h16 mantra user guide the limits tab - houdini · h16 mantra user guide the limits tab the limits...
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H16MantraUserGuide
THELIMITSTABTheLimitsTabcanbefoundundertheRenderingTabontheMantraNode.
Theparametersfoundonthistabcontroltheamountoftimesarayassociatedwithaspecificcomponentisallowedtopropagatethroughascene.Settingtheselimitshasinfluenceoverthefinallookofyoursceneaswellastheamountoftimeitwilltaketherenderyourimagewithoutnoise.
ReflectLimit
Thisparametercontrolsthenumberoftimesaraycanbereflectedinyourscene.
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H16MantraUserGuide
Theaboveexampleshowsaclassic“HallofMirrors”scenariowiththesubjectplacedbetweentwomirrors.Thiseffectivelycreatesaninfiniteseriesofreflections.
Fromthiscameraanglethereflectionlimitsareobviousandhavealargeimpactontheaccuracyofthefinalimage.However,inmostcasesthereflectionlimitwillbesubtler,allowingyoutoreducethenumberofreflectionsinyoursceneandoptimizethetimeittakestorenderthem.
Rememberthatthefirsttimealightsourceisreflectedinanobject,itisconsideredadirectreflection.Therefore,evenwithReflectLimitsetto0,youwillstillseespecularreflectionsoflightsources.
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H16MantraUserGuide
Tocontrolwhathappenswhenthemaximumnumberofreflectionsisexceeded,seetheAtRayLimitparameterontheLimitstab.
RefractLimit
Thisparametercontrolthenumberoftimesarayberefractedinyourscene.
Theaboveexampleshowsasimplescenewithtengridsallinarow.Byapplyingarefractiveshader,wewillbeableseethroughthegridstoanimageofasunsetinthebackground.
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H16MantraUserGuide
Fromthiscameraangle,fortheimagetobeaccurate,therefractionlimitmustmatchthenumberofgridsthatthatareinthescene.However,mostsceneswillnothavethisnumberofrefractiveobjectsallinarowandsoitispossibletoreducetherefractlimitwithoutaffectingthefinalimagewhilealsoreducingthetimeittakestorenderthem.
KeepinmindthatthisRefractLimitreferstothenumberofsurfacesthattheraymusttravelthrough,notthenumberofobjects.
Rememberthatthefirsttimealightsourceisrefractedthroughasurface,itisconsideredadirectrefraction.Therefore,evenwithRefractLimitsetto0,youwillseerefractionsofLightSources.However,sincemostobjectsinyourscenewillhaveatleasttwosurfacesbetweenitandthelightsource,directrefractionsareoftennotevidentinyourfinalrender.
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H16MantraUserGuide
Tocontrolwhathappenswhenthemaximumnumberofrefractionisexceeded,seetheAtRayLimitparametersontheLimitstab.
DiffuseLimit
Thisparametercontrolsthenumberoftimesdiffuserayscanpropagatethroughyourscene.
UnliketheReflectandRefractLimits,thisparameterwillincreasetheoverallamountoflightinyoursceneandcontributetomostglobalillumination.Withthisparametersetabovezerodiffusesurfaceswillaccumulatelightfromotherobjectsinadditiontodirectlightsources.
Inthisexample,increasingtheDiffuseLimithasadramaticeffectontheappearanceofthefinalimage.Toreplicaterealisticlightingconditions,itisoftennecessarytoincreasetheDiffuseLimit.However,sincetheamountoflightcontributionusuallydecreaseswitheachdiffusebounce,increasingtheDiffuseLimitbeyond4doeslittletoimprovethevisualfidelityofascene.Additionally,increasingtheDiffuseLimitcandramaticallyincreasenoiselevelsandrendertimes.
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H16MantraUserGuide
SSSLimit
ThisparametercontrolsthenumberoftimeslightgeneratedfrommaterialswithSub-SurfaceScatteringwillbeincludedintheevaluationofindirectlight.ItisintrinsicallylinkedtotheDiffuseLimit,sincethepropagationofindirectdiffuseraysiswhatallowstheevaluationofnewSSSsamples.
Inthisexample,increasingtheSSSlimitallowsthegreySSSmaterialtoreceiveindirectilluminationfromtheorangeSSSmaterial.YoumayalsonoticearelationshipbetweenDiffuseLimitandSSSLimit–essentially,tomatchthecontributionofindirectlight,youwillusuallyneedoneextraSSSSample.
KeepinmindthatmaterialswithSSSenabledabsorbandscatterlight,sothelightcontributiontootherSSSobjectswilloftenbequitesmall.IncreasingSSSlimitsbeyond2willdolittletoimprovetherealismofafinalrenderbutmayrequiredramaticallymoreSSSsamples.Infact,inmanycasesevenanSSSLimitof1(essentiallylimitingthecontributiontotheobjectitself)willbesufficienttocreatehighlyrealisticrenders.
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H16MantraUserGuide
VolumeLimit
Thisparametercontrolsthenumberoftimesavolumetricraycanpropagatethroughascene.ItfunctionsinasimilarfashiontotheDiffuseLimitparameter.
IncreasingtheVolumeLimitparameterwillresultinmuchmorerealisticvolumetriceffects.Thisisespeciallynoticeableinsituationswhereonlypartofavolumeisreceivingdirectlighting.Also,inorderforavolumetricobjecttoreceiveindirectlightfromotherobjects,theVolumeLimitparametermustbesetabove0.
WiththeVolumeLimitsettovaluesabovezero,thefogvolumetakesonthecharacteristiclightscatteringyouwouldexpectfromlighttravellingthroughavolume.However,aswiththeDiffuseLimit,thelightcontributiongenerallydecreaseswitheachbouncedrayandthereforeusingvaluesabove4doesnotnecessarilyresultinanoticeablymorerealisticimage.
Also,increasingthevalueofthisparametercandramaticallyincreasetheamountoftimespentrenderingvolumetricimages.
OpacityLimit
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H16MantraUserGuide
Asaraytravelsthroughmanytransparentsurfaces,orthroughavolume,itwillcalculatethecumulativeamountofOpacity.WhenthisvalueexceedstheOpacityLimitmantrawillassumeallsurfacesbeyondthispointareopaque.
ThisparameterbehavesinasimilarfashiontoboththeReflectandRefractLimitbutoperatesonaccumulatedvaluesratherthansimplythenumberofsurfacestherayhaspassedthrough.
Intheaboveexample,eachgridhasashaderattachedwithanopacityvalueof0.1.Itisimportanttorememberthatinthiscase“transparent”referstoobjectswhoseopacityislessthan100%anddoesnotincluderefractiveobjectswhichcanappeartransparent.
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H16MantraUserGuide
Intheaboveexample,thesphereofthelefthasanopacityof0.5,withnorefraction.ThesphereontherighthasanOpacityof1withrefractionenabled.YoucanseethattheOpacityLimithasnoeffectontheamountofrefraction,onlyaffectingobjectswhoseopacityvalueislessthan1.
WhilereducingtheOpacityLimitmaysaveasmallamountofrendertime(1–5%)usinglowvaluesmayresultinbandingandotherartifactswhenyourcameraismovingorananimationisevolving.Thiscanbeespeciallynoticeableinsmokesimulationswhereopacityvaluesareconstantlychanging.
ThedefaultvalueforOpacityLimitisquiteaggressive,changingthisvalueshouldbedonecarefullyandtheresultsinspectedacrossarangeofframesinananimatedsequence.
ColorLimit
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H16MantraUserGuide
Thisparametercontrolsthemaximumvalueashadingsampleisallowedtoreturnfromindirectsources.
PhysicallyBasedRenderingcancause“spikes”incolorvalueswhenextremelybrightindirectlightsourcesareundersampled.Thisresultsin“fireflies”inthefinalrenderedimagewhichcanbeverydifficulttoremovewithoutveryhighsamplingrates.
Youcanseeintheexampleabove,thatevenat12x12pixelsamples,the“fireflies”stillremain.AdjustingMinandMaxindirectrayssamplesettingscouldremovethisnoise,butatthecostoflongerrendertimes.
DecreasingtheColorLimitparameterclampsthecolorvaluesintheseindirectsamplesandcanhelptoavoidthese“spikes”.
ReducingthecolorLimitcanbeaneffectivewayofremoving“fireflies”withoutincreasingsamplingrates.However,clampingthevaluesinindirectlightingcanresultinanoverall
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H16MantraUserGuide
reductionintheamountoflightinyourscene.Thisisespeciallyevidentinsceneswhicharemostlyilluminatedbyindirectlight.
ColorLimitDecay
ThisparametercausestheColorLimittodecayasrayspropagatethroughthescene.
SincetheColorLimitparameteractsasclampontheindirectvaluesinyourscene,itcanoccasionallycauseindirectreflectionstoappeartoodim.Todisguisethiseffect,theColorLimitDecaycandecreasethecolorlimitaftereachbounce.Thiswaythedecreaseinlightonlybecomesapparentafterseveralbounceswheretheeffectislessnoticeable.
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H16MantraUserGuide
Intheaboveexample(colorcorrectedwithabrightnessvalueof0.1)youcanseethatbyreducingtheColorLimitvalue,allsecondaryvaluesareclampedtothesameamount.However,bysettingthecolorlimitdecaywithoutadjustingthecolorlimit,thebrightnessofeachreflectionisreducedaftereachbounceproducingamoresubtlereductioninintensity.
SettingtheColorLimitDecayvalueto0.9willcausethecolorlimittobe90%ofitsoriginalvalueafteronebounce,81%aftertwobounces,etc.TheColorLimitwillneverdecaybelowavalueof1,sothissettingwillnotaffectcolorsinthe0-1range.
AtRayLimitThisparameterallowsyoutocontrolhowMantradealswithraysthatreachtheraytracinglimit(ForexampletheReflectLimitorRefractLimit).
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H16MantraUserGuide
Intheaboveexample,therefractLimithasbeensetto2.
Settingthe“AtRayLimit”parameterto“UseBlackBackground”willsimplyrenderblackoncethelimitsarereached.ThisisthedefaultsettingandwillworkinmostscenessincetheReflectorRefractLimitisunlikelytobereached.However,insceneswherethelimitisnoticeableintherenderedimage,theblackcolorcanbequitenoticeableandstandoutagainstthecolorsinthescene.
Inthiscase,itisadvisabletoincreasethelimituntiltheeffectisavoidedorusethesecondoptionforthisparameter“UseDirectLightingasBackgroundColor”.Thiswillreplacetheblackcolorwithwhichevercolororimageisusedinyourdirectlighting,forinstanceanEnvironmentLight.
ForMoreInformationabouthowthesettingsonanEnvironmentLightaffectthisparameterseetheLightingsection.