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EUROGRAPHICS ’99 / P. Brunet and R. Scopigno (Guest Editors) Volume 18 (1999), Number 3 Experiments in Digital Television Philipp Slusallek, Milton Chen, Brad Johanson Computer Science Department, Stanford University Abstract Television has been around for many decades now and has become ubiquitous. Analog TV technology, however, which was developed before the 1940’s, has remained essentially the same over the last 50 years. Alternative digi- tal transmission standards have been developed recently and are in use for satellite and cable transmissions. This technology is now being deployed for terrestrial TV as well. This high-speed infrastructure for digital broadcasts of video, audio, and other data, which will be build over the next few years, suggests that we look more closely at this technology and explore its use from the standpoint of computer graphics. This paper provides an overview of the technology of digital television, discusses its potential and challenges, and presents several experimental applications which demonstrate how computer graphics can provide input and influence the future of digital television. These applications include a virtual VCR, interactive video overlays, technology for the lecture of the future, and interactive video panoramas. 1. Introduction Television (TV) has become a significant part of our life over the last few decades. Available in most households, it is not only a commodity item, which is — contrary to a computer — easily used by anyone, but also a primary source of enter- tainment and information. One of the major characteristics of TV is its broadcast model of data distribution. In contrast to the Internet, TV is mainly used for providing content to large audiences. On ac- count of this, production values are high and therefore con- tent is expensive to create. Broadcast decisions are central- ized and are only weakly influenced by the viewers through feedback on their channel selections. Interactivity in TV is strongly limited and requires a separate back channel — usu- ally a telephone. All this is in strong contrast to the Internet and the World Wide Web. The Internet provides peer-to-peer communica- tion, where anyone can author his or her own content with interaction only limited by the available bandwidth. All this is attracting the interest of an increasingly large part of the public, although time spent using the Internet for entertain- ment and information retrieval is still tiny compared to the number of hours spent in front of TVs. The Internet is not, however, well suited to the simultane- ous distribution of large amounts of data to many places. Ex- periments with video-on-demand have mostly been failures, and the distribution of the Starr Report on the web almost brought the Internet to its knees. While we can expect new technologies, (such as multi-casting) and higher bandwidth, to alleviate some of these problems, the broadcast model of TV seems better suited to the delivery of large data sets to many different consumers (such as the morning news, stock tickers, etc.) than the Internet. With the introduction of digital TV (DTV) 10 16 a high bandwidth digital broadcast infrastructure (20-60 Mbit/s per DTV channel) is being set up in many countries around the world and will become available over the next few years. While it is designed for sending real-time video and au- dio, this digital infrastructure is equally well suited to other kinds of digital data transmission that are compatible with the broadcast model. In this context it is prudent to look into the possibilities and limitations the new medium offers to the computer graphics community and to explore where we can contribute to its development. DTV and the broadcast infrastructure is an interesting new technology from a computer graphics point of view for sev- eral reasons: TV has always provided a particular successful form of “Virtual Reality”. While not technically immersive like a CAVE 6 (in the sense of 3D immersion), TV allows the viewers to imagine themselves in another location by pro- c The Eurographics Association and Blackwell Publishers 1999. Published by Blackwell Publishers, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.
Transcript of Experiments in Digital Television - Computer Graphicsgraphics.stanford.edu/papers/dtv/dtv.pdf · TV...
EUROGRAPHICS’99 / P. BrunetandR. Scopigno(GuestEditors)
Volume18 (1999), Number3
Experiments in Digital Television
Philipp Slusallek,Milton Chen,BradJohanson
ComputerScienceDepartment,StanfordUniversity
AbstractTelevisionhasbeenaroundfor manydecadesnowandhasbecomeubiquitous.Analog TV technology, however,which wasdevelopedbefore the1940’s,hasremainedessentiallythesameover thelast50years.Alternativedigi-tal transmissionstandardshavebeendevelopedrecentlyandare in usefor satelliteandcabletransmissions.Thistechnology is nowbeingdeployedfor terrestrial TV aswell. Thishigh-speedinfrastructure for digital broadcastsof video,audio,andotherdata,which will bebuild over thenext few years, suggeststhatwelook more closelyatthis technology andexplore its usefromthestandpointof computergraphics.This paperprovidesan overview of the technology of digital television, discussesits potentialand challenges,andpresentsseveral experimentalapplicationswhich demonstratehowcomputergraphicscanprovideinputandinfluencethe future of digital television. Theseapplicationsincludea virtual VCR, interactivevideooverlays,technology for thelecture of thefuture, andinteractivevideopanoramas.
1. Introduction
Television(TV) hasbecomeasignificantpartof ourlife overthelast few decades.Availablein mosthouseholds,it is notonly a commodityitem,which is — contraryto a computer— easilyusedby anyone,but alsoaprimarysourceof enter-tainmentandinformation.
One of the major characteristicsof TV is its broadcastmodelof datadistribution. In contrastto theInternet,TV ismainlyusedfor providing contentto largeaudiences.Onac-countof this, productionvaluesarehigh andthereforecon-tent is expensive to create.Broadcastdecisionsarecentral-izedandareonly weakly influencedby theviewersthroughfeedbackon their channelselections.Interactivity in TV isstronglylimitedandrequiresaseparatebackchannel— usu-ally a telephone.
All this is in strongcontrastto theInternetandtheWorldWide Web. The Internetprovidespeer-to-peercommunica-tion, whereanyonecanauthorhis or her own contentwithinteractiononly limited by theavailablebandwidth.All thisis attractingthe interestof an increasinglylarge partof thepublic, althoughtime spentusingthe Internetfor entertain-mentandinformationretrieval is still tiny comparedto thenumberof hoursspentin front of TVs.
TheInternetis not,however, well suitedto thesimultane-ousdistributionof largeamountsof datato many places.Ex-
perimentswith video-on-demandhave mostlybeenfailures,andthe distribution of the StarrReporton the web almostbroughtthe Internetto its knees.While we canexpectnewtechnologies,(suchasmulti-casting)andhigherbandwidth,to alleviatesomeof theseproblems,thebroadcastmodelofTV seemsbettersuitedto the delivery of large datasetstomany differentconsumers(suchasthemorningnews,stocktickers,etc.)thantheInternet.
With the introductionof digital TV (DTV) 10� 16 a highbandwidthdigital broadcastinfrastructure(20-60Mbit/s perDTV channel)is beingsetup in many countriesaroundtheworld and will becomeavailable over the next few years.While it is designedfor sendingreal-time video and au-dio, this digital infrastructureis equallywell suitedto otherkinds of digital datatransmissionthat arecompatiblewiththebroadcastmodel.In thiscontext it is prudentto look intothepossibilitiesandlimitationsthenew mediumoffersto thecomputergraphicscommunityandto explorewherewe cancontributeto its development.
DTV andthebroadcastinfrastructureisaninterestingnewtechnologyfrom a computergraphicspoint of view for sev-eralreasons:
� TV hasalwaysprovided a particularsuccessfulform of“Virtual Reality”. While not technicallyimmersive like aCAVE 6 (in the senseof 3D immersion),TV allows theviewersto imaginethemselvesin anotherlocationby pro-
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viding imagesandsoundsthat suggestthe larger remotelocation.CertainlyTV hasbeenproviding immersive ex-periencesto morepeoplethan any otherVR equipmentto date,mainly by providing highly realistic imagesandconvincing storytelling. Webelieve thatcomputergraph-icscanlearna lot from theseconcepts.
� The recentwave of image-basedtechniquespromisestonarrow thegapbetweenpurelyimage-basedTV andcom-putergraphics.Computergraphicsoffersmany toolsandtechniquesfor extendingthemodelof TV from real-timeand isochronousvideo and audio transmissionto othermodelsfor creatinguserexperiences(suchastextured3Dmodels,panoramas,animatedsprites,etc.).
� WatchingTV hasalways beena purely passive activity(asidefrom channelhopping).With the advent of DTV,new formsof interactionbecomepossible.This is anareawhere computergraphicshas a long tradition and canprovide valuablecontributions.Thesecomputergraphicstechniques,however, needto bereviewedin thecontext ofthe mono-directionalbroadcastmediumthat is providedby DTV.
� TV sets and computerswill technologicallyconverge.DTV receiversrequireconsiderablecomputationalpowerto receive,decode,anddisplaytheDTV program.It seemsreasonablethatsomeof thesecyclescanandwill beusedfor other taskssuchasdisplayingweb pagesor runninginteractiveapplications.Ontheotherhand,computersarealreadyable to act as DTV receivers (seeSection5.1).It remainsto be seenexactly how this convergencewillproceed,but both TV technologyand computergraph-icsshouldbeableprovidesignificantcontributionsin anycase.
In this paperwe explore someof the options for DTVfrom a computergraphicsperspective. Thefirst part of thispaperprovides the basicbackgroundof DTV. We describethebasictechnologyanddiscusscurrentstandardsandsys-tems.In thesecondpart,wepresentfour applications:virtualVCR,videooverlays,lectureof thefuture,andvideopanora-mas.Theseapplicationsexploreseveralaspectsof the largedesignspacethat DTV offers suchas surroundvideo, in-teractivity, non-isochronousdisplay, enhancedinformationdisplay, andmixed-ratevideostreams.All applicationshavebeendevelopedaspart of the ongoingStanford ImmersiveTelevision Project and are running on experimentalDTV-PCs.Our main intentwith this paperis to sparkinterestforthis interestingnew areaof researchandto provideideasandsuggestionsfor furtherwork.
2. Analog TV Technology
Theability to communicatewith otherpeopleover longdis-tanceshasbeenadesirethroughouthumanhistory. Not longafterMorseinventedtheelectrictelegraph,peoplesuggestedthat imagescould also be transmittedelectrically. The de-velopmentof television was a long strugglestartingwith
Figure 1: John L. Baird, one of the TV pioneers, and hismechanical TV in 1925. It providesonly a few scanlinesand couldonly transmitcrudeoutlines,but it alreadyusedthebasicprinciplesof analog TV transmission.
thefirst experimentsaround1880.However, it wasnot un-til early this centurythat inventorsovercamethe immensetechnicalproblems(seeFigure1). It requireda tremendousscientific,technological,andindustrialeffort beforethefirst,reliablyworking electronictelevisioncamerasandreceiverswerebuilt in the 1930sand the first TV standard(NTSC)wasdefinedin 19411� 9.
Essentially, a televisioncamerascansanimageusinghor-izontal lines, converting the light intensitiesalongthe wayinto electrical signals.In black and white TV this signalis directly modulatedon an RF carrier after it has beenaugmentedwith synchronizationinformation. On the re-ceiver sidethesignalis demodulated,anddisplayedby syn-chronouslyscanningthe screenphosphorwith an electronbeamreplicatingtheobservedintensitypattern.
Even for black andwhite TV, compressionby interlacedscanninghadto beusedto squeezetheimmensedatastreaminto a narrow RF band.Interlacedscanningallows the re-fresh rate to be doubledto the ergonomic60 Hz by send-ing only every otherline perscreenupdate.Theadditionofcolor to TV in the1950’s introducedeven morebandwidthproblems.They were solved by significantly compressingthe color signalwithout too much loss in color fidelity byexploiting limitationsin humanvisualperception.Thecom-binedblackandwhiteandcolorsignalsstill hadto fit into thesamesmall RF bandwidthof 6-8 MHz, leaving little roomfor any furtherextensionsor improvementsto thestandard.
As a result, the technologyof the initial TV standard,while brilliant for its time, severely limited the quality ofTV signalsfor many yearsto come.The NTSC standardin particularsufferedfrom this technologylimitation. Sincethe1940’s thebasictechnologyof TV hasremainedessen-tially the same,althoughthe equipmentsthemselves have
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improved dramatically. Given the newnessof the technol-ogy andthedramaticcompetitionandpressureunderwhichearlyTV wasstandardizedandmarketed,it is amazingthatwe have beenableto get by with the sameTV technologyever since.
Nonetheless,the(lack of) quality of videoandaudiohasbeenaconstantnuisanceandmany improvementshavebeensuggested(suchas the many forms of High-Definition TV(HDTV)). Themainproblemfor all of them– andoneof themajor reasonswhy little haschangedfor TV for so long –is the analognatureof TV. Any improvementto the imageand soundquality would have requiredhigher bandwidth,which wasvery difficult to arrangegiven the hugeexistingTV infrastructure.
AnalogHDTV systemsthatwouldoffer significantlybet-ter imageand audio quality have beenproposedin manyvariantsover the last two decades.They always sufferedfrom thebandwidthproblem,becauseof thedifficulty in effi-ciently compressingtheanalogsignal
�. It took considerable
time for the (often strictly analog)TV communityto real-ize thatonly digital processingcouldprovide thenecessarycompressionratiosto make HDTV practical.
3. Digital TV Technology
A majorlimitationof analogTV processingis theconversionof the video imageinto a serialstreamby horizontalscan-ning. This makesit hardto performany verticalprocessingon thesignal.In orderto compressa videoimagewell, bothhorizontalaswell asvertical processingis required.Videocompressionalso relieson taking temporalcoherenceintoaccount,by comparingsuccessive framesandonly sendingthe differenceinformation. All of thosecomputationsaregreatlysimplifiedby breakingthepictureinto blocksof 8 � 8or 16 � 16 pixelseach,whichcaneasilybedonein a digitalrepresentation.
For digital broadcasts,the video and audio streamsareseparatelycompressed,split into packagesof a fixedlength,and thenmultiplexed into a singledigital MPEG transportstream10� 13� 16. Before the DTV transportstreamis broad-casted,it is augmentedwith forwarderrorcorrectionandismodulatedon an RF carrier. Dependingon the DTV andtransmissionsystem,eachDTV channeloffers a payloadbandwidthof roughly20 Mbit/s for terrestrial,30 Mbit/s oncable,and40-60Mbit/s usingsatellitebroadcasting.This isenoughfor distributing at leastoneHDTV or 5-8 standardTV programs.
In additionto audioandvideodata,theMPEG transportstreamcanalsocontainotherdatastreams.Most importantis theprograminformationstreamthat tells a DTV decoder
�The JapaneseHiVision/MUSEstandardactuallyusesa number
of DSPsto performsignalprocessingon theanalogTV signal.
whichvideoandaudiostreamsform oneprogramandshouldbe decodedand renderedtogether. Auxiliary streamssuchasmulti-languageaudio,closedcaption,teletext, andotherarbitrarydatacanalsobeincludedin a DTV stream.
Themainbenefitsof aDTV systemare:
Image and Sound Quality Due to digital error protectionand recovery, imagescontain no snow or other analognoise.However, in casesof significant compressionorpacket loss,blockingartifactsandringingcanoccur(wellknown from JPEG-compressedimages).Similar effectsapplyto sound.
Higher Resolution DTV allows thetransmissionof higherresolutionimages(up to 1920 � 1080interlaced).It turnsout thattheincreasedimagequalityatstandardresolution(480 lines of video for NTSC) in progressive scanmode(i.e. without the interlacingartifacts) is alreadyconsid-eredasignificantqualityimprovementbymostviewers19.It remainsunclearfor now which resolutionis sufficientfor being consideredHDTV. One drawback of HDTVprogramsis that it requiresthe consumerto buy expen-sive high-resolutiondisplaysto take full advantageof thehigherresolution.
Extendibility Due to the very general MPEG transportstream,any new servicescanbe easilyaddedto a DTVsystemwithout compromisingexisting DTV receivers.Also, bettervideoandaudiocompressiontechnologyin-stalledon the headendspromisesto transparentlymakemorebandwidthavailableto otherservicesin the future.New developmentsin scalablevideocodingevenmake itpossibleto incrementallyaddthemissingpixelsof HDTVto existing low resolutionvideostreams7 � 10.
Data Casting A digital TV systemhastheotherimportantadvantagein thatessentiallyany datacanbebroadcasted(datacasting)— videoandaudiobeingonly thetwo mostobvious examples.Examplesof datacastingare simplebroadcastingof datafiles, auxiliary data to a TV show(like overlays and event-driven scriptscontrolling theirappearance,seeSection5.3),andcollectionsof videoandaudiostreamsthattogethermakeupanew TV experience(e.g.videopanoramas,seeSection5.5).Onevery simpleextensionof DTV would be to useanyexcessbandwidth(e.g.at off prime time hours),or evenwholeDTV channelsto broadcastdatasuchaswebpages,news, software updates,etc. DTV receivers with diskscouldstoreor cachesomeof thedatadependingon userpreferenceor subscriptionand make it available on de-mand.It seemsthatdatacastingwill becomeanimportantaspectof DTV networks,althoughit is yet unclearwhichof theseserviceswill becommerciallysuccessful.
Although proprietaryDTV systemshave beenin useforsometime in specificsatellitesystems(e.g.DigiCipher II),the two major DTV systemsnow are the EuropeanDVB(Digital VideoBroadcast) 8 andthe AmericanATSC (Ad-vancedTelevisionSystemsCommittee)systems2. DTV wasfirst introducedfor satellite transmissionand is only now
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alsobeingdeployed for terrestrialbroadcastingwhereit issupposedto replaceanalogTV over thenext decadeor so.
3.1. The DVB System
DVB has been developed as a consistentbasestandardfor the DTV streamthat is augmentedwith standardsthatspecify channelcoding and forward error correctionsuit-ableto thespecificcharacteristicsof transmissionchannelssuch as satellite (DVB-S), cable (DVB-C), and terrestrialbroadcasting(DVB-T). In particular, themodulationscheme(COFDM) for terrestrialbroadcastingsupportsmobile re-ceptionandis well suitedto difficult signalconditions(e.g.ghosting)requiringonly smallsettopantennas.Early on theDVB consortiumincludedfacilities for databroadcasting,interactive services,andconditionalaccessinto their stan-dards.
For the DVB consortiumHDTV hadlow priority on thelist of featuresandhasonly recentlybeenaddedto thestan-dard 17. The DVB consortiumplans to simulcastseparatestandardandhigh-definitionvideostreamsinsteadof requir-ing eachreceiver to be able to decodeand down-convertHDTV streams.Becauseconditionalaccessis well-definedfor DVB, thesamereceiverboxescanbeusedfor freechan-nelsaswell asfor pay-per-view servicesby insertinga spe-cific decryptionPCMCIA/PC-Cardinto thebox.Many sub-sidizedDTV receiversin theUK arenow distributedat low-cost ( $300) by pay-per-view companiesmaking DTV af-fordableto theconsumer.
DVB alreadyis themajorstandardfor mostsatellitesys-temsandmany cableTV systems.TerrestrialbroadcastingusingDVB startedin the UK in last November, with Swe-denandAustraliasoonto follow. Othercountriesstill relyon satellitedistribution of DTV, but mostplan to introduceterrestrialDTV in thenearfuture.
3.2. The ATSC System
TheAmericanATSCsystemstartedoff by concentratingonterrestrialbroadcastingonly. BecauseHDTV hasbeenama-jor motivationfor theintroductionof DTV in theUS,its dis-tribution is actuallymandatedby theUS government.How-ever, stationsplan on broadcastingat most3% of the dailyprogrammingin HDTV.
Interestingly, the FCC adoptedATSC standarddoesnotdefine a specific video format or resolution, leaving thischoiceto theTV networksandreceiver manufacturers.Theconsumer, however, is left with the possibility that his newreceivermaynotactuallybeableto decodeall availablepro-gramming.It still remainsunclearhow well DTV receiverswill beacceptedby consumers,sincetheir pricesin theUSarelikely to behigh,mainlysinceeachboxneedsto beableto decodeandpossiblydown-sampleHDTV programming.It appearsthatsomemanufactureswill ignoreHDTV in fa-vor of low costreceivers.
Although both standardsare basedon similar concepts,thereareenoughdifferencesthatmake DVB andATSCin-compatible:
Modulation Eachsystemusesits own modulationanderrorcorrectionschemefor terrestrialbroadcasting.
Program Information The datastreamsthat describetheavailableprogramsarecompletelyincompatible,so thatneither system would find the other’s programseventhoughbothunderstandthesameMPEGtransportstreamandvideocompressionsyntax.
Audio Different audio encodingschemesare being used.DVB usesMPEG Layer 3, while ATSC usesthe DolbyAC3standard,althoughbothstandardsallow thesame5.1channel,CD qualitysurroundsound10� 12.
Video AlthoughtheMPEG-2compressionscheme10 is thesame,both standardshave chosendifferent MPEG pa-rameters.This may renderthe video streamsincompati-ble with decodersthat are specificallydesignedfor onesystemor theother.
Dueto market constraintsit currentlyseemsunlikely thatthetwo standardswill eventuallyconverge,althoughmanu-facturersmight build dual standardreceivers in the future.Unfortunately, Japanseemsto bedeterminedto developyetanotherDTV standard– ISTV (IntegratedSystemTV) – inadditionto its currentanalogHiVision/MUSEsystem.Thismayintroduceyetanotherstandardinto themix.
Many TV stationsandnetworksseeDTV asanopportu-nity to gain new market share.The situationis particularlyproblematicin the US, becausethe introductionand con-tinuedsupportof anHDTV capableATSCsystemhascostUS TV stationsbillions of dollars for new equipmentanddistribution infrastructure.Thestationsarenow looking fornew sourcesof revenuesto make up for this loss.Althoughfor eachanalogchannela TV stationreceivedanadditionalfreeDTV channelif it promisedto transmitHDTV program-ming,it seemshighly unlikely thata largernumberchannelscancover thecostsof DTV. As a result,many TV networksandstationsarelooking into addingsimpledatacastingtoDTV andaugmentingtheirDTV programwith new userex-periencesthroughinteractive componentsandenhancedvi-sualpresentations.
4. DTV and Computer Graphics
In the following sectionwe explore someof the interest-ing dimensionsof the DTV designspace,suchasthe timedimensionwhere the presentationtime can be decoupledfrom the transmissiontime, the presentationof additionalon-screeninformationlinkedto theDTV program,new userexperiencesthrough extendedand semi-immersive visualpresentation,andfinally theinteractivedimensionof thepre-viously passive mediumof TV. In particularwe concentrateonareaswherecomputergraphicsofferstechniquesandex-periencefor future DTV applications.Concreteapplication
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scenariosandexampleimplementationsarethendescribedin Section5.
4.1. Time Warping
A DTV receiver with enoughdisk spacecouldeasilyactasanintelligentVCRor avideobuffer, thatcan– automaticallyor on command– save your favorite shows andprogramstodisk if you happennot to beat home.Current18 GB disksdrives can provide up to 10 hoursof video dependingoncompression.
Thestrict isochronousnatureof watchingTV canbeelim-inatedby temporarilybuffering the incomingvideo streamof a programona harddisk,while you pick up thephoneorpreparea snack.Thepausedprogramcanthenbecontinuedfrom the stoppedlocationrunningoff the harddisk, whiletheincomingvideocontinuesto bebuffereduntil youhaveachanceto catchupby skippingthenext commercial(seealsoSection5.2).Thisnew techniquerequireslittle supportfroma computergraphicsperspective exceptfor very simpleandintuitive userinterfaces,but offersa basicservicefor manyof thehigherlevel applicationsdescribedbelow.
4.2. Video Overlays and Insets
Thereare many occasionswherea viewer might want togetmoreinformationthanis normallyavailablein thevideostream.A particularlygoodexampleis sportsprogramming,wheresomepeoplewould like to seemoredetailedinforma-tion abouta gameor a player(biography, statistics,relatedevents,merchandise,etc.).This informationcanbeprovidedin videooverlays,which canpopup automaticallyor on therequestof auser. Theusercanthenchooseto browsethein-formationfurther, adjustits display, or dismissit. Theover-lays can be animatedor static, linked to hot spotson thescreen(e.g.to whereaparticularplayeris currentlyvisible),or couldbetexturedinsetsattachedto objectsvisible in thescene(e.g.advertisementsonthefloor of abasketballcourt).Theselatterformsof displayrequirenon-trivial image-basedtechniquesbothfrom thefield of computervision andcom-putergraphicsto properlyoperateandsynchronizewith theprogram.
4.3. Extended Video Presentation
In a traditionalTV programevery detailof thepresentationis predefinedby the TV stationor network. This is a di-rectresultof thechosenrepresentationasafixed,full-screenvideo stream.As soonas we extend the representationofvideo, we offer moreoptionsfor the userto customizethepresentationaccordingto his requirementsandpreferences.
Onegeneraloptionthatis currentlybeingstandardizedaspartof MPEG-4is to split the videoprograminto multiple“Video Objects” (VOB), which neednot have rectangular
shapeandmay be partly transparent.The VOBs are trans-mittedseparatelyacrossthesameDTV streamtogetherwitha default presentationarrangement,which may be three-dimensional.Dependingon the parameters,the user maythen be able to interactand rearrangethis default presen-tationor substitutehis own. Using theseprincipleswe haveimplementeda videopanorama,which is describedin Sec-tion 5.5.
Anotheropportunitythat is offeredby this framework isthe adaptive coding of the different video streams,whichmay have completelydifferent codingand updaterequire-ments.As anexample,whentransmittingalecturetheimageof ablackboardrequiresveryhigh resolutionto bereadable,but only certainregionsneedto beupdateat frequentinter-vals(seeSection5.4).
4.4. Interactivity
The video presentationoutlined in the previous sectional-readyallows theusera limited form of interactionwith theDTV program,but moredegreesof freedomarecertainlyde-sirable.A majorstumblingblock to interactivity is thestrictbroadcastmodelof DTV, which doesnotoffer anintegratedbackchannel.Adding Internetaccesssolves this issueandoffersall theusualinteractioncapabilities.However, it is in-terestingto explore the availableoptionswithout addingabackchannel.
In this mode,only local interactivity can be provided.This interaction is limited to an environment predefinedby the creatorof the program.Examplesare downloadedscriptsthat reactto certaininteractionsby displaying,hid-ing, or modifying previously downloadedmediapresenta-tionsandprogramstate.An exampleapplicationis given inSection5.3,whereoverlayscanreactto usereventsaswellastime eventscomingin from thetransmissionchannel.
4.5. Image-based Modeling and Rendering
While traditionalgeometry-basedapproachescanbeappliedto DTV, image-basedalgorithmsseemto beparticularlywellsuited to the task. We can often directly use input videostreamsandprocessthemappropriatelyto producenew andenhancedDTV experiencesasin thepanoramaexample(seeSection5.5). Sendinga “video” streamof rangeimagesordownloadinga layered-depthimages18 wouldallow a clientto performlocal interactionswith theDTV content.
4.6. Execution Environment
An importantaspectfor developingnew DTV applicationsis the environmentthat is provided by a DTV receiver forexecutingdownloadedsoftware.ThevendorneutralDAVICconsortiumis working on API interfacesfor interactive TVbasedmainly on the DVB standards,but other competingsolutionsexist aswell.
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It still remainsunclearwhat supportthe executionenvi-ronmentwill provide to potentialDTV applications.HowmuchCPUpowerandgraphicssupportarerequiredandhowmuchcan we possiblyexpect to seein a DTV receiver inthefuture?How closelywill a TV setbeintegratedwith thecomputeror will they eventuallybe thesameserver devicewith multipledisplayacrossthehouse?
It seemsthataswecomeupwith new services,theTV setof thefuturewill needto adjustandexpandaccordinglyandmay needto be periodicallyupgradedasis necessarywithcomputerstoday.
5. Experiments in DTV
In the following sectionswe describeongoingwork on anumberof experimentalDTV applicationsthat explore thedesignspaceof DTV. In choosingwhat to persuewe fo-cusedmore on developing new experiencesfor the poten-tial user than on direct applicability in todaysDTV envi-ronment.Nonetheless,all theapplicationshave beenimple-mentedwithin anexperimentalDTV infrastructureandhavebeentestedin abroadcastsetup.
5.1. Infrastructure
The infrastructurefor our experimentsconsistsof several350 MHz Pentium-IIPCsrunningWindows 95 with videoboardsthatprovide colorspaceconversion.In additioneachsystemhasanATSCreceiver card(seeFigure2), which hasbeendesignedby INTEL’s MicroComputerResearchLab.This cardprovides the basicfunctionality for receiving anATSC modulatedRF signaloff the air via a standardroof-top antennaandperformchanneltuning,demodulation,anderror correction.The DTV hardware delivers the raw 19.2Mbit/s MPEG-2transportstreaminto systemmemory. Anyfurtherprocessingis performedin software.
The DTV hardwareis accessedthroughan experimentalAPI thatshieldstheapplicationsoftwarefrom thehardwaredetails.At thelowestlevel this API demultiplexesthetrans-port streamanddeliverspacketsof chosenaudio,video,ordatastreams.Layeredon top of this interfaceare servicesfor decodingand/ordisplayingavideostream,aswell asre-ceiving dataobjects(essentiallyfiles with identificationin-formation).
Sincethehardwareonly providestheraw MPEG-2trans-port stream,all demultiplexing andMPEGdecodingis per-formedin software.Oursystemsallow usto receiveanddis-play a standardTV videostream(720 � 480, interlaced)inrealtime.Decodingof HDTV streamsis anareaof ongoingdevelopment.
An experimentalATSC transmittedset up by INTELsome10 miles from Stanfordhasbeenusedto provide theDTV streams.During developmentthe DTV receiver cardcansimulatethe receptionof a DTV streamby readingthetransportstreamfrom a local file.
Figure 2: Photo of digital/analog TV receivercards buildby Intel’s MicroComputerResearch Lab. Thetop board canreceiveNTSCsignalswhilethebottomboard handlesdigitalATSCsignals.
5.2. Virtual VCR
As partof thesupportinfrastructurewehave implementedavideobuffer thatsupportsVCR-likeoperationssuchasslowmotion,pause,replay, andfastforward.Thebuffer couldbeusedin applicationssuchas personalsportsreplays,self-pacedlectures(seeSection5.4), flexible guidedtours (seeSection5.5), or for downloadinga movie during the nightthatyou wantto watchtomorrow.
To implementthe VCR-like interactive features,a cachemanageris requiredthat is able to buffer a large amountof data and supply it to the decoderin real-time. Sincethememoryrequirementcanbehuge,a main-memory-onlybuffering approachmay be prohibitively costly. The alter-native is usinga memory-diskintegratedcache(MEDIC) 3.Sincethe per-byte disk cost is aboutonehundredthof theper-bytememorycost,MEDIC is economicallyattractive.
MEDIC carefully allocatesa limited amountof memoryto competingtasks,i.e. to receiving new datafrom networkchannels,to writing datato diskasmemoryfills up, to read-ing datafrom disk as needed,and to holding datafor de-codingandplayback.Sincedatais concurrentlywritten tothedisk cacheandreadfrom the disk cache,MEDIC mustintelligently issueIOs to maximizethroughputandto avoidundueconflicts.
With lessthan8 MB of RAM ourvideobuffer cansupportthe interactive operationsdiscussedabove without causingany “jitter”. This resultconfirmsthetheoreticalstudy3. Fig-ure3 andthe accompanying video shows an applicationofvideobuffering for implementinga Virtual VCR.
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Figure 3: A screenshotof the Virtual VCR,where the topwindowshowsthedecodedvideo.Thebottomwindowshowsthe control panel,the current memoryand disk usage, andthereceptionanddecodingprogress.
5.3. Video Overlays
The valueof video overlaysis readily demonstratedby thestocktickers,channellogos,andsportsscoreboardsthatarealreadyfound in today’s analogTV programs.In contrastto analogTV, DTV allows theseoverlaysto becompositedin theviewers’ homesratherthanin theproductionstudios,thus allowing flexibility in the storageandpresentationofthecontentsaswell ascustomizationof theviewing experi-ence.The goal of theoverlay project is to createan infras-tructurefor experimentingwith overlaysaswell asexampleapplications.
Our overlaysaredescribedby a markuplanguagesimilarto HTML. In additionto standardfields,eachobjectdescrip-tor containsa time field. This allows thespecificationof notonly whereandhow objectsshouldappearbut alsowhen.Thedegreeto which viewerscanchangethevisualproper-ties of the overlayscanalsobe specified.For example,anoverlay’s positionandsizecanbe eitherfixedor customiz-ableby theviewer. Eachoverlayobjectaswell asthemarkuplanguageis sentthrougha separateDTV stream.Theover-lay receiver handlesfour tasks:receiving objects,parsingmarkuplanguage,reactingto viewer input, anddisplayingthe overlays.Whena customizableoverlay is received, anicon is displayedin a sliding window at the bottomof thedisplay. Theusercanthenselectthisoverlayto show or hideit, aswell aspositionandresizeit.
An importantnew useof overlaysis context basedadver-tising. In ourvideo,weshow two exampleswheretheinfor-mationfor productsin a featurefilm is madeavailableto theviewersat anopportunemoment.Suchadvertising,coupled
Figure 4: A screenshot of the video overlay project. Thefeature movie is displayedin the center of the screen. Alist of the availableoverlays(movie description,main ac-tressbiography, tuxedoadvertising, and ring advertising)are shownon thebottomof thescreen.Thering advertisingoverlay has just arrived (displayedin lower right corner).This context sensitiveadvertisingis triggered by the moviescenethat showsa similar ring.
with theVirtual VCR technology, will allow theviewerstopausethe movie, considerandpossiblypurchasethe prod-uct,andthenenjoy therestof themovie. Suchadvertisingispotentiallylessdistracting,moreenjoyable,andmoreappro-priatethat currentadvertisementsthat completelyinterrupttheprogram.SeeFigure4 andtheaccompanying video foranexampleof videooverlaysfor DTV.
5.4. Lecture of the Future
StanfordUniversityhasalongtraditionof transmittingmanyof its lectureson a specializednetwork known astheStan-ford InstructionalTelevision Network to which many of thenearbySilicon Valley companiessubscribe.Most transmit-ted lecturesconsist of videos that are switched betweenshowing thelecturer, theblackboard,andslidesor otherma-terialalongwith audio.Thereareseveralproblemswith thisstyleof presentation:
� Theviewercanonlyseewhatthecameraoperatorchoosesto transmit.It is impossibleto look at somematerial inmoredetail.
� Theresolutionof theblackboardimageis often lessthanadequate,renderingsometext unreadable.A higherreso-lution blackboardimagewould beof greathelp.Sinceitscontentsdo not changevery often, even high-resolutionimageswould requirelittle bandwidth.
� For long periodsof the lecture,programsonly show thetalking lecturer. A lot of bandwidthis wastedsincethebackgroundis usuallynot changing.
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In ourprojectweaddressedtheseproblemsfrom multipledirectionsapplying the enhancedcapabilitiesof DTV andimage-basedtechniques.First of all, we have chosento sep-aratelyprocessand transmit the imageof the lecturer, theroom, theblackboard,andany additionalmaterial.Eachofthesesourcematerialshave very distinct video characteris-tics thatwe planto utilize. In the following we give a shortoverview of thewholeproject,but concentratingon theex-tractionof a high-resolutionblackboardimage.
On the head-endwe need to capture multiple videostreamsof the room with the lectureand the blackboard.Wethenneedto segmentandbroadcastthedifferentobjects(muchin thespirit of MPEG-4’svideoobjects14). Onthere-ceiversideweneedto recomposethedifferentvideostreamsinto a singlepresentation,but arenow ableto give theusertheability to customizeit accordingto his preferences.Thisallows him, for instance,to concentratelongeron a black-boardimageor review someearlierslidesagain.
Westartby creatingageometricmodelof thelecturehall,which is augmentedwith projective texturesextractedoc-casionallyfrom a video stream.As a result we can saveconsiderablebandwidthby only transmittingthismodelandthetexturesinfrequentlyinsteadof sendingtheimageof thebackgroundfor eachvideo frame.This approachallows aviewer to freely move within the room andview the class-room from whatever locationhe prefers,not just the anglechosenby thecameraoperator.
In orderto displaythelecturerwithin thismodel,weneedto segmenthim from the background.We currently useasimplesegmentationalgorithm that usesthe known colorsof thebackgroundto distinguishit from the lecturer. Usingthe known camerapositionandthe geometryof the room,we roughly estimatethe positionof the lecturerin front oftheblackboardandplacehis videoimageasa 3D billboardinto thescene.Althoughthis is asimpletechniqueit alreadyprovidesasurprisinglyrealisticview of thelecturewhile us-ingonly afractionof thebandwidthafull videotransmissionof thelecturewould require.
In orderto displaytheblackboardwith highenoughreso-lution to be readable,it is necessaryto useseveral camerasto form a running imageof the blackboardthat is updatedin real time. Sincea single cameracannotcapturethe en-tire boardwith sufficientresolution,weusecamerasthatpanandzoomto areasof interestandintegratethatdatainto therunninghigh-resolutionimageof the board.A singlefixedcamerais usedto obtaina low resolutionreferenceimageoftheentireboardto aidin theintegrationof theimagestreamsfrom thehigherresolutioncameras.It alsoinsuresthatsome-thing canbe saidaboutall of the boardin the casethat anareahasnot yet beenscannedby oneof thehigh-resolutioncameras.
The lectureris segmentedfrom the low resolutioncam-era’s input to obtain the running referenceimage of the
Figure 5: Interior view of a panoramashortlybefore com-pletionof thepainting15.
board,without thelecturerobscuringtheview. Thesegmen-tation problemhereis simplerthanthe previous one,sincewe are eliminating the lecturer, ratherthan extracting himanda conservative algorithmcanbeusedthatmightalsore-moveasmallborderaroundhim.Theremainderof theframeis thencopiedover therunninglow resolutionimage.To de-tect thelecturer, we thresholdtheintensitydifferenceof therunning imageand the next video framebasedon the factthat the blackboardwill staynearly the same.This simpletechniquecanfail andis thereforeaugmentedwith a morerobustbut slower algorithmthatanalyzesthecolor distribu-tion for thoseareasthat have not beenupdatedfor a while(becausewemighthavewrongly identifiedapieceof black-boardasa lecturer).
To decidewhereto point thepanandzoomcameras,wemaintaina “curiosity” bitmap.It is marked whenwe seealargeenoughdifferencein thecorrespondingpixel in thelowresolutioncontrol image(which is updatedin real time, re-gardlessof thepositionsof themobile cameras).Themov-ing camerawill thensweepout thatareaof the image,takehigh-resolutionimages,andcleartheappropriateareasin thecuriositybitmap.
When integrating a high-resolutionimage from a high-resolutioncamerainto the output stream,we computethemappingbetweenthecamera’s imagespaceandboardspaceby identifying markers on the board.After reprojectionofthe high-resolutionimage into the blackboardimage it ismasked with the lecturerandcopiedinto the outputimage.Having a referencestreamwith theentireboardis crucial tointegratingthe high-resolutionimagestaken from arbitrarypositions.
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Figure 6: The camera rig used for capturing the videopanoramas.
We currentlyusea modifiedMPEGencoderto compressthe high resolutionblackboardimageusinga significantlylower frame rate. Ideally, we would like to only transmitthoseareasthathavechanged,but MPEGalreadyhasafairlysmalloverheadfor codingtheseunchangedregions.
5.5. Video-Panorama
In the late 18th and19th century, panoramaswerea popu-lar form of massentertainment.PanoramaswereinventedbyRobertBarker in 1792,asameansto show complete360de-greeviews of interestingenvironments(suchaslandscapes)to the interestedandpaying public 15. Panoramasin thesedayswerecreatedin speciallybuild circularbuildingswitha small viewing platform in themiddleanda paintingon alargecylindrical canvasaroundit. Thesepanoramacanvasesreachedheightsof up to 18m andmeasuredup to 130mincircumference.In order to achieve the correctperspectiveview, the painting (which often took monthsto complete)hadto bewarpedaccordingly(seeFigure5).
Due to the large highly realisticdrawings at a large dis-tancetheviewer, limited verticalviewing,andlimited move-ments on the viewing platform, the human sensesweretricked into believing they wereseeingthe real landscape.This experiencewas often enhancedby placing real 3Dobjects in front of the image that merged into the back-ground(providing otherwisenon-existentparallaxinforma-tion) andby adjustingthelighting conditionsaccordingly. Inthatsense,panoramasprovideda early form of virtual real-ity experienceandcauseddramaticresponsesby theviewersof its time.
Animated“video panoramas”werefirst createdby RaoulBrimoin-Sanson(Cineoramas)in 1897andarestill usedto-day, e.g. in Disneys CircleVision installations.3D IMAXandOMNIMAX 11 arelimited formsof panoramasbut alsoincludestereoeffects.
Figure 7: Thearrangementsof thepolygonsonto which theeightDTV streamsget projected.By adjustingtheposition,orientation,andbrightnessof thepolygons,wecalibratethepanoramato showa seamlesssurroundview.
Panoramasrecently got a new push in the contextof image-basedrendering by the work of Chen andWilliams 5� 4. Here,a cylindrical panoramais createdandasmall view canbe displayedandmoved interactively on amonitor, providing somesenseof thesurroundview of arealpanorama.Thesepanoramas,however, are limited to staticviews points.
Usingtheinfrastructureof DTV, we decidedto tackletheproblemof video panoramasby providing the viewer withan interactive view of a moving panorama.The context ofthisapplicationwouldbeaguidedtourof theStanfordcam-pus.At eachinstantthevideopanoramawouldprovidea360degreesurroundview from thecurrentlocation.As theloca-tion or surroundingschanges,new panoramasbecomevisi-ble. The usershouldalsobe ableto interactively move thecurrentview within the full panorama,sohecanbewatch-ing a particularobject longer thanthe guideanticipatedorhecansimplyenjoy thesurroundingcampus.
In orderto createthe360degreepanoramawehadto cap-ture live video from eight CCD camerasarrangedon a cir-cularplatform.In orderto managetheimmensevideoband-width of roughly240Mbit, two quadvideomergersareusedthatcombinedtheanalogsignalsfrom theeightcamerasintotwo NTSCvideosignals,eachdividedinto quadrantsshow-ing oneof thecameras.Thesetwo videosignalsarecapturedat752x480resolutionusingMotion-JPEGcaptureboardsontwo separatePCs.Finally, the eight video streamsare ex-tractedfrom thecapturedfootage,encodedusingMPEG-2,andmultiplexedinto a singleATSCDTV channel.
Onthereceiverside,apipelineof programswork togetherto displaythis transportstream.TheDTV streamis receivedon a PC,whereit is retransmittedover 100 Mbps Ethernetto anSGI Onyx 2 with eightCPUs.Thestandardreference
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Figure 8: This image showsa single framefrom the videopanorama project. The video panorama consistsof a tourof Stanford campus.At each point in timea full 360degreepanoramais receivedandtheuseris freeto browsearound,but a defaultview is provided.Thedisplayalsoshowsa mapof thecampuswith a pointerindicatingthecurrentposition.By clicking on a location on the map the current viewingdirectionrotatesandlocksontothis feature allowing a userto watch it while thetour continues.
MPEG-2decodersoftwarehasbeenmodifiedso that eightcopiescan be run synchronouslydelivering framesin theYUV format.A masterprocesscontrolstheeightdecoders,andrequeststheframesthatareneededto constructthecur-rentview. Theseframesarethenpassedon to thepanoramaviewer. We have chosento always decodeall eight videostreamsdueto thestartupdelayof abouthalf asecondswhendecodingMPEG-2streamswith largeGOPs(groupof pic-tures).Thisdelaywouldhaveseverelyrestrictedthespeedatwhichviewerscouldhave changedtheir view.
The viewer programis an OpenGLapplicationthat tex-turemapsthevideoframeson to eightrectangularpolygonswhich form anoverlappingoctagonaroundthevirtual cam-era (seeFigure7). The sphericalaberrationof the capturecamerasis correctedby suitablewarpingof the input videoon to eachdisplaypolygon.During display, the viewer ap-plicationdeterminestheframesthatareneededbasedonthecurrentview directionandrequeststhemfrom theMPEG-2decodersystem.TheimagesareloadedasOpenGLtextures,and a color transformationmatrix is usedto convert themfrom YUV formatinto RGBin hardware.For agivencamerarig, theexactarrangementof thedisplaypolygonsneedstobeadjustedsothattheframesappearstitchedtogetherinto aseamlesspanorama.Currently, this is accomplishedby man-ually adjustingtheir positions,but automaticcalibrationisplannedfor a futureversionof theapplication.
In addition to the basicpanoramaviewer, several addi-tional featureswereaddedto makethevideopanoramamoreuseful(seeFigure8 andthevideoclip). Sincethecaptured
materialwasatourof Stanfordcampus,acampusmapis dis-playedin additionto thevideopanoramawindow. Thismapdisplaysthepositionanddirectionof thecurrentview. Theusercanpoint on themapto indicatea new viewing direc-tion, e.g.towardacertainobject,which is thentracked.Thisis useful for getting onesbearingsas the tour progresses.Finally, on commandor in caseswhen the user stopsin-teractions,the view drifts back towardsa predefinedviewsequencechosenby the tour guide.Thus,lazy viewersareguaranteedto be looking in the directionof the objectcur-rently being discussedby the guide on the associatedau-dio track.At any time, theuserhasthe freedomto overridethetour guideor againjoin his tour, very muchlike in real-ity. Combiningthecurrentapplicationwith theVirtual VCRsupport(seeSection5.2)wouldalsoallow a userto staybe-hind andenjoy a particularview beforecatchingup or re-sumingthetour. This is plannedasa futureextension.
6. Conclusions
In this paperwe explored the emerging digital televisiontechnologyandits interactionwith computergraphics.DTVis supposedto provide high bandwidthbroadcastconnec-tions to every householdand offers high-quality, high-resolutionvideostreams.Most interestingfrom a computergraphicsperspective is theability to transmitany otherdatawith theDTV stream,which allows us to extendDTV in anumberof differentdirections.
In theprevioussectionswepresentedseveralapplicationsasexamplesfrom thelargeDTV designspace.Weprovidedthe ability to decouplethe displaytime from the transmis-sion time througha real-timevideobuffer, which is an im-portantbasetechnologythatwe appliedto a numberof theotherapplications.Weshowedhow to provideadditionalin-formation onto the live video screenin the form of videooverlays. Theoverlaysaremanipulatedby smallscriptsthatcontrol their appearanceandallow the userto interactivelybrowsetheir information.In the lecture of thefuture project,we concentratedon using image-basedtechniquesto scanandcombinesmall high-resolutionimagesof a blackboardandtransmitthemasa separatevideostreamto enhancethekey lecturepart. Finally, we presenteda virtual reality ap-plication,usingreal-timevideopanoramasthatprovidenewvisual experiencesand interactivity in the context of DTVtransmissions.
In conclusion,we hopeto have demonstratedthat com-puter graphicsoffers considerableknow-how and technol-ogy in areassuchasimage-basedrendering,interactive ap-plications,andvirtual reality, thatcandirectly contribute tocreatingexciting new DTV applications.As digital televi-sion technologybecomesmore widely available over thenext few monthandyears,weexpectto seemany new devel-opmentsthatmayforever changeour notionof television.
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Acknowledgments
First of all, we would like to thankPat HanrahanandSergeRutman(INTEL) for sharingtheir experience.Many thanksarealsodueto all thestudentswho spentmany hoursin de-signingandimplementingthepresentedprojects.In particu-lar wewouldliketo thankEdwardChang,BenMowery, Ha-reeshKessavan, Brad Nelson,ChuckFraleigh,RustanEk-lund, Bryn Forbes,and Jin Hian Lee for providing directinput to this paper. We alsowant to thankTamaraMunzerfor helpingwith the accompanying video. Finally, we liketo thankIntel, Sony, andInterval Researchfor funding theStanfordImmersiveTelevisionProject,which includesmostof this work.
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