DIBs and Their Use.docx

DIBs and Their Use.docx

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DIBsandTheirUse

DIBsandTheirUse

RonGery

MicrosoftDeveloperNetworkTechnologyGroup

March20,1992

Clicktovieworcopythesampleapplicationfilesforthistechnicalarticle.

Abstract

ThisarticlediscussestheDIB（device-independentbitmap）conceptfromdefinitionandstructuretotheAPIthatusesit.IncludedisasmallsampleapplicationthatillustratessomeofthemostcommonmethodsofusingDIBstodisplayandmanipulatedigitalimages.Functionsdiscussedare GetDIBits, SetDIBits, CreateDIBitmap, SetDIBitsToDevice, StretchDIBits,and CreateDIBPatternBrush.ThisarticledoesnotdiscussusingpaletteswithDIBs.

Overview

ADIB（device-independentbitmap）isaformatusedtodefinedevice-independentbitmapsinvariouscolorresolutions.ThemainpurposeofDIBsistoallowbitmapstobemovedfromonedevicetoanother（hence,thedevice-independentpartofthename）.ADIBisan external format,incontrasttoadevice-dependentbitmap,whichappearsinthesystemasabitmapobject（createdbyanapplicationusing CreateBitmap, CreateCompatibleBitmap, CreateBitmapIndirect,orCreateDIBitmap）.ADIBisnormallytransportedinmetafiles（usuallyusingthe StretchDIBits function）,BMPfiles,andtheClipboard（CF_DIBdataformat）.

ADIBconsistsoftwoparts:

thebitsthemselvesandaheaderthatdescribestheformatofthebits.Theheadercontainsthecolorformat,acolortable,andthesizeofthebitmap.ThecurrentDIBformatsupportsfourcolorresolutions:

1bit,4bit,8bit,and24bit.In1-bit,4-bit,and8-bitDIBs,thepixelsaredefinedbyindexes（oftheappropriatebitresolution）intothecolortable;24-bitpixelsaredescribedas24-bitvalues,1byteeachforred,green,andblue.

TheDIBfunctionsare:

GetDIBits

Translatesadevice-dependentbitmapintotheDIBformat

SetDIBits

TranslatesaDIB'sinformationintodevice-dependentform

CreateDIBitmap

Createsadevice-dependentbitmapinitializedwithDIBinformation

SetDIBitsToDevice

SetsaDIBdirectlytotheoutputsurface

StretchDIBits

MovesarectanglefromtheDIBtoarectangleonthedestinationsurface,stretchingorcompressingasnecessary

CreateDIBPatternBrush

CreatesapatternbrushusingaDIBforthebitmapdescription

DeviceIndependence—What'sItGoodFor?

TransferringcolorbitmapsfromonedevicetoanotherwasnotpossibleinversionsoftheMicrosoft®Windows™graphicalenvironmentearlierthan3.0.WithDIBs,eachdevicedisplaystheimagetotheabilityofitscolorresolution.AnapplicationcanstoreanimageintheDIBformatandthendisplayit,regardlessoftheoutputdevice;anapplicationneednolongercreateaversionofeachimageforeachtypeofdevice.

Thisimagetransferabilitycanbeusedtoprinthalftoneimages.Forexample,the StretchDIBits functioncanpassaDIBdirectlytoanintelligentprinterdriver.Giventhefullcolorinformationoftheimageinsteadofsimplyamonochromeversion（thetraditionalmethod）,thedrivercanusehalftonestoprintarealisticpicture.

BecausetheDIBformatispubliclydefined,anapplicationcanmanipulateitonthefly.Infact,anapplicationcanbuildanimagewithoutanyinteractionwithWindows.IfWindowslacksadrawingprimitive,theapplicationcansimulateitdirectlyintotheDIBinsteadofusingtheexistinggraphicsdeviceinterface（GDI）primitives.Unfortunately,underWindowsversions3.0and3.1,GDIcannotperformoutputoperationsdirectlytoaDIB.

BMPFileFormats

ThefileextensionofaWindowsDIBfileisBMP.Thefileconsistsofa BITMAPFILEHEADER structurefollowedbytheDIBitself.Unfortunately,becausethe BITMAPFILEHEADER structureisneveractuallypassedtotheAPI,noteveryapplicationthatgeneratesBMPfilesfillsoutthedatastructurecarefully.Toaddtothisconfusion,the"proper"definitionofthestructureisatoddswiththedocumentation.Properly,thedatastructurecontainsthefollowingfields:

bfType

A WORD thatdefinesthetypeoffile.Itmustbe'BM'.

bfSize

A DWORD thatspecifiesthesizeofthefileinbytes.TheMicrosoftWindowsSoftwareDevelopmentKit（SDK）documentationclaimsotherwise.Tobeonthesafeside,manyapplicationscalculatetheirownsizesforreadinginafile.

bfReserved1,bfReserved2

WORDsthatmustbesetto0.

bfOffBits

A DWORD thatspecifiestheoffsetfromthebeginningofthe BITMAPFILEHEADER structuretothestartoftheactualbits.TheDIBheaderimmediatelyfollowsthefileheader,buttheactualimagebitsneednotbeplacednexttotheheadersinthefile.

TheDIBheaderimmediatelyfollowsthe BITMAPFILEHEADER structure.

ForacodesamplethatreadsaBMPfile,seethesampleprogram.

TheDIBHeader

Theheaderactuallyconsistsoftwoadjoiningparts:

theheaderproperandthecolortable.Botharecombinedinthe BITMAPINFO structure,whichiswhatallDIBAPIsexpect.

Windowssupportstwovarietiesofheaders:

 BITMAPINFOHEADER and BITMAPCOREHEADER.Ifatallpossible,applicationsshoulduseonly BITMAPINFOHEADERs.The BITMAPCOREHEADER definitionisbasedonthebitmapdefinitionfromPresentationManager™version1.1andissupportedforcompatibility.

DuringaDIBsettingoperation,mostfieldsarealreadyfilledinbywhoevergeneratedtheDIB.Doinga GetDIBits call,however,providesmorecontrol.ThewaytheheaderisfilledinforthisoperationdefinestheresultingDIB,particularlyitscolorresolution.

BITMAPINFOHEADER containsthefollowingfields:

biSize

Shouldbesetto sizeof（BITMAPINFOHEADER）.Thisfielddefinesthesizeoftheheader（minusthecolortable）.IfanewDIBdefinitionisadded,itisidentifiedbyanewvalueforthesize.Thisfieldisalsoconvenientforcalculatingapointertothecolortable,whichimmediatelyfollowsthe BITMAPINFOHEADER.

biWidth,biHeight

Definethewidthandtheheightofthebitmapinpixels.Theyare DWORD valuesforfutureexpansion,andthecodeinWindowsversions3.0and3.1ignoresthehighword（whichshouldbesetto0）.

biPlanes

Shouldalwaysbe1.AllDIBdefinitionsrelyon biBitCount fordefiningthecolorresolution.

biBitCount

Definesthecolorresolution（inbitsperpixel）oftheDIB.Onlyfourvaluesarevalidforthisfield:

1,4,8,and24.Newresolutions（16bit,forexample）maybeaddedinthefuture,butfornowonlythesefourdefineavalidDIB.Choosingtheappropriatevaluewhendoinga GetDIBits isdiscussedbelow.Whenperforminga Set operation,thevalueshouldalreadybedefinedforthebits.

biCompression

Specifiesthetypeofcompression.Canbeoneofthreevalues:

BI_RGB,BI_RLE4,orBI_RLE8.Themostcommonandusefulchoice,BI_RGB,definesaDIBinwhichallisasitseems.Eachblockof biBitCount bitsdefinesanindex（orRGBvaluefor24-bitversions）intothecolortable.TheothertwooptionsspecifythattheDIBisstored（orwillbestored）usingeitherthe4-bitorthe8-bitrunlengthencoding（RLE）schemethatWindowssupports.TheRLEformatsareespeciallyusefulforanimationapplicationsandalsousuallycompressthebitmap.BI_RGBformatisrecommendedforalmostallpurposes.RLEversions,althoughpossiblysmaller,areslowertodecode,notaswidelysupported,andextremelypainfultobandproperly.

biSizeImage

Containsthesizeofthebitmapproperinbytesorthevalue0.Avalueof0indicatesthattheDIBisofdefaultsize.Calculatingthesizeofabitmapisnotdifficult:

biSizeImage=（（（（biWidth*biBitCount）+31）&

~31）>>3）*biHeight:

Thecrazyroundoffsandshiftsaccountforthebitmapbeing DWORD-alignedattheendofeveryscanline.Whennonzero,thisfieldtellsanapplicationhowmuchstoragespacetheDIB'sbitsneed.The biSizeImagefieldreallybecomesusefulwhendealingwithanRLEbitmap,thesizeofwhichdependsonhowwellthebitmapwasencoded.IfanRLEbitmapistobepassedaround,the biSizeImage fieldismandatory.

biXPelsPerMeter,biYPelsPerMeter

Defineapplication-specifiedvaluesforthedesirabledimensionsofthebitmap.Thisinformationcanbeusedtomaintainthephysicaldimensionsofanimageacrossdevicesofdifferentresolutions.GDInevertouchesthesefields.Whennotfilledin,theyshouldbothbesetto0.

biClrUsed

Providesawayforgettingsmallercolortables.Whenthisfieldissetto0,thenumberofcolorsinthecolortableisbasedonthe biBitCount field（1indicates2colors,4indicates16,8indicates256,and24indicatesnocolortable）.Anonzerovaluespecifiestheexactnumberofcolorsinthetable.So,forexample,ifan8-bitDIBusesonly17colors,thenonlythose17colorsneedtobedefinedinthetable,and biClrUsed issetto17.Ofcourse,nopixelcanhaveanindexpointingpasttheendofthetable.

Note:

Thisfieldcannotbeusedduringa GetDIBits operation.GDIalwaysfillsafull-sizecolortable.Thefieldisthereforemoreusefulforpost-processingoperations,whenanapplicationtrimsdownthecontentsoftheDIB.Ifnonzerofora24-bitDIB,itindicatestheexistenceofacolortablethattheapplicationcanuseforcolorreference.

biClrImportant

Specifiesthatthefirst x colorsofthecolortableareimportanttotheDIB.Iftherestofthecolorsarenotavailable,theimagestillretainsitsmeaninginanacceptablemanner.biClrImportantispurelyforapplicationuse;GDIdoesnottouchthisvalue.Whenthisfieldissetto0,allthecolorsareimportant,or,rather,theirrelativeimportancehasnotbeencomputed.

Thecolortableimmediatelyfollowstheheaderinformation.Nocolortableisdefinedfor24-bitDIBs.Thetableconsistsofanarrayof RGBQUAD datastructures.（Thetableforthe BITMAPCOREINFO formatisbuiltwiththe RGBTRIPLEdata structure.）Red,green,andbluebytesare inreverseorder （redswapspositionwithblue）fromtheWindowsconventio