北邮国院视频图像处理Word文档下载推荐.docx

北邮国院视频图像处理Word文档下载推荐.docx

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Reading/writingPGM/PPMimages:

Thefirststeptowardsimageandvideoprocessingisreadingimagesfromfileandwritethemtofile.Thereexistdifferentstandardsthatstoretheinformationindifferentformats;

beforeopeninganimage,knowledgeofthestandardisnecessary.TwowidelyusedimageformatsarePPMandPGM.ThePGMformatisagreyscalefileformatdesignedtobeeasytomanipulate.APGMimagerepresentsagreyscalegraphicimage.Formostpurposes,aPGMimagecanjustbethoughtofanarrayofintegers.Thename"

PGM"

istheacronymof"

PortableGreyMap."

Thename"

PPM"

istheacronymfor"

PortablePixelMap."

Imagesinthisformat（oraprecursorofit）wereoncealsocalled"

portablepixmaps."

Itisahighlyredundantformat,andcontainsalotofinformationthattheHumanVisualSystem（HVS）cannotevendiscern.However,asPGM,PPMisveryeasytowriteandanalyze.Thegoalofthefirstpartoftoday’slabistoletstudentsbecomecomfortablewiththesetwoformats.

YouwillimplementfunctionstoreadandtowritePPMandPGMimages.Thefinaldemonstrationoftheimplementedsoftwarewillbedoneusingwell-knowntestimages:

LENA,BABOON,PEPPERS,etc.YoucanfindPPMandPGMversionsoftheseimagesintheEBU723Uintranetpages.Thewritingfunctionmustaddascommentintheheader:

“imagecreatedbyyour_nameyour_surname”.

Includeinyoursubmissionthefileresultingfromreadingtheimagesprovidedandwritingthembackintheiroriginalformat.

Summarizein5pointstheoperationsnecessarytoreadaPGM/PPMimage:

1.Opentheimageinreadmode.

2.Ignorecommentsinthefile

3.DistinguishtheimageisaPGMorPPM.

4.Readinformationoftheimage.

5.Readmatrix.

Summarizein5pointstheoperationsnecessarytowriteaPGM/PPMimage:

1.Opentheimageinwritemode.

2.Writecommentsinthefile.

3.DecidewrittingaPGMorPPMimage.

4.Writeinformationoftheimage.

5.Writematrix.

WhatisthedifferencebetweentheidentifierP3andP6?

ForidentifierP3,theimageisstoredasASCIIandeachlineislessthan70characters.

ForidentifierP6,theimageisstoredasbinary.

P6formatissmallerthanP3formatandreadP6formatfileisfasterthanP3formatfile.

Exercise1（b）

Formatconversions:

inthispartofthelab,theimageswillbeconvertedfromcolourtogreyscale;

inotherwordsaPPMimagewillbeconvertedtothePGMformat.Youwillimplementafunctioncalled“BUPT_format_converter”whichtransformsimagesfromcolortogrey-scaleusingthefollowingYUVconversion:

Y=0.257R+0.504G+0.098B+16

U=0.439R+0.368G–0.071B+128

V=-0.148R–0.291G+0.439B+128

Whatcomponentrepresentstheluminance,i.e.thegrey-levels,ofanimage?

Yrepresentstheluminance.

Usetheeboxestodisplaytheresultsforthecolourtogrey-scaleconversion.

Lenacolour（RGB）

Lenagrey

Baboongrey

Babooncolour（RGB）

Isthetransformationbetweenthetwocolour-spaceslinear?

Motivateyouranswer

Thetransformationbetweenthetwocolour-spacesislinear.

Transformeachlayer,thencombineeachoutcome.OrTransformlayerstogether.Thesetwooutcomesaresame.

f（ax+by）=af（x）+bf（y）

DisplayintheboxtheLenaimageconvertedtoYUV3channelsformat.

Arethecolorsofthepreviouspicturedistorted?

Ifyeswhy?

Theyaredifferent.BecauseRGBusedforPCandYUVusedforTV.IfweshowYUVimageonPC,itwillbedistorted.

BasedontheformulafortheRGBtoYUVconversion,derivetheformulafortheYUVtoRGBconversion?

R=1.164*（Y-16）+1.596*（V-128）

G=1.164*（Y-16）-0.813*（V-128）-0.391*（U-128）

B=1.164*（Y-16）+2.018*（U-128）

UsetheformulayouderivedattheprevioussteptoconverttheYUVimagebacktotheoriginalRGBformat.Displaytheresultinthebox.

Exercise1（c）

Sub-sampling:

TheHVSisincapableofperceivingcertaindetailsinanimage.ThereforehighcompressionratioscanbeachievedbyexploitingthecharacteristicsoftheHVS,thusdiscardingwhathasalowvisualrelevance.However,thisprocesscanintroducedistortionsinthecompresseddata.AsimplewaytoexploitthecharacteristicsoftheHVScompressionpurposesistosub-sampleanimage.Adrawbackofthisapproachisthatitispossibletoincurinwell-knowproblemsofadiscreterepresentation,suchasaliasing.Thispartofthelabcoverssomesimplesub-samplingoperations.

Implementafunctionthatsub-samplesgreylevelimagesbyafactorn,withnamultipleof2.Thefunctionshouldbeabletosub-sampleindependentlyinthehorizontalandintheverticaldirectionorinbothdirectionsatthesametime.

Displaytheresultsofsu