Seite - 20 - in Document Image Processing
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J. Imaging 2018,4, 27
The background-background probability is a function that needs to be optimized in the
decision-making block, mapping background pixels (paper) from the original image ontowhite
pixelsof thebinary image. Itdependsofall theparametersof theoriginal imagetexture, strengthof
thebacktofront interference (simulatedbythecoefficientα),paper translucidity,etc. foreachRGB
channel. Thus,onecanrepresent thisdependenceas:
P(b/b)=f(α,R,G,B). (1)
Theoptimal threshold tc* foreachchannel is calculated in thedecision-makingblock, the indexc
canbeR,GorB,maximizingP(b/b):
tc*=MaxP(b/b), (2)
subject toagivencriterionP(f/f)≥M.ThecriterionusedherewasM=97%, that is atmost3%of
the foregroundpixelsmaybe incorrectlymapped. During the trainingphase, the best tc*will be
chosenfromthethreechannels,whichbestmaximizes theP(b/b) foreachof the images in the training
set. Thematrixof co-occurrenceprobability is calculatedand thedecisionmaker chooses thebest
binary image. Thedecision-makingblockwas trainedwith32,000synthetic images insuchawayto,
givenareal imagetobebinarized, itfinds theoptimal thresholdparameters.
2.2.GeneratingSynthetic Images
TheDecision-MakingBlockneedstrainingto“learn”about theoptimal thresholdparametersand
thevalueof thekernel tobeusedinthebilateralfilter. Suchtrainingmustbedoneusingcontrolled
imageswhicharesynthesizedtomimic thedifferentdegreesofback-to-front interference,paperaging,
paper translucidity, etc. Figure4presents theblockdiagramfor thegenerationofsynthetic images.
Twobinary imagesofdocumentsofdifferentnature (typed,handwrittenwithdifferentpens,printed,
etc.) are taken: F—frontandV—verso (back). The front image isblurredwithaweakGaussianfilter to
simulate thedigitalizationnoise [1], thehues thatappear inafterdocumentscanning.
Figure4.BlockdiagramoftheschemeforthegenerationofsyntheticimagesfortheDecision-MakingBlock.
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