How to reduce liht leakae and clippin in local-dimmin liquid-crystal displays Seon-Eun Kim (SID Student Member) Joo-Youn An Jon-Ju Hon Tae Wook Lee Chan Gone Kim Woo-Jin Son Abstract In conventional LCDs, the backliht is set to maximum luminance reardless of the imae. For dark scenes, this approach causes liht leakae and power waste. Especially, liht leakae in dark scenes derades the contrast ratio of LCDs; to circumvent this problem, local-dimmin systems have been proposed. In these systems, the LED backliht is divided into several local blocks and the backliht luminance of each local block is controlled individually, and pixel values are adjusted simultaneously accordin to the luminance profile of the dimmed backliht. In this paper, a method of determinin the LED backliht luminance of each local block dependin on the imae is proposed; this method sinificantly improves the imae quality of LCDs. First, we introduce methods of quantifyin liht-leakae at dark ray levels and clippin at briht ray levels. Then, the proposed method to determine the dimmin duty, which controls the LED backliht luminance by compromisin between these two measures, was derived. The proposed alorithm preserves the oriinal imae with little clippin distortion and effectively reduces liht leakae. Keywords Liquid-crystal display (LCD), backliht local dimmin, contrast ratio, power consumption, liht leakae, clippin. DOI # 10.1889/JSID17.12.1051 1 Introduction The liquid-crystal display (LCD) is the most popular display device in the flat-panel-display marketplace due to its excellent performance. However, conventional LCDs have serious liht leakae of the liquid-crystal (LC) because the LC cannot completely obstruct the backliht luminance when the LC is fully closed to display black pixels. 1 In addition, the backliht of conventional LCDs is always at maximum luminance reardless of the imae even when displayin a dark imae. Consequently, dark reions of imaes do not look dark enouh and true black cannot be realized. This problem causes a low contrast ratio and unnecessary power consumption. 2 To reduce the liht-leakae problem, two types of modifications have been proposed. One is to improve the structure of the LC to reduce the transmittance of the LC at low ray levels. However, usin this method alone is not sufficient to increase contrast ratio. The other modification is to dim the backliht when displayin dark scenes. These dimmin techniques can be cateorized into three classes: lobal dimmin, line dimmin, and block dimmin. First, lobal-dimmin methods were proposed. 3 8 These methods consider the imae, then dim all backliht luminance uniformly by an appropriate factor k and enhance the transmittance of the LC by 1/k. However, because briht reions and dark reions occur toether in the imae, lobal dimmin methods do not reduce liht leakae effectively to preserve briht reions. As an alternative, several local (line or block) dimmin methods have been proposed, which divide the backliht into several local blocks, then dim the backliht individually in these blocks. 9 16 When LCDs were first developed, line fluorescent lamps were enerally used as LCD backlihts, so line dimmin methods were proposed. 9,10 Recently, due to the development of liht-emittin-diode (LED) backlihts, block-dimmin methods have received more attention. For local block dimmin, the backlihts are enerally rouped into rectanular blocks and the imae is divided into non-overlappin reions correspondin to the backliht blocks, shown in Fi. 1. Thus, the luminance of backliht blocks can be dimmed independently accordin to the local imae. 11 16 Therefore, because LED backlihts are dimmed only in dark reions, the contrast ratio increases and power consumption decreases. In addition, pixel values are adjusted locally accordin to the backliht luminance, so pixel luminance is preserved. However, by dimmin the backliht to reduce liht leakae, the maximum luminance for the brihtest pixel is decreased, which causes the clippin problem at briht ray levels after pixel compensation. Therefore, in backliht local dimmin, it is important to determine the backliht luminance of each local block to improve the quality of LCDs when briht reions and the dark reions occur in the same block. In this paper, we propose a novel local LED-backliht block-dimmin alorithm to determine the backliht luminance in each local block. This alorithm considers both liht leakae and clippin of local block imaes. We first define a liht-leakae measure and a clippin measure. The proposed alorithm determines backliht luminance of each Expanded revised version of a paper presented at the 15th International Display Workshops held in Niiata, Japan, December 3 5, 2008. S-E. Kim, J-J. Hon, and W-J. Son are with Pohan University of Science and Technoloy, Division of Electrical and Computer Enineerin, San 31, Hyoja-don, Nam-u, Pohan, Gyunbuk, 790-784, Korea; telephone +82-54-279-2789, fax 8686, e-mail: wjson@postech.ac.kr. J-Y. An, Solid Technoloies, Gyeoni-do, Korea. T. W. Lee and C. G. Kim, LG Display, Gyeoni-do, Korea. Copyriht 2009 Society for Information Display 1071-0922/09/1712-1051$1.00 Journal of the SID 17/12, 2009 1051
FIGURE 2 Pixel luminance of the LCD panel. Therefore, a method is required that achieves a trade-off between the Averae and the Max methods. For that reason, historam-based backliht-dimmin methods 9 11 wereproposed.inref.9,whenthedimmin duty d is determined, the inverse of the cumulative distribution function of the entire imae is used; this approach considers the characteristics of the entire imae. In Ref. 10, the historam is used to determine the total error caused by dimmin the backliht, and the value of d that minimizes the total error. In Ref. 11, the major ray levels, small briht objects, and noise pixels of each imae block can be balanced when determinin the LED-backliht luminance. In this paper, we define a liht-leakae measure and a clippin measure for each local block based on the historam. Unlike the conventional historam-based dimmin methods, the proposed backliht-dimmin alorithm balances the two measures by considerin the correlation of inter-blocks. FIGURE 1 The structures of the LED backliht unit and liquid-crystal cells for local block dimmin. local block usin a decision rule that determines optimal backliht luminance by comparin these measures. The proposed alorithm reduces the liht-leakae effectively in dark reions with little clippin in briht reions. This paper is oranized as follows. Section 2 introduces previous backliht local-dimmin methods and the characteristics of LCDs. Section 3 presents the proposed alorithm. Section 4 contains experimental results and Sec. 5 presents conclusions. 2.2 LCD characteristics Generally, the luminance of a pixel is determined by the product of the LC transmittance and the backliht luminance, as shown in Fi. 2. Furthermore, the pixel luminance of LCDs should be set such that luminance of all ray levels meet the ideal amma characteristics, which are represented by the dashed line in Fi. 3. The ideal pixel luminance Y ideal () is defined as Yideal (), = F H G I K J (1) 2 Backround 2.1 Previous work The critical problem of local dimmin is how to choose the luminance of each LED local block accordin to the imae. The simplest method is the Averae method, 14 which determines the LED luminance based on the averae ray level of each imae block. This method has the disadvantae that it may decrease the brihtness and lose information in briht reions. Another method is the Max method, 17 which sets the LED luminance of each block to the maximal ray level of each imae block. The Max method has the disadvantaes that it is very sensitive to noise and that it wastes power; it also suffers from the liht-leakae problem. FIGURE 3 Liht-leakae problem of LCDs; the dashed line is an ideal amma curve and the solid line represents the liht leakae. 1052 Kim et al. / How to reduce liht leakae and clippin in local-dimmin LCDs
for hih ray levels cannot fulfill the ideal amma characteristics even when pixel compensation is executed, so Y(, d) above a threshold Y c (d) is clipped owin to a shortae of backliht luminance, as represented by the solid line in Fi. 5(b), where Y c (d) isivenby d Yc ( d ) = Y (, d ) = Yreal ( ). (6) FIGURE 4 The relationship between pixel luminance and dimmin duty. where is a ray level and γ is an ideal taret amma. However, LCs cause liht leakae at low ray levels, so that conventional LCDs fail to meet the ideal amma characteristics at low ray levels, which is presented by the solid line in Fi. 3. Let us denote the real measured pixel luminance of LCDs as Y real (), which is iven by Y real () =k() +Y ideal (), (2) where k() is the amount of liht leakae at low ray levels. In practice, k() is calculated by the difference between the measured Y real () and the ideal taret Y ideal () asfollows: k() =max[y real () Y ideal (), 0], (3) where max(a, B) means that the larer one between A and B is taken. To reduce the liht leakae, the backliht should be dimmed for blocks that have many low ray levels. However, as the backliht is dimmed, the pixel luminance is also reduced for all ray levels. For LED backlihts, the backliht luminance is controlled by a dimmin duty d, whichis an inteer between 0 (black) and (white). Thus, the effect of d on Y real () should be investiated in detail. To investiate the relationship between d and Y real (), an experiment was conducted in which all pixels were set to thesameraylevel(i.e., 127, 223, or ), and d was varied from 0 to. Then, the pixel luminance was measured in the fixed block. In Fi. 4, the pixel luminance Y(, d) after dimmin backliht is experimentally iven by d Yd (, ) = Yreal(). (4) As a result, if backliht-dimmin alorithms reduce d to eliminate liht leakae, they also reduce Y(,d) foreach ray level, as shown in Fi. 5(a). To compensate for the reduced luminance, ray levels should be adjusted usin pixel compensation. 15 The ray level is enhanced by the compensation factor C(d) as follows: = C(d), (5) where C(d) is obtained by solvin Y(, d) =Y ideal (), which is enerally constituted by a look-up table. However, Y(, d) In addition, althouh the backliht is dimmed, Y(, d) still has liht leakae at low ray levels compared with Y ideal () because Y(0, d) =(d/)k(0) is always larer than Y ideal (0) = 0, when d 0andk(0) 0. Dimmin the backliht reduces liht leakae, but causes clippin at hih ray levels. Thus, determination of the appropriate value of d must compromise between its effects on liht leakae and clippin. 3 Proposed alorithm 3.1 Proposed dimmin duty decision rule The major oal of dimmin the backliht is to reduce liht leakae and thereby increase the contrast ratio. However, clippin should not derade the quality of the imae. For this reason, the alorithm should simultaneously consider FIGURE 5 The results of dimmin the backliht: (a) As a result of dimmin the backliht, pixel luminance is reduced for all ray levels. (b) Some hih ray levels cannot be compensated due to reduced luminance. Journal of the SID 17/12, 2009 1053
artifacts caused by liht leakae and clippin. To consider the effect of these artifacts, we define an error as the difference between the ideal taret luminance and the pixel-compensated luminance after dimmin. Generally, subjective brihtness, which is the intensity as perceived by the human visual system, is a loarithmic function of liht intensity. 18 Thus, two distortion measures based on subjective brihtness are necessary. Because the backliht is divided into i rows and j columns of blocks, these measures must be determined separately for each block. The inverse amma function is applied when luminance is converted to subjective brihtness, so the subjective brihtness error e c (, d i,j ) caused by clippin is iven by where d i,j is the dimmin duty of the block in row i and column j. The clippin measure should consider both the distortion area and the quantity of the subjective brihtness error. Therefore, the clippin measure D c (i, j) isivenby where n i, j () is the number of pixels that have ray level in the (i, j)-th block. Also, the liht-leakae measure D l (i, j) is iven by where L NM b e j -1-1 ec(, di, j) = max Yideal() - Yc( di, j), 0, Dc(, i j) = Â ec(, dij, ) nij, ( ), = 0 Dl(, i j) = Â el(, dij, ) nij, ( ), = 0 L NM e j b el(, dij, ) = max Y(, dij, ) - Yideal(),. (7) (8) (9) (10) Determination of the appropriate value of d must compromise between the conflictin effects of clippin and liht leakae. Thus, d should be determined by comparin D c (i, j) andd l (i, j). If D c (i, j) >D l (i, j), then artifacts caused by clippin at hih ray levels are reconized more easily than artifacts caused by the liht leakae. In that case, d is increased to decrease D c (i, j). On the other hand, if D c (i, j) D l (i, j), d is decreased to reduce D l (i, j). However, the weihts of D c (i, j) andd l (i, j) vary accordin to the characteristics of the imae. As a result, the proposed dimmin duty decision rule to search the optimal dimmin duty d i,j o iteratively is iven by O QP -1-1 0 O QP 3.2 Determinin the weiht parameter The critical problem of the dimmin duty decision rule is how to choose the value of α i,j of each block accordin to the imae. For a iven α i,j, as a dark reion of a block becomes wider, the dimmin duty of the block decreases accordin to Eq. (11). However, when we consider the scene of a dark sky with some briht stars, if the backliht luminance is too small due to a broad dark reion, then the briht stars may look dim. Therefore, if a very dark reion includes briht objects, backliht luminance is increased to preserve the brihtness of objects in this reion. Generally, we assume that the briht objects in a dark reion have the maximum pixel value in the block. Therefore, if maximum luminance of a block is briht and an averae luminance of adjacent blocks is dark, then α i,j must be increased to prevent clippin and preserve details since the diffusion of liht by backliht of adjacent blocks is insufficient. Otherwise, α i,j must be decreased to prevent liht leakae and increase contrast ratio. By this loic, α i,j should be proportional to a ratio of an averae pixel value of adjacent blocks to the maximal pixel value of a current block. Assumin that adjacent blocks are in the M M macro block centerin around the current (i, j)-th block, as shown in Fi. 6, then α i,j is iven by a ij, = MAX(, i j), 1 ( M-1) 2 ( M-1) 2 APL( i -a, j -b) 2 Â Â M a=-( M-1) 2 b=-( M-1) 2 (12) where APL(i, j) is the averae of all pixel values, MAX(i, j) is the maximum of all pixel values in the block in row i and column j and M is the size of the macro block. 3.3 Investiation on the proposed alorithm In some cases, the proposed alorithm may require many iterations to find d i,j o that satisfies D l (i, j) =α i,j D c (i, j). The d i,j o = d i,j if D l (i, j) =α i,j D c (i, j), (11) where α i,j is a weiht parameter for the block in row i and column j. α i,j controls the relative importance of clippin and liht leakae accordin to the imae. The value of d that satisfies Eq. (11) can reduce the liht leakae, thus minimizin the number of sinificant artifacts at hih ray levels. FIGURE 6 AnM M macro block centerin around the (i, j)-th block. 1054 Kim et al. / How to reduce liht leakae and clippin in local-dimmin LCDs
FIGURE 8 Flow diaram of the procedure used in the experiments. FIGURE 7 An efficient method for findin the optimal dimmin duty iteratively, which reduces the complexity of the proposed alorithm. complexity of the proposed alorithm increases as the number of iterations increases. Thus, we present an effective method to find d i,j o. Fiure 7 shows the effective method. The proposed method is presented in six steps for the block in row i and column j as follows: Step 1: initialize d max =, d min =0,andd i,j = APL(i, j). Step 2: calculate α i,j. Step 3: calculate D l (i, j) andd c (i, j) usind i,j. Step 4: if D l (i, j) >α i,j D c (i, j), then d max = d i,j, d min is unchaned, else d min = d i,j, d max is unchaned. Step 5: update d i,j = (d max + d min )/2. Step 6: repeat step 3 step 5 until d max d min =1. where represents roundin off to the nearest inteer. 4 Experimental results The proposed alorithm was implemented in a full-hd 47- in. LCD TV equipped with white LED backlihts, which are rouped into 16 columns of eiht blocks. To verify the proposed alorithm s performance, three methods are comparedinthelcdpanel,suchastheaveraemethod,the Max method, and the proposed method. After dimmin the backliht usin the three methods, the pixel compensation proposed by Ref. 16 was executed, as shown in Fi. 8. In FIGURE 9 The resultin imaes of (a) not usin any dimmin methods; (b) the Averae method; (c) the Max method; (d) the proposed method. Journal of the SID 17/12, 2009 1055
FIGURE 10 Comparison of dimmin duties determined by the Averae method, the Max method, and the proposed method in (a) a briht reion and (b) a dark reion. The weiht parameters are represented in parenthesis. addition, the size of macro block M and the ideal amma γ were set to 3 and 2.2, respectively. Fiure 9 shows the experimental results to compare the liht leakae and details in the dark reion of the three methods. The liht leakae can plainly be reconized when any backliht local dimmin is not applied in Fi. 9(a). In Fi 9(b), the Averae method can reduce liht leakae effectively because of the reduced backliht luminance, but details in dark reions are almost lost. In Fi 9(c), the Max method retains the details, but does not effectively reduce liht leakae that occurs due to the briht backliht. On the other hand, the proposed alorithm preserves most imae details in dark reions and reduces liht leakae sinificantly,asshowninfi.9(d). Fiure 10 shows values of d, which were determined by the three methods in six blocks that form a part of the test imae shown in Fi. 9. The values of d enerated by the proposed method were lower than those of both the Max method and the Averae method in most reions, but were sufficiently lare to minimize clippin distortion when briht objects exist in dark reions. The power consumption of the proposed alorithm and the conventional alorithms are demonstrated in Fi. 11. AscanbeseeninFi.11,thepowerconsumptionrateofthe proposedalorithmisalmosthalfthatofthemaxmethod without loss in brihtness. Because the power consumption rate depends on the imae contents, the power consumption rate of the proposed alorithm is between that of the Max method and that of the Averae method. 5 Conclusions We have presented a noble method of determinin the dimmin duty of each local block by usin the proposed decision rule. The proposed decision rule searches the optimal dimmin duty throuh the comparison between the liht-leakae measure and the clippin measure. Two measures have been introduced to reflect the manitude and occurrence of subjective brihtness error. Compared with conventional methods, the proposed alorithm preserves the oriinal imae with little clippin distortion and reduces liht leakae effectively, so the contrast ratio is improved. In addition, power consumption is reduced due to the reduction of dimmin duty. Acknowledment This work was supported by LG Display, the IT R&D Proram of MKE/MCST/IITA (2008-F-031-01, Development of Computational Photoraphy Technoloies for Imae and Video Contents), the HY-SDR Research Center at Hanyan University under the ITRC Proram of MKE, and the Brain Korea (BK) 21 Proram funded by Ministry of Education, Science and Technoloy, Korea. FIGURE 11 The averae power consumption rates deduced from averae dimmin duties determined by the Averae method, the Max method, and the proposed method. References 1 P. Yeh and C. Gu, Optics of Liquid Crystal Displays (Wiley, New York, 1999), Chap. 5. 2 A. Konno et al., RGB color systems for LED backliht in IPS-LCD TVs, SID Symposium Diest 36, 1380 1383 (2005). 3 I. Hwan et al., Imae synchronized brihtness control, SID Symposium Diest 32, 492 493 (2001). 4 N. Raman and G. Hekstra, Dynamic contrast enhancement of liquidcrystal displays with backliht modulation, Intl. Conf. on Consumer Electronics Diest of Technical Papers, 197 198 (2005). 5 L. Kerofsky and S. Daly, Brihtness preservation for LCD backliht dimmin, J. Soc. Info. Display 14, No. 12, 1111 1118 (2006). 1056 Kim et al. / How to reduce liht leakae and clippin in local-dimmin LCDs
6 J. Stessen and J. Mourik, Alorithm for contrast reserve, backliht dimmin, and backliht boostin on LCD, SID Symposium Diest 37, 1249 1252 (2006). 7 C-F. Hsu et al., Backliht power reduction and imae contrast enhancement usin adaptive dimmin for lobal backliht applications, SID Symposium Diest 39, 776 779 (2008). 8 S.-J. Lee et al., A power reduction method for LCD backliht based on human visual characteristics, Intl. Conf. on Consumer Electronics Diest of Technical Papers, 1 2 (2008). 9 E.Y.Ohet al., IPS-mode dynamic LCD-TV realization with low black luminance and hih contrast by adaptive dynamic imae control technoloy, J. Soc. Info. Display 13, No. 3, 215 219 (2005). 10 P. D. Greef and H. G. Hulze, Adaptive dimmin and boostin backliht for LCD-TV systems, SID Symposium Diest 38, 1332 1335 (2007). 11 H. Pen et al., Hih contrast LCD TV usin active dynamic LED backliht, SID Symposium Diest 38, 1336 1338 (2007). 12 T. Shia et al., Power savins and enhancement of ray-scale capability of LCD TVs with an adaptive dimmin technique, J. Soc. Info. Display 16, No. 2, 311 316 (2008). 13 D.-M. Yeo et al., Smart alorithms for local dimmin LED backliht, SID Symposium Diest 39, 986 989 (2008). 14 W. Lee et al., White LED backliht control for motion blur reduction and power minimization in lare LCD TVs, J. Soc. Info. Display 17, No. 1, 37 45 (2009). 15 F. Lin et al., Inverse of mappin function (IMF) method for imae quality enhancement of hih dynamic rane LCD TVs, SID Symposium Diest 38, 1343 1346 (2007). 16 H. Chen et al., Locally pixel-compensated backliht dimmin on LED-backlit LCD TV, J. Soc. Info. Display 15, No. 12, 981 988 (2007). 17 H. Seetzen et al., Hih dynamic rane display systems, ACM Trans. Graphics 23, No. 3, 760 768 (2004). 18 R. C Gonzalez and R. E. Woods, Diital Imae Processin, 2nd edn. (Prentice Hall, Upper Saddle River, New Jersey, 2002), Chap. 2. Seon-Eun Kim received his B.S. deree in electronic and electrical enineerin from Pohan University of Science and Technoloy (POSTECH), Korea, in 2004. Since 2004, he has been a Research Assistant at the Department of Electronic and Electrical Enineerin, POSTECH, where he is currently workin toward his Ph.D. deree. His research interests include multimedia sinal processin, sinal processin for display, and adaptive sinal processin. Joo-Youn An received his B.S. and M.S. derees in electronic and electrical enineerin from Pohan University of Science and Technoloy (POSTECH), Korea, in 2007 and 2009, respectively. In 2009, he joined Solid Technoloy, Inc., and since then he has been enaed in the research and development of the metropolitan pole repeater. Jon-Ju Hon received his B.S. deree in electronic and electrical enineerin from Pohan University of Science Technoloy (POSTECH), Korea, in 2008. Since 2008, he has been a Research Assistant at the Department of Electronic and Electrical Enineerin, POSTECH, where he is currently workin toward his M.S. deree. His research interests include multimedia sinal processin for display and imae sinal processin. Tae Wook Lee is a Senior Research Enineer at LG Display, Timin Controller Development Team, Paju, Korea. He received the B.S., M.S., and Ph.D. derees from University of Ulsan, Korea in 1998, 2000, and 2004, respectively. His Ph.D. work was on the inner product optimization and its application to imae compression. In 2004, he joined LG Display Co. and since then he has worked on the backliht drivin, LED local dimmin, and timin controller for LCD TVs. Chan Gone Kim is a Chief Research Enineer at LG Display, Timin Controller Development Team, Paju, Korea. He received his B.S. and M.S. derees from Kyunbook National University of Korea. Now, he is responsible for desinin timin controllers as a Leader at the Timin Controller Development Team, LG Display R&D Center. Woo-Jin Son received his B.S. and M.S. derees in electronics enineerin from Seoul National University in 1979 and 1981, respectively, and his Ph.D. deree in electrical enineerin from Rensselaer Polytechnic Institute in 1986. Durin 1981 1982, he worked at Electronics and Telecommunication Research Institute (ETRI), Korea. In 1986, he was employed by Polaroid Corp. as a senior enineer, workin on diital imae processin. In 1989, he was promoted to principal enineerin at Polaroid. In 1989, he joined the faculty at Pohan University of Science and Technoloy (POSTECH), Korea, where he is a professor of electronic and electrical enineerin. His current research interests are in the area of diital sinal processin; in particular, radar sinal processin, sinal processin for diital television and multimedia products, and adaptive sinal processin. Journal of the SID 17/12, 2009 1057