多媒体技术课后题.ppt

Ch3GraphicsandImageDataRepresentations 1 Brieflyexplainwhyweneedtobeabletohavelessthan24 bitcolorandwhythismakesforaproblem Generally whatdoweneedtodotoadaptivelytransform24 bitcolorvaluesto8 bitones Answer Maynotbeabletohandlesuchlargefilesizesornothave24 bitdisplays Thecolorswillbesomewhatwrong however Weneedtoclustercolorpixelssoastobestusethebitsavailabletobeasaccurateaspossibleforthecolorsinanimage Inmoredetail varianceminimizationquantization vmquant mMinimumvariancequantizationallocatesmoreoftheavailablecolormapentriestocolorsthatappearfrequentlyintheinputimageandallocatesfewerentriestocolorsthatappearinfrequently Thereforeifthereareforexamplemanyreds asinaredapple therewillbemoreresolutionintheredpartofthecolorcube AnexcellentimplementationofthisideaisWu sColorQuantizer seeGraphicsGemsvol II pp 126 133 2 Supposewedecidetoquantizean8 bitgrayscaleimagedowntojust2bitsofaccuracy Whatisthesimplestwaytodoso Whatrangesofbytevaluesintheoriginalimagearemappedtowhatquantizedvalues Answer 0to6364to127128to191192to255Thenreconstructionvaluesshouldbetakenasthemiddleoftheseranges i e 3296160224 3 Supposewehavea5 bitgrayscaleimage Whatsizeofordereddithermatrixdoweneedtodisplaytheimageona1 bitprinter Answer 2 5 32levels n 2 1withn 6 thereforeneedD 6 4 Supposewehaveavailable24bitsperpixelforacolorimage However wenoticethathumansaremoresensitivetoRandGthantoB infact 1 5timesmoresensitivetoRorGthantoB Howcouldwebestmakeuseofthebitsavailable Answer ratiois3 3 2 sousebits9 9 6forR G B 5 Atyourjob youhavedecidedtoimpressthebossbyusingupmorediskspaceforthecompany sgrayscaleimages Insteadofusing8bitsperpixel you dliketouse48bitsperpixelinRGB Howcouldyoustoretheoriginalgrayscaleimagessothatinthenewformattheywouldappearthesameastheyusedto visually Answer 48bitsRGBmeans16bitsperchannel sore storetheoldints whichwere 2 8 asnewints 2 16 Butthenthenewvalueshavetobecreatedbymultiplyingtheoldvaluesby2 8 sothate g amid grayisstillamid gray Aswell havetoduplicatetheoldgrayintoallthreeofR G B 6 ForthecolorLUTproblem tryoutthemedian cutalgorithmonasampleimage Explainbrieflywhyitisthatthisalgorithm carriedoutonanimageofredapples putsmorecolorgradationintheresulting24 bitcolorimagewhereitisneeded amongthereds 7 Writedownanalgorithm pseudocode forcalculatingacolorhistogramforRGBdata Answer inthist 256 256 256 imageisanappropriatestructwithintfieldsred green bluefori 0 MAX Y 1 forj 0 MAX X 1 R image x y red G image x y green B image x y blue hist R G B 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 Increasethenumberofbrightpixels weincreasethenumberofpixelsthatmaptotheupperhalfoftheoutputrange 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 Wedon tseeflickeronaworkstationscreenwhendisplayingvideoatNTSCframerate Whydoyouthinkthismightbe Answer NTSCvideoisdisplayedat30framespersec soflickerispossiblypresent Nonetheless whenvideoisdisplayedonaworkstationscreenthevideobufferisreadandthenrenderedonthescreenatamuchhigherrate typicallytherefreshrate 60to90Hz sonoflickerisperceived Andinfactmostdisplaysystemshavedoublebuffers completelyremovingflicker sincemainmemoryismuchfasterthanvideomemory keepacopyofthescreeninmainmemoryandthenwhenwethisbufferupdateiscomplete thewholebufferiscopiedtothevideobuffer 6 Digitalvideouseschromasubsampling Whatisthepurposeofthis Whyisitfeasible Answer Humanvisionhaslessacuityincolorvisionthanithasinblackandwhite onecandistinguishcloseblacklinesmoreeasilythancoloredlines 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 MyoldSoundblastercardisan8 bitcard a Whatisit8bitsof b WhatisthebestSQNR SignaltoQuantizationNoiseRatio itcanachieve Answer 2 Ifasetofearprotectorsreducesthenoiselevelby30dB howmuchdotheyreducetheintensity thepower Answer Areductioninintensityof1000 3 Alossofaudiooutputatbothendsoftheaudiblefrequencyrangeisinevitable duetothefrequencyresponsefunctionofanaudioamplifierandthemedium e g tape a Iftheoutputwas1voltforfrequenciesatmidrange whatistheoutputvoltageafteralossof 3dBat18kHz b Tocompensatefortheloss alistenercanadjustthegain andhencetheoutput onanequalizeratdifferentfrequencies Ifthelossremains 3dBandagainthroughtheequalizeris6dBat18kHz whatistheoutputvoltagenow Hint Assumelog102 0 3 4 Supposethesamplingfrequencyis1 5timesthetruefrequency Whatisthealiasfrequency Answer 0 5timestheTrueFrequency 5 Inacrowdedroom wecanstillpickoutandunderstandanearbyspeaker svoice notwithstandingthefactthatgeneralnoiselevelsmaybehigh Thisisknownasthecocktail partyeffect Thewayitoperatesisthatourhearingcanlocalizeasoundsourcebytakingadvantageofthedifferenceinphasebetweenthetwosignalsenteringourleftandrightears binauralauditoryperception Inmono wecouldnothearourneighbor sconversationwellifthenoiselevelwereatallhigh Statehowyouthinkakaraokemachineworks Hint Themixforcommercialmusicrecordingsissuchthatthe pan parameterisdifferentgoingtotheleftandrightchannelsforeachinstrument Thatis foraninstrument eithertheleftorrightchannelisemphasized Howwouldthesinger stracktiminghavetoberecordedtomakeiteasytosubtractthesoundofthesinger 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 Yes usetwoNoteOnmessagesforonechannelbeforetheNoteOffmessageissent 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 HC1 Whatdoestheinstruction HC103 do Answer Changesthepatchto 4onchannel2 12 InPCM whatisthedelay assuming8kHzsampling Generally delayisthetimepenaltyassociatedwithanyalgorithmduetosampling processing andanalysis Answer SincethereisnoprocessingassociatedwithPCM thedelayissimplythetimeintervalbetweentwosamples andat8kHz thisis0 125msec Ch7LosslessCompressionAlgorithms Exercises4 1 SupposeeightcharactershaveadistributionA 1 B 1 C 1 D 2 E 3 F 5 G 5 H 10 DrawaHuffmantreeforthisdistribution Becausethealgorithmmaygroupsubtreeswithequalprobabilityinadifferentorder youranswerisnotstrictlyunique 2 a Whatistheentropy oftheimagebelow wherenumbers 0 20 50 99 denotethegray levelintensities b ShowstepbystephowtoconstructtheHuffmantreetoencodetheabovefourintensityvaluesinthisimage Showtheresultingcodeforeachintensityvalue c Whatistheaveragenumberofbitsneededforeachpixel usingyourHuffmancode Howdoesitcompareto 3 ArithmeticCodingandHuffmanCodingaretwopopularlosslesscompressionmethods a WhataretheadvantagesanddisadvantagesofArithmeticCodingascomparedtoHuffmanCoding Answer ThemainadvantageofArithmeticCodingoverHuffmanCodingisthatwhereastheminimumcodelengthforasymbolinHuffmanCodingis1 sincewecreateabinarytreewith0or1attachedtoeachbranch inArithmeticCodingthenumberofbitspersymbolcanbefractional b Supposethealphabetis A B C andtheknownprobabilitydistributionisPA 0 5 PB 0 4 PC 0 1 Forsimplicity let salsoassumethatbothencoderanddecoderknowthatthelengthofthemessagesisalways3 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 Supposethealphabetisthesetofsymbols 0 1 Showthedictionary symbolsetsplusassociatedcodes andoutputforLZWcompressionoftheinput0110011 6 Considerthedictionary basedLZWcompressionalgorithm Supposethealphabetisthesetofsymbols a 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 whatistheexacteffectontheDCTvalueF 2 3 Answer ThereisnoeffectonF 2 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 WhichACcoefficient F 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 let scallitDCT 8 Generally wecandefineaDCT NtobeappliedtoNNblocksinanimage DCT Nisdefinedas Givenf i j asbelow showyourworkforderivingallpixelvaluesofF2 u v Thatis showtheresultofapplyingDCT 2totheimagebelow Dividetheimageinto2by2blocks WeonlyneedtoworkoutthefourcoefficientsforF2 u v thenthey llrepeat 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 looking blocky Second althoughtheluminanceandchrominanceimagesareoftencorrelated thereisnoguaranteethattheywillcapturethesameshapeandtextureinformation Dependingontheimagecontent theimagedisplayedmaybehighlydistortedintermsofshapeandtexture 4 a HowmanyprincipalmodesdoesJPEGhave Whataretheirnames b Inthehierarchicalmodel explainbrieflywhywemustincludeanencode decodecycleonthecodersidebeforetransmittingdifferenceimagestothedecodeside c Wha