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4HowcanIunittestaPythonfunctionthat
drawsPDFgraphics?
I’mwritingaCADapplicationthatoutputsPDFfilesusingtheCairographicslibrary.Alotoftheunit
testingdoesnotrequireactuallygeneratingthePDFfiles,suchascomputingtheexpectedbounding
boxesoftheobjects.However,IwanttomakesurethatthegeneratedPDFfiles“look”correctafterI
changethecode.
Isthereanautomatedwaytodothis?HowcanIautomateasmuchaspossible?DoIneedtovisually
inspecteachgeneratedPDF?HowcanIsolvethisproblemwithoutpullingmyhairout?
4.1Answer
I’mdoingthesamethingusingashellscriptonLinuxthatwraps
1. ImageMagick’s
compare
command
2. the
pdftk
utility
3. Ghostscript(optionally)
(Itwouldberathereasytoportthistoa
.bat
BatchfileforDOS/Windows.)
IhaveafewreferencePDFscreatedbymyapplicationwhichare“knowngood”.NewlygeneratedPDFs
aftercodechangesarecomparedtothesereferencePDFs.Thecomparisonisdonepixelbypixelandis
savedasanewPDF.InthisPDF,allunchangedpixelsarepaintedinwhite,whilealldifferingpixelsare
paintedinred.
Thismethodutilizesthreedifferentbuildingblocks:
pdftk
,
compare
(partofImageMagick)andGhostscript.
pdftk
UsethiscommandtosplitmultipagePDFfilesintomultiplesinglepagePDFs:
1
pdftk reference.pdf burst output somewhere/reference_page_%03d.pdf
2
pdftk comparison.pdf f burst output somewhere/comparison_page_%03d.pdf
11
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HowcanIunittestaPythonfunctionthatdrawsPDFgraphics?
12
compare
Usethiscommandtocreatea“diff”PDFpageforeachofthepages:
1
compare
\
2
-verbose
\
3
-debug coder -log g "%u %m:%l %e"
\
4
somewhere/reference_page_001.pdf \
5
somewhere/comparison_page_001.pdf \
6
-compose src
\
7
somewhereelse/reference_diff_page_001.pdf
Ghostscript
Becauseofautomaticallyinsertedmetadata(suchasthecurrentdate+time),PDFoutputisnotworking
wellforMD5hash-basedfilecomparisons.
Ifyouwanttoautomaticallydiscoverallcaseswhichconsistofpurelywhitepages,youcouldalsoconvert
toameta-datafreebitmapformatusingthe
bmp256
outputdevice.YoucandothatfortheoriginalPDFs
(referenceandcomparison),orforthediff-PDFpages:
1
gs
\
2
-o reference_diff_page_001.bmp \
3
-r72
\
4
-g595x842
\
5
-sDEVICE=bmp256
\
6
reference_diff_page_001.pdf
7
8
md5sum reference_diff_page_001.bmp
IftheMD5sumiswhatyouexpectforanall-whitepageof595x842PostScriptpoints,thenyourunittest
passed.
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5HowcanIcompare2PDFsonthe
commandline?
I’mlookingforaLinuxcommandlinetooltocomparetwoPDFfilesandsavethediffstoaPDF
outfile.Thetoolshouldcreatediff-PDFsinabatch-process.ThePDFfilesareconstructionplans,
sopuretext-comparedoesn’twork.
Somethinglike:
1
<tool> file1.pdf f file2.pdf f -o o diff-out.pdf
MostofthetoolsIfoundconvertthePDFstoimagesandcomparethem,butonlywithaGUI.
Anyothersolutionisalsowelcome.
5.1Answer
WhatyouwantcanbeachievedwithusingImageMagick¹’s
compare
command.Andthiswillworkonall
importantoperatingsystemplatforms:Windows,MacOSX,LinuxandvariousUnixvariations.
Thebasiccommandisverysimple:
1
compare file1.pdf file2.pdf delta1.pdf
First,pleasenote:thisonlyworkswellforPDFswhichusethesamepage/mediasize.
ThecomparisonisdonepixelbypixelbetweenthetwoinputPDFs.Inordertogetthepixels,thepages
arerenderedtorasterimagesfirst,bydefaultusingaresolutionof72ppi(pixelsperinch).Theresulting
fileisanimageshowingthe“diff”likethis:
• Eachpixelthatisidenticaloneachinputfilebecomeswhite.
• Eachpixelthatisdifferentbetweenthetwoinputfilesispaintedinred.
• The‘source’file(thefirstonenamedinthecommand)will,forcontext,beusedtoprovideagray-
scalebackgroundtothediffoutput.
¹
http://www.imagemagick.org/
13
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HowcanIcompare2PDFsonthecommandline?
14
TheabovecommandoutputsaPDFfile,
delta.pdf
.ShouldyoupreferaPNGimageoraJPEGimage
insteadofaPDF,simplychangethesuffixofthe‘delta’filename:
1
compare file1.pdf file2.pdf delta2.png
2
compare file1.pdf file2.pdf delta3.jpeg
Insomecasesthedefaultresolutionof72ppiusedtorenderthePDFpagesmaybeinsufficienttouncover
subtledifferences.Or,onthecontrary,itmayover-emphasizedifferenceswhicharetriggeredbyextremely
minimalshiftsofindividualcharactersorlinesoftextcausedbysomecomputationalroundingofreal
numbers.
So,ifyouwanttoincreasetheresolution,addthe
-density NNN
parametertothecommandline.Toget
720ppiimages,usethis:
1
compare -density y 720 file1.pdf file2.pdf delta4.pdf
Note,increasingthedensity/resolutionoftheoutputfilesalsoincreasesprocessingtimeand
outputfileformatsaccordingly.A10-foldincreaseindensityleadstoa100-foldincreaseinthe
numberoftotalpixelsthatneedtobecomparedandprocessed.
Alloftheaboveexamplesdoonlyworkfor1-pagePDFfiles.Formulti-pagePDFsyouneedtoadda
[N]
notationtothefilename,where
N
isthezero-basedpagenumber(page1isnotedas
[0]
,page2as
[1]
,
page3as
[2]
,andsoforth).Thefollowingcomparespage4of
file1.pdf
withpage18of
file2.pdf
:
1
compare file1.pdf[3] file2.pdf[17] delta5.pdf
Ifyoudonotwantthegray-scalebackgroundcreatedfromthesourcefile,useamodifiedcommand:
1
compare file1.pdf file2.pdf -compose e src delta1.pdf
Thismodificationchangestheoutputtopurlyred/white:allpixelswhichareidenticalbetweenthetwo
basefilesarered,identicalpixelsarewhite.
Incaseyoudonotliketheredandwhitedefaultcolorstovisualizethepixeldifferences,youcanaddthe
followingcommandlineparameters:
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HowcanIcompare2PDFsonthecommandline?
15
-highlight-color blue
(changedefaultcolorforpixeldifferencesfrom‘red’to‘blue’)
-lowlight-color yellow
(changedefaultcolorforidenticalpixelsfrom‘white’to‘yellow’)
oranyothercolorcombinationyoudesire.Allowednamesforcolorsinclude
#RRGGBB
valuesforRGB
shades.
Note,ImageMagick’s
compare
commanddoesnotprocessthePDFinputfilesdirectly.
compare
originallywasdesignedtoprocessrasterimagesonly.Youcaneasilytestthisbyreplacingthe
PDFsinabovecommandswithsomeimagefiles–justmakesurethatthefilesare‘similar
enough’togivesensibleresults,andalsoensure,thatthecomparedimagesdohavethesame
dimensionsinwidthandheight.
ToprocessPDFs,ImageMagickneedstoresorttoGhostscriptasits‘delegate’programfor
processingPDFinput.Ghostscriptgetscalledbehindthecurtainsby
compare
inordertocreate
therasterfileswhichthen
compare
doesitsmagicon.
ToseetheexactcommandlineparametersthatImageMagickusesforGhostscriptcall,justadd
a
-verbose
parametertothe
compare
commands.Theoutputontheterminal/consolewillbe
muchmoreverboseandrevealwhatyouwanttoknow.
Examples
I’musingthisverysamemethodforexampletodiscoverminimalpagedisplaydifferenceswhenfont
substitutioninPDFprocessingcomesintoplay.
Itcaneasilybethecase,thatthereisnovisibledifferencebetweentwoPDFs,thoughtheyareextremely
differentinMD5hashes,filesizesorinternalPDFcodestructure.Inthiscasethe
delta1.pdf
outputPDF
pagefromtheabovecommandwouldbecomeall-white.Youcouldautomaticallydiscoverthiscondition,
soyouonlyhavetovisuallyinvestigatethenon-whitePDFsbydeletingtheall-whiteonesautomatically.
Togiveyouamorevisualimpressionaboutthewaythiscomparisonworks,I’veconstructedafew
differentinputfiles.IusedGhostscripttodothis.(TheexactcommandsIusedaredocumentedatthe
endofthischapter.)
Example1
ThefollowingimageshowstwoPDFpagessidebyside.Mostpeoplewillnoticefromaquicklookthe
differencesbetweenthesetwopages:
TwoPDFpageswhichdodiffer–differencescanbespottedbylookingtwice…
Nowusethefollowingcommandstocreateafewdifferentvisualizationofthe‘deltas’:
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HowcanIcompare2PDFsonthecommandline?
17
TwoPDFpageswhichdodiffer–differencescanonlybebespottedbylookingveryclosely.
Nowusethefollowingcommandstocreateafewdifferentvisualizationofthe‘deltas’:
1
compare
file3.pdf file4.pdf
delta5.pdf
2
compare
file3.pdf file4.pdf -compose e src delta6.pdf
3
compare -density y 720 file3.pdf file4.pdf
delta7.pdf
4
compare -density y 720 file3.pdf file4.pdf -compose e src delta8.pdf
Theresultingdifferencesareshowninthefollowingpicture.
Fourdifferentwaystovisualizethedifferencesbetweenthelasttwoinputfiles.Againa72ppiresolutionforthetoptwo
anda720ppiresolutionforthebottomones.The1standthe3rddoshowagrayscalecontextbackground,theothersdo
not.Pleasezoomintospotthefinerpixeldifferencesbetweenthedifferentresolutions…
Again,the72ppi-basedcomparisonofthetwoinputPDFsshowsaclearlyvisible‘pixelization’ofthe
results(toptwoimages).The720ppiversiondoesshowthedifferencesmuchmoreclearly:itisjustthat
thetextisshiftedslightlytotheleftandtothetopinthecaseofthesecondinput.Ifyouzoominenough
intothe720ppiversions,youcanevencountthenumberofpixels:theshiftforeachsinglecharacterof
HowcanIcompare2PDFsonthecommandline?
18
thetextisconstistenlty5pixelstotherightand5pixelstothetop.The72ppiversioncannotbringout
thissubtledifferencesoclearly:atthisresolutiontheshiftisonly1/2pixeltotherightand1/2pixelto
thetop.Thismeansthatforsomecharactersthereisnoshiftoccuringatall,andothercharactersmove
byafullpixelineitherdirection.Thisbecomesclearlyvisibleinthefactthatsomecharactersdonotlook
changedatallwhileothersclearlydo.
Example3
ThefollowingimageshowstwootherPDFdocuments.Canyouspotthedifference?
TwoPDFdocumentswhichdodiffer.Trytospotthedifference!
Creatingvisualizationsinred/whitepixelswillgivethefollowingresults.
HowcanIcompare2PDFsonthecommandline?
19
Fourdifferentwaystovisualizethedifferencesbetweenthelasttwoinputfiles.Againa72ppiresolutionforthetoptwo
anda720ppiresolutionforthebottomones.The1standthe3rddoshowagrayscalecontextbackground,theothersdo
not…
Ifyouhaveaccesstotheoriginaldeltafilesandzoominonno.3youcanclearlyseethatthesecond
documentcontainsachangedprize:goingupby2.000$USbychangetheoriginal‘6’toan‘8’.
Update
Forthoseofyouwhowanttoreproducethecommandsshownabove,you’dalsoneedaccesstothe
samesourcefilesIused.That’seasy:IusedGhostscripttocreatetheseexampleinputPDFs.Herearethe
HowcanIcompare2PDFsonthecommandline?
20
commandsforthis:
1
gs
\
2
-o file1.pdf
\
3
-sDEVICE=pdfwrite
\
4
-g5950x1100
\
5
-c "/Courier r findfont 72 scalefont setfont \
6
30
30
moveto (HELL0, , WORLD\!) show \
7
showpage"
8
9
gs
\
10
-o file1.pdf
\
11
-sDEVICE=pdfwrite
\
12
-g5950x1100
\
13
-c "/Courier r findfont 72 scalefont setfont \
14
30
30
moveto (HELLO, , W0RLD\!) show \
15
showpage"
16
17
gs
\
18
-o file1.pdf
\
19
-sDEVICE=pdfwrite
\
20
-g5950x1100
\
21
-c "/Courier r findfont 72 scalefont setfont \
22
30
30
moveto (Hi, , Universe\!) show \
23
showpage"
24
25
gs
\
26
-o file1.pdf
\
27
-sDEVICE=pdfwrite
\
28
-g5950x1100
\
29
-c "/Courier r findfont 72 scalefont setfont \
30
30.5 30.5 moveto (Hi, , Universe\!) show \
31
show showpage"
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