Super 8 Languages for Making Movies (A Functional Pearl) LEIFANDERSEN,STEPHENCHANG,MATTHIASFELLEISEN,PLT@NortheasternUniversity TheRacketdoctrinetellsdeveloperstocreatelanguages(aslibraries)tonarrowthegapbetweentheterminology ofaproblemdomainandgeneralprogrammingconstructs.Thispearlillustratesthisdoctrinewiththecreation ofarelativelysimpledomain-specificlanguageforeditingvideos. Toproducethevideoproceedingsofa conference,forexample,videoprofessionalstraditionallyuse“non-linear”GUIeditorstomanuallyediteach talk,despitetherepetitivenatureoftheprocess.Asitturnsout,thetaskofvideoeditingnaturallysplitsinto adeclarativephaseandanimperativerenderingphaseattheend. Henceitisnaturaltocreateafunctional- declarative language for the first phase, which reduces a lot of manual labor. The implementation of this user-facingDSL,dubbedVideo,utilizesasecond,internalDSLtoimplementthesecondphase,whichisan interfacetoageneral,low-levelClibrary.Finally,weinjecttypecheckingintoourVideolanguageviaanother DSLthatsupportsprogramminginthelanguageoftypeformalisms. Inshort,thedevelopmentofthevideo editinglanguagecleanlydemonstrateshowtheRacketdoctrinenaturallyleadstothecreationoflanguage hierarchies,analogoustothehierarchiesofmodulesfoundinconventionalfunctionallanguages. CCSConcepts:•Softwareanditsengineering Functionallanguages;Preprocessors;Macrolanguages; Ñ Specificationlanguages;Designingsoftware;•Informationsystems Multimediacontentcreation;•Applied Ñ computing Mediaarts;•Human-centeredcomputing Graphicaluserinterfaces; Ñ Ñ AdditionalKeyWordsandPhrases:Domain-SpecificLanguage,DeclarativeLanguages,VideoEditing,Syntax Elaboration,LanguageOrientedDesign,Movies,IntegratedDevelopmentEnvironment ACMReferenceformat: LeifAndersen,StephenChang,MatthiasFelleisen.2017.Super8LanguagesforMakingMovies.PACMProgr. Lang.1,1,Article30(September2017),28pages. DOI:10.1145/3110274 1 BEINGIAVORDIATCHKI ImaginebeingIavorDiatchki. HeisthefriendlyguywhorecordsallthewonderfulICFPpresen- tations,editsthemintodigestiblevideoclips,andfinallycreatesaYouTubechannelforthewhole conference. Whenhecreatesthevideoclips,hecombinesafeedofthepresenterwiththepresenter’s screen,thesoundfeedforthespeaker,andyetanotheroneforaudiencequestions. Additionally, 30 Diatchkimustaddastartandendsequencetoeachvideoplusvariouswatermarksthroughout. Onceonevideoisputtogether,thesameprocessmustberepeatedforthenextconferencetalkand thenextandsoon. Worse,eventhoughsomeeditingstepsinvolvecreativity,theprocessbecomesso monotonousthatitreducesthecreativespiritforwhenitistrulyneeded. Theproblemcriesoutforadeclarativelanguage,especiallybecausethestateoftheartforvideo editingsuggests(seesection3)thatprofessionalsinthisdomainalreadythink“functionally.” Towit, professionalsspeakof“non-linearvideoediting”(NLVE)tohighlighttheideathattheprocessis Permissiontomakedigitalorhardcopiesofpartorallofthisworkforpersonalorclassroomuseisgrantedwithoutfee providedthatcopiesarenotmadeordistributedforprofitorcommercialadvantageandthatcopiesbearthisnoticeandthe fullcitationonthefirstpage.Copyrightsforthird-partycomponentsofthisworkmustbehonored.Forallotheruses,contact theowner/author(s). ©2017Copyrightheldbytheowner/author(s). 2475-1421/2017/9-ART30$15.00 DOI:10.1145/3110274 PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. 30:2 LeifAndersen,StephenChang,Ma￿hiasFelleisen non-destructive. Technically,theeditingprocessseparatesadescriptivephase—whattheeventual videoissupposedtolooklike,givenexistingtracks—fromtherenderingphase—whichactually createsthevideoclipfromthesedescriptions. ThispearlpresentsVideo(section4),ascriptinglanguageforvideoproduction. Videoturnsvideo editingupsidedown. InsteadofsittingforhoursonendinfrontofsomeNLVEGUI,aprofessional cannowspendafewminutesinfrontofanIDEtocreateaVideoscriptand,voilà,avideoclippops out. AVideoscriptisjustasequenceofexpressions,whichdescribefragmentsofavideoclip,and definitions,whichintroduceconstantsforandfunctionsonvideoclips. Runningsuchascriptturns thisdescriptionintosuitable“assemblycode”foravideorenderer. SpeakingofanIDE,everyoneknowsthatinthisdayandageaprogramminglanguagecomeswith awholesuiteofgadgets,thatis,theprogrammingenvironmentaroundthelanguage. Wetherefore throwinanIDE(section7)andadependenttypesystem(section6),notonlybecausethesemightbe usefulforproducingvideochannels,butbecausearealfunctionalpearldeservesthismuchattention. ThusanAgda-trainedprogrammermayaddtypestoVideomodules. Andbetterstill,Videocode mayincludeanNLVEwidget,whichmayofcoursecomewithembeddedVideocode,whichmay containanotherNLVEwidget,... Didwementionturtlesyet?1 Abstractlyspeaking,VideoonceagaindemonstratesthepoweroftheRacketdoctrine(section2). Racket hosts Video as an embedded domain-specific language. Implementing (section 5) this language in Racket takes only a small effort because of Racket’s powerful language-production language. Indeed,addinganIDEandatypesystemisalsoamatterofafewhoursofthinkingtime andcoding. Inthisspirit,thepearlpointsoutakeydifferencebetweentheconstructionofembedded DSLsinconventionalfunctionallanguagesandtheonetrue-bluekidontheblock,Racket. 2 DRSTRANGELOVE:HOWILEARNEDTOSTOPWORRYINGANDLOVERACKET TheRacketdoctrine(Felleisenetal. 2015)saysthatdevelopersmustfeelempoweredtoeasilycreate anddeployanewlanguagewhenasoftwaredevelopmentsituationdemandsit. Theemphasison languagereflectstheRacketteam’sdeepembraceoftheSapir-Whorfhypothesis. Intheworldof software,thishypothesismeansthatlanguageframesthedeveloper’sabilitytounderstandaproblem andarticulateasolution—ateverystageinthedevelopmentprocess. Philosophically,Racketachievesthisidealwitharadicalemphasisonlinguisticreuse(Krishna- murthi2001). Technically,thisreuseisenabledviaRacket’sdistinctivefeature: amodularsyntax system(Flatt2002). Inthissystem,itiseasytoimportalinguisticconstructasafunction;indeed thesystemblursthedistinctionbetweenlanguagesandlibraries,e.g. Tobin-Hochstadtetal.(2011). Whilealibrarymoduleexportsfunctionswitharelatedpurpose, alanguagemoduleexportsthe constructsofaprogramminglanguage. In Racket, every module must first import its language definition, via a one-line specification. Forexample,#lang racket/base—pronounced“hashlangracketbase”—tellsRacketandafuture readerthatthemoduleiswrittenintheracket/baselanguage. Thisspecificationpointstoafile containingthelanguageimplementationthat,approximatelyspeaking,consistsofasuiteoflinguistic featuresandrun-timefunctions. AdevelopercanthuseditalanguageLinonebufferofanIDEand anLprograminasecondone. Anychangetothefirstisimmediatelyvisibleinthesecondone,just byswitchingbetweentabs. Thus,languagedevelopmentinRacketsuffersfromnopointsoffriction. Developinganewlanguagetypicallystartsfromabaselanguageclosetothedesiredone. From there,aRacketdevelopercreatesanewlanguagewithsomeorallofthefollowingactions: addingnewlinguisticconstructs; ‚ hidinglinguisticconstructs;and ‚ 1Seeen.wikipedia.org/wiki/Turtles_all_the_way_down,lastvisitedFeb20,2017. PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. Super8LanguagesforMakingMovies 30:3 re-interpretinglinguisticconstructs. ‚ Videoexploitsalloftheabove. Here,linguisticconstructsareanyfunctionsorsyntacticextensions addedtoalanguagesuchaslistcomprehensionsorpatternmatching. Forthere-interpretationof linguisticconstructs,Racketdevelopersheavilyrelyonlinguisticinterpositionpoints,thatis,anchors inthesyntaxelaborationprocesswhereaprogrammayinjectadditionalsyntaxtransformations. DuetotheeaseofdevelopingandinstallinglanguagesintheRacketecosystem, thelanguage creation “warhead” is Racket’s distinguishing weapon, akin to Haskell’s type classes or ML’s functors,initsarsenalofsoftware-engineeringtools.Whendevelopersrealizethatitisbesttoexpress themselvesinthelanguageofadomain,theydonothesitatetodevelopamatchingprogramming language. Afterall,domainexpertshavedevelopedthisspecializedterminology(andontology)so thattheycandiscussproblemsandsolutionsefficiently.2 Thedomainofvideoeditingisaparticularlywell-suiteddomainforillustratingtheabovepoints. WhiletheevolutionofthelanguagefollowsthestandardpathfromaveneerforaClibrarytoa full-fledgedlanguage(FowlerandParsons2010), Racketreducesthispathsignificantlyandthis pearldemonstrateshow. BeforewecandescribeVideoanditsimplementation,however,weneedto surveytheworldofeditingvideos. 3 PRIMER Peopleuseso-callednon-linearvideoeditors(NLVEs)tocomposevideoclips(Dancyger2010). Inthecontextoffilmproduction,non-linearmeansnon-destructive,thatis,thesourcevideosdo notdegradeinqualityduetoediting.3 ANLVEisagraphicaltoolwithatimelineoftracks. Each track describes a composition of video clips, audio clips, and effects playing in sequence. The NLVErendersthesetracksbyplayingthemallsimultaneously,placingonetrackontopoftheother. Obviouslyascreendisplayingtheresultcanplayonlyasingletrackforvideo;4byconventionthisis thelastortoptrack. Videoeditorsuseeffectstocompositetracks. Thatis,effectssplicetwoormore trackstogethersothattheyappearonthescreenatthesametime. Technically,therendereruses theseeffectstocombinetracksastheyplayandeitheroutputtheresulttoafileorplayitonascreen. Overtime,professionalshavedevelopedtoolsanddesignpatternstoreducetheamountofrepetitive manuallaborinvideoediting. Theyfrequentlydevelopso-calledmacros—ascriptedsequenceof userinterfaceelements—inlanguagessuchasAppleScript(Cook2007). Someprofessionaltools, such as Adobe Premiere (Jago and Adobe Creative Team 2017), even include an API to create script-style plug-ins directly. Extending tools in this fashion has limits. These kinds of macros areextremelybrittleandfrequentlybreak,evenwithinasingleapplication,becausethesemacro languagesessentiallyspecifydialogboxclickswithoutunderstandingtheunderlyingtools. Using a tool’s official plug-in interface produces reasonably robust scripts but yields plug-ins thataretightlycoupledwithitstool. Theycanbeusedonlywhentheentiretoolchainispresent. Blender(RoosendaalandHess2007),forexample,isonlyscriptablewithaBlender-specificPython interpreterthatrunswhenBlenderislaunched. Alternativeapproachesusegeneral-purposemultimediaframeworkssuchasGStreamer(Taymans etal. 2013)ortheMLTFramework.5 TheseframeworksareAPIsforC-likelanguagesthatprovide datatypesforbuildingandrenderingvideos. Theseframeworksareprimarilyusedintwosituations. First,theyaretheback-endstoNLVEs. Forexample,MLTisthebackendforbothShotcut6 and 2NoteveryDSLisaweekormonth-longproject.Turnstile(Changetal.2017)tookalmostayearofdevelopmenttime! 3Digitaleditorsachievethisresultbyoperatingonreferencestovideos,ratherthanoperatingonthevideosthemselves. 4Audiotrackscanactuallybeplayedsimultaneously. 5mltframework.org/ 6shotcutapp.com/ PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. 30:4 LeifAndersen,StephenChang,Ma￿hiasFelleisen Kdenlive.7 Second,professionalsusetheseframeworkstobatch-processvideos,particularlywhen interactivedevelopmentisnotdesired. Theappealoftheseframeworkscomesfromtheirabilitytocreateabstractions,suchasfunctions, tohandleotherwiserepetitivetasks.Usingtheseframeworksquicklybecomescumbersome,however, whenthereisaneedtocombineinteractiveandprogrammaticworkflows, asisthecaseforthe creationofconferencerecordings. Thus,studiostendtostickwithNLVEsandusetheseframeworks onlysparingly. Professionalsalsousedomain-specificlanguagesforvideoediting. TheseDSLsprimarilyfallinto twocategories: XML-basedDSLsandscripting-basedDSLs. XMLDSLssuchasMLTXMLand thenow-deprecatedSMIL(Bultermanetal. 2008)offerdeclarativelanguagesforprocessingvideos. Theselanguagesgenerallydonothavefunctionsoranyothertypeofabstraction,however,andthus professionalstendnottodealwiththeseXMLlanguagesdirectlywhenvideoediting. Rather,NLVEs usetheselanguagesasafileformattosavevideoprojects. Scripting-basedDSLssuchasAVISynth8aredeclarativeandsupportfunctionsandotherabstrac- tions,buthavetheirownlimitations.Theytypicallysupportonlythesimplestoftaskssuchasplaying videos in sequence with transitions and minor visual effects. They also tend to lack any formal grammarsanduseasmallscriptforaparser. Finally,theylackmanycodereusefeatures. AVISynth, forexample,allowsprogrammerstocreatesimplefunctionsbutcomeswithoutanycontrolflow constructssuchasconditionalbranching. 4 THEPRODUCERS The literature survey in the preceding section suggests that non-linear video editing distinctly separatesthedescriptionofavideoclipfromtherenderingactiononit. Specifically,aneditor(asin thetool)needsadescriptionofwhatthefinalvideoshouldlooklikeintermsofthegivenpieces. Theactionofrenderingthisvideoisadistinctsecondstep. Goingfromthisassessmenttoalanguage design requires one more idea: abstraction. For example, a description of a video composition shouldbeabletouseasequencecomprehensiontoapplyawatermarktoallimages. Similarly,a professionalmaywishtocreateonemoduleperICFPpresentationinordertomakeupacomplete ICFPchannelinamodularfashion. Andofcourse,thelanguagemustallowthedefinitionanduseof functionsbecauseitisthemostcommonformofabstraction. TheVideolanguagegetstotheheartofthedomain. EachVideoprogramisacompletemodule thatinterminglesdescriptionsofvideoclipsandauxiliarydefinitions. ItdenotesaRacketmodule thatexportsaplaylistdescriptionofthecompletevideo. OnewaytouseaVideomoduleistocreate avideowitharenderer. AdifferentwayistoimportitintoasecondVideomoduleandtoincorporate itsexportedplaylistintoanothervideo. Figure1displaysasimpleVideoscript. Itconsistsoffiveexpressions,eachdescribingapiece ofavideoclip. Rightbelowthethirdpartofthevideodescription,italsocontainstwodefinitions, whichintroduceonenameeachsothattheprecedingmultitrackdescriptiondoesnotbecometoo deeplynested. Whentherendererturnsthisscriptintoanactualvideo,itturnsthefivepiecesinto sequenceofimages,takingintoaccountthetransitionsbetweenthefirstandsecondfragmentand thefourthandthefifth. Inessence,thescriptinfigure1assemblesthevisualpartofasimpleconferencevideo. What ismissingistheaudiopart. Naturally,aVideoprogrammershouldabstractoverthissequenceof expressions,plustheaudioprocessing,andcreateasuitablelibrary. Figure2showswhatthesame scriptroughlylookslikeafteraVideoprogrammerhasencapsulatedanabstractionoverthescriptin 7kdenlive.org/ 8avisynth.nl PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. Super8LanguagesforMakingMovies 30:5 01 #lang video 02 03 (image �splash.png� #:length 100) 04 05 (fade-transition #:length 50) 06 07 (multitrack (blank #f) 08 (composite-transition 0 0 1/4 1/4) 09 slides 10 (composite-transition 1/4 0 3/4 1) 11 presentation 12 (composite-transition 0 1/4 1/4 3/4) 13 (image �logo.png� #:length (producer-length talk))) 14 15 ; where 16 (define slides 17 (clip �slides05.MTS� #:start 2900 #:end 80000)) 18 19 (define presentation 20 (playlist (clip �vid01.mp4�) 21 (clip �vid02.mp4�) 22 #:start 3900 #:end 36850)) 23 24 (fade-transition #:length 50) 25 26 (image �splash.png� #:length 100) time Figure1: AfirstVideoscript 01 #lang video 02 03 (require �conference-lib.vid�) 04 05 (conference-talk video slides audio 125) 06 ; where 07 (define slides (clip �slides05.MTS� #:start 2900 #:end 80000)) 08 (define video (playlist (clip �vid01.mp4�) (clip �vid02.mp4�) 09 #:start 3900 #:end 36850)) 10 (define audio (playlist (clip �capture01.wav�) (clip �capture02.wav�))) time Figure2: AVideodescriptionofaconferencetalk PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. 30:6 LeifAndersen,StephenChang,Ma￿hiasFelleisen figure1asautilitylibrary. Thisprogramusestheimportedconference-talkfunctiontocombine arecordingofthespeaker,acaptureoftheslides,andtheaudio. Asmentioned,line1oftheprogram specifiesthatthismoduleiswrittenintheVideolanguage. Line3importsthelibrarythatdefines theconference-talkfunction. Line5producesthevideothatthismoduledescribes. Finally,the remainderisasequenceofdefinitionsthatintroduceauxiliaryconstants. Figure3showstheessenceoftheutilitylibrary,alsowrittenasaVideomodule. Explainingits constructionintroducesenoughofVideo’sprimitivesandcombinatorstogetasenseofwhattherest ofthelanguagelookslike. FirstweexplainVideo’sprimarylinguisticmechanisms,modulesand functions(section4.1). Then,wedescribebasicproducers(section4.2): images,clips,colorsandso on,followingupwithadiscussionofthebasicsofhowtocombinetheseproducersintoplaylists andmultitracks(sections4.3,4.5). Tomakecompellingexamples,weintroducetransitions,filters, andproperties(sections4.4,4.6,4.7). Finally,wedescribetheinterfacefordisplayingvideosand renderingVideoprograms(section4.8). 4.1 EssentialVideo Videomodulesconsistofaseriesofinterleavedexpressions,definitions,andimport/exportforms; functions have the same shape as modules but without the import/export forms. Video enforces differentscopingrules,andassignsslightlydifferentmeaningtotheseconstructs,thanRacketdoes. Inbothcases,definitionsarevalidintheentirescope—thatis,theentiremoduleortheentirefunction body. Theexpressionsmustdescribevideoplaylists. Modulesandfunctionsdifferinthattheformer providesavideo,whilethelatterreturnsone. Furthermore,Videomodulesarefirst-orderentities thatcanbecompiledseparately,whilefunctionsareactuallyfirst-classvalues. Now,takeasecondlookatfigure3. Lines5through7showthefunctionheader. Therestofthe codedescribesthefunctionbody(lines7–28). FunctionsinVideoaredeclarative;inparticular,line 8istheproducerreturnedbythisfunction. TheremainingsubsectionsexplaintheVideolanguagein sufficientdetailtounderstandtherestofthiscode. Specifically,weexplainindividualfeaturesofthe languageandhowtheyimprovethevideoeditingprocess. 4.2 Producers TheproduceristhemostbasicbuildingblockforaVideoprogram. Aproducerevaluatestoadata structurethatdenotessomesortofmultimediaobject: audioclips,videoclips,pictures,andsoon. Combinationsofproducersarethemselvesproducers,andtheycanbefurthercombinedintoyet morecomplexproducers. The simplest type of producer, clip, incorporates traditional video files. The clip producer convertsthefileintoasequenceofframes. Developersusecliptoimportrecordingsfromfiles, suchasaconferencetalk,intoscripts: #lang video (clip �talk00.mp4�) time Unlike clip, image creates an infinite stream of frames. Video’s combination forms truncate thesestreamstofitthelengthofotherproducers. Additionally,developerscanusethe#:length keywordwhentheywantaspecificnumberofframes: #lang video (image �splash.png� #:length 3) time PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. Super8LanguagesforMakingMovies 30:7 01 #lang video 02 03 (provide conference-talk) 04 05 ; Describes an edited conference video with appropriate feeds 06 ; Producer Producer Producer Positive-Integer -> Producer 07 (define (conference-talk video slides audio offset) 08 (multitrack clean-video clean-audio) 09 ; where 10 (define clean-audio 11 (playlist (blank offset) 12 (attach-filter audio 13 (envelope-filter 50 #:direction �in) 14 (envelope-filter 50 #:direction �out)))) 15 (define spliced-video 16 (multitrack (blank #f) 17 (composite-transition 0 0 1/4 1/4) 18 slides 19 (composite-transition 1/4 0 3/4 1) 20 video 21 (composite-transition 0 1/4 1/4 3/4) 22 (image �logo.png� #:length (producer-length talk)))) 23 (define clean-video 24 (playlist (image �splash.png� #:length 100) 25 (fade-transition #:length 50) 26 spliced-video 27 (fade-transition #:length 50) 28 (image �splash.png� #:length 100)))) Figure3: AVideofunctiondefinition 4.3 Playlists Avideoisusuallyacompositionofmanyproducers. Videoprovidestwomainwaysforcombining them: playlistsandmultitracks. Roughlyspeaking,playlistsplayclipsinsequence,whilemultitracks playclipsinparallel. Anyproducercanbeputinaplaylist,includinganotherplaylist. Each clipintheplaylistplaysinsuccession. Frequently,videocamerassplitrecordingsintomultiple files. Withplaylist,developerscaneasilystitchthesefilestogethertoformasingleproducer: #lang video (playlist (clip �talk00.MTS�) time (clip �talk01.MTS�)) Developers cut playlists and other producers to desired lengths with the #:start and #:end keywords. Thiscapabilityisincludedbecausevideorecordingsfrequentlystartbeforeatalkbegins andendafterthetalkfinishes;seefigure4a. Recallthatwhiledefineislocatedbelowthevideo description,itsscopeincludestheprecedingexpressions. 4.4 Transitions Jumpingfromoneproducerinaplaylisttoanothercanberatherjarring. Scriptscanreducethis effectwithtransitions: fading, swiping, etc. Thesetransitionsmergethetwoadjacentclipsina playlistandareplaceddirectlyinsideof(possiblyimplicit)playlists. Expandingontheexample PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. 30:8 LeifAndersen,StephenChang,Ma￿hiasFelleisen #lang video (image �logo.jpg� #:length 100) talk (image �logo.jpg� #:length 100) ; where time (define talk (playlist (clip �talk00.MTS�) (clip �talk01.MTS�) #:start 100 #:end 8000)) Figure4: (a). Exampleofcuttingaplaylist #lang video (image �splash.png� #:length 100) (fade-transition #:length 50) talk (fade-transition #:length 50) time (image �splash.jpg� #:length 100) ; where (define talk (cid:1)(cid:2)elided(cid:2)(cid:3)) Figure4(continued): (b). Examplewithfadingtransition #lang video (multitrack (clip �slides.mp4�) (composite-transition 0 0 1/4 1/4) talk) time ; where (define talk (cid:1)(cid:2)elided(cid:2)(cid:3)) Figure4(continued): (c). Exampleusingmultitracks fromfigure4a,fadetransitionsareusedtosmooththetransitionfromlogotothetalk. Figure4b illustratesthispoint. Every transition in a playlist actually shortens the length of its frame sequence, because transitionsproduceoneclipforeverytwoclipstheyconsume. Additionally,aplaylistmaycontain multipletransitions. Suchaplayliststillspecifiesauniquebehaviorbecauseplaylisttransitions areassociativeoperations.Thus,multipletransitionsplacedinasingleplaylistdescribethedesired clipwithoutanysurprises. 4.5 Multitracks Multitracksplayproducersinparallel. Likeplaylists,theyemploytransitionstocompositetheir producers. Syntactically, multitrackissimilartoplaylist. Themultitrackformconsistsof PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. Super8LanguagesforMakingMovies 30:9 #lang video (multitrack (blank #f) (composite-transition 0 0 1/4 1/4) (clip �slides.mp4�) (composite-transition 1/4 0 3/4 1) time talk) ; where (define talk (cid:1)(cid:2)elided(cid:2)(cid:3)) Figure4(continued): (d). Exampleusinginlinedtransitionsinamultitracks asequenceofproducersandcreatesanewmultitrackproducer. Again,transitionsareincluded withinthesequencetocombinetracks;seefigure4c. Thisexampleusescomposite-transition, whichplacesoneproducerontopoftheother. Thefourconstantsspecifythecoordinatesofthe top-leftcorneroftheproducerandthescreenspacethatthetopproducertakes. Here,theproducer followingthetransitionappearsinthetop-lefthandofthescreenandtakesuponequarterofthe widthandheightofthescreen. Transitionswithinamultitrackarenotassociative;instead,multitrackinterpretstransitionsin lefttorightorder. Videosthatrequireadifferentordercanembedamultitrackinsideofanother one,becauseamultitrackisitselfaproducer. Usingmultipletransitionsallowproducerstoappear sidebysideratherthanjustontopofeachother. Expandingontherunningexampleinfigure4c, figure4ddescribesaconferencevideowheretherecordingofthepresentergoesinthetopleftwhile theslidesgoontheright. Thisexampleaddsacomposite-transitiontothepreviousexample andplacestheslidesandtherecordingoverasingleblankproducer. Blankproducersareempty slidesthatactaseitherabackgroundforamultitrackorafillerforaplaylist. Inthiscase,the blankproducerisprovidingabackgroundthattheslidesandcamerafeedsappearon. 4.6 Filters Filtersaresimilartotransitions,buttheymodifythebehaviorofonlyoneproducer. Inotherwords, filtersarefunctionsfromproducerstoproducers. Amongothereffects,filterscanremovethecolor fromacliporchangeaproducer’saspectratio. Conferencerecordingsfrequentlycaptureaudioand videoonseparatetracks. Beforesplicingthetrackstogether,adevelopermayaddanenvelopefilter toprovideafadeeffectforaudio. Ascriptmayusefunctionapplicationnotationtoapplyfiltersorattachthemtoproducerswiththe #:filterskeyword. Figure4eshowsanexampleofafilterbeingattachedtoanaudiotrackthatis itselfcompositedwiththevideoofthecompositedtalkinfigure4d. 4.7 PropertiesandDependentClips Producersusetwotypesofpropertiestostoreinformation: implicitandexplicitproperties. Implicit propertiesareinnatetoclips,forexample,lengthanddimensions. Explicitpropertiesmustbeadded bytheprogramitself. ThepropertiesAPIcomeswithtwofunctions: (set-property producer key value)createsanexplicitproperty.Itreturnsanewproducer ‚ withkeyassociatedwithvalue. PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017. 30:10 LeifAndersen,StephenChang,Ma￿hiasFelleisen #lang video (multitrack composited-talk (clip �0000.wav� #:filters (list (envelope-filter 50 #:direction �in) (envelope-filter 50 #:direction �out)))) time ; where (define composited-talk (cid:1)(cid:2)elided(cid:2)(cid:3)) Figure4(continued): (e). Exampleofaddingaudiotracks #lang video (multitrack (blank #f) (composite-transition 0 0 1/4 1/4) (clip �slides.mp4�) (composite-transition 1/4 0 3/4 1) talk time (composite-transition 0 1/4 1/4 3/4) (image �logo.png� #:length (get-property talk �length))) ; where (define talk (cid:1)(cid:2)elided(cid:2)(cid:3)) Figure4(continued): (f). Exampleofaddingawatermark (get-property producer key)returnsthevalueassociatedwithkey. Ifthepropertyisset ‚ bothimplicitlyandexplicitly,theexplicitpropertyhaspriority. Explicit properties provide a protocol for communicating information from one clip to another. Implicitpropertiesexistforthesamepurposeexceptthattheystoreinformationthatisimplicitly associatedwithaproducer,suchasitslength. Forexample,aconferencevideomayhavetocome withawatermarkthatisthesamelengthasthecapturedconferencetalk. Ascriptthatperformsthis operationcanbefoundinfigure4f. 4.8 FromProgramstoVideos AVideomodulemaybeincorporatedintoaprogramorunderstoodasastand-aloneprogram. In thefirstcase,anotherVideoorRacketmodulemayrequiretheVideomoduleandincorporateits exportintoitsowncode. Inthesecondcase,ausercanhandtheVideoscripttoarendererthateither playsthevideoonascreenorsavesittoafile. Bydefault,aVideomoduleimplicitlyprovidesaproducer. Anymodulethatwantstousethis implicitlycreatedvideoimportsitwiththeexternal-videoform. Foranexample,considerthis two-linemoduleandwhatitdenotes: PACMProgr.Lang.,Vol.1,No.1,Article30.Publicationdate:September2017.