16.5. COLORFIELDSTEREOGRAMS
259
Figure16.18:AColoreldImageofHouses
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260
CHAPTER16. RANDOMDOTSTEREOGRAMS
Figure16.19: ADepthImage
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16.5. COLORFIELDSTEREOGRAMS
261
Figure16.20:TheStereogramfromFigures16.18and16.19
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262
CHAPTER16. RANDOMDOTSTEREOGRAMS
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111112222222222222220000000000000000000000
00000000000001111111112222222222222220000000000000000000000
00000000000001111111112222222222222220000000000000000000000
00000000000000000000002222222222222220000000000000000000000
00000000000000000000002222222222222220000000000000000000000
00000000000000000000002222222222222220000000000000000000000
00000000000000000000002222222222222220000000000000000000000
00000000000000000000002222222222222220000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
Figure16.21:ACharacterDepthFile
theletters\writecode"asthespecialrepeatingpattern(justliketheboyand
houseabove). Thishasapatternlengthof9,soifprintedat10characters
perinch,itiseasytoviewandseethedepth.
Figure16.22illustratesthedierencesincoloreldstereograms.Shorten-
ingapatternissimpleandisthesameasinrandomdotstereograms. The
algorithmremovesaletterofthepattern. Thechangeiswhenlengthening
thepattern. Theinsertedelement t cannotbejustany character (inearlier
examplesanAwasinsertedintothe0though9pattern). Theinsertedel-
ementmustbefromthespecialpattern(inthiscasealetterinthepattern
writecode).
Listings16.3and16.4showsthescstereo andspstereoprograms. . These
dierfromcstereoandpstereoshownearlieronlyintheroutinesthatlengthen
the patternandmake the nalsubstitutions. . These e newprograms use a
s
lengthen
pattenandaspecial
substitutionroutinevicetheformerlengthen
pattern
andrandom
substitutionroutines.Thesenewroutinesensurethatonlyele-
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16.5. COLORFIELDSTEREOGRAMS
263
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcdewritcdewritcdodewritcdodewritcdodew
writecodewritcodewritcdewritcdewritcdodewritcdodewritcdodew
writecodewritcodewritcdewritcdewritcdodewritcdodewritcdodew
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
Figure16.22:ACharacterColoredStereogram
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264
CHAPTER16. RANDOMDOTSTEREOGRAMS
mentsfromthespecialpatternareused.
16.6 Conclusions
Thischapterhasdescribedhowstereogramswork andshownhowtomake
yourown. Therearemanywaystoexperimentwiththese.Filterthedepth
lestoblurorroundo theedgesofobjects. . Trymakingdepthleswith
dierentcommercialpackages. Usedierentpatternlengthsandoutputde-
vices. Thisisafuntopicandmakingstereogramscanbecomemoreaddic-
tivethanplayingDoom. Thecstereoprogramallowsyoutomakecharacter
stereogramsthatyoucane-mail. Sendhiddenmessages s throughthe mail,
experiment,andhavefun.
16.7 Reference
16.1\HiddenImages: MakingRandomDotStereograms,"BobHankinson,
AlfonsoHermida,QueCorporation,1994.
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Chapter17
Steganography: Hiding
Information
17.1 Introduction
Steganographyistheartofhidinginformation.Itincludestechniquestohide
animage,atextle,andevenanexecutableprograminsidea\cover"image
withoutdistortingthecoverimage.Thispaperwilldiscussthebasicideasof
steganographyandshowhowtohidetextonanimageviawatermarkingand
hideanimageinanimage.Sourcecodewillbepresentedtoimplementthese
techniques. Extensionstotheseideasarealsoavailableforthoseinterested
in augmenting the code shown. . Further r information n on steganography y is
availablein[17.1]andrelatedwebsites.
17.2 HiddenWriting
ThewordsteganographycomesfromtheGreekandliterallymeans\hidden
writing." People e have e used steganography y through the centuries to o hide
messages. Themessages s arehiddeninplainsight,becausetheyarevisible
topeoplewhoknowwheretolook.
Consider thesentence \Where realinterestingtechnical l exchanges s can
overcomedullentertainment."
Therstletterofeachwordspellsthemessage\writecode."Thisisnot
hiddenwell. Betterhidingmethods s usethesecondorthird letter of each
wordortherst letteroftherstword,secondletter of thesecondword,
265
266
CHAPTER17. STEGANOGRAPHY:HIDINGINFORMATION
etc.
Steganographyandcryptographyarecloselyrelated.Cryptographyscram-
blesamessagetoproducesomethingthatlooksscrambled.The\writecode"
examplecouldbescrambledtobe\xsjufdpef"(replaceeachletterwiththe
letterthatfollowsitinthealphabet). Thescramblesometimesencourages
pryingeyeswhoseeitasachallengetounscramble. Steganographyinstead
hides a a message in a cover r message. . The e result t looks s like e something in-
nocent,so prying eyes oftendismissit. . Lawyers s andlibertariansdebateif
steganographyiscloseenoughtocryptographytoregulateitsuse.Todate,
steganographyremainsunregulated.
17.3 Watermarking
Awatermarkaddsinformationtoadocumentorimagebyplacingalogoor
sealinplainsight. Thewatermarkprotectstheowner’srightsbyshowing
ownership. TV V broadcasters commonly y dothis by placingtheir logo in n a
cornerofthebroadcastpicture. Awatermark k canbehiddeninanimage.
Hidingthewatermarkdoesnotchangetheappearanceoftheimage. This
protectstheowner’srights,withoutdisturbingtheimage.
Figures17.1through17.4showanexampleofhidingawatermark.Figure
17.1showsaboyandFigure17.2showsawatermark.Thewatermarkiswhite
wordsonablackbackground.Itispossibletousemorecomplexwatermarks,
butwhiteonblacksimpliestheprogram.
Figure17.3istheresultoflayingthewatermarkontopoftheboyimage.
Avalueof20wasaddedtoeachpixeloftheboyimagewherethewatermark
imagewaswhite.Thisexampledidnothidethewatermark.
Figure17.4showstheresultofhidingthewatermarkontheboyimage.
Avalueof2wasaddedtoeachpixeloftheboyimagewherethewatermark
imagewaswhite.Thissmallincreaseisnotvisibletothecasualobserver.
It is simple to o recover the watermark k by y subtracting the original l boy
image(Figure17.1)fromFigure17.4.
Listing17.1showsthesourcecodethathidesawatermarkinanimage
andrecoversit. Therstpart t oflisting17.1isthehidingprogram. . After
interpretingthe command line,the code ensures the images are the same
size,andallocatestwoarraystoholdtheimages.Thehidingoperationadds
afactortotheimagewhenthewatermarkimageisnon-zero. Thelastfew
linesofcodewritetheresulttoaleandfreethememoryallocatedforthe
17.3. WATERMARKING
267
Figure17.1:TheOriginalBoyImage
Figure17.2:TheWatermarkImage
268
CHAPTER17. STEGANOGRAPHY:HIDINGINFORMATION
Figure17.3:OverlayingtheWatermarkontheBoyImage
Figure17.4:HidingtheWatermarkontheBoyImage
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