高动态范围图像压缩
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HIGHDYNAMICRANGEIMAGECOMPRESSIONUSINGBASEMAPCODING
TakuyaFUJIKI†NicolaADAMI⋆TakaoJINNO‡MasahiroOKUDA†
†TheUniversityofKitakyushu⋆BresciaUniversity‡ToyohashiUniversityofTechnology
ABSTRACT
AstheHighdynamicrange(HDR)imagesgenerallyhave
morethan10bit/1024colorsperchannel,itsenormousdata
sizeoftenneedstobereducedwhentransmittingorstoring
theimages.Thusdevelopmentforfunctionalcompression
isoneofimportantresearchtopics.Recentlyalotoftech-
niquesfortheHDRimagecompressionarebeingsuggested,
andseveraltwo-layercodingalgorithmswhichseparatelyen-
codealowdynamicrange(LDR)imageandaresidualimage
havebeenstudied.However,thosemethodsareinefficientin
codingperformance.Inthisarticle,wesuggestanewtwo-
layercodingalgorithmfortheHDRimages,whichrealizes
two-layereddynamicrange.Ourmethodencodesabasemap,
whichisablurredversionoftheHDR,andLDRimagepro-
ducedbythebasemap.Ouralgorithmsignificantlyimproves
acompressionperformance.
IndexTerms—Highdynamicrangeimage,Two-layer
encoding,Tonemapping
1.INTRODUCTION
Thedynamicrangeofthehumanvisualsystem(HVS)
reachesmorethan200dB,andtheratioofshadowsand
highlightscanbecome100∼120dBinarealscene.How-
ever,thedynamicrangeofmostconsumercamerasisabout
80dBgenerallyathighestandcannotrecordalltheradiance
informationofthescenes.TheHDRimageisdevelopedto
recordallthebrightnessinformationofthevisiblerangeof
thescene.Itcansupportthewholedynamicrangeandthe
colorgamutthatthehumanvisualsystemperceives,andbe
utilizedinmanyfields,forexample,imagebasedlighting,
in-vehiclecameras,surveillancesystems,andsoon.
Generally,theHDRimagehasmorethan10bitcolors
perchannel,andhasenormousdataquantity.Thereforethe
dataprocessingofHDRimagesneedsenormoustransmission
effortanddiskspace.Hence,thedevelopmentoftheeffec-
tivecompressiontechniqueoftheHDRimageisoneofthe
importantresearchtopics.ManyHDRencodingtechniques
havebeensuggestedsofar.Spaulding[1]andWardetal.[2]
encodeinformationindependentlytotheLDRandresidual
image(whichisdifferenceorratiobetweenLDRandHDR
images),insteadofencodingtheHDRimagedirectly.Itis
calledthetwo-layercoding.Inthetwolayercodingnoeffort
isneededtoextracttheLDRimageinthedecoder.However
itisinefficientinasenseofcodingperformancebecausethequantizationerroroftheLDRimagebringstheextraenergy
totheresidualimageinhighfrequencies.IftheoriginalLDR
isusedtocalculatetheresidualimage,theRDcurveiseasily
saturatedathighbitratesduetoitsinconsistency.
Inthisarticle,wesuggestanewtwo-layercodingalgo-
rithmfortheHDRimage,usingReinhard’stonemapping
[3].Inanencoderside,torealizethetwo-layercoding,one
encodestheLDRandbasemapthatincludesfewerhigh-
frequencycomponentsthantheresidualimage,whichmakes
thecompressionefficiencysignificantlyimproved.Weex-
plainaboutconventionaltwo-layercodingforHDRinSec-
tion2.Section3.1introducessummaryoftheproposedtech-
nique.Insection3.2,weelaborateourcodingmethod.Fi-
nallyweshowthevalidityofourmethodwithcomparingto
conventionalmethodinSection4.
2.CONVENTIONALMETHOD
Spauldingetal.[1]proposesatwo-layerencodingforgamut
extendedimages.ThisisatechniquefortheRAWimageen-
codingofthedigitalcamera.Theyextendthedynamicrange
andcolorgamutbyapplyingtheinverseofnonlinearitytothe
LDRimages,andthenencodethedifferencebetweentheex-
tendedimagesandtheoriginalRAWimages.Laterthisstrat-
egywasappliedtocodingoftheHDRimages[2]andHDR
video[4]inthefieldofComputerGraphics.Inthesemethods,
atonemapped24bit(LDR)imageisfirstencodedbyJPEG
andthenthedifferenceinformationbetweentheLDRandthe
originalHDRisencodedandstoredinauser-availablebuffer
oftheJPEGwrapper.Thedifferenceinformationisobtained
bydividingHDRbyLDR.
Ir=IHDRILDR(1)
Intheconventionalmethod,thisILDRandthedifference
informationIrarecodedbyJPEG.InthispaperwecallIraresidualimage.Asfollows,inadecodingside,itsinverse
operationisperformedtorestoreIHDR.
I′HDR=I′LDR·I′r(2)
Theprime(’)meansanimagethatincludesquantization
erroraddedbythecompression.
Thistechniquemaintainsbackwardcompatibilitywiththe
conventionalLDRimageformat.Inthedecoderside,the
Fig.1.Theflowoftheproposaltechnique:IHDR:HDRimage,ILDR:TonemappedLDRimage,B:Basemapimage,2′
:imagewithquantizationerror,rgb2y:Theconversionprocessfromacolorimagetoaluminanceimage,LPF:Low-pass
filtering,TMO:Tonemappingoperator,ITMO:Inversetonemappingoperator
LDRimagethattheencoderpreparescanbeobtainedwith-
outcarryingoutatonemappingoperation.Whentheresidual
imageisalsodecoded,theHDRimagecanalsoberestored.
However,thistechniquehassomedisadvantages.First,
acompressionefficiencydeterioratessincetheerrorimage
Ircontainesmuchenergyinhighfrequencies.Thesecond
disadvantageisthesaturationofSNR.SinceILDRin(1)is
usedasapredictedimageofIHDR,inordertoavoiddriftin
decoderside,ingeneralyoushouldusethedecodedimage
I′LDRinsteadofILDRwithquantizationerror.However,the
quantizationerrorinI′LDRbringssomeextraenergyinhigh
frequencyiestoIr,whichalsocausesdegradationofSNR.
Thisproblemismainlyremarkableinalowluminanceregion.
Forthesereasons,intheconventionalmethod,uncompressed
ILDRisusedasthepredictedvalueofIHDR.Asaresult,RD
curveiseasilysaturatedasabitrateincreasesduetothemis-
match.Toovercometheproblems,ourmethodusesanovel
two-layercodingalgorithmfromtheabove,anditcansolve
thesetwoproblems.
3.PROPOSEDTECHNIQUEALGORITHM
3.1.Summaryandprocedure
Theschematicdiagramoftheproposaltechniqueisshownin
Fig.1.Inourmethod,theLDRimageisobtainedbyatone
mappingoperatorbasedonthebasemapBthatisobtained
bylow-passfilteringtheHDRimage.Thatis,intheencoder
side,twoimagesarecoded,thebasemapBandtheLDR
imageILDRobtainedbythetonemappingwithB.Inthe
decoderside,onecandecodeonlyILDR.Additionallyby
decodingBandapplyingitsinversetonemappingoperation
(ITMO),theHDRimagecanberestored.Thedetailofthe
tonemappingandtheproceduresofencoder/decodersidein
ourtwo-layercoding,areshownintherestofthissection.
3.2.Tonemapping
IngeneraltheHDR’sdynamicrangeiswiderthanconven-
tionaloutputdevices.Hence,anoperationcalled”tonemap-
ping”isoftennecessary,inwhichthedynamicrangeoftheHDRimageisreducedtodisplayableranges.Manytone
mappingoperatorshavebeenproposedinthelastdecade.
Manyofthemarebasedontherod/conephotoreceptorre-
sponseoftheHVS.Itiswell-knownthattherod/conere-
sponsecanbeapproximatedbyNaka-Rushtonphotoreceptor
model(alsoknownasMichaelis-MentenmodelorHillfunc-
tion)[5]:
R=In
In+cn,(3)
whereRisthephotoreceptorresponseandIisintensitystim-
uli,whichcorrespondstotheLDRandHDRimages,respec-
tively.cisaconstantthatcontrolsthesaturationlevelofthe
curve.Inourcodingframework,thistypeoftonemappingis
used.Mostofthelocaltonemappinginthiscategorycanbe
usedinourmethod[5].Amongthem,weadoptReinhard’s
tonemapping[3]whichisoneofthemostwellknownmeth-
odsduetoitshighquality[7][8].
Reinhardetal.hasproposedthefollowingfunctionasa
simpletonemapoperator,whichconvertstheHDRintothe
LDR.
ILDR(x)=IHDR(x)
1+B(x,s(x))(4)
B(x,s(x))istheimagethatisobtainedbyapplyinglocally
adaptiveGaussianfiltersG(x,s(x))
B(x,s(x))=IHDR(x)∗G(x,s(x)).(5)
s(x)isascaleparameterequivalenttoastandarddeviation
whichvariesdependingonpixels.Ass(x)issmall,theeffect
ofaGaussianfilterbecomessmaller.Tomaintainthelocal
contrast,s(x)issetsmallforsmoothareaandlargearound
edges.Fordetails,see[3].
3.3.ENCODER
Theconventionalmethods[1][2]encodetheresidualimage
Ir.AsmentionedatSec.1,thequantizationnoisespropagate
toIrwhenthequantizedLDRimageisusedin(1).Itworsens
thecodingefficiency.Furthermore,theRDcurveissaturated
athighbitrateswhentheoriginalLDRimageisused.Onthe