ScannersCapture Devices - Home Department of Computer scannerscapture设备计算机家系

FromAnalogtoDigitalWhatareDigitalImages?Electronicsnapshotstakenofasceneorscannedfromdocumentssamplesandmappedasagridofdotsorpictureelements(pixels)pixelassignedatonalvalue(black,white,grays,colors),representedinbinarycodecodestoredorreduced(compressed)readandinterpretedtocreateanalogversionCULBiasonImageCapture:Createrichimagesthatareusefulovertimeinthemostcost-effectivemanner.SetconversionrequirementsgreaterthanimmediateapplicationPromotereuseofcontentEnablesharingofcomparableandtrustedresourcesacrossdisciplines,users,andinstitutionsWhyRichDigitalMasters?PreservationOriginalmayonlywithstandonescanMaintenanceofdigitalfilesCostOnescanmaybeallthatisaffordableConversioncostsdwarfedbyothercostsAccessManyfromoneThericherthefile,thebetterthederivativeintermsofqualityandprocessibilityHowtodeterminewhat’sgoodenough?ConnoisseurshipofdocumentattributesIdentifykeyinformationcontentObjectivelycharacterizeormeasureattributes:size,detail,tone,andcolorAppreciateimagingfactorsaffectingqualityandcostTranslatebetweenanaloganddigitalEquatemeasurementstodigitalequivalenciesandcorrespondingmetrics,e.g.,detailsize

resolution

MTFCUL’sApproachtoImaging:“NoMore,NoLess〞ImagerequirementsandcostimagequalityandutilitydesiredpointofcaptureDigitalImageQualityis

GovernedBy:resolutionandthresholdbitdepthcolormanagementimageenhancementcompressionandfileformatsystemperformanceResolutionDeterminedbynumberofpixelsusedtorepresenttheimageIncreasingresolutionincreaseslevelofdetailcapturedandgeometricallyincreasesfilesizezoominEffectsofResolution600dpi300dpi200dpi9ThresholdSettinginBitonalScanning definesthepointonascalefrom0to255atwhichgrayvalueswillbeinterpretedeitherasblackorwhiteEffectsofThresholdthreshold=100threshold=6011BitDepthDeterminedbythenumberofbinarydigits(bits)usedtorepresenteachpixel1-bit8-bit24-bitBitDepthincreasingbitdepthincreasesthelevelofgrayorcolorinformationthatcanberepresentedandarithmeticallyincreasesfilesizeBitdepth,dynamicrange,andcolorappearanceUtilizingSufficientBit-Depth3-bitgray8-bitgrayUtilizingSufficientBitDepth8-bitcolor24-bitcolorBitDepthvs.DynamicRangeTherangeoftonaldifferencebetweenlightestlightandthedarkestdarkMappingTonesCorrectly:UseofHistogramsRepresentingColorAppearanceBalancedColorColorShiftTowardsRedImageEnhancementImageeditingtomodifyorimproveanimagefilters(brightness,contrast,sharpness,blur)toneandcolorcorrectionUseraisesconcernsaboutfidelityandauthenticityEffectsofFiltersnofiltersusedmaximumenhancementImageEditingCompressionreducesfilesizeforprocessing,storage,transmission,anddisplayimagequalitymaybeaffectedbythecompressiontechniquesusedandthelevelofcompressionappliedCompressionVariableslosslessversuslossycompressionproprietaryvs.openschemeslevelofindustrysupportbitonalvs.gray/colorseeattributesofcommoncompressiontechniquesat:/preservation/tutorial/presentation/table7-3.htmlEffectsofJPEGCompression300dpi,8-bitgrayscaleuncompressedTIFFJPEG18.5:1compressionCompression/FileFormatComparisonGIF(lossless)FileSize:60KBJPEG(lossy)FileSize:49KBimagescourtesyofEdisonPapersFileFormatsConsistofboththebitsthatcompriseimageinformationandheaderinformationonhowtoreadandinterpretthefileImagequalityaffectedbyformatsupportfor:BitdepthCompressiontechniquesColormanagementHardware,software,andnetworksupportSeecommonfileformatschartat:/preservation/tutorial/presentation/table7-1.htmlEquipmentusedanditsperformanceovertimescannerswithsamestatedfunctionalitycanproducedifferentresultsFactorsaffectingimagequality:Optical,mechanical,andsensingcomponentsCalibrationAgeofequipmentEnvironment300dpi,scannerA300dpi,scannerBVariationsinImageQualityduetoScannerPerformanceCorrelatingDocumentAttributestoImageRequirementsExample:DeterminingResolutionRequirementsWhat’syoufinestfeature?What’syourqualityrequirement?What’syourimagingapproach?CaseStudy:BrittleBooksVariables:featuresize,quality,imagingapproachfixedmetric:smallestlowercaseletterQIvalues:8(excellent),5(good),3.6(marginal)Resolutionkeytotextcapture,e.g.,dpiBitonalQIformulafortextDPI=3QI/.039h600dpi1-bitcaptureadequatelypreservesinformationalcontentofcleanlyproducedtextandsupportsimageprocessing(e.g.,OCR)TextualDocumentsMayRequireTonalCapturePagesbadlystainedPagesexhibitlowcontrastbetweentextandbackgroundFinefeaturesnotfullyresolvedPagescontaincomplexgraphics,colororimportantcontextualinformationGray/colorQIformulafortextDpi=2QI/.039hDeterminingResolutionRequirementsbeyondTextStrokewidthFinestscaleVisualperceptionDefiningDetailasStrokeEdge-basedrepresentationsVariables:featuresize,quality,imagingapproachfixedmetric:widthoffinestline,stroke,dot,ormarkingQIvaluesbasedonsamplingfrequency:2(excellent),1.5(good),1(marginal)ResolutionandbitdepthkeytoqualityDefiningDetailasStrokeQIformulasforstrokeGray/color:dpi=QI/.039w

QI=dpix.039wBitonal:dpi=(1.5QI)/.039wAdequatelyRenderedStrokeInadequatelyRenderedStrokeExampleManuscriptpagewithfinestsignificantstrokemeasuring.2mm,whichmustbefullycapturedGray/color:dpi=QI/.039wQI=2,w=.2Dpi=2/[.039(.2)]=256dpiDefiningDetailasScaleSmallestsignificantscaleorrepeatablepattern,e.g.,knotsinarugvs.strandsinthethreadCanresultinveryhighresolutionrequirements(e.g.,photographs,bookillustrations)Halftones:scanat4timesthescreenruling,utilizespecialdescreen/rescreening,orscaningrayscale(400dpiisagooddefault)HalftoneScannedat150DPIHalftoneScannedat400DPIDefiningDetailBasedonVisualPerceptionHumaneyecandetectdetailsapproximately1/215inchwideFinerdetailsareopticallyaveragedUsingtwopixelrule,visualperceptionrequirementsmetat430dpiIllustratedBookStudy:400dpigrayscalerecommendation/preservation/illbk/AdComm.htmy.DetailRepresentedat400dpi

TranslatingBetweenDigitalResolutionandScannerPerformanceDetailcapturegovernedbyresolution,threshold,bit-depth,andsystemperformanceSamplingresolution(DPI)isnotatrueindicatorofimagequality,althoughitmaysufficeforscanninginthe300-600dpirangeMonitorEmergingSystemPerformanceMetrics(e.g.,MTF)AligningDocumentAttributeswithDigitalRequirementsIdentifykeydocumentattributesTone,color,anddetailCharacterizethem,ifpossiblethroughobjectivemeasurementsDeterminequalityrequirementsandtolerancelevelsTranslatebetweenanaloganddigitalandbetweenscanningrequirementsandscanningperformanceAligni