22.7 Colorintheprintingindustryandseparation
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Intheprevioussequenceofcommandswefirstmakeacopyoftheinputfile
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onto
save.tex
.Nextwerunthefile
pdftoscrtext.tex
through
pdflatex
,whichgeneratestheout-
put file
pdftoscrtext.pdf
,containingitssource
pdftoscrtext.tex
asanPDFobject. Now we
run
pdftosrc
whichcreates
pdftoscrtext.tex
,overwritingthe original file. Tocheckthatthe ex-
tractedsourceandtheoriginalarethesamewelistedthefilecharacteristicsbeforeandaerrunning
theprograms.Finally,wemakeacheckfordifferencesandthe
diff
utilityconfirmsthattherearenone.
Whentheobject orsourcecannotbefound
pdftosrc
issuesanerrormessage(seethemanualfor
moredetails).
22.6.6.6 Toolsforhandlingimages
pdftoppm
converts a PDF file to a series of bitmaps (one for each page). By default color portable
pixmaps (PPM)areobtained. Specifyingthe
-gray
optionwillgenerateportablegraymaps (PGM),
while with the
-mono
optionmonochrome portablebitmaps (PBM) willbe created. eresolution
canbespecifiedwiththeoption
-r number
(default150dpi).
pdftopbm
letsyoudirectlygeneratemonochromeimages.isprogramalsotakesthe
-r
option.
pdfimages
readspagesofaPDFfile,extractstherawimages(ignoringpossibleadditionaltransfor-
mationspresentinthePDF)andwritesthemascolorportablepixmaps(PPM),monochromeportable
bitmaps(PBM),or,whenthe
-j
optionisspecified,asJPEGfiles(thisonlyworksforimagesencoded
inDCTformat,theotherswillbesavedasPPMorPBM).
22.7 Color inthe printingindustryand separation
eprintingindustrygenerallyuseseithertheCMYKcolormodelforfull-colorprintingandspotcol-
orsforoneortwo-colorwork.elatterarenormallyspecifiedaccordingtotheMunsellandPantone
charts;morerecently,theFocoltoneandTrumatchsystemshavebecomecommonforcolormatching.
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22
POSTSCRIPTANDPDFTOOLS
PostScriptisalmostuniversallyusedforhighqualitytypesetoutput,andLevel2ofthePostScript
languageoffers fullsupport for color, with not onlythe simpleRGB, CMYK, and HSB models, but
alsotheCIEsystemand variousspecialcolorspaces.edetailsofthesesystems,andalgorithmsfor
convertingbetweencolormodels,arediscussedexhaustivelyin [2],pp.210–248.ereareveryuseful
discussions of colorinPostScript in[12]and [13]; [4]alsohasusefulmaterial. Ifyou readthe older
books, you shouldnotethatfullLevel2PostScript provides anumberof importantnew commands
thatconsiderablyeasepreparationofcolorseparations.
IssuesencounteredwhenworkingwithT
E
Xintheprofessionalprintingindustryarediscussedin
ausefularticlebyMichaelSoa[17].[5]alsotreatscolorseparationfromaT
E
Xperspective,while[18]
describeshowtodoseparationsatthelevelofbitmapoutputfromT
E
X.
eLAT
E
Xusercanbelargelyshieldedfromthecomplicationsoftheactualprinting.Adocument
containingcolormaterialcanbetypesetandrunthroughadriverlike
dvips
tocreateacolorPostScript
documentthatcanbepreviewedonscreenorprintedonacolorprinter.emainproblemisthatthe
colorrenderingandprecisionof thecommoninkjet printers isnotgoodenoughtopreparecopyfor
professionalprinting;however, theyareverywellsuitedtotransparencies,handouts,postersetc.,and
itiswellworththeefforttodevelopyouruseofLAT
E
Xcolorforthistypeofwork.
22.7.1 Colorseparation
Toproducea“real”bookorjournalusingoffsetprinting,theprinterwillrequirefourversionsofeach
pagethatcontain,respectively,thegraylevelscorrespondingtotheproportionsofcyan,magenta,yel-
low,andblack.Eachpageisoverprintedfourtimeswitheachofthecolors.Figure22.10,containingthe
fiveOlympicringsatthetopandamulti-coloredellipseatthebottom,showshowaddingthevarious
colorinksgivesacoloredpictureitsfinalform.Westartedbyapplyingthecyanink(tople),andthen
addedthemagenta(topright),yellow(bottomle),andfinallytheblackinks(bottomright)toobtain
thepictureinfullcolor.efourseparatestagesoftheprocess,andthecumulativeeffect,areshown.
Colorworkisusuallytypesetonspecialfilmandtohightolerances,sinceeachpageisoverprinted
fourtimesandregistrationmustbeexact.Sometypesettingsystemscanproducethefourseparations
automatically,butmorecommonlythisisdonebyPostScriptmanipulation.Professional-qualitytype-
settingpackages and sophisticated graphicsmanipulationpackages like
AdobeIllustrator
,
CorelDraw
and
AdobePhotoshop
handlecolorseparationdirectly,whileAdobe’s
Separator
programcanworkon
arbitrarycolorPostScript.eeffectivenessofprogramslikethisdependsonhowcolorisspecifiedin
thePostScriptfileproduced,andPostScriptLevel2providesagreatdealofsupportforthis.
Whenpreparingmaterialforseparation,theuserneedstounderstandsomespecificissues:
• customor“spot”colors;
• knockoutandoverprint(seeSection22.7.3);
• trapping.
PrintjobscommonlydifferentiatebetweenworkthatcanbehandledwiththeCMYKmodel(pro-
cess color)andworkspecifyingaprecisecolor(customcolor).Whileprocesscolorpagesareprinted
fourtimeswithproportionsoftheappropriatecolor,pageswithacustomcolorareprintedseparately,
withtheinkspecificallyrequested(commonlyfromstandardsetslikePantone).eLAT
E
Xcolorpack-
agedoesnotdirectlysupportspotcolor.
Let usconsiderthesimpleexampleofadrawingofa yellow circleonabluebackground(likein
Fig.22.11).It must bedecidedat somepoint whether tocreate this bygettingthe right values ofC,
M,YandK¹andthenprintingthemsuperimposedononeanother(thisiscalledoverprinting),orby
¹enormalprintingorderisinfactfirstyellow,thencyan,magenta,andfinallyblack.
182
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22.7 Colorintheprintingindustryandseparation
          CMYK separations
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
ThefourCMYKseparations,cumulatively
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
1
October4,1993
/1
Figure22.10:eseparationofcolorsintheCMYKmodel.
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22
POSTSCRIPTANDPDFTOOLS
Exa.
22-7-1
Figure22.11:Simple
PSTricks
colorexample
leavinga circular hole inthebluebackgroundwhere theyellow circlewillgo (thisis knockout). e
CMYKsystem(andthedefaultPostScriptcolormodel)assumesoverprinting(sincethewholesystem
works onanadditiveprinciple), but custom colors naturallyrequirea knockout, sincetheyspecifya
precisecolorthatshouldnotbemixedwithanythingelse.InPostScript,the
setoverprint
operator
canbeusedtoturnoverprintingonoroff;thiscouldbeusedinmoresophisticated colorhandlingin
advi-to-PostScriptdriver.
Whether we use knockout or overprinting, wewill encounter another problem, registration of
successive printings ona piece of paper. Eventhe most precise machineryhas difficultyinaligning
materialthatisspecifiedinscaledpointsataresolutionofover2500dpi.Inpractice,therefore,careful
colorworkusestrapping. Returningtotheexampleofayellowcircleonabluebackgroundwithtrap-
ping,weeitherslightlydecreasethesizeoftheholeintheblue(achoketrap)orincreasethesizeofthe
circle(aspreadtrap),andthuscreateaslightoverlaptoavoidregistrationproblems.
Itisbeyondthescopeofthisbooktodealwiththefullramificationsofhigh-qualitycolorprinting
(such as the specificationof screens), and we stronglysuggest that readers takeprofessional advice
beforeembarkingonlarge-scalefullcolorprojectswithLAT
E
X.WhileMikeSoa([17])withaprivate
dvidriver,andothersusing
Textures
onaMacintosh,haveshownthatLAT
E
Xcandofull-scalecolorwork,
userswhoneedrelativelysimplecolorprinting(suchasatwo-colorbookinwhichallheadingsareblue
andalltableshaveabluetintedbackground)candoverywellwithoutrecoursetocommercialsoware.
CMYKcolorseparationscanbemadewiththe
color
packageand
dvips
byusingonlyPostScriptLevel1
operators;wedescribethisinthefollowingsection.
22.7.2 ColorseparationusingL
A
T
E
Xanddvips
eprincipleofattainingsimple
dvips
separationsfromT
E
Xoutputisthateachoutputpageisproduced
fourtimes,usingthe
-b 4
command-lineswitch,or
b 4
inaconfigurationfile,andaheaderfilethat
184
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22.7 Colorintheprintingindustryandseparation
redefines thecoloroperatorsforeach ofthe fourpages. eheaderfile
colorsep.pro
(distributed
with
dvips
,maintainedbySebastianRahtz,andlargelyderivedfrom[12]and[4];seeSection22.7.2.1)
usesthe
bop-hook
handle(seeSection22.2.6onpage111)toincrementacounteratthebeginningof
eachpage,thuscheckingwhetheraC,M,YorKpageisbeingproduced.e
setcmykcolor
operator
isthenredefinedtoproducejustoneofthefourcolors,ingray,andRGBcolorsareconvertedtoCMYK
beforegoingthroughthesameprocess.e
setgray
operatorisactivatedonlyontheblack(K)page.
Unfortunately,colorbitmapimagesarenothandledbythissystem.
eoutputfrom theseparationfortheexampleof
Warning! issimulatedinFigure22.12;here
theboxissetin“ForestGreen”, whoseCMYKvalueis“0.9100.880.12”. Noticethat the“M”pageis
blank,asneitherthegreenboxnortheblacktextneedsanymagenta.
22.7.2.1 edvipscolorseparationheaderfile
isisthecontentsofthefile
colorsep.pro
,distributedwith
dvips
,whichredefinesPostScriptcolor
operatorstogenerateCMYKseparations.Itstartsbydefiningsomeusefulcommands:
1
%!
2
/seppages
0
def
3
userdict begin
4
/Min {% 3 items on stack, find the smallest
5
2 copy lt { pop }{ exch pop } ifelse
6
2 copy lt { pop }{ exch pop } ifelse
7
} def
Weneedtoconvertcolorspecifiedinthegray,HSBorRGBmodestoCMYKforthefourseparations¹
Grayisstraightforward;HSBissimplyavariantwayofexpressingRGB:
8
/SetGray {
9
1 exch sub systemdict begin adjustdot setgray end
10
} def
11
/sethsbcolor {systemdict begin
12
sethsbcolor currentrgbcolor end
13
userdict begin setrgbcolor end}def
RGBitselfisharder(see[2,Section7.2.3]onhowtoconvertRGBtoCMYK).Inparticular,wesubstract
theamountofeachcolorfrom1,andfindtheminimumblack:
14
/ToCMYK
15
{
16
3 { 1 exch sub 3 1 roll } repeat
17
3 copy
Min
18
blackUCR sub
19
dup 0 lt {pop 0} if
20
/percent_UCR exch def
Nowwesubtractthatblackundercolorfromeachcolorandworkouttheblackitself:
21
3 { percent_UCR sub 3 1 roll } repeat
22
percent_UCR 1.25 mul % 1 exch sub
23
} def
¹ForconversionsamongthevariousdevicecolorspacesknowntoPostScript,see[2,Section7.2].
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22
POSTSCRIPTANDPDFTOOLS
ePostScriptstackshouldnowhavetheC,M,Y,andKvalues.
e
CM
commandprintscropmarks:
24
/cX 18 def
25
/CM{gsave TR 0 cX neg moveto 0 cX lineto stroke
26
cX neg 0 moveto cX 0 lineto stroke grestore}def
ecriticalpart is thedefinitionof
bop-hook
,whichisexecuted at thestartofeach page; this goes
aroundacycleoffourpages,doingadifferentseparationoneach.Belowweassumethateachpageis
duplicatedfourtimesusingthe
-b 4
optionof
dvips
:
27
/bop-hook{cX dup TR
28
seppages 1 add
29
/seppages exch def
30
seppages 5 eq { /seppages
1
def } if
31
seppages 1 eq {
32
/ColorName (CYAN) def
33
CYAN setupcolor
34
/WhichColor 3 def } if
35
seppages 2 eq {
36
/ColorName (MAGENTA) def
37
MAGENTA setupcolor
38
/WhichColor 2 def } if
39
seppages 3 eq {
40
/ColorName (YELLOW) def
41
YELLOW setupcolor
42
/WhichColor 1 def } if
43
seppages 4 eq {
44
/ColorName (black) def
45
black setupcolor
46
/WhichColor 0 def } if
Setcropmarksoneachpagewiththeseparationnameprinted:
47
gsave .3 setlinewidth
48
3 -7 moveto
49
/Helvetica findfont 6 scalefont setfont
50
ColorName show
51
0 0 CM
52
vsize cX 2 mul sub dup hsize cX 2 mul sub dup isls{4 2 roll}if 0 CM
53
exch CM 0
54
exch CM
55
grestore 0 cX -2 mul TR isls
56
{cX -2 mul 0 TR}if
57
} def end
58
/separations 48 dict def
59
separations begin
60
/cmykprocs [
61
{ pop pop
pop SetGray
}
% cyan
62
{ pop pop exch pop SetGray
}
% magenta
63
{ pop 3 1 roll pop pop SetGray
} % yellow
64
{ 4 1 roll pop pop pop SetGray
} % black
65
] def
66
/rgbprocs [ %def
186
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22.7 Colorintheprintingindustryandseparation
67
{ ToCMYK pop pop pop SetGray }
68
{ ToCMYK pop pop exch pop SetGray }
69
{ ToCMYK pop 3 1 roll pop pop SetGray }
70
{ ToCMYK 4 1 roll pop pop pop SetGray
}
71
] def
72
/screenangles [ %def
73
105
% cyan
74
75
% magenta
75
0
% yellow
76
45
% black
77
] def
78
end
ecommand
setupcolor
takes0, 1, 2, or3asitsargument,forcyan, magenta,yellow,and black,
respectively:
79
/CYAN 0 def
/MAGENTA 1 def
80
/YELLOW 2 def
/black 3 def
81
/setupcolor{ %def
82
userdict begin
83
dup separations /cmykprocs get exch get
84
/setcmykcolor exch def
85
dup separations /rgbprocs get exch get
86
/setrgbcolor exch def
87
separations /screenangles get exch get
88
currentscreen
89
exch pop 3 -1 roll exch
90
setscreen
91
/setscreen { pop pop pop } def
Weredefine
setgray
sothatitshowsonlyontheblackseparation:
92
/setgray {
93
WhichColor 0 eq
94
{systemdict begin adjustdot setgray end}
95
{pop systemdict begin 1 setgray end}
96
ifelse}def
97
end
98
} bind def
99
/adjustdot { dup 0 eq { } { dup 1 exch sub .1 mul add} ifelse } def
Finally,wesetthepercentagesofundercolorremoval:
100
/magentaUCR .3 def
101
/yellowUCR .07 def
102
/blackUCR .4 def
ForgreatercontrolovertheseparationsGrahamFreeman’s
aurora
packagecanalsobeusedwith
dvips
oranyotherdvi-to-PostScriptprogramthat supportsthedownloadingofheaderfiles.isalso
worksbyredefiningPostScriptcolorcommands,butthepagesforeachdifferentC,M,Y,andKcolor
areproducedwithseparateheaderfiles.Onemainheaderfile(
aurora.pro
)isdownloadedtogether
with oneof
cyan.pro
,
magenta.pro
,
yellow.pro
,or
black.pro
.usthe
dvips
command line
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187
22
POSTSCRIPTANDPDFTOOLS
Cyan
Warning!
Magenta
Warning!
Yellow
Warning!
black
Warning!
Figure22.12:Exampleofsimplecolorseparations.
Magenta
Cyan
Yellow
Black
Figure22.13:Colorseparationofabitmapimageusing
aurora
.
Magenta
Cyan
Yellow
Black
Figure22.14:Detailofcolorseparationofabitmapimage.
toputthemagentaportionsofafile
text.dvi
intoaPostScriptfilecalled
text-magenta.ps
would
be
dvips text -h aurora.pro -h magenta.pro -o text-magenta.ps
Manyformsofcolorbitmap images arehandledby
aurora
,butnotthefullrangeofpossibilitiessup-
portedbythePostScriptLevel2
colorimage
command;specifically,compoundcolordoesnotwork.
Figure22.13 shows what the separations for an imagelook likeusing
aurora
.e familiar canine is
shownwithagrayshadeforthequantitiesofcyan,magenta,yelloworblack.eeffectisevenclearer
inFigure22.14,asmallportionofthesamepicture(partofthebarrelatthedog’sneck).
Acustomcolor canbeproducedbypreciselydefiningtheC, M, Y,andK componentsandpro-
vidinga matchingheader file for the color that specifies a point in CMYK space of the separation
you want. Ifthecurrent colorinthedocument fits thecolor,thentheobjectisprinted.ismethod
ofselectingcustomcolorsfor output requiresthatthenumbersspecifyingthecustomcolorprecisely
matchthecolorusedwithinthedocument. Ifthecolordoesnotmatch,nooutputisproduced,while
withthenormalsystemthecomponentofthecurrentprocesscolordeterminestheamountofoutput
toproduce.
188
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22.7 Colorintheprintingindustryandseparation
22.7.3 Overprinting
Asexplainedpreviously,color printinginvolvesgeneratingoneimage“plate”foreach“printingink”,
most oen, infour-color mode, cyan, magenta, yellow and black(CMYK). Sometimesone or more
spotcolorsareadded.
Whenprintingblackover acoloredbackground,colorseparationsowareusuallysetstheother
platestowhite.is,however,resultsinanymisregistrationontheoffsetprintergeneratingdistracting
slivers of white. With a light background this effect can be more or less ignored. If theeffect is too
visibleandithastobecorrected,onecanuseoverprinting,i.e.,oneletsthecolorcontinueunderneath
theblack.
SiepKroonenbergdevelopedthe
overprint
package(availablefrom
http://tex.aanhet.net/
overprint/
),whichsetsandunsetsoverprinting.Itiscompatiblewithboth
dvips
and
pdftex
.Asmall
examplefollows.Notethatthe
overprint
packageloadsthe
color
package.
Exa.
22-7-2
text
text
text
\usepackage{overprint}
% All colors explicitly defined as cmyk colors
\definecolor{mg}{cmyk}{0,1,0,0}
\definecolor{cn}{cmyk}{1,0,0,0}
\definecolor{bk}{cmyk}{0,0,0,1}
\color{bk}\Large
\colorbox{cn}{text}
\colorbox{mg}{\overprint text}
\colorbox{cn}{text}
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