电子信息类专业英语(西电第二版)Unit-17-Images-and-Televisions43页P

Unit17ImagesandTelevisionsUnit17ImagesandTelevisionsPassageAFundamentalConceptsPassageBCompression/DecompressionTechniquesPassageCTelevisionUnit17ImagesandTelevisionsPassageADigitalImageFundamentals1.DigitalImageResolutionAdigitalimageismadeupofmanyrowsandcolumnsofpixels.Forgrayscaleimages,eachpixelisassignedanumberthatrepresentsthegrayshadeassignedtothatpixel.Thelargerthenumberofpixelsinanimage,andthelargerthenumberofavailablegrayscalelevels,thebettertheresolutionoftheimage.Figure17.1isan17-bitimages,with217＝256possiblegrayscalelevels.Thenumberofrowandcolumnare303×228.Unit17ImagesandTelevisionsFigure17.1An303×228×17DigitalImageUnit17ImagesandTelevisions2.HistogramsThegrayscalespresentinadigitalimagecanbesummarizedbyitshistogram(seeFigure17.2)．Thehistogramreportsthenumberofpixelsforeachgrayscalelevelpresentintheimage,asabargraph.Whenanimageusesonlyasmallportionoftheavailablegrayscalelevels,histogramequalizationcanbeusedtospreadouttheusageofgrayscalelevelsovertheentireavailablerange［1］．Thisprocedurere-assignsgrayscalelevelssothatimagecontrastisimproved.Unit17ImagesandTelevisionsFigure17.2AHistogramUnit17ImagesandTelevisions3.AdditionandSubtractionofImagesDigitalimagescanbeaddedandsubtractedpixel-by-pixel.Addingtwoimagescancombinetwosetsofobjectsintoasingleimage.Moreover,severalnoisyimagesofthesamescenecanbeaveragedtogethertoreducetheeffectofnoise.Imagesubtraction,ontheotherhand,canbeusedtoremoveanunwantedbackgroundfromanimage.Subtractionoftwotime-lapsedphotographswillshowwheremotionhasoccurredbetweenthetwo.Unit17ImagesandTelevisionsWhentwoimagesareaddedorsubtracted,theresultantmatrixwillfrequentlycontainillegalgrayscalevalues.Forexample,whenapixelinone17-bitimagehasthegrayscalelevel129andthecorrespondingpixelinasecond17-bitimagehasthegrayscalelevel201,thesumpixelis129+201＝330.Thisisoutsidethelegalrangeforan17-bitimage,whichmayonlycontaingrayscalelevelsbetween0and255.Whenthesametwoimagesaresubtracted,thedifferencepixelis129-201=-72,againavalueoutsidethelegalrange.Forthesereasons,scalingfollowsmostimagearithmetic.Scalingtotherange［0,GSLmax］maybeaccomplishedasfollows:max)(Min)(Max)(Min-n]Qutput[m,n]Output[m,ScaledGSLOutputOutputOutputUnit17ImagesandTelevisions4.WarpingandMorphingWarpingandmorphingaredigitalimagetechniquesthatarefindingapplicationnotonlyinentertainmentbutalsoinmedicalimaging.Warpingstretchesorre-shapesanobjectinanimage,whilemorphingtransformsoneimageintoanother.Thesetransformationsmaybeaccomplishedbymarkingcontrolpoints,controllines,ortrianglesinasourceimageandchoosingtheirnewpositionsinadestinationimage.Thetransitionbetweensourceanddestinationimagesisthenaccomplishedbysmoothlytransformingnotonlythecontrolelementlocations,butalsotheircolors.Thelocationsandcolorsofpixelsnotexplicitlymarkedascontrolelementsaredeterminedbythelocationsandcolorsofthecontrolelementsthatarenearest.Unit17ImagesandTelevisions5.ImageFilteringDigitalimagescanbefilteredusingtwo-dimensionalconvolutionwithaconvolutionkernel.WhenanN×NimageisfilteredbyanM×Mconvolutionkernel,(M-1/2)rowsandcolumnsoneachsideoftheimagearelosttoboundaryeffects.Lowpassfiltersblurimages,highpassfiltersemphasizesharpchangesingrayscalelevel,andedgefilterslocateedgesinanimage.［2］Unit17ImagesandTelevisions6.DilationandErosionDilationaddsalayerofpixelstoallobjectsinanimage.Erosionremovesonelayerofpixelsfromallobjects.Whendilationisfollowedbyerosion,gapsinbrokenboundariesidentifiedthroughedgedetectioncanbefilledin.Conversely,whenerosionisfollowedbydilation,spotsofnoiseinanimageareremoved.Successfullydetectingtheedgesinanimageisthefirststeptowardsconfidentidentificationofobjectboundariesandthenobjectsrecognition.Fromboundaryinformation,shapecharacteristicslikeperimeterandareacanbecalculated,whichcanbeusedtoclassifyanobject.Unit17ImagesandTelevisions7.ImageSpectraTwo-dimensionalFFTsareusedtoanalyzethespectraofdigitalimages.Justasintheone-dimensionalcase,atwo-dimensionalspectrumcomprisesamagnitudespectrumandaphasespectrum.Thephasespectrumcarriesthebestinformationaboutthelocationsoftheobjectsintheimage.［3］Whenallmagnitudesaresettoone,thephasesalonestillshowafacsimileoftheoriginalimage.Whenallphasesaresettozero,themagnitudesaloneshownotraceofit.ImagespectraformthebasisforbothCT(computedtomography)andMRI(magneticresonanceimaging)scandisplays.CTscansareX-raystakeninmanydirectionsinasingleplaneofanobject.［4］MRIscansdependinsteadonthemagneticpropertiesofanobjectplacedinavaryingmagneticfield.Bothtypesofscanspermitnon-invasiveinvestigationsofthree-dimensionalobjects.Unit17ImagesandTelevisions8.ImagecompressionInpartduetotheInternet,digitalimagesaretransmittedfromplacetoplacemoreoftenthanever.Tosavetimeandbandwidth(space),bothimagesandotherfilesareoftencompressedbeforebeingtransmitted.Losslesscompressionmeansthatafileiscompactedwithoutlosinganyinformation,sothatthereconstructedfileisidenticaltotheoriginal.［5］Lossycompressionmeansthatsomeinformationfromtheoriginalfileisirretrievablylost,butgenerallythereconstructedfileisextremelyclosetotheoriginal.Thecompressionratioi