OpenShadingLanguageforBlender

ByMichelAnders

Copyright2013,2016MichelAnders

BlenderMarketEdition

BlenderMarketEdition,LicenseNotes

Thisebookislicensedforyourpersonalenjoymentonly.Thisebookmaynotbere-soldorgivenawaytootherpeople.Ifyouwouldliketosharethisbookwithanotherperson,pleasepurchaseanadditionalcopyforeachrecipient.Ifyou’rereadingthisbookanddidnotpurchaseit,oritwasnotpurchasedforyouruseonly,thenpleasereturntoBlenderMarketandpurchaseyourowncopy.

Thankyouforrespectingthehardworkofthisauthor

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TableofContents

Intro 4WhatareOSLshadersgoodfor? 6Readinghints 8Codeavailability 8Averysimpleshader 9Datatypes 15Controlstructures 19Points,vectorsandnormals 26Mappingvectors 29Patterns 33Polkadots 38Leopardshader 43Diamondplate 54Symmetryoperations 59Victorianeracastironcovers 69Ripples 80Concreteshader 86Citruspeel 95Ceilingplaster 101Amountainshader 106Lotsofprettypictures 111Hedgeshader 115Griddisplays 123DebuggingOSLcode 129TheOSLpreprocessor 131Additionalinformation 133

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Intro

Whoshouldreadthisbook?

Anyonewhowantstoextendhisorherskillsetindesigningmaterials.BlendersnodesystemisalreadyprettypowerfulbutifyouwanttohavetotalcontroloverthematerialsyoudesignforBlendersCyclesrendererorifyouwanttohavesomespecificnodestoeaseyourworkflow,learningOpenShadingLanguageisamust.AlsomanyshaderswrittenfortheRenderman(tm)ShadingLanguagearefairlyeasytoporttoOSL.

Whatkindoftopicswillbecovered?

Thisbookfocusesonwritingpracticalshaderswhichmeansyoucandownloadthecodeanduseitasisor,evenbetter,learnfromitbyadaptingittoyourneeds.OftenevendetailsinthecodeareexplainedtolowerthelearningcurvebutontheotherhandsomespecialisttopicsnotspecifictoOSLarenotcovered(anexampleisregularexpressions).Eachshaderthatwedevelopiscoveredinitsownsectionandspecificareasofinterestarelistedatthebeginningofeachsection.

IsOpenShadingLanguagerestrictedtoBlender?

Inprincipleno:CurrentlyV-RaysupportsOSLandAutodeskBeastwillalsosupportOSLandmaybeotherpaid3Dapplicationswillsupportitonceitgainsmomentum,butatthismomentBlenderistheonlyopensourceapplicationthathasadaptedOSL.ThismightwellchangeinthefutureasOSLiswelldocumentedandcanbereadilyadaptedtootherrenderers.

WhatskillsdoIneedtobenefitfromthisbook?

YoushouldbebefairlycomfortableinusingBlender.YouneednotbeaanexperiencedartistbutyoushouldknowyourwayaroundandknowhowtocreatematerialsfortheCyclesrendererwithBlender'snodesystem.Youshouldalsoknowhowtoprogram.Againyouneedn'tbeaseasonedsoftwaredeveloperbutitcertainlyhelpsifyouknowhowtoprogramshortpiecesofcodeinanyprocedurallanguagelikeC,C++,Java,Pythonoreven(visual)Basic.OpenShadingLanguageisaC-likelanguageandquiteeasytolearn.Itdoesn'trelyonconceptslikeobjectorientationnordoesitdependonhardtomasterthingslikepointersanditsdatatypesarelimitedbutadequateforthejob.Inshort,logicalthinkingwillgetyoualongwayandtheintroductorychapterswillintroducethemostimportantideas.IfyoucanprogramashortprogramthatliststhefirsttenevenintegersintheprogramminglanguageofyourchoiceOSLwillposenoproblem.

Besidesprogramming,writingshadersinvolvessomemathandgeometryaswellbutnothingabovehighschoolmath.Wewillexplaineachmathissueweencounterandwillconcentrateonhowtouse

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thingswithoutdelvingintothesmalldetails.Whereappropriatelinkstorelevanttopicsonthewebwillbeprovided.

WherecanIfindmoreaboutOpenShadingLanguage?

ThesinglemostimportantsiteistheGitHubrepositoryofOSL.NotbecauseyouneedtogetthesourcecodebutforemostbecauseitistherepositoryoftheOSLLanguageSpecification.OnceyoumasteredlearningOpenShadingLanguageforBlenderyouwillfindtheLanguagespecificationavaluablereference.

IntheappendicesIlistsomeadditionalsiteswhichareworthvisitingtolearnmoreaboutOSL.

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WhatareOSLshadersgoodfor?WhenshouldyouconsiderprogrammingashaderinOSL?Youmightdoitforfunofcoursebuteventhenyouneedtobalancethecostsversustheprofits.Afterallittakestimetolearnanewlanguageandimplementingandmaintainingshaderstakessomeefforttoo.SowhenistherereallyaneedtoimplementashaderinOSL?Thefollowingcriteriamayhelpyoudecide:

AshaderwouldbeimpossibleorveryhardtocreateinBlender'snodesystem

Thisisthemostclearcutcriterion.Blender'snodesystemisimmenselypowerfulbutstillsomethingsareimpossible.ForexampleOSLprovidesnoisetypeslikeGabornoisethatyoumightwanttousebutthatBlender'snodesystemdoesnotprovide(yet).Regularpatternsarealsoquitedifficultorimpossibletogeneratewithnodesbecausethenumberofregulartextureinputsislimitedtowave,gradient,checkerandbrick.Evenifpossibleanodesystemmightneedanenormousamountofnodes.Likewiserandomdistributionsofregularelementslikeripplesonapondcausedbyraindropsarehard.Basicallyanythingthatisnaturallyexpressedasarepetition(aprogrammedloop)orhasmanydecisionstomake(manyif/elsestatements)isgenerallyeasiertoprogramthantoconstructfromnodes.

Ashaderbasedonnodeswouldtaketomuchmemory

Ifyourshaderwoulduseimagetexturesyouwouldneedlargeonestobeabletozoomincloseandgetgoodresultseveniftheshadedpartcoversonlyafewpixels.AproceduralshaderlikeoneimplementedinOSLwouldtakethesameamountofmemoryregardlesswhetheryouneedhighorlowresolution.Ofcourseit'softeneasiertouseimagetexturesbutifsizemattersyoumightwanttocreatesuchtexturesprogrammatically.Beingabletoproduceatextureatanylevelofdetailmightalsopreventaliasingeffectsandadditionallyprogrammedshadersoftendon'tdependonuv-unwrapping.Notdependingonuv-mapssavestimebecauseyoudon'thavetocreateonebutalsomightgetridofseams.(whenuv-unwrappingyouhavetomakesurethatseamsareinvisibleorlineupwiththeedgesofatileableimage.)

Ashaderbasedonnodeswouldbetooslow

Blender'snodesystemisn'tslowatallbutifyouneedverymanynodestoimplementyourshader,asolutionthatisbundledinasingleOSLshadermightbequiteabitfaster.Ontheotherhand,OSLshadersarecurrentlylimitedtoexecutionontheCPUandthereforecannotbenefitfromthepotentiallymustfasterGPU.

SotherealanswertothequestionofwhentouseanOSLshaderis,aswithmostnon-trivialquestions:"itdepends".Itispossiblyeasiesttocobbletogetherafirstapproximationoftheshaderyouhaveinmindfromnodes.Ifindoingsothenumberofnodesneededincreasesquicklyoryou

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encounterthingsthatcannotbedoneatall,considerwritinganOSLnode.

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ReadinghintsTypesettingabookcontainingafairamountofsourcecodeisfiendishlydifficult,especiallybecauseyouhavetoallowfortheverysmallwidthofsomereadingdevices.Thereforethecodeisformattedtofitveryshortlinewidthsbutthatmightattimesnotbeverycomfortabletoread,inwhichcaseIrecommendlookingattheoriginalsourcecode(seebelow)whichisgenerallynotformatted.

Itmightalsobeagoodideatoturnoffoverridingthestylesheetsassociatedwiththise-book.Manye-readers(notablyAldiko)dothisbydefault,whichisfinefornon-fictionbutforbookswithlotsofillustrationsthisdoesn'tlookgood.Disablingthisoverrideletyouenjoythebookbetter.Inmostreadersitisstillpossibletoseparatelysetthesizeofthetext.

CodeavailabilityThesourcecodeshowninthebookisfreelyavailableonGitHub:https://github.com/varkenvarken/osl-shadersThesourcecodeislicensedasopensourceunderaGPLv3license.

Note:allexampleshadersinthisbookarepresentasan.oslfileintheShadersdirectorybutforeachshaderfilethereisalsoanaccompanying.blendfilewiththesamenamethatdemonstratesitsuse.Simplybyopeningthisfileandselecting'Rendered'asdisplaymodeinthe3dviewwillshowcasetheshader.(WhenopeningthefileBlendersetsthedisplaymodetosolidsoitisnecessarytoresetthistorenderedtoseetheeffect.)

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AverysimpleshaderFocusareas

howtoprepareBlendertouseOpenShadingLanguagecreatingyourfirstOSLshadertroubleshooting

PreparingBlender

BecausemosterrorsthatmayoccurwhenworkingwithOSLwillappearintheconsole,itisagoodideatomakesuretheconsoleisvisible.Forunix-likesystemslikeLinuxorOSXthisisdonebystartingBlenderfromacommandline(forexampleanxterm).ForWindowsaconsolecreatedafterBlenderhasstartedbyselectingWindow -> Toggle console.

DonotconfusethisconsolewithBlendersbuilt-inPythonconsole,thatisacompletelydifferentandunrelatedentity.

OSLiscurrentlyonlyavailableforCycles(notforBlendersinternalrenderer)sowehavetocheckifallprerequisitesaremet.

OpenanewBlenderfilewithjustasingleobject,alampandacamera.

Ifyouhaven'tchangedyourstartuppreferencesthedefaultstartupwilldooralternativelyyoumaydownloadandopenblank.blendfromtheShadersdirectoryinthecodeaccompanyingthisbook,

verifythatyouhaveselectedCyclesasyourrendererasshowninthescreenshot

verifythatyouwillusetheCPUtorender

(nottheGPUasOSLiscurrentlyonlyavailableontheCPU)File -> User Preferences -> System

,

verifythatOSLisenabled

ChecktheOpenShadingLanguageboxintheproperties.

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Toverifythateverythingisworkingasexpectedyoumustbeabletoworkwiththenodeeditor.Asuggestedlayoutisshownbelow(thatisthelayoutthatblank.blendhas)

blank.blendhasadefaultnodebasedmaterialalreadyassignedtoitsplanebutiftheobjectinyourstartupfilehasnomaterialyoucansimplyassignanewonebyselectingtheobjectandclickingNewinthenodeeditor.

CreatingyourfirstOSLshader

Ifyouhavewrittenashaderthefollowingstepsareneededtouseitaspartofanodebasedmaterial:

addanewscriptnode,pointittothecodeyouhavewritten,connectthesocketstoothernodes.

Letshaveacloserlookateachofthesesteps.

TouseashaderasanewnodeinBlendersCyclesrendererwemustaddaScriptnodetoanodebasedmaterial.ClickAdd->Script->Scriptinthenodeeditormenuandanewnodeisaddedto

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yourmaterial.Itdoesn'thaveanyinputoroutputsocketsyetbutitdoesletyouchooseeitheranexternalscript(=OSLcodethatresidesinatextfileondisk)oraninternalscript(=OSLcodepresentinoneofBlenderstexteditorbuffers)

Eachmethodisfinebutbothhavetheirprosandcons.Anexternalfileiseasiertodistributeonitsownandmaybemaintainedaspartofaseparatecollectionofshaderswhileashadercontainedinthetexteditorisapartofyour.blendfileandwillbesavedwiththerestofyourscene.BlenderstexteditorissufficientforsmallshadersbutforlargershadersusinganexternaleditorisoftenpreferableandifyoudownloadshadersfromtheShadersdirectoryinthecodeaccompanyingthisbookyoucanselectExternalandusethefilebrowserthatopenstopointtothedownloadedfile.

SelectExternalandclickonthetextfieldtoopenafilebrowser,chooseShaders/firstshader.oslandconfirmyourchoiceoralternatively,selectInternal,copythecodeshownbelowtotheinternaltexteditorandclickonthetextfieldtoselectyourinternaltextfile.

Whenyouselecteitheraninternaltextbufferoranexternalfile,theshaderiscompiledautomaticallyandifeverythingwentwellsomeinputandoutputsocketswillappear.

Ifsomethingwentwrongwhilecompilingtheshaderashortmessageisshowninthetopmenubar

andpossiblyadditionalinformationisshownontheconsole.Ifthathappens(andifyouwriteashaderyourselfinsteadofusingadownloadedsampleshaderthisisverylikelytohappenbecausewritinganewnon-trivialshaderwithoutanysyntaxerrorsinonegoishighlyimprobable),fixtheerrorsandclicktherecompilebutton(thebuttonontherightsideofascriptnodenexttothetextfield.

firstshader.oslisashaderwithjustoneinputsocket,acolor(andhenceshownwithayellowdot)andoneoutputsocket,alsoacolor.Youmayconnectitsoutputsockettothecolorinputofthe

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diffuseshaderofyourmaterialasshowninthescreenshot.

Ifyounowselect'Rendered'inthe3dviewmenu

youseethattheshaderhastakenthedefaultbrightwhiteinputcoloranddarkeneditsignificantly.Letshavealookathecodeforfirstshader.oslandseehowthishascomeabout:

01. shader darken(02. color In = 1,03. output color Out = 104. ){05. Out = In * 0.5;06. }

Notethatthelinenumbersprecedingthecodeareforreferenceonlyandnotpartoftheshadercode.

Thefirstlinedeclaresagenericshadercalleddarken(therearemoretypesofshaderinOSLbutnotallarecurrentlysupportedbyBlender.Mostshadersinthisbookaredefinedasgenericshaderswiththeshaderkeyword.Whenweencounterdifferentshaderslaterontheirdifferenceswillbeexplained).

AshaderlookslikeafunctionwithparametersandthesecondlinedefinesainputparametercalledIn.Thetypeofthisparameteriscolor(oneofthebasictypesofOSL)anditsdefaultvalueis1,meaningthatallthreecomponentsofthecolor(red,greenandblue)willbesettoonemakingthedefaultcolorbrightwhite.

ThenextlinedefinesaparameterOut,whichisalsoacolorwiththedefaultsetto1,i.e.white.Itismarkedasanoutputparameterbytheoutputkeyword.Whentheshaderiscompiled,Blenderadds

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socketstothenodeforeachparameter.Inputparameterswillappearassocketsontheleftsideofthenodeandoutputparameterswillappearontheright.Blenderwillcolorthesesocketsbasedontheirtypes,withyellowforcolorparameters.

Allparametersmustbedefinedwithadefault.Thedefaultofaninputparameterisusedifnoothernodesareconnectedtothissocket.Ifaninputsocketisnotconnecteditsvaluecanbechangedbytheenduserbyclickingonit.Dependingonthetypeofsocketasuitableeditorwillpopup,forexampleacolorpickerifitisacolor.Avalueofanunconnectedinputsocketcanevenbeanimated(byinsertingakeyframebyhoveringoveritwiththemouseandpressingI),justlikealmosteverythingelseinBlender.

Thedefaultvalueofanoutputsocketisusedifnowhereinthebodyoftheshaderavalueisassignedtoit.

Thebodyofthisshaderconsistofasingleline(line5)thatmultipliestheInvalueby0.5andassignsittotheOutparameter.Multiplyingacolorbyasinglevaluewillmultiplyeachcomponentbythatvaluesointhisexamplewedarkenallcolorcomponentsbythesameamount.

ThisexampleshaderisofcourseaboutassimpleasashadercanbeandmaybenotallthatusefulbutitdoesshowhowBlendertakescareofmuchoftheworkofmakinganewnodeusablebyaddingsocketswithappropriateeditorstothenewnodebasedonthetypesoftheinputandoutputparameters.

Troubleshooting

Whenyouusescriptnodesafewthingsmaygowrongbesideyourscripthavingsyntaxerrors.Mostconfigurationerrorswon'tevenshowupaserrorsontheconsoleandoftenallyouseethenisthatyourmaterialappearsblackwhenrendered.Themostcommononesare:

youcannotaddascriptnode

whenyouclickaddinthenodeeditorandyoudon'thaveachoicelistedcalled'Script'youprobablyforgottoselectCyclesasyourrenderenginebecausetheinternalrendererdoesn'tsupportscriptnodes.(InBlenderversionsbefore2.68therewasaScriptentryfortheinternaleditorbutclickingithadnoeffect)

compilationgoeswellbutthematerialshowsupasauniformcolor

youforgottochecktheOSLbuttonintherenderoptions

thereisnoOSLbuttonintherenderoptions

thisoptionisonlyavailableifyouuseCyclesasyourrenderengineandselectCPUrendering

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DatatypesFocusareas

Simpledatatypes(numbers,strings,vectors,colors)Compounddatatypes(arrays)Userdefineddatatypes(structs)

OSLhasseveraldatatypesthatmaybeusedtorepresentnumericalvaluesandstringsbutalsothingslikepoints,vectororcollectionsofthesevalues.Thissectiongivesabriefoverviewofthemostimportantissuestogetyoustartedquickly.Thisoverviewisbynomeanscomplete:fordetailsyoubestrefertochapter5oftheOSLlanguagespecification.

Simpledatatypes

int

anintrepresentspositiveandnegativeintegervalues.Allintegeroperationsfoundinmostprogramminglanguagesaresupported(includingmodulo%andthebitwiseoperatorsand&,or|andxor^).Integersareatleast32bitswide(i.e.mayholdvaluesabout±2000,000,000butdependingonyourplatformmuchlarger64bitwideintegersmaybeimplemented.Don'tdependonthisifyouwanttowriteportableshaders!).Anintisalsousedasabooleanvaluewhere0equalsfalseandanyothervaluetrue(OSLdoesn'thaveanexplicitbooleantype.Infactallsimpletypesmaybeusedasbooleanvalues,eachwithitsownrulesforwhichvaluesareconsideredtrue.Inthisbookwesticktointvaluesforbooleans.RefertotheOSLlanguagespecificationfordetails).

example:

int a = 4;int b = a * 5;

float

afloatrepresentspositiveandnegativefractionalvalues.Itcanatleastrepresentvaluesassmall1e-35orasbigas1e35with7decimalsofprecision(tobepreciseitisatleasta32bitfloatconformingtotheIEEEspecification.Onsomeplatformsthismaybea64bitfloatwithmoreprecisionandawiderrangebutdon'tdependonitifportabilityisimportant).Anywhereafloatisusedanintmaybeusedaswell;itwillbeautomaticallyconvertedtoafloat.Alltheusualoperationsaresupportedforfloatsandthereiswholehostofstandardfunctionsavailableaswell,includingtrigonometricfunctions,exponentiation,etc.

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example:

float a = -1.4;float b = sin(a*1e-3);

string

astringisasequenceofcharacters.Itsmainuseisinspecifyingfilenamesoftextures.Theoperationsarechieflylimitedtocomparisonsbutafairnumberofstandardfunctionsareprovidedaswell,includingsupportforregularexpressions.

example:

string a = "filename.png";

void

Voidisnotavaluepersebutusedtoexplicitlysignifytheabsenceofavalue,forexampleaswithafunctionthathasonlysideeffectsbutdoesn'treturnanything.

pointlikedatatypes

pointlikedatatypeshaveincommonthattheyallconsistofthreefloatingpointvaluesandthattheyallshareacommonsetofoperationslikemultiplicationandaddition.Theydifferinsubtledetailsundersomeconditionsbutfornowwecantreatthemasequalbutwithanamethatrelatestotheirpurpose.Thecomponentsofapointlikedatatypecanbeaccessedwithanindexthatstartsa0forthefirstcomponent.Operationsbetweenapointliketypeandafloatwillperformtheoperationonallthreecomponentsindividually.Thisincludesassignment:inthecodebelowweassignthevalue7toallcomponentsofthevariableb.

example:

point a = point(1,0,0);point b = point(0,1,0);vector v = a - b;normal n = cross(a,b);a[0] = 3;b = 7;b = b + v * 5;

point

apointisusedtorepresentalocationorpositionintheworld,suchasthelocationoftheobject

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beingshaded.Youcanaddtopointstotranslatethem,multiplytoscalethemorcallthebuilt-infunctionrotate()torotatethemaroundanaxis

vector

avectorrepresentsadirectionoralinesegmentbetweenpoints.Itispossibletotranslate,scaleandrotatetheminthesamewayaspointsandOSLprovidesanumberoffunctionstoperformcommonvectoroperationsincludingcalculationitslength,normalizingitandcalculatingthecrossanddotproductoftwovectorstonameafew.

Thesefunctionsarejustashappytoacceptpointsornormalsbutmostoftenwethinkofthesefunctionsinthecontextofvectors.Manyshadersinthisbookusevectorsandsomebasicunderstandingofthemisnecessarybutinmostcasestheiruseisexplainedindetail.EspeciallythefirstfewshadersthatwewillencounterinthethenextchapterintroducenotonlyhowtouseOSLasalanguagebutwillimplementafewshadersthatperformsomebasicvectoroperationsjusttogetfamiliar.

normal

anormalisavectorthatdenotesadirectionperpendiculartosomething,oftenafaceofamesh.Anythingyoucandowithapointoravectoryoucandowithanormalaswellbutbecausethedirectionperpendiculartoasurfaceplayssuchanimportantroleindesigningshadersitsusefultohaveaseparatenameforsuchavector.(Whenweencounterclosures(=lightscatteringfunctions)wewillseethattheseclosuresdefinewhichfractionoftheincominglightisreflectedrelativetothenormal.Forexamplewhenlightisreflectedfromaperfectmirrortheangleofthereflectedlightrelativetothesurfacenormalisthesameastheangleoftheangleoftheincominglightrelativetothesurfacenormal)

color

acolorisnotpointlikeinthesensethatithasageometricalsignificancebutitdoeshavethreecomponents(hererepresentingthered,blueandgreencomponentsofacolor)andsharesoperationswiththegeometricaltypeslikeindexing,addition,etc.Inmanycasesnoneofthecomponentswillhaveavaluegreaterthanonebutthisisnotmandatory.

matrix

Amatrixisnotpointlikeatallbutacollectionof4x4floatsthatismainlyusedtotransformpointlikedatatypes.BecauseOSLprovidesalotofstandardfunctionsforpointliketypessuchasrotationandtransformationsbetweendifferentvectorspaces,wewillnotusethematrixtypeinthisbook.

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aggregatetypes

Ifwewanttomanipulatemorethanonesimplevalueasoneitemweneedsomewaytoaggregatethem.ForthispurposeOSLoffersarraysandstructs.

array

Anarrayisalistofitemsofthesametype.Eachofthosecanbeaccessedbyusinganindexinthesamewayascomponentsofpointliketypesmaybeaccessed.Thelengthofanarrayisfixedonceitisdeclared.

example:

float a[4];float b[3] = { 3, 4, 5 };point c[5];a[2]=b[1]*8;int alength = arraylength(a);

Notethatthefirstelementofanarrayhasanindexof0.Inthefirstlinewedefineanarrayawithfourelements.Thebarrayisdefinedwiththreeelementsandtheyareeachinitializedtoavalue.Asthecarrayshows,arraysarenotlimitedtofloatvaluesbutmaybepoint-liketypesaswell.

struct

Astructisacollectionofitemsofdifferenttypes.Eachofthosecanbeaccessedbyname.Astructissomewhatlikeanewtype:Onceyouhavedefinedastructyoucanuseittodeclarevariables.

example:

struct ray{ point start; vector direction;};

ray view = { point(0,0,0), vector(0,0,1) };view.point[2] = 5;

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ControlstructuresFocusareas

makingdecisions,if/elsestatementsrepeatingactivities,forandwhileloops

Likeanyprogramminglanguageworthitssalt,OSLhasitsshareofcontrolstructurestomakedecisionsandrepeatactions.ThesecontrolstructuresarealmostidenticaltothosefoundinCorC++andaredocumentedindetailinthelanguagespecification.Inthissectionwegiveashortoverviewinthecontextofsomesimpleshaderstogetafeelingforwhatispossible.

Leftorright,youdecide

Makingdecisionsandgeneratingdifferentoutputbasedonsomeconditionispresentinalmosteveryshaderbutthemosttrivial.Inourexampleshaderwewanttoproduceanoutputvalueof1ifaninputvalueliesbetweentogivenlimits.Suchashadernodemightbeusedforexampletoproducesharpedgedareasfromasmoothnoiseinput.Anexampleisshownintheimagetogetherwithanodesetup.

Thecodethatimplementsthisshaderisshownbelow:

ThecodeisavailableinShaders/range.osl

01. shader range(02. float Value = 0,03. float Low = 0,04. float High = 0,05. 06. output float Fac = 0

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07. ){08. if( Value >= Low && Value <= High ){09. Fac = 1;10. }11. }

Theshaderhasthreeinputparameters:theValuewewanttotestandtheLowandHighvaluesthatspecifytherangethatwillreturnavalueof1.Theresult,either0or1,isreturnedinthesingleoutputparameterFac.

Thebodyofthecodeconsistsofasingleifstatement(line8).Anifstatementconsistsofanexpressionbetweenparenthesesandbody.Thebodyconsistsofasinglestatementoragroupofstatementsbetweencurlybracesandisonlyexecutediftheexpressionoftheifstatementevaluatestonon-zero.Becauseifstatementsmaybenested(thebodymaycontainotherifstatements)itmightbecomeunclearwhatthestructureofthecodeissointhisbookwealwaysusethecurlybraces,evenifthereisjustasinglestatementinthebodytoclearlyindicatewhichpieceofcodebelongstowhichifstatement.

Theexpressionofanifstatementmaybearbitrarilycomplex.Herewecheckifthevalueislargerorequaltothelowerlimitandcombinethiswithachecktoseeifthevalueislessthanorequaltotheupperlimit.The&&operator(logicaland)indicatesthatbothconditionsshouldbetrue.AlistofalloperatorsthatcanbeusedcanbefoundintheOSLlanguagespecification,chapter6.Ifexpressionsarecomplexortheprecedenceofoperatorsmakesitnecessary,parenthesesmaybeusedtogrouppartsoftheexpression.Inthiscaseweassignthevalue1totheoutputparameterFaciftheconditionistrue.

Inthepreviousexampleweassignedadefaultvaluetoanoutputparameterandchangedthisvalueifaconditionwasmet.Sometimeshoweverwewanttotakedifferentactionsiftheconditionisnotmet.Forthispurposetheifstatementhasanoptionalelsepart.

Intheshaderpresentedbelowwewanttoproducethreedifferentoutputcolors,dependingonaninputvaluebeingbelow,insideoraboveagivenrange.

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Thecodefortheshaderisgivenbelowandcomparedtotherangeshaderthisshaderhasthreeextrainputparameters,thecolorswewanttochoosefrom.Thesingleoutputparameteristhechosencolor.

CodeavailableinShaders/colorrange.osl

01. shader colorrange(02. float Value = 0,03. float Low = 0,04. float High = 0,05. color LowColor = 0,06. color MidColor = 0.5, // grey07. color HighColor = 1,08. 09. output color Color = 010. ){11. Color = LowColor;12. if( Value >= Low && Value <= High ){13. Color = MidColor;14. } else {15. if( Value > High ){16. Color = HighColor;17. }18. }19. }

ThefirstlineassignsLowColortotheoutputparameterasadefault.Theifstatementagaincheckswhethertheinputvalueisinthespecifiedrangebutifthisnotthecasetheelsepart(line14)isexecuted.Thiselsepartcontainsagainanifstatementthatchecksiftheinputvalueisabovetheuppervalueiftherangeandifso,assignsHighColortotheoutputparameter.Notethatinthis

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exampleagainnoneofthecurlybracesintheifstatementswereneededbutweusedthemanywaytogetherwithconsistentindentationtoavoidconfusion.

IfearImightberepeatingmyself

Makingdecisionsisafundamentalfeatureofaprogramminglanguageandsoistheabilitytorepeatanactionagivennumberoftimes.TothisendOSLofferstheforstatementwhichisusedinthenextshadertoproduceagivennumberofstripes.

CodeavailableinShaders/stripes.osl

01. shader stripes(02. point Pos = P,03. int Number = 4,04. 05. output float Fac = 006. ){07. float x = mod(Pos[0],1);08. 09. int i;10. for(i=1; i <= Number; i++){11. if( x < (float)i/Number ){12. Fac = i % 2;13. break;14. }15. }16. }

Theinputparametersofthestripesshaderarethepositionbeingshaded(Pos)andthenumberofstripeswewanttoproduce(Number).ThesingleoutputparameterFacwillbeeither0or1foralternatingstripes.

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Inthebodyoftheshaderwemakesurethatthevalueofthexcoordinateisintherange[0,1]bycalculatingthexcomponentofthepositionbeingshadedmoduloone.Thisguaranteesthatforlargervaluesofxthepatternthatwegenerateforthe[0,1]rangeisrepeated.

Theforstatementrepeatsthestatementsinitsbodyasoftenasspecifiedbythethreesemi-colonseparatedexpressionsbetweenparentheses(line10).Aswiththebodyoftheifstatementthecurlybracesareoptionalifthebodyconsistsofasinglestatementbutinthisbookwealwaysusethem.

Thefirstexpressionfollowingtheforkeywordinitializesthecontrolvariablei.Thesecondexpressionisaconditionandaslongasthisconditionistruethebodyoftheforstatementisexecuted.Thethirdexpressionisevaluatedaftereachexecutionofthebodyandisusedtoincrementthecontrolvariableby1.(Theexpressionsinaforstatementarenotlimitedtoonesaffectingacontrolvariable.Detailscanbefoundinthelanguagespecification)

Thebodyoftheforstatementitselfconsistsofasingleifstatement.Theexpressioninthisifstatementchecksifthexcoordinateissmallerthantheupperlimitofthestripewearecurrentlychecking.Forexample,ifNumberis4(wewantfourstripes)thefirsttimewecheckthisexpressionisx < 1/4(istartscountingfromonebecausethatisthevaluewegaveitintheinitializationexpressionoftheforstatement).Thesecondtimearoundthisexpressionisx < 2/4(becauseafterthefirstexecutionofthebodyoftheforstatementthecontrolvariableiwasincrementedby1).Notethatwehadtocasttheivariabletoafloattomakesurethatthedivisionresultedinafraction.Hadwenotdonethiswewouldhavebeendividingintegersandwewouldhavegotthevalue0insteadofafraction.

Iftheexpressionevaluatestotrue,weassigntheoutputvariableFacthevalueofthecontrolvariableimodulo2.Thiswillalternatebetween0an1dependingonwhetheriisevenorodd.Iftheexpressionistruewenotonlyassignavaluetotheoutputparameterbutstopexecutingtheforstatementaltogether.Thisisdonewiththebreakstatement.Ifwewouldexecutetheremaining

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iterationsofforloopthetestwouldevaluatetotrueagain(ifxissmallerthan1/4itisalsosmallerthan2/4,3/4,etc.)soweneedtobreakoutofthelooptopreventassigningthewrongvaluetotheoutputparameter.

Wearenotalwaysinthepositiontodeterminebeforehandhowmanyrepetitionsareneeded.Inthosecasesitmightbebettertouseawhileloop,whichwillexecuteitsbodyaslongasanexpressionistrue.

Inthisexampleshaderwedonotwanttoproduceafixednumberofstripesbutstripsthatgetprogressivelysmalleruntiltheyaredeemedsmallenough.Thislowerlimitonthewidthofthestripsisaninputparameter.

Thecentralideaintheimplementationpresentedhereistocheckifthex-coordinateisbelowacertainlimitandifnot,extendthislimitwithavaluedxandcheckagain.OneachsuccessiveiterationthisvaluedxisdiminishedbyaquarteruntilitissmallerthaninputparameterLimit.

CodeavailableinShaders/smallerstripes.osl

01. shader smallerstripes(02. point Pos = P,03. float Limit = 0.01,04. 05. output float Fac = 006. ){07. float x = mod(Pos[0],1)*2;08. float dx = 0.5;09. float xlimit = dx;10. 11. float ActualLimit =

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12. Limit>0.001 ? Limit : 0.01;13. while( dx >= ActualLimit ){14. if( x < xlimit ){15. break;16. }17. Fac = abs(Fac-1);18. dx *= 0.75;19. xlimit += dx;20. }21. }

TopreventanendlessloopwecheckifLimitislargerthansomesmallvalueandifso,assignittothevariableActualLimit.Ifnotwetake0.001asasafeminimum.Therearetwothingsworthnotinghere:WeneedthisextravariableActualLimitbecauseaninputparameterlikeLimitisreadonly,wecannotchangeitsvalue.Alsoyoumightwonderwhatthestrangeexpressionontherighthandsideoftheassignmentdoes(line12).Thisisinfactjustanifstatementdisguisedasanexpression,anidiomdirectlycopiedfromtheClanguage.Thevalueoftheexpressiona ? b : cisequaltobifaistrue(nonzero)andcifaisfalse.

Thewhilestatementconsistsofanexpressionbetweenparenthesesandabody(line13).Thebodyisexecutedaslongastheexpressionistrue.Thebodymightconsistofasinglestatementorseveralstatementsenclosedincurlybraces.Forclaritywewillalwaysusecurlybraces,justaswedoforifandforstatements.

Thebodyofthewhileloopcontainsanifstatementthatchecksifthex-coordinateisbelowthecurrentlimit.Ifso,webreakoutofthewhileloop.SincetherearenostatementsfollowingthewhileloopthismeansthattheoutputparameterFacwillholdwhatevervalueitisholdingatthatinstant.Therefore,ifthex-coordinatewasnotbelowthecurrentlimitwemustnotonlyextendthelimitagainstwhichwewillcheckinthenextiterationbutalsoflipthevalueofFac.Facisafloatingpointvalueandtheexpressionabs(Fac-1)isonewaytoconvertavaluefrom0to1andviceversa.

Thefinaltwolinesofthebodyshortenthewidthdxofthestripeby25%andaddthiswidthtothexlimitvariable.

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