Media Studies Chapter 3 Homework Open the image with Adobe Photoshop, and convert this image into

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Worksheet: JPEG Compression Artifact

ApplicationProgramRequired

AdobePhotoshop5.5orlater



Objectives

1. TotakeacloserlookattheJPEGartifact.

2. TolearnwhereJPEGartifactismostvisible.



Introduction

JPEGisacommonfileformatforWebimages.ManydigitalcamerasalsosavedigitalphotographsdirectlyinJPEG

format,which,forsomecameras,maybetheonlychoiceofthefileformat.



HaveyounoticedanyuglyartifactinsomeJPEGimages?IfyouarepostingyourportfolioofartworkasJPEGimages

ontheWeb,youshouldunderstandhowtheJPEGcompressionaffectsyourimagequalityandhowtominimizethe

artifact.

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Theexampleimageyouwilluseinthisexercisehasmanysolidcolorareasandedgeswheresharpcolorchanges

occur.ItisnotthetypeofimagesthatJPEGcompressionisbestforbecausetheJPEGcompressionartifactismore

pronounce.However,thiswillmakeiteasyforyoutoseetheartifactinthisexercise.

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Instructions

LookunderJPEGCompressionArtifact.ThisexampleimageisaTIFfilewithnoimagecompression.Theimageismade

upoflargeareasofsolidcolorsandgradients.



I.CompressionQualityandFileSize

1. OpentheimagewithAdobePhotoshop,andconvertthisimageintoJPEGfileswithdifferentcompressionlevel.

2. IfyouhaveaccesstoPhotoshop5.5uptoCS2,chooseFile>SaveforWeb…

IfyouhavePhotoshopCS3orlater,chooseFile>SaveforWeb&Devices…

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3. Clickthe4‐Uptab(Figure4andFigure5,onthelastpageofthisworksheet),soyoucanseetheoriginalTIF

imageandthreeoptimizedimagesofdifferentJPEGcompressionsettingssimultaneously.

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4. Clickoneachofthesethreedifferentoptimizedimages,andselectJPEGfromtheOptimizedFileFormat

dropdownmenu.ThenselectfromtheCompressionQualitydropdownLow,Medium,andMaximum.

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1.Notedowninthetablethefilesizesandthetimetotransferthefileoveraspecificnetwork

connectionspeed:



Compression

Quality

FileSize

(KB)

TransferTime(sec.)atwhat

NetworkConnectionSpeed

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2.Whichcompressionqualityamongthethreeinthetableabovegivesthebestimage

quality?

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3.Whichcompressionqualityamongthethreeinthetableabovegivestheworstimage

quality?

Page2of6

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5. ZoomintheimagestolookcloselyandcomparetheoriginalTIFimagewiththethreeoptimizedimages.Doyou

seethedifference?



A.Attheintersectionofverydifferentcolors

B.Withinasolidcolorawayfromtheedgeofthecolor

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6. SelecttheoptimizedimagethathastheCompressionQualitysettoMaximum;youcanalsodirectlysetthe

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II.TakeaCloserLookattheJPEGImage

Openthismax.jpgfileinPhotoshop.ItmaylookthesameastheoriginalTIFimagetothenakedeyes.However,

somecolorsarealtered,especiallythepixelsaroundtheedgeswheresharpchangesofcolorsoccur.Trythese:

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1. UsetheMagicWandselectiontool,withitsTolerancesettozero,clicktoselectasolidcolorareaintheoriginal

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2. Now,trythesameonthemax.jpg.Howistheshapeoftheselection?Mostofthemiddleareaisintheselection

buttheedgearoundthesolidcolorisnot.And,theselectionoutlineisalittlejagged(Figure1b).



 

(a)(b)

Figure1

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5.Whycanyoutellfromtheobservationintheabovestepsthatthemax.jpgisnotexactly

thesameastheoriginalTIF?(First,whatdoessettingtheToleranceofMagicWandtozero

means?)



Lowercompressionqualitysettingscanimposeveryvisiblealternationsontheimage,buttheMaximumsetting

causesonlyindiscerniblealterations.TheMaximumsettinggiveslargerfilesizebutitisstillmuchsmallerthanthe

originalTIF.

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Buthowwouldthisaffectthewayyoucreateyourdigitalimages?Thisaffectshowyouusetheimage.Forexample,

anyimagesthatareintendedforfurthereditingshouldnotbekeptinJPEGformatbecauseeverytimeyousavea

JPEGfile,itgetsalteredevenifyouhavenotchangedanything.(Seethefollowingsections)

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III.ResavingaJPEGEvenat100%WillAltertheImage

2. Closethefileandopenthemax2.jpginPhotoshop.



SincethecompressionqualityissettoMaximum,thealternationcausedbytheJPEGartifactisindiscernible.Toseeif

max2.jpgisreallyalteredfrommax.jpg,trythefollowing:



i. UsetheMagicWandselectiontool,againwithitsTolerancesettozero,clicktoselectasolidcolorareain

themax.jpg.Filltheselectionwithadistinctivecolor,suchasblack.(Edit>Fill…)

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IV.ResavingaJPEGWillLowertheQuality

OpentheoriginalTIFfileinPhotoshopandsaveitasJPEGatthecompressionqualitysettozero.Namethenewfile

asmax0.jpg.



Openthismax0.jpginPhotoshop.Zoomintheimageto600%.Youwillseethatthecolorsseemtobegroupedin

squareblocks(Figure2).



Figure2

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6.Doallthesquareblockshavethesamepixeldimension?



7.Whatisthepixeldimensionofthesquareblocks?

(Therearemanywaystofindoutthepixeldimensionofthesquareblocks.Onewayistousea

RectangularMarqueeTooltomakeaselectionaroundasquareblock.Then,lookattheInfopalette

forthewidthandheightoftheselection.)



Whyarethecolorsgroupedinsquareblocksofsuchapixeldimension?ItisrelatedtotheJPEGcompression

Page4of6

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A.recovered,andisasgoodas,oralmostasgoodas,themax.jpg–theJPEGsaved

directlyfromtheoriginalTIFatthemaximumcompressionquality.

B.worsethanmax0.jpg,oratleastthereisnotanyobservableimprovementover

max0.jpg.

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VI.SummaryandConclusions

9.True/False:SavinganimageasJPEGatlowercompressionqualityforWebimagescanreduce



11.True/False:ThequalityofanimagefilethatissavedasJPEGoverandoveratthemaximum



12.True/False:AnimagethatissavedasJPEGatanycompressionqualitycanberecoveredtothe

originalqualityanytimebyre‐savingitinanotheruncompressedfileformatsuchasPhotoshop

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16.Nameonefileformat,inadditiontoPSD(Photoshop),thatwillnotcausedegradationinimage



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Figure3.SaveforWeb…dialogboxinAdobePhotoshopCC2014.

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Figure4.SaveforWeb&DevicesdialogboxinAdobePhotoshopCS5.1.

Page6of6

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Figure5.SaveforWeb&DevicesdialogboxinAdobePhotoshopCS3.