A Clipping Reduction Algorithm Using Backlight Luminance Compensation for Local Dimming Liquid Crystal Displays

Transcription

1 A Clipping Rection Algorith Uing Backlight Linance Copenation for Local Diing Liqi Crytal Diplay Jong-J Hong, Seong-En Ki, Stent Mebe IEEE, Woo-Jin Song, Mebe IEEE Abtract Local iing techniqe cae clipping artifact in liqi crytal iplay. To overcoe the proble aociate with the artifact, the backlight linance hol be ajte, bt thi proce increae power conption. In thi paper we propoe a novel backlight linance copenation algorith to rece clipping artifact by iniizing the increent of backlight linance. 1 Inex Ter Clipping artifact, local iing, backlight linance, power aving I. INTRODUCTION Liqi crytal iplay (LCD) have been increaingly e in iplay e to their low cot, lack of raiation eiion, high vieo qality playback an light weight. In conventional LCD, the backlight provie a contant light orce an liqi crytal control the pixel brightne by changing their tranittance. Howeve the backlight i flly trne on to provie contant linance on the whole panel. Thi ean that the backlight i a ignificant orce of power conption. Moreove light leakage an low contrat ratio (CR) are alo erio proble becae the light cannot be obtrcte copletely when iplaying very ark iage. The local iing backlight techniqe enable LCD to preent iage with high CR an low power conption [1]- [9] by iviing the backlight into everal local block an olating the inivially accoring to the inpt iage, th proviing rece backlight linance for the iplay. In the proce of a local iing backlight techniqe (Fig. 1) the pixel ata of the inpt iage are e to olate the backlight iing ty ignal which eterine the backlight linance to rece light leakage an power conption. Backlight olation rece the linance of the iplaye iage by recing backlight linance. To copenate for the rece linance of the iplaye iage, the pixel ata of the inpt iage t be ajte accoring to the backlight 1 Thi work wa pporte by LG Diplay, the IT R&D Progra of MKE/MCST/IITA (2008-F ), the HY-SDR Reearch Center at Hanyang Univerity ner the ITRC Progra of MKE, an the Brain Korea (BK) 21 Progra fne by Minitry of Ecation, Science an Technology, Korea. The athor are with Diviion of Electrical an Copter Engineering, Pohang Univerity of Science an Technology, Pohang, , Korea (eail: wjong@potech.ac.kr). Fig. 1. The proce of a local iing backlight techniqe. linance [1]. After the pixel ata are ajte, the iplaye iage i not exactly the ae a the inpt iage becae the linance of the iplaye iage cannot excee the backlight linance [2]. Therefore, the linance at oe high gray level cannot be copenate an the gray level are clippe. Thi i calle the clipping artifact [1]. Iage with clippe pixel look nnatral an oetie exhibit everal contor [10]. Therefore, in the local iing backlight techniqe, the clipping artifact t be rece. Conventional backlight olation algorith foc on recing either the backlight linance or the clipping artifact. In the average [4] an qare root [5] algorith, the backlight linance i eterine by the average linance an qare root of the average linance of the inpt iage repectively; th the backlight linance i effectively rece in the ark region of the iage. Howeve the rection in the backlight linance i extreely large, o the clipping artifact in the bright region increae. In the ax algorith [4], the backlight linance i eterine by the axi linance of the inpt iage; th the backlight linance i bright enogh to cover the clipping artifact in the bright region. Howeve the backlight linance i too enitive to the noie fro high gray level an the benefit of the local iing backlight, i.e. low CR an power conption, are rece. Clipping can be rece by increaing the backlight linance, bt thi increae relt in aitional power conption an a ecreae in CR. Therefore, we propoe a novel backlight linance copenation (BLC) algorith to rece the clipping artifact by ing the allet poible backlight increae.

2 II. PROPOSED ALGORITHM A. Backlight Diing an Backlight Linance LCD are et to eet an ieal target linance at each gray level (or ieal pixel linance) when the backlight i flly trne on (Fig. 2, ahe line). Howeve LCD have a light leakage proble e to the iperfect natre of liqi crytal, o the actal linance i affecte by the pixel tranittance which cae light leakage at low gray level (Fig. 2, circle). Therefore, we nee a echani to control the backlight linance when an inpt iage i iplaye on the LCD. Fig. 4. Die pixel linance an rece light leakage accoring to the ie backlight linance. Dahe line: ieal target linance; epty rectangle, circle an epty triangle: ie pixel linance of Fig. 3 a, b an c, repectively. In general, aing that the backlight linance B( of block ( i nifor, the iing tie,, of everal rroning block an their inflence can be evalate a [6]. Fig. 2. Linance v. gray-level, ahe line: ieal target linance; epty circle: pixel tranittance. A a relt, light cannot be obtrcte perfectly when iplaying very ark pixel, o the ark iage oe not look ark enogh [6]. Moreove in conventional LCD the backlight i alway trne on at axi linance. Coneqently, a hge percentage of the power conption can be attribte to the backlight. To rece light leakage an power conption, the backlight can be ivie into local block an the backlight linance of each block can be olate inepenently accoring to the local iage content (Fig. 3), bt thi trategy rece linance of all pixel (Fig. 4). Fig. 3. Exaple of local iage block: (a): bright; (b): ile; (c): ark. Pixel linance i eterine by pixel tranittance in aition to backlight linance. Iproving pixel tranittance i not coniere here. We are interete in eviing the ot appropriate etho of controlling backlight linance to rece light leakage an backlight power conption. B c ( i j ), (1) ( r where = (M-1)/2, M i the ize of the block ak incling neighboring bloc c i the backlight block prea fnction (BSF) coefficient which reflect the inflence between the block ( an each neighboring block; c epen on the backlight trctre an it i eterine experientally. B. Pixel Copenation an Clipping artifact After the backlight i ie an the ie backlight linance i evalate, the pixel vale are copenate to achieve the ieal target linance. In the ieal target linance, the axi linance i 1 which i the backlight linance when the backlight i flly trne on. However in pixel linance after backlight iing, the axi linance i ecreae to the level of the ie backlight linance. In other wo after pixel vale are copenate, the iplaye iage an inpt iage are not exactly the ae becae the linance of the iplaye iage cannot excee the backlight linance [2]. Therefore, the linance at oe high gray level cannot achieve the target linance an the high gray level are clippe to a vale of 255. A hown in Fig. 5, gray level between g c +1 an 255 are clippe. g c i the critical gray level, which i the axi gray level to be copenate a the ieal target linance. If g c becoe highe the clippe pixel are rece o that the copenate linance approache the ieal target linance a hown in Fig. 6. Therefore, g c eterine the clipping level which ean that a low vale of g c ha the potential to ake erio clipping

3 255 N n j, g, (4) ggc 1 Fig. 5. Clipping: high gray level are clippe when the pixel vale are copenate. where n(j,g) i a hitogra of block ( at gray level g in the given inpt iage. The clipping eare can be ifferent between two local block even if the critical gray level are ientical. Thi i becae the critical gray level i eterine by everal rroning block fro the evalate backlight linance in (1) an (3) while the hitogra i only eterine by the ( block. A hown in Fig. 7 an Fig. 8, the two local block of the iage have the ae critical gray level, bt one of the block ha a large portion of pixel whoe gray level are above the critical gray level. In thi cae, the clipping eare of the block col be larger than that of another block. Fig. 6. Rece clipping a a relt of increae g c. artifact bt high one oe not. A entione above, the ie backlight cae clipping an the relationhip between the critical gray level an backlight linance after iing can be expree ing the ieal target linance. Fig. 7. Exaple of noralize hitogra an aller clipping eare in a block. B( gc f gc 255 (2) where f(g) i the apping fnction fro the gray level g to the ieal target, which i a onotonically increaing fnction an ally a gaa fnction a hown in (2), an g c ( i the critical gray level of the ( block when the backlight linance i B( a evalate in (1). A a relt, the critical gray level of a block i highly relate to the backlight linance of the block. Fro (2), the critical gray level can be erive a: 1/ B( B( g f c (3) D. Clipping Meare an Opti Backlight Linance Now, we can qantify the clipping artifact by efining a clipping eare N a the nber of clippe pixel a follow: Fig. 8. Exaple of noralize hitogra an larger clipping eare in a block. We can rece the clipping artifact at block ( by retricting the clipping eare N( to be le than a threhol (TH). TH i the axi nber of allowable clippe pixel in a block which can be a preefine paraeter epening on the anfactrer reqireent. To

4 atify the retriction, the critical gray level hol be increae becae N( ecreae when g c ( i increae a hown in (4). A the critical gray level an the backlight linance are cloely relate a hown in (2), the backlight linance alo increae when the critical gray level i increae. Howeve the increental change in backlight linance cae aitional light leakage an aitional backlight power conption. Therefore the increent of the critical gray level or the backlight linance hol be iniize. Uing thi logic, g c ( i increae ntil N( i le than TH. In other wo the i g c ( i obtaine by axiizing N( in (4) bject to N( TH ch that: g c arg ax gc ggc 1 bject to n j, g TH. ggc 1 n j, g, The oltion for g c ( i iteratively obtaine. g c ( i controllable by ajting B( a hown in (3). Therefore, the vali oltion that we want i relate to backlight linance. A copatible atche backlight linance B ( correponing to g c ( can be obtaine by ing (2). The final relt, B ( i the i backlight linance at block ( on the clipping contraint. Specifically, the backlight linance hol be larger than the i backlight linance to prevent the gray level fro being clippe bt the vale hol be a cloe a poible. C. Backlight Linance Copenation (BLC) The final tage i to copenate for backlight linance by ajting the iing tie of entire local block in the iplay to atify the i backlight linance conition, B ( at each block. Thi proce i calle backlight linance copenation (BLC). To atify the B ( reqireent, all the iing tie of M M neighboring block rroning ( hol be copenate in cooperation becae the backlight linance i inflence by neighboring block at the ae tie a hown in (1). Mltiplying the iing tie by oe caling factor can copenate for the backlight linance. In thi pape we are interete in jt increaing in cae where the caling factor hol be larger than or eqal to 1. ( (5) i j ) ( i j ), 1 (6) where ( i the caling factor that eterine the ratio by which iing ty (i+j+) i increae an the reltant (i+j+) i the i iing ty of the BLC. To copenate for the backlight linance of block (, not only the iing ty of block ( bt alo the rroning (i+j+) block hol be increae a hown in (1). Therefore, or objective i to eterine the caling factor that iniize the increent of the iing tie while atifying the reqireent that the copenate backlight linance hol be larger than or eqal to the i backlight. The ai of the BLC i to fin the i caling factor a follow: { } [, ] arg in {, (, )} r r i j r, [, ] bject to r (7) ( i j ) 1, c, ( ( i j ) B. r To iniize the increent of the iing tie, they hol be increae accoring to the agnite of the BSF coefficient. In other wo the i cenario i to increae the iing ty of a block which ha a larger BSF coefficient becae it increent i aller than one that ha a aller BSF coefficient. Therefore, the inex of the BSF coefficient () hol be iplifie an thi inex iplification relt in inex rebiling of the caling factor an iing tie. We e the following inex rebiling etho which i a ifferent poitional repreentation of the M M block that rron (. Inice r an are rebilt a k an. Firt, the block are orte in the ecreaing orer of the BSF coefficient. k i the block inex in thi lit, th c i iply rebilt a c k. In thi pape k i calclate on the aption that M i a vale of 3 an each c ha the ae vale when both r an are not zero. Then, for block that have the ae BSF coefficient c k, the block are orte in a ecreaing orer accoring to iing ty. i the block inex in thi lit (a in Fig. 9). A ary of inex rebiling efinition i hown in Table I. Fig. 9. Exaple of inex rebilt BSF coefficient an the iing tie of the rroning block. The block poitional notation in the iing ty i oitte, i.e. 0,0 in the figre ean 0,0(. TABLE I DEFINITION OF INDEX REBUILDING Inex () () BSF coefficient c c k Diing ty (i+ j+) ( Scaling factor ( ( Inex range r(=)=0,±1,,± k=0,1,, =0,1,,(2k+1) 2 -(2k-1) 2 =8k By ing the rebilt inice, the contraine iization proble in (7) can be rewritten by

5 { arg bject to } k [0, ], [0,8k ] in { k, } k[0, ], [0,8 k ] k 0 0 8k 8k c k0 k 0 k, 1, ( k ( B. Hence, the i caling factor are obtaine by olving the reforlate contraint iization proble. The backlight linance can be viewe a the area of a rectangle with a vale of 1 in with an B( in height (Fig. 10 (a)). A hown in (1), the backlight linance i evalate by ing the BSF coefficient weighte by iing tie an the area in the iagra i eqal to the total area of the 3 eparate long rectangle whoe with i the BSF coefficient an height i the iing ty (Fig. 10 (b)). Now, the iization proble i to fin the ini increent in total height that atifie the area, B (. The increent of height i generate when the iing ty i ltiplie by the caling factor. (a) (b) Fig ienional iagra of the contraine iization proble in (8). (8) B B( 0,0 1. (9) c 0 Howeve thi increae height, 0,0 (. 0,0 (, ight excee the axi vale of ax (=255). If thi happen, the increae height i fixe a ax an the rpl, c 0. [ 0,0 (. 0,0 (- ax ], i copenate for by ajting the neighboring rectangle in tep 2. The rpl i ae a a ppleent of rectangle with a with of c 1, a hown a in Fig. 11 (c). The ratio of height in the ppleent i 1,0 (-1 an it area i c 1. [ 1,0 (-1]. [ 1,0 (+ 2,2 (]. A the area of the rpl an ppleent in tep 2 are eqal, 1,0 ( i acqire by ing the eqality. In thi way, the general oltion of the i caling factor 0( in tep 2 can be acqire a: 0 1 [ k1,8( k1) k1,8( k1) ax c / c. k1 k 0,0 8k t 0 t ] (10) Here, the height of the ppleental rectangle can alo excee ax. In contrat to the ajtent of neighboring rectangle to copenate for the rpl in tep 2, the rpl in tep 3 i copenate for in rectangle that have with becae it i ore effective than covering the rpl in the next rectangle that have a aller with. The area of the rpl an ppleent are c 1. [ 1,0 (. 1,0 (- ax ] an c 1. [ 1,1 (- 1,0 (]. 1,1 (, repectively. In a iilar way to tep 2, the general oltion of the i caling factor ( in tep 3 can be acqire a: k, k, 1 1,2,,8k. k, 1 8k t k, 1 k, t ax, (11) (a) (b) (c) () Fig. 11. Soltion of the contraine iization proble in (8) in 1- ienional iagra. To iniize the increent change of the total height, the bet approach i to only increae the height of the center rectangle becae it with c 0 i the larget aong the BSF coefficient. Therefore, in tep 1, the hortfall of the backlight linance relative to the i i relieve by aing a ppleental rectangle of c 0 in with, a hown in Fig. 11 (a) an (b). A the vale of the hortfall in backlight linance, B (-B(, an the ppleental area of the rectangle, c 0 [ 0,0 (-1]. 0,0 (, are eqal, the oltion of the i caling factor in tep 1 i acqire by: If the height of the ppleental rectangle oe not excee ax, the ret of the caling factor are a hown below:, t 1, 2,,8. (12) t ( k The caling factor whoe firt inex are larger than k are nity (=1). III. EXPERIMENTAL RESULTS We have tete the propoe algorith ing three aple iage. Saple iage with a reoltion were ilate with a local iing backlight yte of 16 8 block ing MATLAB R2008a. The iage were ilate by ing APL, Max [5] an the propoe algorith

6 (TH=0.0001) hown in Fig. 12. The backlight power conption percentage, total clipping eareent ( of clipping eare of each local block) an clipping ratio of iage in Fig. 12 are hown in Table II an Table III, repectively. The clipping ratio i erive a: Total clipping eare Clipping ratio 100(%). Total nber of pixel (13) For a bright iage (a contryie hot), the backlight power conption wa 57.07, an 63.35% an the clipping ratio wa , an % when ing APL, Max an the propoe algorith, repectively a hown in Table II (a) an Table III (a). The APL algorith i effective for recing power conption bt it cae a erio clipping artifact. The Max algorith i effective in recing the clipping artifact bt it increae power conption. On the other han, the propoe algorith i ore effective in recing the power conption than the Max algorith, epite the fact that it i till effective at clipping rection. For a ark iage (a torehoe), the propoe algorith ave on power conption conierably copare to the Max algorith an rece the clipping artifact when copare to the APL algorith a hown in Table II (b) an Table III (b). TABLE II POWER CONSUMPTIONS OF IMAGES TO PERCENTAGE IN FIG. 12 Iage (a) (b) (c) Inpt iage 100 % - exact 100% - exact 100% - exact APL Max Propoe TABLE III TOTAL CLIPPING MEASURES AND CLIPPING RATIOS OF IMAGES IN FIG. 12 Iage (a) (b) (c) Inpt iage 0 (0%)-exact 0 (0%)- exact 0 (0%)- exact APL 39228(1.8918) 11335(0.5466) (7.4198) Max 356(0.0172) 3765(0.1816) 46(0.0022) Propoe 79(0.0038) 37(0.0018) 71(0.0034) For a high contrat iage (a treet venor), the APL algorith rece the backlight linance by too ch which relt in evere clipping artifact a hown in Fig. 12 (c) an Table III (c). To copenate the backlight linance, the propoe algorith increae the backlight linance enogh to contrain the clipping artifact, th the power conption i increae ignificantly. Howeve the propoe algorith till ha the avantage on power conption when copare to the Max algorith a hown in Table II (c). The propoe algorith i iize to rece the clipping artifact bject to INPUT IMAGE APL [5] MAX [5] PROPOSED (a) (b) (c) Fig. 12. Saple iage: (a) ark iage (torehoe), (b) bright iage (contryie) (c) high contrat iage (treet venor).

7 iniize the increent of the backlight linance. Coniering all apect, we i not want to excle the etho of eterining the threhol (TH) fro the propoe fraework becae another threhol col proce better relt accoring to the brightne of the inpt iage, an we leave the invetigation of thi a a ftre work. IV. CONCLUSION We have propoe a novel BLC algorith which eterine the ial increent of backlight linance by olving the propoe contraint iniization proble to rece the clipping artifact. Experiental relt how that the propoe algorith rece the clipping artifact in a cot-effective anner copare to the Max algorith. ACKNOWLEDGMENT The athor wol like to thank the reearcher of LG Diplay Korea for their pport. REFERENCES [1] C.-C. Lai an C.-C. Ta Backlight Power Rection an Iage Contrat Enhanceent Uing Aaptive Diing for Global Backlight Application IEEE Tran. Coner Electronic, vol. 54, no. 2, pp , May [2] H. Cho an O.-K. Kwon, A Backlight Diing Algorith for Low Power an High Iage Qality LCD Application, IEEE Tran. Coner Electronic, vol. 55, no. 2, pp , May [3] T. Fnaoto, T. Kobayah an T. Mrao, High-Pictre-Qality Techniqe for LCD Televiion: LCD-AI, Proc. International Diplay Workhop, pp , [4] H. Seetzen, W. Heirich, W. Sterzlinge G. Wa L. Whitehea, M. Trentacote, A. Ghoh, an A. Vorozcovy, High Dynaic Range Diplay Syte, ACM Tranaction on Graphic, pp , [5] H. Chen, J. Sng, T. Ha an Y. Par Locally Pixel-Copenate Backlight Diing for Iproving Static Contrat on LED Backlight LCD, SID2007 Diget of Technical pape May 20-25, 2007, pp [6] C.-C. Lai an C.-C. Ta Backlight Power Rection an Iage Contrat Enhanceent Uing Aaptive Diing for Global Backlight Application, IEEE Tran. Coner Electronic, vol. 54, no. 2, pp , May [7] T. Lee, J. Lee, C. Ki, S. Kang, An ical feeback yte for local iing backlight with RGB LED, IEEE Tran. Coner Electronic, vol. 55, no. 4, pp , Nov [8] T. Shiga, S. Kwahara, N. Takeo an S. Mikohiba, Aaptive Diing Techniqe with Optically Iolate Lap Grop, SID2005 Diget of Technical pape pp , [9] W-C. Cheng an M. Pera, Power iniization in a backlit TFTLCD iplay by concrrent brightne an contrat caling, IEEE Tran. Coner Electronic, vol. 50, no. 1, pp , Feb [10] Y. Ki, Contrat enhanceent ing brightne preerving bihitogra eqalization, IEEE Tran. Coner Electronic, vol. 43, no. 1, pp. 1-8, Feb Seong-En Ki (S 08) wa born in Changwon, Korea, on Agt 9, He receive hi B.S. egree in electronic an electrical engineering fro Pohang Univerity of Science an Technology (POSTECH), Korea, in Since 2004, he ha been a Reearch Aitant at the Departent of Electronic an Electrical Engineering, POSTECH, where he i crrently working towar the Ph.D. egree. Hi reearch interet incle ltieia ignal proceing, ignal proceing for iplay, an aaptive ignal proceing. Woo-Jin Song (M 86) wa born in Seol, Korea, on October 23, He receive hi B.S. an M.S. egree in electronic engineering fro Seol National Univerity in 1979 an 1981, repectively an hi PhD egree in electrical engineering fro Renelaer Polytechnic Intitte in Dring , he worke at Electronic an Teleconication Reearch Intitte (ETRI), Korea. In 1986, he wa eploye by Polaroi Corporation a a enior enginee working on igital iage proceing. In 1989, he wa proote to principal engineering at Polaroi. In 1989, he joine the faclty at Pohang Univerity of Science an Technology (POSTECH), Korea, where he i a profeor of electronic an electrical engineering. Hi crrent reearch interet are in the area of igital ignal proceing, in particla raar ignal proceing, ignal proceing for igital televiion an ltieia proct, an aaptive ignal proceing. BIOGRAPHIES Jong-J Hong wa born in Haong, Korea, on Noveber 27, He receive hi B.S. egree in electronic an electrical engineering fro Pohang Univerity of Science Technology (POSTECH), Korea, in Since 2008, he ha been a Reearch Aitant at the Departent of Electronic an Electrical Engineering, POSTECH, where he i crrently working towar hi MS egree. Hi reearch interet incle ltieia ignal proceing for iplay an iage ignal proceing.