A Binarization Algorithm specialized on Document Images and Photos

Transcription

1 A Bnarzaton Algorthm specalzed on Document mages and Photos Ergna Kavalleratou Dept. of nformaton and Communcaton Systems Engneerng Unversty of the Aegean Abstract n ths paper, a new method for document mages or photos bnarzaton s presented. The method s smple, fast and robust and approprate for normal as well as for specal cases of documents lke photos, hstorcal documents etc. The proposed method s appled to problematc cases of documents and t s compared to other tradtonal methods. where x and y are the horzontal and vertcal coordnates of the mage, and r can take any value between 0 and whle r= stands for whte colour and r=0 stands for black colour. Our ntenton lays on the transform of the ntermedate gray tones to ether black (r=0) for foreground or whte (r=) for background.. ntroducton The transformaton of greyscale mages to black and whte, s a common problem of last decades. Many methods have been proposed as for mages as for documents mages. However, the ncreased necesstes (automatc hstorcal document processng, dgtal camera photos of documents etc) requre new approaches. One of the older methods n mage bnarzaton s Otsu s [3], based on the varance of pxel ntensty. Bernsen [] calculates local thresholds usng neghbours. Nblack [2] uses local mean and standard devaton. Sauvola [4] presents a method specalzed on document mages that apples two algorthms n order to calculate a dfferent threshold for each pxel. n the next secton, t s gven the descrpton of the algorthm, whle n secton 3 the algorthm s analysed n detal. Our expermental results and a short comparson wth tradtonal bnarzaton methods are descrbed n secton 4. Fnally, our conclusons are provded n secton Algorthm Descrpton As nput, we assume grayscale document mages or photos where the tones of the foreground (characters, graphcs, etc) outrange over the background. As example, consder the hstorcal document of fgure. Our mages are descrbed by the equaton: = r, r [0,], () Fgure : Hstorcal Documents The algorthm s based on the fact that a document mage ncludes very few pxels of useful nformaton (foreground) compared to the sze of the mage (foreground+background). As an ndcaton, n table, t s gven the amount of black pxels n relaton wth all the pxels of the mage concernng the cleaned verson of the mentoned mage n black and whte (shown n fgure 2), after the applcaton of our algorthm. Table : The amount of black pxels n relaton wth all the pxels of the mage. Total sze (pxels) Amount of black pxels Rate (%) The fact s that rarely the amount of black pxels exceeds the 0% of the total pxels n the document. Takng advantage of ths fact, we assume that the average value of the pxel values of a document mage s determned manly by the background even f the document s qute clear. Ths clam s supported from

2 fgure 3, where are depcted the hstogram of the above example. n the same fgure two thresholds, of our method and Otsu s method, as well as the average value n each case are gven. t s obvous that the average value s always on the background sde, consderng ether threshold. 2. Subtracton of the T from all the pxels of the mage. 3. Hstogram equalzaton. 4. Repetton of steps -3 tll the cleanng of the document. 5. Bnarzaton of the fnal mage. n the next secton, we analyze each of the above steps gvng the necessary mathematcal relatons and examples. 3. Algorthm Analyss Fgure 2: The document of fgure after the applcaton of our algorthm Consderng the equaton () the calculaton of the T, threshold used n -th repetton for an MxN document mage, s gven by the formula: x y T = (2) MxN where (x, s the mage at the -th repetton. Keepng n mnd that s stand for background and 0s for foreground the equaton used for the subtracton and provdes the after-subtracton and before-equalzaton mage s s: s = T + ( x,, (3) n each repetton, durng the subtracton, a lot of pxels are moved to the sde of the background and the rest of the pxels are fadng. n fgure 4, the mage that corresponds to the T threshold of the document mage of fgure s shown as well as the mage after the frst subtracton. Fgure 3: The hstogram of the documents of fgure. Thresholds extracted wth the proposed method (--), Otsu s method (- ) and the average value of the pxels ( ). Usng ths fact our method conssts of two procedures that are appled alternately. n the frst part the average colour value of the mage s calculated and then subtracted from the mage, whle n the second part of the algorthm we perform hstogram equalzaton, thus the remanng pxels would expand and take up all of the grayscale tones. The nput of the algorthm s assumed to be grayscale documents lke the one of fgure whle the output wll be black and whte document mages lke the one of fgure 2. Our algorthm conssts of the followng steps:. Calculaton of the average pxel value (T ) of the mage. (a) (b) Fgure 4: a) the mage that corresponds to the T = threshold of the document mage of fgure, b) the mage fg. after the frst subtracton and before the hstogram equalzaton. After the subtracton step, we adjust the ntensty of the mage by usng the hstogram and extendng the values to all the colour range from 0 to. Snce the s (background) shouldn t be changed, and the rest of the

3 pxel values should extend from 0 to. The relaton we use for the equalzaton s: s = E (4) where s s gven by the equaton (3) and E s the mnmum pxel value n the mage s durng -th repetton, just before the hstogram equalzaton. n fgure 5, t s shown the document mage of fg. before and after the frst hstogram equalzaton. The correspondng hstograms are dsplayed below the mages. Please note that the hstograms have been scaled approprately n order to show more detal. The maxmum value s shown on the upper rght corner. Fgure 5: the document mage of fg. before and after the frst hstogram equalzaton and the correspondng hstograms. The whole procedure s repeated the necessary tmes tll the document mage s satsfactorly cleaned. Each repetton removes more stans from the mage. The number of repettons depends on the mage and the ntensty of any exstent stans, crumples, lghtng effects on the mage. Snce n every repetton the mage and the hstogram are changng, t s useful to be able to transform the threshold values nto values of the ntal hstogram, n order to have a clear mage of the progress n our experments. Combnng the equatons (3) and (4) we extract a relaton between the fnal and ntal mage durng a repetton of our algorthm: T = E (5) where s the mage after the -th repetton havng used the correspondng thresholds T and E for the subtracton and equalzaton n the repetton, as t has been descrbed above. Thus, usng the equaton (5) and makng the necessary replacements for n repettons, the correspondng ntal value T o, n the ntal hstogram, of the fnal threshold T f wll be: n n n (6) T = T E E + T E + T 0 f = j = j= = 2 The termnaton of the algorthm, the specfcaton of the necessary repetton tmes, s a problem that occuped us long (fx number of repettons, values of the above mentoned thresholds T and E, as well as the varaton of them or the nter se dfference were concerned as crtera). Fnally, we notced that all the documents had a smlar behavour wth most changes at the frst -3 repettons and very slow modfcaton afterwards. The necessary amount of repettons depends very much on the document. as well as on the requred result, and t never exceeded the 20 th repetton n our experments. However, the process after the frst repettons s very slow. n fgure 6 we show the thresholds on the hstogram of the fgure durng the 4 frst repettons, whle n fgure 7 a detal of the document s presented. Thus, we could say that more than one stage could be accepted. Fnally, after many experments, we conclude that the amount of transformed pxels n each repetton s a more objectve measure. After numerous experments, we suggest as a threshold the frst repetton that the transformed pxels are less than 3 of the document sze. Havng already concluded to the rght fnal stage, we bnarze the mage by turnng all the pxels that are not already whte (value ) to black (value 0) j= Fgure 6: The correspondng thresholds on the hstogram of fg.. n crcle, you can see the result accordng to the suggested threshold 3 of the document sze. j

4 Fgure 7: A detal of the fgure n the ntal stage (top-left) and durng the 4 frst repettons (from left to rght and top to bottom). n rectangle, the suggested threshold result ( th repetton). 4. Expermental Results As mentoned before n our experments we used a set of about 00 document mages and photos n grey scale. Snce, to the best of our knowledge, there s no known database, n order to compare the results we used the methods descrbed n [,3,4]. Some results are shown n fgures 8-0, tryng to demonstrate the performance of the proposed method n typcal cases of document mages and photos. Due to the smplcty of the algorthm, the computatonal cost s very low n comparson to other algorthms approprate for document mages. For example, the computatonal cost for the mages of the fgures, 9 and 0 s 22, 2 and 8 seconds, respectvely. The algorthm has been mplemented n Matlab n Pentum M processor.60 Ghz. 5. Concluson n ths paper we presented a method approprate for the bnarzaton, of document mages and photos. The method makes use of the fact that the pxels that compose the text n an document, usually, do not exceed the 0% of ts sze. Ths allowed us to buld an algorthm that conssts of two successve stages appled alternatvely on the mage. The results have been compared wth other methods and are qute satsfactory. The advantages of our algorthm s: smplcty, snce t doesn t requre any further pre-processng procedure and s based on a smple technque, low computatonal cost due to ts smplcty, and robustness, snce t gves the capablty to the user to succeed the desrable result n grayscale or bnarzed fnal mage. Acknowledgments We would lke to thank professor N.Papamarkos for provdng the algorthms used for the comparson wth other methods n our experments, from hs personal lbrary References [] Bernsen, J."Dynamc thresholdng of grey-level mages", Proc. 8th nternatonal Conference on Pattern Recognton (CPR8), pp , Pars, France, October 986. [2] Nblack, W. An ntroducton to Dgtal mage processng", pp 5-6, Prentce Hall, 986. [3] Otsu, N. A threshold selecton method from gray-level hstograms. EEE Trans. Systems Man Cybernet. pp , 9 (), 979. [4] Sauvola, J., Petkanen, M., Adaptve Document mage Bnarzaton, Pattern Recognton, pp , 33 (2000).

5 (a) (b) (c) Fgure 8: Bnarzaton of the document of fgure by the methods of (a) Otsu s, (b) Bernsen s, (c) Sauvola s. Fgure 9 (from left to rght): orgnal photo, our approach, Bernsen s and Otsu s. Fgure 0 (from left to rght): orgnal photo, our approach and Bernsen s.