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H16MantraUserGuide
SAMPLINGANDNOISEWhengeneratinganimage,Mantramustdetermineacolorvalueforeachpixelbyexaminingthescenebehindtheimageplane.Mantraachievesthisbysendingoutanumberofraysfromthecamera’spositionuntiltheyhitanobjectinthescene.Everytimearayhitsanobject,itwillreturnsomepieceofinformationabouttheobject(Itscolor,forinstance).Thisprocesscanbroadlybedescribedas“Sampling”thescene.
Samplingonceperpixel,however,cancause“aliasing”whereinformationislostbetweensamples.Thisisparticularlyevidentinsceneswithmorevarietyinshapesandcolors.
Increasingthenumberofsamplesperpixelgivesan“anti-aliased”imagewhichbetterrepresentstheactualscene.
H16MantraUserGuide
Thesamplesdescribedabovecouldbedescribedas“Primary”rays(orpixelsamples).Theycandeterminetheoverallqualityoftheimagebeingrendered,especiallywithregardtotheshapeandaccuracyoftheobjectsinthescene.
Forotheraspectsofanimage,likelighting,reflectionsandrefractions,moreraysmustbecastintothescene,originatingfromthehitlocationoftheprimaryrays.Foreachprimaryray,Mantrawillfireatleastone“Secondary”ray.Thesesecondaryrayscanbedividedintotwotypes-DirectandIndirect.
DirectandIndirectRaysDirectRayscanbedescribedasrayswhichdealwithLights.Thisgenerallymeansthattheraystravelfromsomepositioninthescenetowardthevariouslightsources.Theseraysdetermineifasurfaceisinshadow,andifnot,lightinginformationcanbecalculated.
Thesame“aliasing”problemsdescribedpreviouslycanexistwiththesedirectrays,resultinginanoisyimage.Youwillusuallyfindnoisefromdirectsourcesshowingupwhenrenderingspecular
H16MantraUserGuide
highlightsorthesoftedgesofshadowscastfromarealights.Inthesecases,itmaybenecessarytosendmoredirectrays.
WhenevaluatingtheeffectofsendingmoreDirectRaysinyourrenders,itcansometimesbechallengingtoseparateonesourceofnoisefromanother.Addingthe“DirectLighting(percomponent)”imageplanewillallowyouviewthedirectcontributionofeachcomponentseparately.
Whenattemptingtooptimizethenumberofdirectraysinyourscene,the“DirectSamples”imageplanecanbeadded.Thisplanewillshowyouthenumberofdirectraysusedthroughoutyourimagedisplayedasintensity.
H16MantraUserGuide
IndirectRayscanbedescribedasrayswhichdealwithobjectsandtheirsurfaceproperties.Thisgenerallymeansthatraystravelfromsomepositioninthesceneindirectionsdeterminedbytheshaderattachedtotheobject.Refractionrayswilltravel“through”objects,ReflectionRayswillbounce,andDiffuseRayswillscatterinarandomdirectionwithinahemisphericaldistribution.
Withindirectrays,“aliasing”canbemuchworsethanwithDirectraysandusuallythegreatestcauseofnoiseinarender.Generallyspeaking,small,verybrightfeatureswillcausethemostnoiseinindirectsamples–softreflectionsofverybrightspecularhighlightsonotherobjects,forexample.Inthesecases,itmaybenecessarytosendmoredirectrays.
H16MantraUserGuide
WhenevaluatingtheeffectofsendingmoreIndirectRaysinyourrenders,itcansometimesbechallengingtoseparateonesourceofnoisefromanother.Addingthe“IndirectLighting(percomponent)”imageplanewillallowyouviewthedirectcontributionofeachcomponentseparately.
Whenattemptingtooptimizethenumberofindirectraysinyourscene,the“IndirectSamples”imageplanecanbeadded.Thisplanewillshowyouthenumberofindirectraysusedthroughoutyourimage.
H16MantraUserGuide
VOLUMESSamplingvolumetricobjectsrequiresadifferentapproachthansamplingsurfaces.Whiledirectraysarestillused,theymustsamplethevolumemultipletimeswhiletravellingthroughthevolume.Indirectraysbehaveinasimilarfashion,sentmultipletimesastheray“steps”throughthevolumetricobject.Thismeansthatsamplingvolumesisamuchmoreexpensiveprocessthansamplingasurface.
Asaraytravelsthroughavolume,itmovesforwardataratedeterminedbytheVolumeQuality.Ateachstep,itevaluatestheshaderandaccumulatesthedensityofthevolume.Becausethese
H16MantraUserGuide
densityvaluescanvarydrasticallythroughoutthevolume,nearbyrayscancalculatesubstantiallydifferentvalues,introducingnoiseintotherender.Inthesecases,itmaybenecessarytosendoutmoredirectandindirectraysortotakesmallerstepsthroughthevolume.
Evenatlowsamplingrates,itcanbecostlytorendercleanimagesofvolumetricdata.Thisisbecausetheshadingisrunforeverystepthroughthevolume.However,mantrahasavarietyofwaystooptimizevolumerenderingwhichcandecreaserendertimeswithoutsacrificingdetail.
Oneoptimization,knownasStochasticTransparency,decouplestheaccumulationofdensityvaluesfromtheshadingsamples.Thismeansthattheamountofsamplingcanbegreatlyreducedasvariationsinlightinginformationislessobviousthanvariationsindensity.
H16MantraUserGuide
WhenevaluatingtheeffectofsendingmoreDirectandIndirectraysinyourrenders,itcansometimesbechallengingtoseparateonesourceofnoisefromanother.Addingthe“DirectLighting(percomponent)”and“IndirectLighting(percomponent)”imageplaneswillallowyouviewthedirectandindirectvolumecontributionsseparately.
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