Gif89a Specification.docx

Gif89a Specification.docx

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Gif89aSpecification

CoverSheetfortheGIF89aSpecification

DEFERREDCLEARCODEINLZWCOMPRESSION

Therehasbeenconfusionaboutwhereclearcodescanbefoundinthe

datastream.Asthespecificationsays,theymayappearatanytime.There

isnotarequirementtosendaclearcodewhenthestringtableisfull.

Itistheencoder'sdecisionastowhenthetableshouldbecleared.When

thetableisfull,theencodercanchosetousethetableasis,makingno

changestoituntiltheencoderchoosestoclearit.Theencoderduring

thistimesendsoutcodesthatareofthemaximumCodeSize.

Aswecanseefromtheabove,whenthedecoder'stableisfull,itmust

notchangethetableuntilaclearcodeisreceived.TheCodeSizeisthat

ofthemaximumCodeSize.Processingotherthanthisisdonenormally.

Becauseofalargebaseofdecodersthatdonothandlethedecompressionin

thismanner,weaskdevelopersofGIFencodingsoftwaretoNOTimplement

thisfeatureuntilatleastJanuary1991andlateriftheyseethattheir

particularmarketisnotreadyforit.ThiswillgivedevelopersofGIF

decodingsoftwaretimetoimplementthisfeatureandtogetitintothe

handsoftheirclientsbeforethedecodersstart"breaking"onthenew

GIF's.Itisnotrequiredthatencoderschangetheirsoftwaretotake

advantageofthedeferredclearcode,butitisfordecoders.

APPLICATIONEXTENSIONBLOCK-APPLICATIONIDENTIFIER

TherewillbeaCourtesyDirectoryfilelocatedonCompuServeinthePICS

forum.ThisdirectorywillcontainApplicationIdentifiersforApplication

ExtensionBlocksthathavebeenusedbydevelopersofGIFapplications.

Thisfileisintendedtohelpkeepdevelopersthatwishtocreate

ApplicationExtensionBlocksfromusingthesameApplicationIdentifiers.

Thisisnotanofficialdirectory;itisforvoluntaryparticipationonly

anddoesnotguaranteethatsomeonewillnotusethesameidentifier.

E-MailcanbesenttoLarryWood（forummanagerofPICS）indicatingthe

requestforinclusioninthisfilewithanidentifier.

GRAPHICSINTERCHANGEFORMAT（sm）

Version89a

（c）1987,1988,1989,1990

Copyright

CompuServeIncorporated

Columbus,Ohio

CompuServeIncorporatedGraphicsInterchangeFormat

DocumentDate:

31July1990ProgrammingReference

TableofContents

Disclaimer.................................................................1

Foreword...................................................................1

Licensing..................................................................1

AbouttheDocument.........................................................2

GeneralDescription........................................................2

VersionNumbers............................................................2

TheEncoder................................................................3

TheDecoder................................................................3

Compliance.................................................................3

AboutRecommendations......................................................4

AboutColorTables.........................................................4

Blocks,ExtensionsandScope...............................................4

BlockSizes................................................................5

UsingGIFasanembeddedprotocol..........................................5

DataSub-blocks............................................................5

BlockTerminator...........................................................6

Header.....................................................................7

LogicalScreenDescriptor..................................................8

GlobalColorTable.........................................................10

ImageDescriptor...........................................................11

LocalColorTable..........................................................13

TableBasedImageData.....................................................14

GraphicControlExtension..................................................15

CommentExtension..........................................................17

PlainTextExtension.......................................................18

ApplicationExtension......................................................21

Trailer....................................................................23

QuickReferenceTable......................................................24

GIFGrammar................................................................25

Glossary...................................................................27

Conventions................................................................28

InterlacedImages..........................................................29

Variable-Length-CodeLZWCompression.......................................30

On-lineCapabilitiesDialogue..............................................33

1

1.Disclaimer.

Theinformationprovidedhereinissubjecttochangewithoutnotice.Inno

eventwillCompuServeIncorporatedbeliablefordamages,includinganylossof

revenue,lossofprofitsorotherincidentalorconsequentialdamagesarising

outoftheuseorinabilitytousetheinformation;CompuServeIncorporated

makesnoclaimastothesuitabilityoftheinformation.

2.Foreword.

ThisdocumentdefinestheGraphicsInterchangeFormat（sm）.Thespecification

givenheredefinesversion89a,whichisanextensionofversion87a.

TheGraphicsInterchangeFormat（sm）asspecifiedhereshouldbeconsidered

complete;anydeviationfromitshouldbeconsideredinvalid,includingbutnot

limitedto,theuseofreservedorundefinedfieldswithincontrolordata

blocks,theinclusionofextraneousdatawithinorbetweenblocks,theuseof

methodsoralgorithmsnotspecificallylistedaspartoftheformat,etc.In

general,anyandalldeviations,extensionsormodificationsnotspecifiedin

thisdocumentshouldbeconsideredtobeinviolationoftheformatandshould

beavoided.

3.Licensing.

TheGraphicsInterchangeFormat（c）isthecopyrightpropertyofCompuServe

Incorporated.OnlyCompuServeIncorporatedisauthorizedtodefine,redefine,

enhance,alter,modifyorchangeinanywaythedefinitionoftheformat.

CompuServeIncorporatedherebygrantsalimited,non-exclusive,royalty-free

licensefortheuseoftheGraphicsInterchangeFormat（sm）incomputer

software;computersoftwareutilizingGIF（sm）mustacknowledgeownershipofthe

GraphicsInterchangeFormatanditsServiceMarkbyCompuServeIncorporated,in

UserandTechnicalDocumentation.ComputersoftwareutilizingGIF,whichis

distributedormaybedistributedwithoutUserorTechnicalDocumentationmust

displaytothescreenorprinteramessageacknowledgingownershipofthe

GraphicsInterchangeFormatandtheServiceMarkbyCompuServeIncorporated;in

thiscase,theacknowledgementmaybedisplayedinanopeningscreenorleading

banner,oraclosingscreenortrailingbanner.Amessagesuchasthefollowing

maybeused:

"TheGraphicsInterchangeFormat（c）istheCopyrightpropertyof

CompuServeIncorporated.GIF（sm）isaServiceMarkpropertyof

CompuServeIncorporated."

Forfurtherinformation,pleasecontact:

CompuServeIncorporated

GraphicsTechnologyDepartment

5000ArlingtonCenterBoulevard

Columbus,Ohio43220

U.S.A.

CompuServeIncorporatedmaintainsamailinglistwithallthoseindividualsand

organizationswhowishtoreceivecopiesofthisdocumentwhenitiscorrected

2

orrevised.Thisserviceisofferedfreeofcharge;pleaseprovideuswithyour

mailingaddress.

4.AbouttheDocument.

ThisdocumentdescribesindetailthedefinitionoftheGraphicsInterchange

Format.Thisdocumentisintendedasaprogrammingreference;itis

recommendedthattheentiredocumentbereadcarefullybeforeprogramming,

becauseoftheinterdependenceofthevariousparts.Thereisanindividual

sectionforeachoftheFormatblocks.Withineachsection,thesub-section

labeledRequiredVersionreferstotheversionnumberthatanencoderwillhave

touseifthecorrespondingblockisusedintheDataStream.Withineach

section,adiagramdescribestheindividualfieldsintheblock;thediagrams

aredrawnvertically;topbytesinthediagramappearfirstintheDataStream.

Bitswithinabytearedrawnmostsignificantontheleftend.Multi-byte

numericfieldsareorderedLeastSignificantBytefirst.Numericconstantsare

representedasHexadecimalnumbers,precededby"0x".Bitfieldswithinabyte

aredescribedinorderfrommostsignificantbitstoleastsignificantbits.

5.GeneralDescription.

TheGraphicsInterchangeFormat（sm）definesaprotocolintendedfortheon-line

transmissionandinterchangeofrastergraphicdatainawaythatis

independentofthehardwareusedintheircreationordisplay.

TheGraphicsInterchangeFormatisdefinedintermsofblocksandsub-blocks

whichcontainrelevantparametersanddatausedinthereproductionofa

graphic.AGIFDataStreamisasequenceofprotocolblocksandsub-blocks

representingacollectionofgraphics.Ingeneral,thegraphicsinaData

Streamareassumedtoberelatedtosomedegree,andtosharesomecontrol

information;itisrecommendedthatencodersattempttogrouptogetherrelated

graphicsinordertominimizehardwarechangesduringprocessingandto

minimizecontrolinformationoverhead.Forthesamereason,unrelatedgraphics

orgraphicswhichrequireresettinghardwareparametersshouldbeencoded

separatelytotheextentpossible.

ADataStreammayoriginatelocally,aswhenreadfromafile,oritmay

originateremotely,aswhentransmittedoverad