多媒体技术课后题.ppt

Ch3GraphicsandImageDataRepresentations,1.Brieflyexplainwhyweneedtobeabletohavelessthan24-bitcolorandwhythismakesforaproblem.Generally,whatdoweneedtodotoadaptivelytransform24-bitcolorvaluesto8-bitones?Answer:Maynotbeabletohandlesuchlargefilesizesornothave24-bitdisplays.Thecolorswillbesomewhatwrong,however.,Weneedtoclustercolorpixelssoastobestusethebitsavailabletobeasaccurateaspossibleforthecolorsinanimage.Inmoredetail:varianceminimizationquantizationvmquant.mMinimumvariancequantizationallocatesmoreoftheavailablecolormapentriestocolorsthatappearfrequentlyintheinputimageandallocatesfewerentriestocolorsthatappearinfrequently.Thereforeifthereareforexamplemanyreds,asinaredapple,therewillbemoreresolutionintheredpartofthecolorcube.AnexcellentimplementationofthisideaisWusColorQuantizer(seeGraphicsGemsvol.II,pp.126-133).,2.Supposewedecidetoquantizean8-bitgrayscaleimagedowntojust2bitsofaccuracy.Whatisthesimplestwaytodoso?Whatrangesofbytevaluesintheoriginalimagearemappedtowhatquantizedvalues?Answer:,0to6364to127128to191192to255Thenreconstructionvaluesshouldbetakenasthemiddleoftheseranges;i.e.,3296160224,3.Supposewehavea5-bitgrayscaleimage.Whatsizeofordereddithermatrixdoweneedtodisplaytheimageona1-bitprinter?Answer:25=32levels=n2+1withn=6;thereforeneedD(6),4.Supposewehaveavailable24bitsperpixelforacolorimage.However,wenoticethathumansaremoresensitivetoRandGthantoBinfact,1.5timesmoresensitivetoRorGthantoB.Howcouldwebestmakeuseofthebitsavailable?Answer:ratiois3:3:2,sousebits9:9:6forR:G:B.,5.Atyourjob,youhavedecidedtoimpressthebossbyusingupmorediskspaceforthecompanysgrayscaleimages.Insteadofusing8bitsperpixel,youdliketouse48bitsperpixelinRGB.Howcouldyoustoretheoriginalgrayscaleimagessothatinthenewformattheywouldappearthesameastheyusedto,visually?,Answer:48bitsRGBmeans16bitsperchannel:sore-storetheoldints,whichwere28,asnewints216.Butthenthenewvalueshavetobecreatedbymultiplyingtheoldvaluesby28,sothate.g.amid-grayisstillamid-gray.Aswell,havetoduplicatetheoldgrayintoallthreeofR,G,B.,6.ForthecolorLUTproblem,tryoutthemedian-cutalgorithmonasampleimage.Explainbrieflywhyitisthatthisalgorithm,carriedoutonanimageofredapples,putsmorecolorgradationintheresulting24-bitcolorimagewhereitisneeded,amongthereds.,7.Writedownanalgorithm(pseudocode)forcalculatingacolorhistogramforRGBdata.Answer:inthist256256256;imageisanappropriatestructwithintfieldsred,green,bluefori=0.(MAX_Y-1)forj=0.(MAX_X-1)R=imagexy.red;G=imagexy.green;B=imagexy.blue;histRGB+;,Ch4ColorinImageandVideo,Exercise3,1.Considerthefollowingsetofcolor-relatedterms:(a)wavelength(b)colorlevel(c)brightness(d)whitenessHowwouldyoumatcheachofthefollowing(morevaguelystated)characteristicstoeachoftheaboveterms?,(a)luminance)brightness(b)hue)wavelength(c)saturation)whiteness(d)chrominance)colorlevel,2.Whatcolorisoutdoorlight?Forexample,aroundwhatwavelengthwouldyouguessthepeakpowerisforaredsunset?Forblueskylight?Answer:450nm,650nm.,3.(a)Supposeimagesarenotgammacorrectedbyacamcorder.Generally,howwouldtheyappearonascreen?Answer:Toodarkatthelow-intensityend.,(b)Whathappensifweartificiallyincreasetheoutputgammaforstoredimagepixels?(WecandothisinPhotoshop.)Whatistheeffectontheimage?Answer:Increasethenumberofbrightpixelsweincreasethenumberofpixelsthatmaptotheupperhalfoftheoutputrange.Thiscreatesalighterimage.andincidentally,wealsodecreasehighlightcontrastandincreasecontrastintheshadows.,Ch5FundamentalConceptsinVideo,1.NTSCvideohas525linesperframeand63.6secperline,with20linesperfieldofverticalretraceand10.9sechorizontalretrace.(a)Wheredoesthe63.6seccomefrom?Answer:,(b)Whichtakesmoretime,horizontalretraceorverticalretrace?Howmuchmoretime?Answer:,2.Whichdoyouthinkhaslessdetectableflicker,PALinEuropeorNTSCisNorthAmerica?Justifyyourconclusion.Answer:PALcouldbebettersincemorelines,butisworsebecauseoffewerframes/sec.,3.SometimesthesignalsfortelevisionarecombinedintofewerthanallthepartsrequiredforTVtransmission.(a)Altogether,howmanyandwhatarethesignalsusedforstudiobroadcastTV?Answer:5R,G,B,audio,sync;cansay“blanking”instead,too.,(b)HowmanyandwhatsignalsareusedinS-Video?WhatdoesS-Videostandfor?Answer:Luminance+chrominance=2+audio+sync=4Separatedvideo(c)HowmanysignalsareactuallybroadcastforstandardanalogTVreception?Whatkindofvideoisthatcalled?Answer:1Composite,4.OnesometimeshearsthattheoldBetamaxformatforvideotape,whichcompetedwithVHSandlost,wasactuallyabetterformat.Howwouldsuchastatementbejustified?Answer:Betamaxhasmoresamplesperline:500,asopposedto240.,5.WedontseeflickeronaworkstationscreenwhendisplayingvideoatNTSCframerate.Whydoyouthinkthismightbe?Answer:NTSCvideoisdisplayedat30framespersec,soflickerispossiblypresent.Nonetheless,whenvideoisdisplayedonaworkstationscreenthevideobufferisreadandthenrenderedonthescreenatamuchhigherrate,typicallytherefreshrate60to90Hzsonoflickerisperceived.,(Andinfactmostdisplaysystemshavedoublebuffers,completelyremovingflicker:sincemainmemoryismuchfasterthanvideomemory,keepacopyofthescreeninmainmemoryandthenwhenwethisbufferupdateiscomplete,thewholebufferiscopiedtothevideobuffer.),6.Digitalvideouseschromasubsampling.Whatisthepurposeofthis?Whyisitfeasible?Answer:Humanvisionhaslessacuityincolorvisionthanithasinblackandwhiteonecandistinguishcloseblacklinesmoreeasilythancoloredlines,whichsoonareperceivedjustamasswithouttextureasthelinesmoveclosetoeachother.Therefore,itisacceptableperceptuallytoremoveagooddealofcolorinformation.Inanalog,thisisaccomplishedinbroadcastTVbysimplyassigningasmallerfrequencybandwidthtocolorthantoblackandwhiteinformation.Indigital,we“decimate”thecolorsignalbysubsampling(typically,averagingnearbypixels).Thepurposeistohavelessinformationtotransmitorstore.,7.WhatarethemostsalientdifferencesbetweenordinaryTVandHDTV?Answer:Morepixels,andaspectratioof16/9ratherthan4/3.WhatwasthemainimpetusforthedevelopmentofHDTV?Immersion“beingthere”.Goodforinteractivesystemsandapplicationssuchasvirtualreality.,8.Whatistheadvantageofinterlacedvideo?Whataresomeofitsproblems?Answer:Positive:Reduceflicker.Negative:Introducesserratededgestomovingobjectsandflickersalonghorizontaledges.,9.Onesolutionthatremovestheproblemsofinterlacedvideoistode-interlaceit.Whycanwenotjustoverlaythetwofieldstoobtainade-interlacedimage?Suggestsomesimplede-interlacingalgorithmsthatretaininformationfrombothfields.Answer:Thesecondfieldiscapturedatalatertimethanthefirst,creatingatemporalshiftbetweentheoddandevenlinesoftheimage.,Themethodsusedtoovercomethisarebasicallytwo:non-motioncompensatedandmotioncompensatedde-interlacingalgorithms.Thesimplestnon-motioncompensatedalgorithmiscalled“Weave”;itperformslinearinterpolationbetweenthefieldstofillinafull,“progressive”,frame.Adefectwiththismethodisthatmovingedgesshowupwithsignificantserratedlinesnearthem.,Abetteralgorithmiscalled“Bob”:inthisalgorithm,onefieldisdiscardedandaafullframeisinterpolatedfromasinglefield.Thismethodgeneratesnomotionartifacts(butofcoursedetailisreducedintheresultingprogressiveimage).,Inavertical-temporal(VT)de-interlacer,verticaldetailisreducedforhighertemporalfrequencies.Other,non-linear,techniquesarealsoused.Motioncompensatedde-interlacingperformsinter-fieldmotioncompensationandthencombinesfieldssoastomaximizetheverticalresolutionoftheimage.,Ch6BasicsofDigitalAudio,Exercise1,1.MyoldSoundblastercardisan8bitcard.(a)Whatisit8bitsof?(b)WhatisthebestSQNR(SignaltoQuantizationNoiseRatio)itcanachieve?,Answer:,2.Ifasetofearprotectorsreducesthenoiselevelby30dB,howmuchdotheyreducetheintensity(thepower)?Answer:Areductioninintensityof1000.,3.Alossofaudiooutputatbothendsoftheaudiblefrequencyrangeisinevitable,duetothefrequencyresponsefunctionofanaudioamplifierandthemedium(e.g.,tape).(a)Iftheoutputwas1voltforfrequenciesatmidrange,whatistheoutputvoltageafteralossof3dBat18kHz?(b)Tocompensatefortheloss,alistenercanadjustthegain(andhencetheoutput)onanequalizeratdifferentfrequencies.Ifthelossremains3dBandagainthroughtheequalizeris6dBat18kHz,whatistheoutputvoltagenow?Hint:Assumelog102=0.3.,4.Supposethesamplingfrequencyis1.5timesthetruefrequency.Whatisthealiasfrequency?Answer:0.5timestheTrueFrequency.,5.Inacrowdedroom,wecanstillpickoutandunderstandanearbyspeakersvoice,notwithstandingthefactthatgeneralnoiselevelsmaybehigh.Thisisknownasthecocktail-partyeffect.Thewayitoperatesisthatourhearingcanlocalizeasoundsourcebytakingadvantageofthedifferenceinphasebetweenthetwosignalsenteringourleftandrightears(binauralauditoryperception).Inmono,wecouldnothearourneighborsconversationwellifthenoiselevelwereatallhigh.Statehowyouthinkakaraokemachineworks.,Hint:Themixforcommercialmusicrecordingsissuchthatthe“pan”parameterisdifferentgoingtotheleftandrightchannelsforeachinstrument.Thatis,foraninstrument,eithertheleftorrightchannelisemphasized.Howwouldthesingerstracktiminghavetoberecordedtomakeiteasytosubtractthesoundofthesinger(whichistypicallydone)?,Answer:Forthesinger,leftandrightisalwaysmixedwiththeexactsamepan.Thisinformationcanbeusedtosubtractoutthesoundofthesinger.Todoso,replacetheleftchannelbythedifferencebetweentheleftandtheright,andboostthemaximumamplitude;andsimilarlyfortherightchannel.,6.ThedynamicrangeofasignalVistheratioofthemaximumtotheminimumabsolutevalue,expressedindecibels.Thedynamicrangeexpectedinasignalistosomeextentanexpressionofthesignalquality.Italsodictatesthenumberofbitspersampleneededtoreducethequantizationnoisetoanacceptablelevel.Forexample,wemaywanttoreducethenoisetoatleastanorderofmagnitudebelowVmin.Supposethedynamicrangeforasignalis60dB.Canweuse10bitsforthissignal?Canweuse16bits?,7.SupposethedynamicrangeofspeechintelephonyimpliesaratioVmax/Vminofabout256.Usinguniformquantization,howmanybitsshouldweusetoencodespeechtomakethequantizationnoiseatleastanorderofmagnitudelessthanthesmallestdetectabletelephonicsound?Answer:Vmin=Vmax/256.ThequantizationnoiseisVmax=2exp(n),ifweusenbits.Thereforetogetquantizationnoiseaboutafactorof16belowtheminimumsound,weneed12bits.,8.Perceptualnonuniformityisageneraltermfordescribingthenonlinearityofhumanperception.Thatis,whenacertainparameterofanaudiosignalvaries,humansdonotnecessarilyperceivethedifferenceinproportiontotheamountofchange.(a)Brieflydescribeatleasttwotypesofperceptualnonuniformitiesinhumanauditoryperception.(b)WhichoneofthemdoesA-law(or-law)attempttoapproximate?Whycoulditimprovequantization?,Answer:(a):(1)Logarithmicresponsetomagnitude,(2)differentsensitivitytodifferentfrequencies,(b):A-law(or-law)approximatesthenon-linearresponsetomagnitude.Itmakesbetteruseofthelimitednumberofbitsavailableforeachquantizeddata.,9.Supposeasignalcontainstonesat1,10,and21kHzandissampledattherate12kHz(andthenprocessedwithanantialiasingfilterlimitingoutputto6kHz).Whattonesareincludedintheoutput?Hint:Mostoftheoutputconsistsofaliasing.Answer:1kHz,12-10=2kHz,and2*12-21=3kHztonesarepresent.,10.(a)CanasingleMIDImessageproducemorethanonenotesounding?Answer:No.(b)Isitpossibleformorethanonenotetosoundatonceonaparticularinstrument?Ifso,howisitdoneinMIDI?Answer:YesusetwoNoteOnmessagesforonechannelbeforetheNoteOffmessageissent.,2019/12/13,53,可编辑,(c)IstheProgramChangeMIDImessageaChannelMessage?Whatdoesthismessageaccomplish?BasedontheProgramChangemessage,howmanydifferentinstrumentsarethereinGeneralMIDI?Why?Answer:Yes.Replacespatchforachannel.128,sincehasonedatabyte,whichmustbein0.127.,(d)Ingeneral,whatarethetwomainkindsofMIDImessages?Intermsofdata,whatisthemaindifferencebetweenthetwotypesofmessages?Withinthosetwocategories,listthedifferentsubtypes.Answer:ChannelMessagesandSystemMessages.Channelvoicemessages,Channelmodemessages,Systemreal-timemessages,Systemcommonmessages,Systemexclusivemessages.Channelmessageshaveastatusbytewithleadingmost-significant-bitset,and4bitsofchannelinformation;Systemmessageshavethe4MSBsset.,11.(a)Giveanexample(inEnglish,nothex)ofaMIDIvoicemessage.Answer:NoteOn(b)Describethepartsofthe“assembler”statementforthemessage.Answer:opcode=Noteon;data=note,orkey,number;data=“velocity”=loudness.,(c)WhatdoesaProgramChangemessagedo?SupposeProgramchangeishex“PB=0.4;PC=0.1.Forsimplicity,letsalsoassumethatbothencoderanddecoderknowthatthelengthofthemessagesisalways3,sothereisnoneedforaterminator.i.HowmanybitsareneededtoencodethemessageBBBbyHuffmancoding?Answer:6bits.HuffmanCode:A-0,B-10,C-11;orA-1,B-00,C-01.,ii.HowmanybitsareneededtoencodethemessageBBBbyarithmeticcoding?,4、(a)WhataretheadvantagesofAdaptiveHuffmanCodingcomparedtotheoriginalHuffmanCodingalgorithm?(b)AssumethattheAdaptiveHuffmanCodingisusedtocodeaninformationsourceSwithavocabularyoffourletters(a,b,c,d).Beforeanytransmission,theinitialcodingisa=00,b=01,c=10,d=11.AsintheexampleillustratedinFig.7.7,aspecialsymbolNEWwillbesentbeforeanyletterifitistobesentthefirsttime.,Fig.7.11istheAdaptiveHuffmanTreeaftersendinglettersaabb.Afterthat,theadditionalbitstreamreceivedbythedecoderforthenextfewlettersis01010010101.i.Whataretheadditionallettersreceived?ii.DrawtheadaptiveHuffmantreesaftereachoftheadditionallettersisreceived.,Answer:(a)Likeanyotheradaptivecompressionalgorithms,itismoredynamic,thereforeoffersbettercompressionandworksevenwhenpriorstatisticsofthedatadistributionisunavailableasitisinmostmultimediaapplications.Italsosavestheoverheadsincenosymboltableneedstobetransmitted.(b)(i)Theadditionallettersreceivedare“b(01)a(01)c(0010)c(101)”.(ii)Thetreesareasbelow.,5、Considerthedictionary-basedLZWcompressionalgorithm.Supposethealphabetisthesetofsymbols0,1.Showthedictionary(symbolsetsplusassociatedcodes)andoutputforLZWcompressionoftheinput0110011,6、Considerthedictionary-basedLZWcompressionalgorithm.Supposethealphabetisthesetofsymbolsa,b,c.Showthedictionary(symbolsetsplusassociatedcodes),outputforLZWcompressionoftheinput,LossyCompressionAlgorithms,1、Whentheblocksizeis8,thedefinitionoftheDCTisgiveninEq.(8.17).(a)Ifan88grayscaleimageisintherange0.255,whatisthelargestvalueaDCTcoefficientcouldbe,andforwhatinputimage?(Also,statealltheDCTcoefficientvaluesforthatimage.)Answer:WhentheimageisallWHITE,i.e.,allpixelshaveI=255.ThelargestcoefficientistheDCvaluewhichis8*255=2,040.Allothers(ACvalues)arezero.(b)Ifwefirstsubtractthevalue128fromthewholeimageandthencarryouttheDCT,whatistheexacteffectontheDCTvalueF2,3?Answer:ThereisnoeffectonF2,3.Infact,noeffectonanyACvalues.,(c)Whywouldwecarryoutthatsubtraction?Doesthesubtractionaffectthenumberofbitsweneedtocodetheimage?Answer:Theideahereistoturnitintoazeromeanimage,sowedonotwasteanybitsincodingthemeanvalue.(Thinkofan8*8blockwithintensityvaluesrangingfrom120to135.)(d)Woulditbepossibletoinvertthatsubtraction,intheIDCT?Ifso,how?Answer:Afterdecoding,simplyadd128backtoallpixelvalues.,ImageCompressionStandards,1.(a)JPEGusestheDiscreteCosineTransform(DCT)forimagecompression.i.WhatisthevalueofF(0,0)iftheimagef(i,j)isasbelow?ii.WhichACcoefficientF(u,v)isthelargestforthisf(i,j)?Why?IsthisF(u,v)positiveornegative?Why?,Answer:i.8timesaverage-intensity=8*110=880.ii.|F(1,0)|isthelargest,becausetheintensityvaluechangeissimilartoahalfcosinecycleverticallywithinthe8*8block.F(1,0)isnegative,becausethephaseofthechangeisoffby180degrees.(Orsimplyput,itisopposite.),(b)Showindetailhowathree-levelhierarchicalJPEGwillencodetheimageabove,assumingthati.TheencoderanddecoderatallthreelevelsuseLosslessJPEG.ii.Reductionsimplyaverageseach22blockintoasinglepixelvalue.iii.Expansionduplicatesthesinglepixelvaluefourtimes.,AssumingP1modeofLosslessJPEG(i.e.,taketheimmediateprecedingpixelasthepredictedvalue),thenthecodewordsgeneratedare:,2.InJPEG,theDiscreteCosineTtransformisappliedto8*8blocksinanimage.Fornow,letscallitDCT-8.Generally,wecandefineaDCT-NtobeappliedtoNNblocksinanimage.DCT-Nisdefinedas:Givenf(i;j)asbelow,showyourworkforderivingallpixelvaluesofF2(u;v).(Thatis,showtheresultofapplyingDCT-2totheimagebelow.),Dividetheimageinto2by2blocks.WeonlyneedtoworkoutthefourcoefficientsforF2(u,v),thentheyllrepeat.F2(0,0)=0,becauseaverageintensityiszero.F2(1,0)=0,becausenochangevertically.F2(0,1)=,=200,F2(u,v):,3.Supposeweviewadecompressed512*512JPEGimagebutuseonlythecolorpartofthestoredimageinformation,nottheluminancepart,todecompress.Whatdoesthe512*512colorimagelooklike?AssumeJPEGiscompressedusinga4:2:0scheme.,Answer:Withoutallcomponents,wecannotrestorethecolorimage.Assumingthatweareonlyrecoveringthemonochromepart,thenfirst,eachpixelisanenlarged(duplicated)versionofasubsampled2*2block,sotheimageis“pixellated”lookingblocky.Second,althoughtheluminanceandchrominanceimagesareoftencorrelated,thereisnoguaranteethattheywillcapturethesameshapeandtextureinformation.Dependingontheimagecontent,theimagedisplayedmaybehighlydistortedintermsofshapeandtexture.,4.(a)HowmanyprincipalmodesdoesJPEGhave?Whataretheirnames?(b)Inthehierarchicalmodel,explainbrieflywhywemustincludeanencode/decodecycleonthecodersidebeforetransmittingdifferenceimagestothedecodeside.(c)WhatarethetwomethodsusedtodecodeonlypartoftheinformationinaJPEGfile,sothattheimagecanbecoarselydisplayedquicklyanditerativelyincreasedinquality?,Answer:(