SNR-Scalability-by-transform-coefficient-refinemen

SNRScalabilitybyTransformCoe±cientRe¯nementforBlock-BasedVideoCodingTillHalbach‡andThomasR.Fischer†‡DepartmentofTelecommunicationsNorwegianUniversityofScienceandTechnology(NTNU)Trondheim,Norway†SchoolofElectricalEngineeringandComputerScienceWashingtonStateUniversity(WSU)PullmanWA,USAABSTRACTVarioustechniquesforSNRscalabilityinhybridblock-basedvideocodingexistintheliteratureandindifferentstandards.Anewapproachbasedontransformcoefficientrefinementisproposedinthisarticle.Thecoefficientdifferenceiscomputedafterquantization,subsequentlyentropy-encoded,andtransmittedforreconstructionofahigh-qualitylayeratthedecoder.Theapproachachievesinmostcasesonlyamoderateincreaseinbitrateascomparedtootherschemes.Thebitrateofourtwo-layerframeworkconvergestowardstherateofasingle-layersystemasthequalitydifferencebetweenthetwolayersincrease.Thegainscomeatthecostofincreasedcomputationalcomplexityandmemoryrequirements.Keywords:SNR/qualityscalability,block-basedhybridvideocoding,bitstreamlayering,H.26L1.INTRODUCTIONCurrently,newvideocompressionalgorithmsarebeingstandardizedbytheJointVideoTeam,ajointeffortofITU-T’sVideoCodingExpertsGroupandISO/IEC’sMovingPicturesExpertsGroup.Theprojectwilleventuallyevolveastwonew–andidentical–standards,RecommendationH.264andInternationalStandard11172-10,alsoknownasMPEG-4AdvancedVideoCoding.ThestandardizationprocessismorecommonlyaddressedbyitsprojectnameH.26LandhasatthemomentthestatusDraftInternationalStandard.Itisexpectedtoberatifiedbysummer2003.H.26Lis,amonganumberofotherapplications,beingdevelopedforbroadcastingandstreamingframeworks.Onaclient-serverbasis,theseapplicationsoffertheirservicesusuallyatdifferentlevels,i.e.withdifferentimagequalitiesfordifferentclients.However,featureslikescalabilitybyspatialresolutionorqualityhavesofarnotbeenstandardizedinH.26L.Consideringthelatterscheme,thisarticleproposesanewtechniquewhichissuitableforallhybridblock-basedvideocodecs.Thearticleisstructuredasfollows.First,webrieflyintroduceH.26Landexplainitsbasicfunctionality.WethengiveanoverviewofdifferentapproachestoSNRscalabilityinpreviousresearchandstandards,followedbytheproposalforanewtechniquebasedoncoefficientrefinement.Afterthat,wediscussimplementationissuesindetailandpresentsimulationresults.Thearticlecloseswithconcludingremarks.2.DESCRIPTIONOFH.26LH.26LisverysimilartopriordigitalvideocompressionstandardslikeH.26xandMPEG-x.Itisablock-basedhybridcodingscheme,i.e.aspatialortemporalpredictionofthecurrentsignaliscomputedandsubtractedfromtheoriginal.Then,thepredictionerroristransformed,quantizedandentropy-encoded.H.26Lconsistslikeitsprecessorsofseveralsetsofalgorithms,alsocalledprofiles,ofwhichonehastobechosenforaspecificapplication.Wewillconcentrateinthefollowingonthebasicoperationmode,whichisspecifiedbytheBaselineprofile.There,thetransformisaDCT-approximating4£4integertransform.Thetransformcoefficientsarethenprocessedbyuniformscalarquantizationandthereafterencodedbycontext-adaptivevariable-lengthcoding,resultinginabinarycodestream.ThechoiceamongAtthetimeofwriting,TillHalbachwason[e-mail]fi[e-mail]52differentavailablequantizersismadebypassingaquantizationparameter(QP)totheencodingsystem.TheQPspecifiesthequantizer’sstepsizeandinfluencesherebyboththeencoder’soutputrateaswellasthequality/PSNRofthereconstructedsequence.AnincreaseofQPbyoneleadstoanapproximately12%largerstepsize.Toreducecomplexityrequirements,H.26Lworksonso-calledmacroblocks(MBs)which–assumingaYCbCrcolorspaceand4:2:0chrominance(chroma)subsampling–consistofone16£16luminance(luma)blockandtwo8£8blocksforthecolorcomponents.AgroupofMBsformsinturnaslice,thesmallestindependentlydecodabledataunit.TheencoderhastodetermineapredictionmodeforeachMBwithregardtoanerrorcriterion.Thisisreferredtoasmodedecisionanddoneindependentlyforthelumaandchromasignalswiththeexceptionthat,ifalumaMBisINTRA-coded,thenalsothepredictionsforbothcorrespondingchromaMBshavetobeintra-framepredictions.Inordertoadaptcloselytotheinputsignal,thecodecfollowsasub-MB/blockconcept.InINTRA(I)coding,alumaMBcaneitherbepredictedasawhole,16£16-pixelblock,orissplitinto16blocksofsize4£4.INTER(P)codingemploysahierarchicalsplitwherealumaMBcanbecutintohalvesbydividingitsdimensionshorizontally,vertically,orboth.Thesameschememayinturnbeappliedtotheresultingonetofour8£8blocks.AnH.26Lsystemcanherebyadapttothedetailoftheinputimageandcapturemotionacrossframes.Incontrasttoe.g.H.263,standardizationofscalabilitybyqualityhasnotbeenconsideredinH.26Lsofar.However,SNRscalabilityisaveryimportantfeaturewhichmakesthesystemveryflexibleforapplicationsinbroadcastingandstreaming,wheredifferentchannelbandwidthsareavailablewithseveralreceivers.Itisfurtherofadvantageinerror-proneenvironmentsforgracefuldegradationofthequalityofthereconstructedpicturesincaseoftransmissionerrors.3.SNRSCALABILITYSNR-scalablesystemsoffermultiplerepresentationsofthevisualdataatdifferentqualitylevels.Thisisachievedbyseverallayers,eachofwhichrepresentstheoriginalsignalwithacertainquality.