81%aftertwobounces,etc.TheColorLimitwillneverdecaybelowavalueof1,sothissettingwillnotaffectcolorsinthe0-1range.

AtRayLimitThisparameterallowsyoutocontrolhowMantradealswithraysthatreachtheraytracinglimit(ForexampletheReflectLimitorRefractLimit).

H16MantraUserGuide

Intheaboveexample,therefractLimithasbeensetto2.

Settingthe“AtRayLimit”parameterto“UseBlackBackground”willsimplyrenderblackoncethelimitsarereached.ThisisthedefaultsettingandwillworkinmostscenessincetheReflectorRefractLimitisunlikelytobereached.However,insceneswherethelimitisnoticeableintherenderedimage,theblackcolorcanbequitenoticeableandstandoutagainstthecolorsinthescene.

Inthiscase,itisadvisabletoincreasethelimituntiltheeffectisavoidedorusethesecondoptionforthisparameter“UseDirectLightingasBackgroundColor”.Thiswillreplacetheblackcolorwithwhichevercolororimageisusedinyourdirectlighting,forinstanceanEnvironmentLight.

ForMoreInformationabouthowthesettingsonanEnvironmentLightaffectthisparameterseetheLightingsection.