Seite - 23 - in Document Image Processing

J. Imaging 2018,4, 27 14. Pun[26] 15. Shanbhag[27] 16. Triangle [28] 17. Wu-Lu[29] 18. Yean-Chang-Chang[30] 19. Intermodes [31] 20. Minimum(variationof [31]) 21. Ergina-Local [32] 22. Sauvola [33] 23. Niblack[34] Aground-truth image foreach“real”worldone isneeded toallowaquantitativeassessment of the quality of the final binary image. Only theDIBCOdataset [10] had ground-truth images available. Thismakes theassessment taskof real-world imagesextremelydifficult [35].All caremust be takentoguarantee the fairnessof theprocess. Theground-truth images for theotherdatasetswere generatedbyapplyingthe23algorithmsaboveandthebilateralalgorithmtoall the test images in the Nabuco[7]andLiveMemory[36]datasets.Visual inspectionwasmadetochoosethebestbinaryimage inablindprocess, aprocess inwhich thepeoplewhoselected thebest imagedidnotknowwhich algorithmgenerated it. To increase thedegreeof fairness and thenumberoffilteringpossibilities, the threecomponent imagesproducedbytheDecisionMakingblockwereallanalyzed. Thebinary imageschosenusing themethodologyabovewent throughsalt-and-pepperfilteringandwereusedas ground-truth imagefor theassessmentbelow.All theprocessingtimefigurespresented in thispaper are fromIntel i7-4510U@2.00GHzx2,8GBRAM,runningLinuxMint18.264-bit.Allalgorithmswere codedinJava,possiblybytheirauthors. 3.1. TheNabucoDataset The Nabuco bequest encompasses about 6500 letters and postcards written and typed by JoaquimNabuco [7], totalingabout30,000pages. Suchdocumentsareofgreat interest towhoever studiesthehistoryoftheAmericas,asNabucowasoneofthekeyfiguresinthefreedomofblackslaves, andwas thefirst BrazilianAmbassador to theU.S.A. Thedocuments ofNabucoweredigitalized by the second author of this paper and the historians of the JoaquimNabuco Foundation using a table scanner in 200dpi resolution in true color (24 bits per pixel), back in 1992 to 1994.Due to seriousstorage limitations then, imagesweresavedin the jpegformatwith1%loss. Thehistorians in theproject concludedthat150dpiresolutionwouldsuffice torepresentall thegraphicalelements in the documents, but choice of the 200-dpi resolutionwasmade to be compatiblewith the FAX deviceswidelyusedthen.About200of thedocuments in theNabucobequestexhibitedback-to-front interference. The15document imagesused in thisdatasetwerechosenforbeingrepresentativeof the diversityofdocuments insuchauniverse. Table 1presents thequantitative results obtained for all thedocuments in thisdataset. P(f/f) stands for the ratio between thenumber of foregroundpixels in the original imagemappedonto blackpixelsandthenumberofblackpixels in theground-truth image. Similarly,P(b/b) isproportion between thenumber of backgroundpixels in theoriginal imagemappedontowhitepixels of the binary imageandthenumberofwhitepixels in theground-truth image. Thefigures forP(b/b)and P(f/f)are followedby“±”andthevalueof thestandarddeviation. The timecorresponds to themean processingtimeelapsedbythealgorithmtoprocess the images inthisdataset. Theresultswereranked inP(b/b)decreasingorder. Theresultspresented inTable1showsthebilateralfilter in thirdplace for thisdataset in termsof imagequality,however thestandarddeviation ismuchlower thanthe twofirst. That implies that its quality ismorestable for thevariousdocument images in thisdataset. Figure5presents thedocument 23