電子信息專業(yè)英語（第二版）課件：Images and Televisions

ImagesandTelevisionsPassageAFundamentalConcepts

PassageBCompression/DecompressionTechniques

PassageCTelevision

PassageADigitalImageFundamentals

1.DigitalImageResolution

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

2.Histograms

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

Figure17.2AHistogram

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.Thetransition

betweensourceanddestinationimagesisthenaccomplishedbysmoothlytransformingnotonlythecontrolelementlocations,butalsotheircolors.Thelocationsandcolorsofpixelsnotexplicitlymarkedascontrolelementsaredeterminedbythelocationsandcolorsofthecontrolelementsthatarenearest.

5.ImageFiltering

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

locateedgesinanimage.［2］

6.DilationandErosion

Dilationaddsalayerofpixelstoallobjectsinanimage.Erosionremovesonelayerofpixelsfromallobjects.Whendilationisfollowedbyerosion,gapsinbrokenboundariesidentifiedthroughedgedetectioncanbefilledin.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.

當一幅圖像只使用了可用灰度級的一小部分時,可以使用直方圖均衡的方法將灰度級的使用擴展到整個可用的范圍。［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,邊緣濾波器則對圖像邊緣進行定位。［3］Thephasespectrumcarriesthebestinformationaboutthelocationsoftheobjectsintheimage．相位譜攜帶著圖像中目標位置的信息。［4］CTscansareX-raystakeninmanydirectionsinasingleplaneofanobject.

CT技術是使用X射線從不同方向對目標的某個平面進行掃描。［5］Losslesscompressionmeansthatafileiscompactedwithoutlosinganyinformation,sothatthereconstructedfileisidenticaltotheoriginal．無損壓縮是不損失任何信息地將文件進行壓縮,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幾個重要系數上。Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)灰度圖像(2)直方圖均衡

(3)圖像對比度 (4)結果矩陣

(5)邊緣檢測 (6)行程編碼

(7)無損壓縮 (8)低通濾波器

(9)哈夫曼編碼 (10)數字圖像壓縮

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:圖像出現在視網膜上會保留幾毫秒,然后消失。如果一個圖像序列以50幅圖像每秒逐行掃描顯示,人們就不會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生運動畫面的。

(2)所有壓縮系統(tǒng)均需要兩種算法:一個在信源壓縮數據,一個在信宿對數據進行解壓。在文獻當中,這兩個算法分別被稱做編碼算法和解碼算法。

(3)JPEG(聯合圖像專家組)用于壓縮連續(xù)色調的靜止圖像(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。攝像機常用1.85∶1或者2.35∶1的寬高比。

(5)MPEG代表運動圖像專家組,它是對數字壓縮格式視聽信息(如電影、錄像、音樂)編碼的一組標準的統(tǒng)稱。與其他視頻、音頻的編碼格式相比,MPEG的主要優(yōu)勢是相同質量下的文件要小得多。這是因為MPEG使用了非常復雜的壓縮技術。

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數來確定。

·intermsof…意為“根據…”。

[2]?Inimageprocessing,aprocedurecalledprincipalcomponentshasbeendesignedtoidentifycorrelationbetweenimagebandsandtocreateanewsetoftransformedbandsthatrepresentanewcolorspaceinwhichthenewimagebandsareuncorrelated.

在圖像處理過程中，一個稱為主分量的過程被設計用以確認圖像頻譜之間的相關性，并產生一組新的變換頻譜，其中新圖像頻譜互不相關地表示一個新的彩色空間。

·that引導定語從句修飾bands。inwhich引導定語從句修飾space。

[3]?Usingthesevectors,clustersofvectorsarederivedusingaderivedspectraldistancemeasure.使用這些矢量，用一個導出的頻譜距離可推導出矢量簇。

·第二個using相當于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.fractalcompressionPassageCTelevision

Atelevisionpictureisbuiltupgraduallybymovingaspotoflightacrossanddownascreeninarasterpattern.

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

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

ThevideosignalcontainslineandfieldsyncpulsestomakesurethattheTVreceiverstartsanewlineandanewframeatthesametimeastheTVcamera.

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