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14. APPENDIX:COLORTUTORIAL
71
Device-dependentcolor
DisplaythesameRGBdataonthreedifferentmonitors,sidebyside,andyouwillgetanoticeablydifferent
colorbalanceoneachdisplay. Thisisbecauseeachmonitoremitsaslightlydifferentshadeandintensity
ofred,green,andbluelight. RGBisanexampleofadevice-dependentcolormodel—thecoloryouget
dependsonthedevice. Thisalsomeansthataparticularcolor—representedassayRGB87,146,116on
onemonitor—mighthavetobespecifiedasRGB98,123,104onanothertoproducethesamecolor.
Device-independentcolor
Afullphysicaldescriptionofacolorwouldrequirespecifyingtheexactspectralpowerdistributionofthe
lightsource. Fortunately,thehumaneyeandbrainarenotsosensitiveastorequireexactreproductionof
aspectrum.Mathematical,device-independentcolormodelsexistthatdescribefairlywellhowaparticular
colorwillbeseenbyhumans.Themostimportantdevice-independentcolormodel,towhichallotherscan
berelated,wasdevelopedbytheInternationalLightingCommittee(CIE,inFrench)andiscalledXYZ.
InXYZ,Xisthesumofaweightedpowerdistributionoverthewholevisiblespectrum.SoareYandZ,each
withdifferentweights.Thusanyarbitraryspectralpowerdistributioniscondenseddowntojustthreefloating
pointnumbers.Theweightswerederivedfromcolormatchingexperimentsdoneonhumansubjectsinthe
1920s.CIEXYZhasbeenanInternationalStandardsince1931,andithasanumberofusefulproperties:
twocolorswiththesameXYZvalueswilllookthesametohumans
twocolorswithdifferentXYZvalueswillnotlookthesame
theYvaluerepresentsallthebrightnessinformation(luminance)
theXYZcolorofanyobjectcanbeobjectivelymeasured
ColormodelsbasedonXYZhavebeenusedformanyyearsbypeoplewhoneedaccuratecontrolofcolor
—lightingengineersforfilmandTV,paintanddyestuffsmanufacturers,andsoon. Theyarethusproven
inindustrialuse.Accurate,device-independentcolorstartedtospreadfromhigh-end,specializedareasinto
themainstreamduringthelate1980sandearly1990s,andPNGtakesnoticeofthattrend.
Calibrated,device-dependentcolor
Traditionally,imagefileformatshaveuseduncalibrated,device-dependentcolor. Iftheprecisedetailsof
theoriginaldisplaydeviceareknown,itbecomespossibletoconvertthedevice-dependentcolorsofapar-
ticularimagetodevice-independentones. Makingsimplifyingassumptions,suchasworkingwithCRTs
(whicharemucheasierthanprinters),allweneedtoknowaretheXYZvaluesofeachprimarycolorandthe
CRT_gamma.
SowhydoesPNGnotstoreimagesinXYZinsteadofRGB? Well,tworeasons. First,storingimagesin
XYZwouldrequiremorebitsofprecision,whichwouldmakethefilesbigger.Second,allprogramswould
havetoconverttheimagedatabeforeviewingit. Whethercalibratedornot,allvariantsofRGBareclose
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PNG(PORTABLENETWORKGRAPHICS)SPECIFICATION
enoughthatundemandingviewerscangetbywithsimplydisplayingthedatawithoutcolorcorrection. By
storingcalibratedRGB,PNGretainscompatibilitywithexistingprogramsthatexpectRGBdata,yetprovides
enoughinformationforconversiontoXYZinapplicationsthatneedprecisecolors.Thus,wegetthebestof
bothworlds.
Whatarechromaticityandluminance?
Chromaticityisanobjectivemeasurementofthecolorofanobject,leavingasidethebrightnessinformation.
Chromaticityusestwoparametersxandy,whicharereadilycalculatedfromXYZ:
x
= X X / / (X + + Y Y + + Z)
y
= Y Y / (X X + + Y Y + + Z)
XYZcolorshavingthesamechromaticityvalueswillappeartohavethesamehuebutcanvaryinabsolute
brightness.Noticethatx,yaredimensionlessratios,sotheyhavethesamevaluesnomatterwhatunitswe’ve
usedforX,Y,Z.
TheYvalueofanXYZcolorisdirectlyproportionaltoitsabsolutebrightnessandiscalledtheluminanceof
thecolor.WecandescribeacoloreitherbyXYZcoordinatesorbychromaticityx,yplusluminanceY.The
XYZformhastheadvantagethatitislinearlyrelatedto(linear,gamma=1.0)RGBcolorspaces.
Howarecomputermonitorcolorsdescribed?
The“whitepoint”ofamonitoristhechromaticityx,yofthemonitor’snominalwhite,thatis,thecolorpro-
ducedwhenR=G=B=maximum.
It’scustomarytospecifymonitorcolorsbygivingthechromaticitiesoftheindividualphosphorsR,G,and
B,plusthewhitepoint.Thewhitepointallowsonetoinfertherelativebrightnessesofthethreephosphors,
whichisn’tdeterminedbytheirchromaticitiesalone.
Notethattheabsolutebrightnessofthemonitorisnotspecified.Forcomputergraphicswork,wegenerally
don’tcareverymuchaboutabsolutebrightnesslevels.InsteadofdealingwithabsoluteXYZvalues(inwhich
X,Y,Zareexpressedinphysicalunitsofradiatedpower,suchascandelaspersquaremeter),itisconvenient
toworkin“relativeXYZ”units,wherethemonitor’snominalwhiteistakentohavealuminance(Y)of1.0.
Giventhisassumption,it’ssimpletocomputeXYZcoordinatesforthemonitor’swhite,red,green,andblue
fromtheirchromaticityvalues.
WhydoescHRMusex,yratherthanXYZ?Simplybecausethatishowmanufacturersprinttheinformation
intheirspecsheets!Usually,thefirstthingaprogramwilldoisconvertthecHRMchromaticitiesintorelative
XYZspace.
WhatcanIdowithit?
IfaPNGfilehasthegAMAandcHRMchunks,thesource_RGBvaluescanbeconvertedtoXYZ.Thislets
you:
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14. APPENDIX:COLORTUTORIAL
73
doaccurategrayscaleconversion(justusetheYcomponent)
converttoRGBforyourownmonitor(toseetheoriginalcolors)
printtheimageinLevel2PostScriptwithbettercolorfidelitythanasimpleRGBtoCMYKconversion
couldprovide
calculateanoptimalcolorpalette
passtheimagedatatoacolormanagementsystem
etc.
HowdoIconvertfromsource_RGBtoXYZ?
Makeafewsimplifyingassumptionsfirst,likethemonitorreallyisjetblackwithnoinputandthegunsdon’t
interferewithoneanother. Then,giventhatyouknowtheCIEXYZvaluesforeachofred,green,andblue
foraparticularmonitor,youputthemintoamatrixm:
Xr Xg Xb
m =
Yr Yg Yb
Zr Zg Zb
HereweassumeweareworkingwithlinearRGBfloatingpointdataintherange0..1.Ifthegammaisnot
1.0,makeitsoonthefloatingpointdata.Thenconvertsource_RGBtoXYZbymatrixmultiplication:
X
R
Y = = m m G
Z
B
Inotherwords,X = = Xr*R R + + Xg*G + Xb*B,andsimilarlyforYandZ.Youcangotheotherwaytoo:
R
X
G = = im Y
B
Z
whereimistheinverseofthematrixm.
Whatisagamut?
Thegamutofadeviceisthesubsetofvisiblecolorswhichthatdevicecandisplay. (Ithasnothingtodo
withgamma.)ThegamutofanRGBdevicecanbevisualizedasapolyhedroninXYZspace;thevertices
correspondtothedevice’sblack,blue,red,green,magenta,cyan,yellowandwhite.
Differentdeviceshavedifferentgamuts,inotherwordsonedevicewillbeabletodisplaycertaincolors(usu-
allyhighlysaturatedones)thatanotherdevicecannot. ThegamutofaparticularRGBdevicecanbedeter-
minedfromitsR,G,andBchromaticitiesandwhitepoint(thesamevaluesgiveninthecHRMchunk).The
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74
PNG(PORTABLENETWORKGRAPHICS)SPECIFICATION
gamutofacolorprinterismorecomplexandcanonlybedeterminedbymeasurement. However,printer
gamutsaretypicallysmallerthanmonitorgamuts,meaningthattherecanbemanycolorsinadisplayable
imagethatcannotphysicallybeprinted.
Convertingimagedatafromonedevicetoanothergenerallyresultsingamutmismatches—colorsthatcan-
notberepresentedexactlyonthedestinationdevice. Theprocessofmakingthecolorsfit,whichcanrange
fromasimplecliptoelaboratenonlinearscalingtransformations,istermedgamutmapping. Theaimisto
produceareasonablevisualrepresentationoftheoriginalimage.
Furtherreading
References[COLOR-1]through[COLOR-5]providemoredetailaboutcolortheory.
15 Appendix:SampleCRCCode
ThefollowingsamplecoderepresentsapracticalimplementationoftheCRC(CyclicRedundancyCheck)
employedinPNGchunks.(SeealsoISO3309[ISO-3309]orITU-TV.42[ITU-V42]foraformalspecifica-
tion.)
ThesamplecodeisintheANSICprogramminglanguage.NonCusersmayfinditeasiertoreadwiththese
hints:
&
BitwiseANDoperator.
ˆ
Bitwiseexclusive-ORoperator.(Caution:elsewhereinthisdocument,ˆrepresentsexponentiation.)
Bitwiserightshiftoperator.Whenappliedtoanunsignedquantity,ashere,rightshiftinsertszeroesat
theleft.
!
LogicalNOToperator.
++
“n++”incrementsthevariablen.
0xNNN
0xintroducesahexadecimal(base16)constant.SuffixLindicatesalongvalue(atleast32bits).
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15. APPENDIX:SAMPLECRCCODE
75
/* Table of CRCs of all 8-bit t messages. . */
unsigned long crc_table[256];
/* Flag: has the table been n computed? ? Initially false. */
int crc_table_computed = = 0;
/* Make the table for a fast t CRC. . */
void make_crc_table(void)
{
unsigned long c;
int n, k;
for (n = 0; n < 256; n++) ) {
c = (unsigned long) n;
for (k = 0; k < 8; k++) ) {
if (c & 1)
c = 0xedb88320L ˆ ˆ (c c » 1);
else
c = c » 1;
}
crc_table[n] = c;
}
crc_table_computed = 1;
}
/* Update a running CRC with h the e bytes buf[0..len-1]-the e CRC
should be initialized d to o all 1’s, and the transmitted d value
is the 1’s complement t of f the final running CRC (see the
crc() routine below)). . */
unsigned long update_crc(unsigned d long g crc, unsigned char r *buf,
int len)
{
unsigned long c = crc;
int n;
if (!crc_table_computed)
make_crc_table();
for (n = 0; n < len; n++) ) {
c = crc_table[(c ˆ buf[n]) ) & & 0xff] ˆ (c » 8);
}
return c;
}
/* Return the CRC of the e bytes s buf[0..len-1]. */
unsigned long crc(unsigned d char r *buf, int len)
{
return update_crc(0xffffffffL, , buf, , len) ˆ 0xffffffffL;
}
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76
PNG(PORTABLENETWORKGRAPHICS)SPECIFICATION
16 Appendix:OnlineResources
(ThisappendixisnotpartoftheformalPNGspecification.)
ThisappendixgivesthelocationsofsomeInternetresourcesforPNGsoftwaredevelopers.Bythenatureof
theInternet,thelistisincompleteandsubjecttochange.
Archivesites
ThelatestreleasedversionsofthisdocumentandrelatedinformationcanalwaysbefoundatthePNGFTP
archivesite,ftp://ftp.uu.net/graphics/png/.ThePNGspecificationisavailableinseveralfor-
mats,includingHTML,plaintext,andPostScript.
Referenceimplementationandtestimages
A reference implementation in portable C is available from m the e PNG G FTP archive e site,
ftp://ftp.uu.net/graphics/png/src/. The e reference e implementation is s freely y usable in
allapplications,includingcommercialapplications.
Testimagesareavailablefromftp://ftp.uu.net/graphics/png/images/.
Electronicmail
ThemaintainersofthePNGspecificationcanbecontactedbye-mailatpng-info@uunet.uu.netor
atpng-group@w3.org.
PNGhomepage
ThereisaWorldWideWebhomepageforPNGathttp://quest.jpl.nasa.gov/PNG/.Thispage
isacentrallocationforcurrentinformationaboutPNGandPNG-relatedtools.
17 Appendix:RevisionHistory
(ThisappendixisnotpartoftheformalPNGspecification.)
ThePNGformathasbeenfrozensincetheNinthDraftofMarch7,1995,andallfuturechangesareintended
tobebackwardscompatible.TherevisionssincetheNinthDraftaresimplyclarifications,improvementsin
presentation,andadditionsofsupportingmaterial.
On1October1996,thePNGspecificationwasapprovedasaW3C(WorldWideWebConsortium)Recom-
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18. REFERENCES
77
Atthattime,itwasawaitingpublicationasanInformationalRFC.
ChangessincetheTenthDraftof5May,1995
Clarifiedmeaningofasuggested-palettePLTEchunkinatruecolorimagethatusestransparency
ClarifiedexactsemanticsofsBITandallowedsampledepthscalingprocedures
ClarifiedstatusofspacesintEXtchunkkeywords
Distinguishedprivateandpublicextensionvaluesintypeandmethodfields
Addeda“CreationTime”tEXtkeyword
MacintoshrepresentationofPNGspecified
Addeddiscussionofsecurityissues
Addedmoreextensivediscussionofgammaandchromaticityhandling,includingtutorialappendixes
Clarifiedterminology,notablysampledepthvs.bitdepth
Addedaglossary
Editingandreformatting
18 References
[COLOR-1]
Hall,Roy,IlluminationandColorinComputerGeneratedImagery.Springer-Verlag,NewYork,1989.
ISBN0-387-96774-5.
[COLOR-2]
Kasson,J.,andW.Plouffe,“AnAnalysisofSelectedComputerInterchangeColorSpaces”,ACM
TransactionsonGraphics,vol11no4(1992),pp373-405.
[COLOR-3]
Lilley,C.,F.Lin,W.T.Hewitt,andT.L.J.Howard,ColourinComputerGraphics. CVCP,Sheffield,
1993.ISBN1-85889-022-5.
Alsoavailablefrom
URL:http://info.mcc.ac.uk/CGU/ITTI/Col/colour_announce.html
[COLOR-4]
Stone,M.C.,W.B.Cowan,andJ.C.Beatty,“Colorgamutmappingandtheprintingofdigitalimages”,
ACMTransactionsonGraphics,vol7no3(1988),pp249-292.
78
PNG(PORTABLENETWORKGRAPHICS)SPECIFICATION
[COLOR-5]
Travis,David, EffectiveColorDisplays—TheoryandPractice. AcademicPress, , London,1991.
ISBN0-12-697690-2.
[GAMMA-FAQ]
Poynton,C.,“GammaFAQ”.
URL:http://www.inforamp.net/%7Epoynton/Poynton-colour.html
[ISO-3309]
InternationalOrganizationforStandardization,“InformationProcessingSystems—DataCommuni-
cationHigh-LevelDataLinkControlProcedure—FrameStructure”,IS3309,October1984,3rdEdi-
tion.
[ISO-8859]
InternationalOrganizationforStandardization,“InformationProcessing—8-bitSingle-ByteCoded
GraphicCharacterSets—Part1:LatinAlphabetNo.1”,IS8859-1,1987.
Alsoseesamplefilesat
ftp://ftp.uu.net/graphics/png/documents/iso_8859-1.*
[ITU-BT709]
InternationalTelecommunicationsUnion,“BasicParameterValuesfortheHDTVStandardfortheStu-
dioandforInternationalProgrammeExchange”,ITU-RRecommendationBT.709(formerlyCCIR
Rec.709),1990.
[ITU-V42]
International Telecommunications Union, , “Error-correcting g Procedures for r DCEs s Using
Asynchronous-to-SynchronousConversion”,ITU-TRecommendationV.42,1994,Rev.1.
[PAETH]
Paeth,A.W.,“ImageFileCompressionMadeEasy”,inGraphicsGemsII,JamesArvo,editor. Aca-
demicPress,SanDiego,1991.ISBN0-12-064480-0.
[POSTSCRIPT]
AdobeSystemsIncorporated,PostScriptLanguageReferenceManual,2ndedition.Addison-Wesley,
Reading,1990.ISBN0-201-18127-4.
[PNG-EXTENSIONS]
PNGGroup,“PNGSpecial-PurposePublicChunks”.Availableinseveralformatsfrom
ftp://ftp.uu.net/graphics/png/documents/pngextensions.*
[RFC-1123]
Braden,R.,Editor,“RequirementsforInternetHosts—ApplicationandSupport”,STD3,RFC1123,
USC/InformationSciencesInstitute,October1989.
URL:ftp://ds.internic.net/rfc/rfc1123.txt
19. CREDITS
79
[RFC-1521]
Borenstein,N.,andN.Freed,“MIME(MultipurposeInternetMailExtensions)PartOne:Mechanisms
forSpecifyingandDescribingtheFormatofInternetMessageBodies”,RFC1521,Bellcore,Innosoft,
September1993.
URL:ftp://ds.internic.net/rfc/rfc1521.txt
[RFC-1590]
Postel, J., “MediaType RegistrationProcedure”, , RFC1590, , USC/InformationSciences Institute,
March1994.
URL:ftp://ds.internic.net/rfc/rfc1590.txt
[RFC-1950]
Deutsch,P.andJ-L.Gailly,“ZLIBCompressedDataFormatSpecificationversion3.3”,RFC1950,
AladdinEnterprises,May1996.
URL:ftp://ds.internic.net/rfc/rfc1950.txt
[RFC-1951]
Deutsch,P.,“DEFLATECompressedDataFormatSpecificationversion1.3”,RFC1951,AladdinEn-
terprises,May1996.
URL:ftp://ds.internic.net/rfc/rfc1951.txt
[SMPTE-170M]
SocietyofMotionPictureandTelevisionEngineers,“Television—CompositeAnalogVideoSignal
—NTSCforStudioApplications”,SMPTE-170M,1994.
19 Credits
Editor
ThomasBoutell,boutell@boutell.com
ContributingEditor
TomLane,tgl@sss.pgh.pa.us
Authors
Authors’namesarepresentedinalphabeticalorder.
MarkAdler,madler@alumni.caltech.edu
80
PNG(PORTABLENETWORKGRAPHICS)SPECIFICATION
ThomasBoutell,boutell@boutell.com
ChristianBrunschen,cb@df.lth.se
AdamM.Costello,amc@cs.berkeley.edu
LeeDanielCrocker,lee@piclab.com
AndreasDilger,adilger@enel.ucalgary.ca
OliverFromme,fromme@rz.tu-clausthal.de
Jean-loupGailly,gzip@prep.ai.mit.edu
ChrisHerborth,chrish@qnx.com
AlexJakulin,Aleks.Jakulin@snet.fri.uni-lj.si
NealKettler,kettler@cs.colostate.edu
TomLane,tgl@sss.pgh.pa.us
AlexanderLehmann,alex@hal.rhein-main.de
ChrisLilley,chris@w3.org
DaveMartindale,davem@cs.ubc.ca
OwenMortensen,104707.650@compuserve.com
KeithS.Pickens,ksp@swri.edu
RobertP.Poole,lionboy@primenet.com
GlennRanders-Pehrson,glennrp@arl.milorrandeg@alumni.rpi.edu
GregRoelofs,newt@pobox.com
WillemvanSchaik,willem@gintic.gov.sg
GuySchalnat
PaulSchmidt,pschmidt@photodex.com
TimWegner,71320.675@compuserve.com
JeremyWohl,jeremyw@anders.com
TheauthorswishtoacknowledgethecontributionsofthePortableNetworkGraphicsmailinglist,thereaders
ofcomp.graphics,andthemembersoftheWorldWideWebConsortium(W3C).
TheAdam7interlacingschemeisnotpatentedanditisnottheintentionoftheoriginatorofthatschemeto
patentit. TheschememaybefreelyusedbyallPNGimplementations. Thename“Adam7”maybefreely
usedtodescribeinterlacemethod1ofthePNGspecification.
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