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14HowtorecognizePDFformat?
Givenastreamofbytes,howcanItellifthisstreamcontainsaPDFdocumentorsomethingelse?
Iamusing.NETandC#butitdoesnotmatter.
14.1Answer
Italldependsofhowwell/reliableyouwantthedetectionworking.
Heremyselectionofmostimportantbits+piecesfromthe756pagelongofficialdefinition,straightfrom
thehorse’smouth(PDF32000:1-2008¹):
AbasicconformingPDFfileshallbeconstructedoffollowingfourelements(seeFigure2):
• A A one-lineheaderidentifyingtheversionofthePDF specificationtowhichthefile
conforms
• Abodycontainingtheobjectsthatmakeupthedocumentcontainedinthefile
• Across-referencetablecontaininginformationabouttheindirectobjectsinthefile
• Atrailergivingthelocationofthecross-referencetableandofcertainspecialobjects
withinthebodyofthefile
[….]
ThefirstlineofaPDFfileshallbeaheaderconsistingofthe5characters%PDF–followedby
aversionnumberoftheform1.N,whereNisadigitbetween0and7.Aconformingreader
shallacceptfileswithanyofthefollowingheaders:*
%PDF-1.0
%PDF-1.1
%PDF-1.2
%PDF-1.3
%PDF-1.4
%PDF-1.5
%PDF-1.6
%PDF-1.7
[…]
IfaPDFfilecontainsbinarydata,asmostdo(see7.2,“LexicalConventions”),theheaderline
shallbeimmediatelyfollowedbyacommentlinecontainingatleastfourbinarycharacters—
thatis,characterswhosecodesare128orgreater.Thisensuresproperbehaviouroffiletransfer
applicationsthatinspectdatanearthebeginningofafiletodeterminewhethertotreatthe
file’scontentsastextorasbinary.
¹
http://www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf#page46
51
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HowtorecognizePDFformat?
52
Trailer
[….]Thelastlineofthefileshall contain onlytheend-of-filemarker,%%EOF.Thetwo
precedinglinesshallcontain,oneperlineandinorder,thekeywordstartxrefandthebyte
offsetinthedecodedstreamfromthebeginningofthefiletothebeginningofthexrefkeyword
inthelastcross-referencesection.
Summary
Twoofthemostimportantthingstoremember:
(a)Thefirst‘headerline’
1
%PDF-1.X
[whereXin0..7]mustappearonalineofitsownbefollowedbyanewline.Thislinemustappearwithin
thefirst4096Bytes,notnecessarilyontheveryfirstline.Theprecedinglinesmaycontainnon-PDF
content,butprinterjoblanguangecommands(PJL)orcomments.
(b)TheverynextlinemustbefourbinarybytesifthePDFcontainsbinarydata.
Justparsingfor‘%PDF-1.’,relyingonthisandnotlookingforanythingelse,hasbittenalotofpeople
already….
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C# WinForms Viewer: Load, View, Convert, Annotate and Edit PDF
Draw PDF markups. PDF Protection. • Sign PDF document with signature. • Erase PDF text. • Erase PDF images. • Erase PDF pages. Miscellaneous.
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15CanIquerythedefaultsettings
Ghostscriptusesforanoutputdevice
(suchas‘pdfwrite’or‘tiffg4’)?
Inthisanswerto‘GhostscriptcommandlineparameterstoconvertEPStoPDF’;itisstatedthat
thedefaultresolutionforthe
pdfwrite
deviceofGhostscriptis720x720,whichIinitiallyfound
unbelievable!
IsthereawaytolistthedefaultoptionsofaGhostscriptdevice?
http://stackoverflow.com/a/3461186/277826
15.1Answer
SinceGhostscriptisafull-blownPostScriptinterpreter,youcanalsosendPostScriptsnippetstoitwhich
donotcausethedrawingofpageelements,butwhichqueryitforitsinternalstate.
IfyouwanttoknowwhatthedefaultsettingsoftheDisplayare,whenyouaskitvia
gs some.pdf
tojust
displayaPDFonscreen,youcouldtrythis:
Samplecommandline(Linux,Unix,MacOSX):
1
gs
\
2
-c "currentpagedevice {exch ==only ( ( ) print == } forall"
OnWindowsthisbecomes:
1
gswin32c.exe
^
2
-c "currentpagedevice e {exch h ==only y ( ) ) print == = } forall"
Theresultisalistof
/SomeName somevalue
pairswhichdescribethesettingsusedforrenderingpagesto
thecurrentscreen.
54
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CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
55
ThisissobecauseusuallythedisplayisthedefaultdeviceforGhostscripttosenditsoutputto.Nowyou
maynoticethatyou’llseeanemptyGhostscriptwindowpopup,whichyou’llhavetoclose….Ah,how
aboutaddingsomeoptionstoavoidthepopupwindow?
1
gs
\
2
-o /dev/null
\
3
-dNODISPLAY
\
4
-c "currentpagedevice e {exch h ==only y ( ) ) print == = } forall"
Or,onWindows:
1
gswin32c.exe
^
2
-o nul
^
3
-dNODISPLAY
^
4
-c "currentpagedevice e {exch h ==only y ( ) ) print == = } forall"
Butthiswillchangethequeryreturnvalues,becauseyou(unintentionally)changedtheoutputdevice
settings:
1
gs -c c "currentpagedevice e {exch h ==only y ( ) ) print == = } forall" " | | grep Resolution
Result:
1
HWResolution [86.5426483 86.5426483]
2
/.MarginsHWResolution [1152.0 0 1152.0]
Comparethisto
1
gs
\
2
-o /dev/null
\
3
-dNODISPLAY
\
4
-c "currentpagedevice {exch ==only ( ( ) print == } forall" " \
5
| grep Resolution
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CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
56
Result:
1
/HWResolution [72.0 72.0]
2
/.MarginsHWResolution [72.0 72.0]
So,pleaseavoidthistrap.Isuccessfullyfellintoitafewyearsago,anddidn’tevennoticeitforquitea
longtime…
NowassumingyouwanttoqueryforthedefaultsettingsofthePDFwritingdevice,runthisone:
1
gs
\
2
-o /dev/null
\
3
-sDEVICE=pdfwrite
\
4
-c "currentpagedevice {exch ==only ( ( ) print == } forall" " \
5
| tee e ghostscript-pdfwrite-default-pagedevice-settings.txt
You’llnowhaveallsettingsforthe
pdfwrite
deviceina*.txtfile.Andyoumayrepeatthatwithsome
otherinterestingGhostscriptdevicesandthencomparethemforalltheirdetailleddifferences:
1
for _dev in
\
2
pswrite ps2write pdfwrite
\
3
tiffg3 tiffg4 tiff12nc tiff24nc tiff32nc c tiff48nc c tiffsep p \
4
jpeg jpeggray jpegcmyk
\
5
png16 png16m png256 png48 pngalpha a pnggray y pngmono;
\
6
do
\
7
gs
\
8
-o /dev/null
\
9
-sDEVICE=${_dev}
\
10
-c "currentpagedevice {exch ==only ( ) ) print == = } } forall" " \
11
| sort
\
12
| tee e ghostscript-${_dev}-default-pagedevice-settings.txt; \
13
done
It’sratherinterestingtocomparethesettingsfor,say,the
pswrite
and
ps2write
deviceslikethis(andalso
discoverparameterswhichareavailablefortheone,butnottheotherdevice).Onemethodthatgivesyou
aquickoverviewistoapplythecommandlinetool
sdiff
tothetwotextfiles:
1
sdiff -sbB ghostscript-ps{2,}write-default-pagedevice-settings.txt
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CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
57
OnmyMacOSXsystem,thisyieldsthefollowingresult(leftcolumn:
ps2write
,rightcolumn:
pswrite
output):
1
/AllowIncrementalCFF false
<
2
/AllowPSRepeatFunctions true
<
3
/AutoFilterColorImages true
| /AutoFilterColorImages s false
4
/AutoFilterGrayImages true
| /AutoFilterGrayImages s false
5
/AutoPositionEPSFiles true
<
6
/CalCMYKProfile (None)
| /CalCMYKProfile e ()
7
/CalGrayProfile (None)
| /CalGrayProfile e ()
8
/CalRGBProfile (None)
| /CalRGBProfile e ()
9
/CannotEmbedFontPolicy /Error
| /CannotEmbedFontPolicy y /Warning
10
/CenterPages false
<
11
/ColorImageDownsampleType /Bicubic
| /ColorImageDownsampleType e /Subsample
12
/ColorImageResolution 600
| /ColorImageResolution n 150
13
/CompatibilityLevel 1.2
<
14
/CompressEntireFile false
<
15
/CompressFonts true
<
16
/CoreDistVersion 5000
<
17
/CreateJobTicket false
<
18
/DSCEncodingToUnicode []
<
19
/DetectDuplicateImages true
<
20
/DoNumCopies false
<
21
/DocumentTimeSeq 0
<
22
/DocumentUUID ()
<
23
/DownsampleColorImages true
| /DownsampleColorImages s false
24
/DownsampleGrayImages true
| /DownsampleGrayImages s false
25
/DownsampleMonoImages true
| /DownsampleMonoImages s false
26
/EmitDSCWarnings false
<
27
/EncryptionR 0
<
28
/FirstObjectNumber 1
<
29
/FitPages false
<
30
/GrayImageDownsampleType /Bicubic
| /GrayImageDownsampleType /Subsample
31
/GrayImageResolution 600
| /GrayImageResolution n 150
32
/HaveCIDSystem false
<
33
/HaveTransparency true
<
34
/HaveTrueTypes true
<
35
/HighLevelDevice true
<
36
/ImageMemory 524288
| /ImageMemory 500000
37
/InstanceUUID ()
<
38
/IsDistiller true
<
39
/KeyLength 0
<
40
> /LanguageLevel l 2.0
41
/MaxClipPathSize 12000
<
42
/MaxInlineImageSize -1
<
43
/MaxShadingBitmapSize 256000
<
44
/MaxViewerMemorySize -1
<
45
/MonoImageDownsampleThreshold 1.5
| /MonoImageDownsampleThreshold 2.0
46
/MonoImageDownsampleType /Bicubic
| /MonoImageDownsampleType /Subsample
47
/MonoImageResolution 1200
| /MonoImageResolution n 300
48
/Name (ps2write)
| /Name (pswrite)
49
/NoEncrypt ()
<
CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
58
50
/OffOptimizations 0
<
51
/Optimize true
<
52
/OutputDevice /ps2write
| /OutputDevice e /pswrite
53
/OwnerPassword ()
<
54
/PDFA false
<
55
/PDFACompatibilityPolicy 0
<
56
/PDFEndPage -1
<
57
/PDFStartPage 1
<
58
/PDFX false
<
59
/PDFXBleedBoxToTrimBoxOffset [0.0 0.0 0.0 0 0.0]<
60
/PDFXSetBleedBoxToMediaBox true
<
61
/PDFXTrimBoxToMediaBoxOffset [0.0 0.0 0.0 0 0.0]<
62
/ParseDSCComments true
<
63
/ParseDSCCommentsForDocInfo true
<
64
/PatternImagemask false
<
65
/Permissions -4
<
66
/PreserveCopyPage true
<
67
/PreserveDeviceN true
<
68
/PreserveEPSInfo true
<
69
/PreserveHalftoneInfo true
| /PreserveHalftoneInfo o false
70
/PreserveOPIComments true
| /PreserveOPIComments s false
71
/PreserveOverprintSettings true
| /PreserveOverprintSettings s false
72
/PreserveSMask false
<
73
/PreserveSeparation true
<
74
/PreserveTrMode false
<
75
/PrintStatistics false
<
76
/ProduceDSC true
<
77
/ReAssignCharacters true
<
78
/ReEncodeCharacters true
<
79
/RotatePages false
<
80
/SetPageSize false
<
81
/UCRandBGInfo /Preserve
| /UCRandBGInfo o /Remove
82
/UsePrologue false
<
83
/UserPassword ()
<
84
/WantsToUnicode false
<
85
/sRGBProfile (None)
| /sRGBProfile ()
Howtointerpretthisoutput?
• Lineswithparamkeyentriesonbothhalvesindicate:thereisthesamekeyavailableforbothoutput
devices,buteachoneusesadifferentdefaultvalue.
• Lineswithanentryforonehalfonlyindicate:thisparameterkeyisunknowntotheotheroutput
device.
Oneexampleisthe
/GrayImageResolution
key:
ps2write
hasthissetto
600
bydefaultwhereas
pswrite
uses
150
.Anotherexampleis
/LanguageLevel
:
ps2write
hassetitto
2.0
,while
pswrite
doesn’tknow
aboutthissetting.(ItproducesPostScriptlanguagelevel1only).Thethirdexampleis
/CompressFonts
:
ps2write
willcompressfontsbydefault.(Youcouldoverridethis,byspecifyingadifferentbehavioron
thecommandlineandforceuncompressedfontsinthePostScriptoutput.)
pswrite
doesnotsupportthis
settingatall.
CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
59
15.2Update
AsyoumayimaginethisisalsoagreatwaytocompaaredifferentGhostscriptversions,andtrackhow
defaultsettingsmayhavechangedfordifferentdevicesinrecentreleases.Thisisespeciallyinterestingif
youwanttofindoutaboutallthenewlyimplementedcolorprofileandICCsupportwhichisnowpresent
inGhostscript.
Also,toavoidthereturnofjust
-dict-
forcertainkeyvalues,usethe
===
insteadof
==
macro.
===
acts
like
==
butalsoprintsthecontentofdictionaries.
Sohereistheexampleoutputforthe
pdfwrite
device.Remember,Ghostscript’s
pdfwrite
deviceismeant
toprovidemostlythesamefunctionalityasAdobeAcrobatDistiller(withtheadditionalfeaturethatit
doesnotonlyacceptPostScriptasinput,butalsoPDFs,soyoucansortofredistillexistingPDFfilesin
ordertorepair,improveorotherwisemanipulatethem).Therefore,Ghostscript’s
pdfdevice
honorsmost
ofthe
setdistillerparams
operatorwhichtheoriginalDistilleralsosupports.Thisisthecommandto
use:
1
gs
\
2
-o /dev/null
\
3
-sDEVICE=pdfwrite
\
4
-c "currentpagedevice {exch ==only ( ( ) print === } forall" " \
5
| sort
Onmysystem,thisproducesthefollowingoutput.Iincludeithereinfull,becausethisbookwillalso
serveasmypersonallookupreferenceforcertaininfo–inthisisoneIdoneedquitefrequently:
1
/%MediaDestination 0
2
/%MediaSource 0
3
/.AlwaysEmbed []
4
/.HWMargins [0.0 0.0 0 0.0 0.0]
5
/.IgnoreNumCopies false
6
/.LockSafetyParams false
7
/.MarginsHWResolution [720.0 720.0]
8
/.MediaSize [612.0 0 792.0]
9
/.NeverEmbed [
\
10
/Courier /Courier-Bold d /Courier-Oblique e /Courier-BoldOblique
\
11
/Helvetica /Helvetica-Bold d /Helvetica-Oblique /Helvetica-BoldOblique \
12
/Times-Roman
\
13
/Times-Bold /Times-Italic /Times-BoldItalic
\
14
/Symbol /ZapfDingbats
\
15
]
16
/ASCII85EncodePages false
17
/AllowIncrementalCFF false
18
/AllowPSRepeatFunctions false
19
/AlwaysEmbed []
CanIquerythedefaultsettingsGhostscriptusesforanoutputdevice(suchas‘pdfwrite’or‘tiffg4’)?
60
20
/AntiAliasColorImages false
[*]
21
/AntiAliasGrayImages false
[*]
22
/AntiAliasMonoImages false
[*]
23
/AutoFilterColorImages true
24
/AutoFilterGrayImages true
25
/AutoPositionEPSFiles true
26
/AutoRotatePages /PageByPage
27
/BeginPage {--.callbeginpage--}
28
/Binding /Left
[*]
29
/BitsPerPixel 24
30
/BlueValues 256
31
/CalCMYKProfile (None)
[*]
32
/CalGrayProfile (None)
[*]
33
/CalRGBProfile (None)
[*]
34
/CannotEmbedFontPolicy /Warning
[*]
35
/CenterPages false
36
/ColorACSImageDict << /Blend 1 1 /VSamples s [2 1 1 1 2] ] /QFactor r 0.9 9 /HSamples [2 1 1 1 1 2] ] >>
37
/ColorConversionStrategy /LeaveColorUnchanged
38
/ColorImageDepth -1
39
/ColorImageDict
<< /Blend 1 1 /VSamples s [2 1 1 1 2] ] /QFactor r 0.9 9 /HSamples [2 1 1 1 1 2] ] >>
40
/ColorImageDownsampleThreshold 1.5
41
/ColorImageDownsampleType /Subsample
42
/ColorImageFilter /DCTEncode
43
/ColorImageResolution 150
44
/ColorValues 16777216
45
/Colors 3
46
/CompatibilityLevel 1.4
47
/CompressEntireFile false
48
/CompressFonts true
49
/CompressPages true
50
/ConvertCMYKImagesToRGB false
51
/ConvertImagesToIndexed true
52
/CoreDistVersion 5000
53
/CreateJobTicket false
[*]
54
/DSCEncodingToUnicode []
55
/DefaultRenderingIntent /Default
56
/DetectBlends true
[*]
57
/DetectDuplicateImages true
58
/DeviceGrayToK true
59
/DeviceLinkProfile ()
60
/DoNumCopies false
61
/DoThumbnails false
[*]
62
/DocumentTimeSeq 0
63
/DocumentUUID ()
64
/DownsampleColorImages false
65
/DownsampleGrayImages false
66
/DownsampleMonoImages false
67
/EmbedAllFonts true
68
/EmitDSCWarnings false
[*]
69
/EncodeColorImages true
70
/EncodeGrayImages true
71
/EncodeMonoImages true
72
/EncryptionR 0
73
/EndPage {--.callendpage--}
[*]
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