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1、JPEG图像格式结构JPEGimageformatstructure2009-02-23,10:40microprocessorstoragesequencehaspositivesequence(bigendian)andreverse(littleendian)points・Positivesequencestoragemeansthatthehighbyteisstoredbeforethelowbyte,andthereverseorderisthelowbyte,intheprevioushighby
2、te.Forexample,thesixteenhexadecimalnumberisA02B,andthepositivesequeneestorageisA02B,andthereverseorderstorageis2BA0.Motorola(Motorola)microprocessorsusepositivesequencestorage,whileIntel(Intel)microprocessorsusereverseorder.ThebytesintheJPEGfilearelistedinth
3、epositivesequence・TheJPEGCommitteedefinesmanytags(marker)todefineandidentifyimagedataanditsrelatedinformationwhenformulatingJPEGstandards,buttheauthordoesnotfindtheJPEGCommittee'sexplicitdefinitionoftheformatoftheJPEGfileinterchange・UntilDecember1998,fromthe
4、analysisofspecificJPGimagesontheInternet,theJPEGFileInterchangeFormat(JPEGFile,Interchange,Format,JFIF)wasusedwidely,andtheversionnumberwas1.02.ItwasproposedinSeptember1992byEricHamilton,whoworksatC~CubeMicrosystems・Inaddition,thereareformatssuchasTIFF,JPEG,
5、butbecausethisformatiscomplex,mostapplicationssupporttheformatofJFIFfileinterchange.ThecolorspaceusedbytheJPEGfileisthecolorspacedescribedbytheCCIR601recommendation(seeChapterseventh).Inthiscolorspace,thelevelofeachcomponentandeachpixelisspecifiedaslevel255,
6、representedby8bitcode・WhenconvertingfromRGBtoYCbCrspace,usethefollowingexacttransformationrelationships:Y=256*E‘yCb二256*[E‘Cb]+128Cr二256*[E‘Cr]+128ThebrightnesslevelsE'yandthecolordifferencelevelsE'CbandE'CbaretheparametersdefinedbyCCIR601,respectively.Since
7、therangeofE'yis0~l,therangeofE'CbandE'Cbis-0・5~+0.5,sothemaximumvalueofY,CbandCrmustbeclampedto255.Thus,thetransformationrelationshipbetweenRGBandYCbCrneedstobecalculatedinthefollowingway.(1)convertfromRGBtoYCbCrTheYCbCr(class256)componentcanbecalculateddire
8、ctlyfromtheRGBcomponentrepresentedby8bits:Y二0.299R+0.587G+0.114BCb二-0.1687R-0.3313G+0.5B+128Cr=0.5R-0.4187G一0.0813B+128ItshouldbenotedthatnotallimagefileformatsareinR0,GO,B0,TheorderofRn,Gn,andB
