《電子信息類專業(yè)英語 》課件-第10章

Unit10ImagesandTelevisionsPassageADigitalImageFundamentalsPassageBCompression/DecompressionTechniquesPassageCTelevision

PassageADigitalImageFundamentals

1.DigitalImageResolution

Adigitalimageismadeupofmanyrowsandcolumnsofpixels.Forgrayscaleimages,eachpixelisassignedanumberthatrepresentsthegrayshadeassignedtothatpixel.Thelargerthenumberofpixelsinanimage,andthelargerthenumberofavailablegrayscalelevels,thebettertheresolutionoftheimage.Figure17.1isan17-bitimages,with217＝256possiblegrayscalelevels.Thenumberofrowandcolumnare303×228.Figure10.1An303×228×17DigitalImage

2.Histograms

Thegrayscalespresentinadigitalimagecanbesummarizedbyitshistogram(seeFigure17.2)．Thehistogramreportsthenumberofpixelsforeachgrayscalelevelpresentintheimage,asabargraph.Whenanimageusesonlyasmallportionoftheavailablegrayscalelevels,histogramequalizationcanbeusedtospreadouttheusageofgrayscalelevelsovertheentireavailablerange［1］．Thisprocedurere-assignsgrayscalelevelssothatimagecontrastisimproved.

Figure10.2AHistogram

3.AdditionandSubtractionofImages

Digitalimagescanbeaddedandsubtractedpixel-by-pixel.Addingtwoimagescancombinetwosetsofobjectsintoasingleimage.Moreover,severalnoisyimagesofthesamescenecanbeaveragedtogethertoreducetheeffectofnoise.Imagesubtraction,ontheotherhand,canbeusedtoremoveanunwantedbackgroundfromanimage.Subtractionoftwotime-lapsedphotographswillshowwheremotionhasoccurredbetweenthetwo.

Whentwoimagesareaddedorsubtracted,theresultantmatrixwillfrequentlycontainillegalgrayscalevalues.Forexample,whenapixelinone17-bitimagehasthegrayscalelevel129andthecorrespondingpixelinasecond17-bitimagehasthegrayscalelevel201,thesumpixelis129+201＝330.Thisisoutsidethelegalrangeforan17-bitimage,whichmayonlycontaingrayscalelevelsbetween0and255.Whenthesametwoimagesaresubtracted,thedifferencepixelis129-201=-72,againavalueoutsidethelegalrange.

Forthesereasons,scalingfollowsmostimagearithmetic.Scalingtotherange［0,GSLmax］maybeaccomplishedasfollows:

4.WarpingandMorphing

Warpingandmorphingaredigitalimagetechniquesthatarefindingapplicationnotonlyinentertainmentbutalsoinmedicalimaging.Warpingstretchesorre-shapesanobjectinanimage,whilemorphingtransformsoneimageintoanother.Thesetransformationsmaybeaccomplishedbymarkingcontrolpoints,controllines,ortrianglesinasourceimageandchoosingtheirnewpositionsinadestinationimage.

Thetransitionbetweensourceanddestinationimagesisthenaccomplishedbysmoothlytransformingnotonlythecontrolelementlocations,butalsotheircolors.Thelocationsandcolorsofpixelsnotexplicitlymarkedascontrolelementsaredeterminedbythelocationsandcolorsofthecontrolelementsthatarenearest.

5.ImageFiltering

Digitalimagescanbefilteredusingtwo-dimensionalconvolutionwithaconvolutionkernel.WhenanN×NimageisfilteredbyanM×Mconvolutionkernel,(M-1/2)rowsandcolumnsoneachsideoftheimagearelosttoboundaryeffects.Lowpassfiltersblurimages,highpassfiltersemphasizesharpchangesingrayscalelevel,andedgefilters

locateedgesinanimage.［2］

6.DilationandErosion

Dilationaddsalayerofpixelstoallobjectsinanimage.Erosionremovesonelayerofpixelsfromallobjects.Whendilationisfollowedbyerosion,gapsinbrokenboundaries

identifiedthroughedgedetectioncanbefilledin.Conversely,whenerosionisfollowedbydilation,spotsofnoiseinanimageareremoved.

Successfullydetectingtheedgesinanimageisthefirststeptowardsconfidentidentificationofobjectboundariesandthenobjectsrecognition.Fromboundaryinformation,shapecharacteristicslikeperimeterandareacanbecalculated,whichcanbeusedtoclassifyanobject.

7.ImageSpectra

Two-dimensionalFFTsareusedtoanalyzethespectraofdigitalimages.Justasintheone-dimensionalcase,atwo-dimensionalspectrumcomprisesamagnitudespectrumandaphasespectrum.Thephasespectrumcarriesthebestinformationaboutthelocationsoftheobjectsintheimage.［3］Whenallmagnitudesaresettoone,thephasesalonestillshowafacsimileoftheoriginalimage.Whenallphasesaresettozero,themagnitudesaloneshownotraceofit.

ImagespectraformthebasisforbothCT(computedtomography)andMRI(magneticresonanceimaging)scandisplays.CTscansareX-raystakeninmanydirectionsinasingleplaneofanobject.［4］

MRIscansdependinsteadonthemagneticpropertiesofanobjectplacedinavaryingmagneticfield.Bothtypesofscanspermitnon-invasiveinvestigationsofthree-dimensionalobjects.

8.Imagecompression

InpartduetotheInternet,digitalimagesaretransmittedfromplacetoplacemore

oftenthanever.Tosavetimeandbandwidth(space),bothimagesandotherfilesareoftencompressedbeforebeingtransmitted.Losslesscompressionmeansthatafileiscompactedwithoutlosinganyinformation,sothatthereconstructedfileisidenticaltotheoriginal.［5］

Lossycompressionmeansthatsomeinformationfromtheoriginalfileisirretrievablylost,butgenerallythereconstructedfileisextremelyclosetotheoriginal.Thecompression

ratioistheratiooftheoriginalfilesizetothecompressedfilesize.

Onesimplecompressionschemeisrun-lengthencoding,whichcodesmorethanthreerepetitionsofanumberasthreecopiesofthenumberfollowedbyacountoftheadditionalcopiesneeded.AnothercompressionschemeisHuffmanencoding,whichusesshortercodestorepresentthemostcommonsignalelements,andlongercodestorepresenttheleastcommonsignalelements.

JPEG,anextremelycommonimagecompressionscheme,usesthediscretecosinetransform(DCT)toconcentratemostoftheinformationaboutan17×17sub-blockofanimageintoafewsignificantcoefficients.［6］

Itthenusesbothrun-lengthencodingandHuffmanencodingtoprovidefurthercompression.

Notes

［1］Whenanimageusesonlyasmallportionoftheavailablegrayscalelevels,histogramequalizationcanbeusedtospreadouttheusageofgrayscalelevelsovertheentireavailablerange.

當(dāng)一幅圖像只使用了可用灰度級(jí)的一小部分時(shí),可以使用直方圖均衡的方法將灰度級(jí)的使用擴(kuò)展到整個(gè)可用的范圍。

［2］Lowpassfiltersblurimages,highpassfiltersemphasizesharpchangesingrayscalelevel,andedgefilterslocateedgesinanimage．

低通濾波器使圖像變得模糊,而高通濾波器突出了圖像的灰度銳變,邊緣濾波器則對(duì)圖像邊緣進(jìn)行定位。

［3］Thephasespectrumcarriesthebestinformationaboutthelocationsoftheobjectsintheimage．

相位譜攜帶著圖像中目標(biāo)位置的信息。

［4］CTscansareX-raystakeninmanydirectionsinasingleplaneofanobject.

CT技術(shù)是使用X射線從不同方向?qū)δ繕?biāo)的某個(gè)平面進(jìn)行掃描。

［5］Losslesscompressionmeansthatafileiscompactedwithoutlosinganyinformation,sothatthereconstructedfileisidenticaltotheoriginal．

無損壓縮是不損失任何信息地將文件進(jìn)行壓縮,重建得到的文件和原文件是完全一樣的。

［6］JPEG,anextremelycommonimagecompressionscheme,usesthediscretecosinetransform(DCT)toconcentratemostoftheinformationaboutan17×17sub-blockofanimageintoafewsignificantcoefficients.

JPEG是一種極其常用的圖像壓縮方法,該方法使用離散余弦變換將圖像中17×17小塊的大部分信息集中到少數(shù)幾個(gè)重要系數(shù)上。

Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)灰度圖像(2)直方圖均衡

(3)圖像對(duì)比度 (4)結(jié)果矩陣

(5)邊緣檢測(cè) (6)行程編碼

(7)無損壓縮 (8)低通濾波器

(9)哈夫曼編碼 (10)數(shù)字圖像壓縮

2.PleasetranslatethefollowingphrasesintoChinese.

(1)edgefilter

(2)magnitudespectrum

(3)objectrecognition

(4)phasespectrum

(5)CT(ComputedTomography)

(6)MRI(MagneticResonanceImaging)

(7)DCT(DiscreteCosineTransform)

(8)highpassfilters

(9)lossycompression

3.TranslatethefollowingsentencesintoEnglish.

(1)人眼具有這樣的特性:圖像出現(xiàn)在視網(wǎng)膜上會(huì)保留幾毫秒,然后消失。如果一個(gè)圖像序列以50幅圖像每秒逐行掃描顯示,人們就不會(huì)覺得看到的是一幅離散的圖像。所有視頻系統(tǒng)都是利用這一原理產(chǎn)生運(yùn)動(dòng)畫面的。

(2)所有壓縮系統(tǒng)均需要兩種算法:一個(gè)在信源壓縮數(shù)據(jù),一個(gè)在信宿對(duì)數(shù)據(jù)進(jìn)行解壓。在文獻(xiàn)當(dāng)中,這兩個(gè)算法分別被稱做編碼算法和解碼算法。

(3)JPEG(聯(lián)合圖像專家組)用于壓縮連續(xù)色調(diào)的靜止圖像(例如照片)。它是ITU、ISO和IEC共同支持的圖像專家開發(fā)出來的。

(4)寬高比是圖像的寬度和高度之比。常規(guī)電視的寬高比是4∶3。高清晰度電視使用16∶9的寬高比。攝像機(jī)常用1.85∶1或者2.35∶1的寬高比。

(5)MPEG代表運(yùn)動(dòng)圖像專家組,它是對(duì)數(shù)字壓縮格式視聽信息(如電影、錄像、音樂)編碼的一組標(biāo)準(zhǔn)的統(tǒng)稱。與其他視頻、音頻的編碼格式相比,MPEG的主要優(yōu)勢(shì)是相同質(zhì)量下的文件要小得多。這是因?yàn)镸PEG使用了非常復(fù)雜的壓縮技術(shù)。

4.Answerthequestions.

(1)Whatisthefunctionofsubtractionofimages?

(2)Whatisthefunctionofwarping?

(3)Whatisdilation?

(4)Comparelosslesscompressiontolosscompression.

PassageBCompression/DecompressionTechniques

Numerousmethodshavebeendevelopedforthecompressionofdigitalimagedata.Oneoftheprincipaldriversforthisdevelopmentisthetelevisionindustrywherequalityimagedatamustbetransferredtoreceiversusingrelativelysimpleequipment.Thedevelopmentofhighdefinitiontelevisionisfurtherfocusingtheattentionofindustryanduniversityscientiststowardproblemsofdatareductionanddigitaltransmission.

Theprincipalevaluationcriteriafortheanalysisofcompressedversusuncompressedimageryiswhetherapersoncantellthedifferencebetweentheimages.AmoreimplementalmeasureistheRootMeanSquare(RMS)errorbetweentheoriginalimageandtheimagethathasbeencompressed.Compressionratesmaybegeneratedbydeterminingthesizeofthecompressedimageintermsofnumberofbitsperimagepixelfortheoriginalimage.[1]

Hereweonlyconsiderscompressionofsinglehighresolutionmulti-spectralimages.Highercompressionrateswillbeachievedinamotionsequencewhereframetoframevariationsmaybequantifiedandonlythechangesfromareferenceimageneedbecoded.

Therearetwogeneraltypesofcompression:(1)loss-less,and(2)?loss.Loss-lesscompressionmeansthatyoucanachieveacertaincompressionfactorandbeabletoexactlyreproducetheoriginalimage.Losscompressionontheotherhandallowssomeloss,buthasthepotentialformuchhighercompressionrates.Nomatterwhattechniquethatyouuse,theexactrateisverydependentonthecomplexityoftheimagethatyouareanalyzing.Forexample,thenormalbestthatcanbeachievedwithloss-lessencodinginarateof2bitsperpixel.

Infact,forsomeLand-satsceneswithurbanareasandmanysmallfarms,thefactorof2bitsperpixelmaynotbeabletoachieved.ThesametechniqueappliedtoaLand-satimageofthemid-westwherelargefieldsoccurandfewshadowsexistinimagesmightproduceamuchbettercompression.

Oneloss-lesstechniqueisknownasrunlengthencoding.Thecompressionalgorithmprocesseseachlineofinputimagerylookingforregionsinwhichdatavaluesarethesame.Iftenpixelsintheoriginalimagehaveavalueof10,thenthesamedatamayberepresentedasadatavalue,10,andamultipliersayinghowmanytimesthevalueisrepeatedbeforeachangedvalue.Huffmanencodingfollowsasimilarprocess.Theseloss-lesstechniquesaregenerallycalledentropycodingtechniques,andhaveapplicationindocumentimaging,desktoppublishing,andGIS.Itshouldbenotedthatentropycodingdoesnotworkexceptionallywellintherepresentationofremotesensingimages.

Inremotesensingimageryitiswellknownthattheremaybesignificantcorrelationbetweendifferentbandsofmulti-spectraldata.Inimageprocessing,aprocedurecalledprincipalcomponentshasbeendesignedtoidentifycorrelationbetweenimagebandsandtocreateanewsetoftransformedbandsthatrepresentanewcolorspaceinwhichthenewimagebandsareuncorrelated.[2]Theprocedurealsoprovidesameasureofthepercentoftheoriginalvariationpresentintheoriginalimageasfoundineachofthenewtransformedbands.ForLand-satTMdata,threetofourofthetransformedimagesrepresent98percentofthevarianceintheoriginalimages;therefore,acompressionfactorof2wouldbeachievedwithlittleloss.

Anothertypeoftransformcodingdoesnotinvolvearotationofthecolorspace,butinsteadrepresentsimagesintermsofspatialfrequencyofcertainbasefunctions.Fouriertransformsmapanimageintoaspatialfrequencyimagebaseonsinandcosinefunctions.AfastcomputerimplementationoftheDiscreteFourierTransform(DFT)isknownasaFastFourierTransform(FFT).DiscreteCosineTransforms(DCT’s)mapthesameimagetoaspatialfrequencyimagebasedonlyonthecosinefunction.

Eachpixelmayberepresentedbyaseriesoftrigonometricfunctionsandcoefficientsderivedfromtheimages.Ifalltermsofthetransform’strigonometricfunctionsareused,compressionisminimal.Asmoretermsaredeleted,compressiongoesup,buttheresultingcompressedimagedevelopscertainartifactsoftheprocedure.

VectorQuantification(VQ)isatypeofencodingthatdefinesavectorrepresentationofnon-overlappingareablockswithinanimage.Avectorconsistsofvaluesrepresentingthedatavaluesforeachpixelwithintheregion.Usingthesevectors,clustersofvectorsarederivedusingaderivedspectraldistancemeasure.[3]Acodebookconsistingoftheclusteredvectorsisstored,representingthecharacteristicsoftheimage.Thisprocessisnumericallyintensiveandmaybeiterative.ThedecoderforVQtakesanimagevectorandcomparesittostoredvectorsinthecodebook.

Aselectionismadebasedonminimizationofadistortionfunctionbetweenthenewvectorandthecodebook.TheVQtechniquecangenerallyachievecompressionratiosofbetween20tooneand35toonewithlittleobservabledistortion.TheVQtechniqueisanexampleofasymmetriccompressioninthatconsiderablymoretimeisspentderivingthecodebookthanindecompressingviaacodebooklookup.Sincedifferentimagesmayhavedifferentcharacteristics,arobustcodebookisnecessarytosuccessfullycodeanddecodeLand-satandothersatelliteimages.AVQtechniqueusingbetweenchannelcorrelationaswellasspatialcorrelationachieveshighercompressionrateswithlosslessthanindependentbandVQ.

Thelasttypeofcompressionconsideredisfractalcompression.BasedonMandelbrotsetswhichtakeadvantageofaselfsimilar,scalingindependent,statisticalfeatureofnature,fractalcompressionanddecompressioninvolvesaclusteringapproachtofindingregionswhichexhibitthesamecharacteristicsasasampleregionwithoutregardtorotationandscale.Regionswithinanimagearerelatedtonumerousotherregionswithinthesameimage,withthisduplicationofinformationbeingthebasisofthecompressionpotential.

Fractalcompressioncanachievecompressionratiosofupto80toonewithonlymoderatelossofinformation.Thefractaltechnique,liketheVQtechniqueisalsoasymmetric.Hardwareimplementationofthedecompressionoffractalimageshasachievedreal-timerates.

NOTES

[1]?Compressionratesmaybegeneratedbydeterminingthesizeofthecompressedimageintermsofnumberofbitsperimagepixelfortheoriginalimage.

壓縮比可以通過原始圖像每像素所需壓縮的比特?cái)?shù)來確定。

·intermsof…意為“根據(jù)…”。

[2]?Inimageprocessing,aprocedurecalledprincipalcomponentshasbeendesignedtoidentifycorrelationbetweenimagebandsandtocreateanewsetoftransformedbandsthatrepresentanewcolorspaceinwhichthenewimagebandsareuncorrelated.

在圖像處理過程中，一個(gè)稱為主分量的過程被設(shè)計(jì)用以確認(rèn)圖像頻譜之間的相關(guān)性，并產(chǎn)生一組新的變換頻譜，其中新圖像頻譜互不相關(guān)地表示一個(gè)新的彩色空間。

·that引導(dǎo)定語從句修飾bands。inwhich引導(dǎo)定語從句修飾space。

[3]?Usingthesevectors,clustersofvectorsarederivedusingaderivedspectraldistancemeasure.

使用這些矢量，用一個(gè)導(dǎo)出的頻譜距離可推導(dǎo)出矢量簇。

·第二個(gè)using相當(dāng)于byusing。

EXERCISES

1.?True/False.

(1)?Loss-lessandlossaretwogeneraltypesofcompression.()

(2)?Nomatterwhattechniquethatyouuse,theexactcompressionrateisnotverydependentonthecomplexityoftheimagethatyouareanalyzing.()

(3)?ThedecoderforVQtakesanimagevectorandcomparesittostoredvectorsinthecodebook.()

2.Fillintheblanks.

(1)?Loss-lesscompressionmeansthatyoucanachieveacertain

andbeabletoexactlyreproducetheoriginalimage.

(2)AfastcomputerimplementationoftheDiscreteFourierTransformisknownasa

.

(3)VectorQuantification(VQ)isatypeofencodingthatdefinesa

ofnon-overlappingareablockswithinanimage.

(4)?Thefractaltechnique,liketheVQtechniqueisalso

.

(5)

compressioncanachievecompressionradiosofupto80toonewithonlymoderatelossofinformation.

3.?Chosethebestanswer.

(1)?Whatkindofcompressionalgorithmprocesseseachlineinputimagerylookingforregionsinwhichdatavaluesarethesame?

a.runlengthencoding b.VectorQuantification

c.fractalcompression

(2)?Whichtypeofcompressiontakeadvantageofaselfsimilar,scalingindependentandstatisticalfeatureofnature?

a.runlengthencoding b.VectorQuantification

c.fractalcompression

PassageCTelevision

Atelevisionpictureisbuiltupgraduallybymovingaspotoflightacrossanddownascreeninarasterpattern.

Thevideosignalcausesthebrightnessofthespottovaryinproportiontotheintensityoflightintheoriginalimage.Themovementofthespotacrossthescreeniscontrolledbythelinescansignal.Eachtimethespotreachestherightsideofthescreen,itisblankedandmovedrapidlybacktotheleftsidereadytostartthenextline.Thisrapidmovementbacktoastartingpositionisknownasfly-back.Eachcompleteimageorframerequiresaminimumof500linestogiveapictureofacceptablequality.ThepresentEuropeanTVsystemuses625linesperframe.

Themovementofthespotdownthescreeniscontrolledbythefieldscansignal.Whenthespotreachesthebottomofthescreen,itisblankedandmovedrapidlybacktothetopofthescreen.Theframemustbescannedatleastfortytimespersecondtopreventthescreenfromflickering.ThepresentEuropeanTVsystemhasaframescanrateof50?Hz.

ThevideosignalcontainslineandfieldsyncpulsestomakesurethattheTVreceiverstartsanewlineandanewframeatthesametimeastheTVcamera.

Toallowthevideosignaltobetransmittedusingasmallerrangeoffrequencies,eachframeistransmittedintwoseparatehalves,knownasfields.Thefirsttimethespottravelsdownthescreenitdisplaysthefirstfield,whichconsistsoftheodd-numberedframelines.Thesecondtimethespottravelsdownthescreenitdisplaysthesec