Seite - 3 - in Document Image Processing

Journal of Imaging Article Non-LocalSparseImageInpaintingforDocument Bleed-ThroughRemoval MuhammadHanif*,AnnaTonazzini,PasqualeSavinoandEmanueleSalerno Instituteof InformationScienceandTechnologies, ItalianNationalResearchCouncil, 56124Pisa, Italy; anna.tonazzini@isti.cnr.it (A.T.);pasquale.savino@isti.cnr.it (P.S.); emanuele.salerno@isti.cnr.it (E.S.) * Correspondence:muhammad.hanif@isti.cnr.it Received: 14 January2018;Accepted: 26April2018;Published: 9May2018 Abstract:Bleed-throughisa frequent,pervasivedegradation inancientmanuscripts,which iscaused byinkseepedfromtheoppositesideof thesheet. Bleed-through,appearingasanextra interfering text, hinders document readability andmakes it difficult to decipher the information contents. Digital image restoration techniqueshavebeensuccessfully employed to removeor significantly reduce thisdistortion. Thispaperproposesa two-steprestorationmethodfordocumentsaffected bybleed-through,exploiting informationfromtherectoandverso images. First, thebleed-through pixels are identified, based on anon-stationary, linearmodel of the two texts overlapped in the recto-versopair. In thesecondstep,adictionary learning-basedsparse image inpaintingtechnique, with non-local patch grouping, is used to reconstruct the bleed-through-contaminated image information.Anovercompletesparsedictionaryislearnedfromthebleed-through-freeimagepatches, which is thenusedtoestimateabefittingfill-in for the identifiedbleed-throughpixels. Thenon-local patchsimilarity isemployedin thesparsereconstructionofeachpatch, toenforce the local similarity. Thanks to the intrinsic image sparsity and non-local patch similarity, the natural texture of the backgroundiswell reproduced in thebleed-throughareas, andevenapossibleoverestimationof thebleedthroughpixels iseffectivelycorrected, so that theoriginalappearanceof thedocument is preserved.Weevaluate theperformanceof theproposedmethodontheimagesofapopulardatabase ofancientdocuments, andtheresultsvalidate theperformanceof theproposedmethodcomparedto thestateof theart. Keywords:ancientdocumentrestoration;imageinpainting;bleed-throughremoval;sparserepresentation 1. Introduction Archival, ancient manuscripts constitute the primary carrier of most authentic information startingfromthemedievalera, servingashistory’sowncloset, carryingstoriesofenigmatic,unknown placesor incredibleevents that tookplace in thedistantpast,manyofwhicharestill toberevealed. Thesemanuscriptsareofgreat interestandimportance forhistorians,andprovide insight intoculture, civilisation, eventsandlifestylesofourpast.With thepassageof time, thesedocumentshavebeen exposedtodifferenttypesofprogressivedegradations,suchasspotsorinkfading,duetofragilenature of thewritingmedia,andbadstorageorenvironmentalconditions. Thisdegradationprocess limits the useof theseancientclassics, andsomeof thedeteriorateddocumentshadaverynarrowescapefrom totalannihilation. Specifically, in themanuscriptswrittenonbothsidesof thesheet,often the inkhad seepedthroughandappearsasanunpleasantdegradationpatternonthereverseside. Inkpenetration through thepaper ismainlydue toaging, humidity, inkchemicalpropertiesorpaperporosity [1], andcanrangefromfainttosevere. Intheliterature, thiskindofdegradationistermedasbleed-through, and impairs the legibility and interpretationof thedocument contents [2]. Therefore, it is of great significance to remove the bleed-through contamination and restore the integrity of the original J. Imaging 2018,4, 68 3 www.mdpi.com/journal/jimaging