A Modified Stripe-RGBW TFT-LCD with Image-Processing Engine for Mobile Phone Displays

Transcription

1 1628 IEEE Transactins n Cnsumer Electrnics, Vl. 53, N. 4, NOVEMBER 27 A Mdified Stripe-RGBW TFT-LCD with Image-Prcessing Engine fr Mbile Phne Displays Chih-Chang Lai and Ching-Chih Tsai, Senir Member, IEEE Abstract - This paper presents a Mdified Stripe-RGBW (MS-RGBW) clr filter structure t keep the same high reslutin, and btain a higher brightness in cmparisn with cnventinal RGB clr filters. An image-prcessing engine is als designed t achieve sharp text image fr thin-film-transistr (TFT) LCD with the MS-RGBW clr filter. In MS-RGBW clr filter structure, each pixel with three sub-pixels is the same area t that in the cnventinal RGB stripe clr filter, and each rw shifts tw sub-pixels. The image-prcessing engine cnsists f tw new algrithms: RGB-RGBW mapping algrithm and sub-pixel rendering algrithm. The RGB-RGBW mapping algrithm btains a new RGBW image data withut distrtin in hue and saturatin. The sub-pixel rendering algrithm transfers RGBW data int a MS-RGBW clr filter structure in rder t achieve sharp text image. Numerus simulatin results are prvided t illustrate the merits and perfrmance f the prpsed techniques. The usefulness f the prpsed techniques fr mbile phne displays is exemplified by cnducting several experimental results n a 2.2-inch TFT LCD. Index Terms: Brightness, clr filter, image-prcessing engine, rendering, RGBW. Py Py R G B R G B R G B R G B R G B R G B R G R G R G W B W B W B R G R G R G W B W B W B Py R G B W R G B W R G B W R G B W R G B W R G B W I. INTRODUCTION Recently, the TFT-LCD market fr the mbile applicatins has been raised up extrardinarily. Since the mbile display devices require many functins such as still picture, game, mving picture, and navigatin, the demands fr the LCD panels with high reslutin and brightness have been cnstantly grwing up. In the same size f LCD panels in Fig. 1, the higher the reslutin f the panel is, the lwer the brightness is. The brightness is intimately linked with the backlight. T increase the brightness f the LCD panel, mre LEDs t prvide backlight shuld be needed at the cst f increasing pwer cnsumptin and price. Fr that reasn, many researchers have cnsidered the RGBW display technlgy in Fig. 1 as a prmising slutin [1-5]. These RGBW displays have imprved luminance and high cntrast while cmpared with equivalent RGB displays [2]. The authrs in [1, 2] used a vertical stripe RGBW (VS RGBW) in Fig. 1 and a checkerbard RGBW in Fig. 1. Hwever, the VSRGBW increased ne-third f ttal data Ching-Chang Lai and Ching-Chih Tsai ( cctsai@dragn.nchu.edu.tw) are with the Department f Electrical Engineering, Natinal Chung-Hsing University, Taichung, Taiwan. Ching-Chang Lai is als with Wintek Crpratin, Taichung, Taiwan. This wrk was in part supprted by the M.O.E., Taiwan, under the ATU plan. (e) Fig. 1. Cnventinal clr filter structures. RGB Stripe. VSRGBW. Checkerbard RGBW. Kdak s RGBW. (e) PenTile RGBW. channels and reduced the aperture rate f each pixel with 4 sub-pixels, and the checkerbard RGBW clr filter structure required the duble scan lines and decreased the aperture rati f each pixel with its 4 sub-pixels. Arnld et al. [3] prpsed a RGBW clr filter structure in Fig. 1 that each pixel had 4 hrizntal pixels and each sub-pixel area was equal t that in RGB stripe; bviusly the structure reduced its reslutin. Researchers in [4, 5] described a PenTile RGBW clr filter structure in Fig. 1(e) with the benefits, such as reducing the data channels number and enhancing the aperture rati. Hwever, the cntinuus lines in this PenTile RGBW clr filter under lw reslutin (<22 ppi) became discntinuus. T circumvent the afrementined shrtcmings, an MS-RGBW clr filter Cntributed Paper Manuscript received September 11, /7/$2. 27 IEEE

2 C.-C. Lai and C.-C. Tsai: A Mdified Stripe-RGBW TFT-LCD with Image-Prcessing Engine fr Mbile Phne Display 1629 Py R G B R G B R G B Py R G B W R G B W R R G B R G B R G B B W R G B W R G B Fig. 2. Clr filter structures. RGB Stripe. MS-RGB W. Fig. 5. Structure f prpsed TFT LCM with MS-RGBW and image prcess engine. Fig. 3. Blck diagram f the prpsed image-prcessing engine. Fig. 6. Structure f driver ICs with prpsed image-prcessing engine. Fig. 4. A white dt f MS-RGBW. A basic arrangement f MS-RGBW. structure in Fig. 2 is prpsed in this paper t imprve the brightness and retain the same high reslutin. Image-prcessing engine is usually used fr mapping riginal RGB image data t the crrespnding RGBW data, and rearranging these RGBW image data t match with their wn sub-pixel clr filters. In ding s, any image-prcessing engine requires tw schemes: RGB-RGBW mapping algrithm and sub-pixel rendering algrithm. Image-prcessing engines have already been applied in mst f mdern display prducts [1-5]. RGB- RGBW mapping algrithms were prpsed fr their wn RGBW clr filter structure displays [1-5]. Hwever, these appraches mentined in [1-5] cannt be directly applied t ur prpsed MS-RGBW clr filter withut mdificatin. Sub-pixel rendering algrithm has been applied t MS-RGBW structure in [6], but its mixing effect has been shw t blur text display. In this paper, a new image-prcessing engine in Fig. 3 tgether with its wn RGB-RGBW mapping and sub-pixel rendering algrithms is presented fr ur prpsed clr filter structure. The prpsed image-prcessing engine will be shwn useful in reducing image distrtin and achieving sharp text image. The remainder f the paper is utlined as fllws. Sectin II describes advantages f the MS-RGBW clr filter structures. Sectin III prpses the RGB-RGBW mapping algrithm fr cnverting the RGBW data. Sectin IV describes the sub-pixel rendering algrithm fr matching the RGBW data t the MS-RGBW clr filter. Sectin V presents and discusses the experimental results. Sectin VI cncludes the paper. II. MS-RGBW COLOR FILTER STRUCTURE Fig. 2 cmpares the clr filter structures f the RGB Stripe and MS-RGBW. The MS-RGBW has the same sub-pixel size as the RGB Stripe. As shwn in Fig. 2, each pixel in MS-RGBW cnsists f three sub-pixels, and has fur kinds f arrangements, such as RGB, WR BWR and GBW. A simple example f GBW pixel arrangement in MS-RGBW is shwn in Fig. 4. Such a GBW pixel cmbines the surrunding fur R sub-pixels fr btaining the white pixel n a black backgrund. The similar idea hlds fr ther thee kinds f pixel arrangements, in rder t keep the same aperture rati and reslutin. These pixel-arrangement designs are cmpleted achieved by repeating arrangement f eight sub-pixels (e.g., the first rw is RGBW, and the secnd rw is BWRG). As depicted in Table I, the MS-RGBW structure has the fur merits: it uses identical TFT arrays f cnventinal RGB Stripe, has the same channels f driver ICs with prpsed image-prcessing engine in Fig. 5, keeps high reslutin f 24x32 pixels, and enhances 5% brightness. The image-prcessing engine is embedded int the driver ICs between the RAM and the utput channels in Fig. 6. Fig. 7 respectively depicts the simulatin results f RGB stripe, PenTile RGBW and MS-RGBW. As can be seen in Fig. 7, MS-RGBW has sharper and mre cntinuus texts than PenTile RGBW.

3 163 IEEE Transactins n Cnsumer Electrnics, Vl. 53, N. 4, NOVEMBER 27 Table I. Perfrmance Cmparisn between RGB Stripe, VS RGBW, Checkerbard RGBW, Kdak s RGBW and prpsed MS-RGBW. 2.2 inch QVGA TFT LCD Clr filter RGB VS RGBW Checkerbard Kdak s Prpsed structure Stripe RGBW RGBW MS-RGBW Data channel number 24x3 24x4 24x2 24x3 24x3 Scan channel number x Pixel pitch 141x x141 (um) 141x x x141 Sub-Pixel pitch (um) 47x x x7.5 47x141 47x141 Aperture rati 6% 47% 47% 6% 6% Transmittance rati 7% 8.2% 8.2% 1% 1% Reslutin 24x32 24x32 24x32 18x32 24x32 Fig. 7. Text cmparisns using different clr structures. RGB stripe. PenTile RGBW. MS-RGBW. III. RGB-RGBW MAPPING ALGORITHM The bjective f this sectin is t map the RGB image data int the RGBW image data pixel by pixel in Fig. 8. In designing this RGB-RGBW mapping algrithm, the hue and the saturatin f the riginal clr are preserved and the crrespnding luminance is enhanced. The algrithm takes the fllwing steps. Step1: input R, G and B image data f each pixel. Step2: btain the mapping gain M frm (1). Min( R, M (1) Max( R, where Min ( R, and Max ( R, dente the minimum and maximum f R,G and B, respectively. Step3: calculate the parameter W frm (2). W Min( R, (2) Fig. 8. Blck diagram f RGB-RGBW mapping algrithm. Step4: generate new R, and B image data f each pixel frm (3). R 1 M G B 1 M R W * G W 1 M B W On the basis f the HSV clr space, it can be shwn that the image data, R, G, B and W, are n distrtin in hue and saturatin. Using (4) and (5), G B (1 M) G(1 M) B GB H H RB (1 MR ) (1 MB ) RB S (3) (4) Min ( R, G, B) (1 M ) * Min ( R, 1 1 Max ( R, G, B) (1 M ) * Max ( R, Min ( R, 1 S Max ( R, ne shws that the image btained frm the RGB-RGBW mapping algrithm has the same hue and saturatin as in the riginal image pixel by pixel. By using (6), V Max( R, G, B) (1 M ) * Max( R, (1 M ) * V (6) it is shwn that the enhanced luminance is (1+M) times the riginal ne. Fig. 9 respectively depicts the simulatin results f the prpsed RGB-RGBW mapping algrithm. In Figs. 9 and, the resulting images are clearly brighter than their riginal nes. Furthermre, thrugh the simulatin results in Table II, the prpsed methd is shwn capable f lw clr distrtin ( E ab <1.) in the Lab clr space, where E ab dentes the averaged errr f ttal pixels in terms f a and b frm (7). Besides, let E Lab represent the averaged errr f ttal pixels in terms f L, a, and b via (8). a () n a () n b () n b () n 2 2 1/ 2 m p p Eab( ave) (7) m n1 L() n L () n a () n a () n b() n b () n /2 m p p p ELab ( ave) (8) n1 m where L p a p bp ( n) and L a b ( n) are respectively the values f L, a, and b at the nth pixel f the transfrmed and riginal images. As shwn in Table II, the significant difference between ELab ( ave) and E ab ( ELab ( ave) > E ab ) is definitely attributed t the prpsed MS-RGBW which is designed t enhance image brightness. (5)

4 C.-C. Lai and C.-C. Tsai: A Mdified Stripe-RGBW TFT-LCD with Image-Prcessing Engine fr Mbile Phne Display 1631 Fig. 1. Fur kinds f MS-RGBW pixel definitins. RGB pixel. GBW pixel. BWR pixel. WRG pixel. (e) Fig. 9. Simulatin results,, and (e) riginal images,, and (f) transfrmed images using the prpsed RGB -RGBW mapping algrithm. (f) Fig. 11. Rendering image data (R, G, B, W) t the MS-RGBW clr filter structure. RGB pixel. GBW pixel. BWR pixel. WRG pixel. Table II. Clr distrtin cmparisns between Figs. 9,, (f) and Figs. 9,, (e). (ave E ab Figures ) E Lab Fig 9. vs Fig Fig 9. vs Fig Fig 9. (f) vs Fig 9. (e) Fig. 12. Simulatin results f letter C. Cnventinal algrithm [6]. Prpsed sub-pixel rendering algrithm.

5 1632 IEEE Transactins n Cnsumer Electrnics, Vl. 53, N. 4, NOVEMBER 27 Fig. 13. Illustratin f the experimental MS-RGBW LCM fr the prpsed clr filter structure and image-prcessing engine. Step3: tw mapping sub-pixels data are directly put int MS-RGBW sub-pixels data. Step4: the image cntent f a sharing sub-pixel is used t fuse with that f its right-side rendering sub-pixel data using the fllwing equatin. P ( i 1) Min( R ( i), R ( i 1)) r P ( i 1) Min( W ( i), W ( i 1)) w P ( i 1) Min( B ( i), B ( i 1)) b P ( i 1) Min( G ( i), G ( i 1)) g (1) where Pr ( i 1), Pw ( i 1), Pb ( i 1), Pg ( i 1) are the image data at the (i+1)-th pixel f the prpsed MS-RGBW clr filter in Step 4; R () i, G ( i ), Bb () i and W () i dente the image data at the i-th sharing sub-pixel, while R ( i 1), G ( i 1), B ( i 1) and W ( i 1) represent the image data at the (i+1)-th rendering sub-pixel. Fig. 12 respectively shws the simulatin results f the prpsed sub-pixel rendering algrithm. The result in Fig.12 reveals that the prpsed sub-pixel rendering algrithm gives a sharper and smther text image while cmparing t the methd in [6]. Fig. 14. Experimental FPGA-based driving system fr RGB stripe and MS-RGBW LCM. IV. SUB-PIXEL RENDERING ALGORITHM This sectin is devted t the image data (R, G, B and W) rendering t the MS-RGBW clr filter structure. Each pixel f the MS-RGBW defines three sub-pixels shwn in Fig. 1, such as (Rs, Gs, Bs), (Gs, Rs, Ws), (Bs, Ws, Rs), and (Ws, Rs, Gs). In Fig. 11, the sub-pixel rendering algrithm fr MS-RGBW clr filter structure is presented in fur steps: Step1: (R, G, B, W) image data f each pixel is decmpsed int ne rendering sub-pixel data, tw mapping sub-pixels data, and ne sharing sub-pixel data. Step2: the image cntent f a rendering sub-pixel is used t fuse with that f its lift-side sharing sub-pixel data using (9). Pr() i Min( R( i1), R()) i Pw() i MinW ( ( i1), W()) i (9) Pb() i Min( B( i1), B()) i P () i Min( G ( i1), G ()) i g where Pr(), i Pw(), i Pb(), i Pg() i are the image data at the i-th pixel f the prpsed MS-RGBW clr filter in Step 2; R () i, G ( i ), Bb () i and W () i dente the image data at the i-th rendering sub-pixel, while R ( i 1), G ( i 1), B ( i1) and W ( i1) represent the image data at the (i-1)-th sharing sub-pixel. V. EXPERIMENTAL RESULTS AND DISCUSSION The aim f this sectin is t examine the perfrmance and merit f the prpsed MS-RGBW and image-prcessing engine by using a 2.2-inch 24x3x32 reslutin panel with MS-RGBW clr filter in Fig. 13. The experimental setup is equipped with ne standard QVGA driver IC withut any image-prcessing engine, and a FPGA (field-prgrammable-gate-array)-based cntrller in Fig.14. The FPGA-based cntrller btains image data frm its flash memry t cntrl the driver ICs in the TFT-LCM. Tw kinds f flash memry in the FPGA-based cntrller were used; ne stred the prcessed image, and the ther memrized the riginal image. The driver IC btained the prcessed image data and shwed them n the panel. In the experiments, these prcessed image data using the prpsed image-prcessing algrithm were dne in advance by a persnal cmputer. The panel cmbines the identical TFT array t RGB stripe panel and the MS-RGBW clr filter. In rder t facilitate the experimentatin prcess, the experimental driver IC adpted the cmmercial RGB stripe driver IC with 24x3x32 utput channels. The driver IC btained the image data frm the prpsed image prcess engine and then displayed them n the panel. In cmparisn with the left-side riginal pictures, the MS-RGBW panel with image-prcessing engine generates a higher brightness and a better cntrast rati than the RGB stripe panel in Fig. 15. Furthermre, the result in Fig. 16 clearly indicates that the prpsed methd is capable f prviding sharper images than the cnventinal ne [6].

6 C.-C. Lai and C.-C. Tsai: A Mdified Stripe-RGBW TFT-LCD with Image-Prcessing Engine fr Mbile Phne Display 1633 and a shaper image than the cnventinal ne. The prpsed RGB-RGBW mapping algrithm easily btains a clr mapping withut hue and saturatin distrtin in the pixel level. Numerus simulatin results have been prvided t cnfirm the afrementined merits f the prpsed techniques. Thrugh experimental results, the prpsed techniques have been shwn useful and effective fr 2.2-inch TFT LCDs with MS-RGBW clr filter structure. ACKNOWLEDGEMENTS The authrs wish t acknwledge Mr. Jyun-Sian Li and Mr. Ching-Fu Hsu at Wintek Crpratin, Taiwan, fr preparing the simulatin and experimental results. REFERENCES [1] (e) (f) Fig. 15. Experimental results., and (e) Original images and RGB stripe clr filter., and (f) Transfrmed images using the prpsed image-prcess engine and MS-RGBW clr filter. Fig. 16. Experimental results f letter C. Cnventinal algrithm [6]. Prpsed sub-pixel rendering algrithm. VII. CONCLUSIONS This paper has presented a MS-RGBW clr filter structure with a nvel image-prcessing engine. The image-prcessing engine is equipped with tw new algrithms: RGB-RGBW mapping algrithm and sub-pixel rendering algrithm. The prpsed techniques nt nly retain the same high reslutin, but als btain a higher brightness B. W. Lee, C. Park, S. Kim, T. Kim, Y. Yang, J. Oh, J. Chi, M. Hng, D. Sakng, and K. Chung, TFT-LCD with RGBW Clr System, SID23 Digest f Technical Papers, 24, pp [2] B. W. Lee, K. Sng, Y. Yang, C. Park, J. Oh, C. Chai, J. Chi, N. Rh, M. Hng, and K. Chung, Implementatin f RGBW Clr System in TFT-LCDs, SID24 Digest f Technical Papers, 24, pp [3] A. D. Arnld, T. K. Hatwar, M. V. Hettel, P. J. Kane, M. E. Miller, M. J. Murdch, J. P. Spindler and S. A. Van Slyke, Full-Clr AMOLED with RGBW Pixel Pattern, Prceedings f the 24th Internatinal Display Research Cnference, 24, pp [4] H. J. Yn, J. H. Lee, K. P. Hng, J. Y. Chun, B. Y. Ryu, J. M. Jun and J. Y. Lee, Develpment f the RGBW TFT-LCD with Data Rendering Innvatin Matrix(SRIM), SID25 Digest f Technical Papers, 25, pp [5] C. Y. Tsai, Y. C. Tsai, Y. J. Chang, W. C. Chang and D. L.P. Ting, Advanced Transmissive-LCDs with High Reflectance in RGBW, Prceeding f the 13th Internatinal Display Wrkshps, 26, pp [6] S.T. L and R. S. Weng, Methd and apparatus fr fur-clr data cnverting, US patent # , 26. Chih-Chang Lai received the B.S. degree in Electrical Engineering frm Natinal Taiwan Ocean University in 1997 and M.S. degree in Electrical Engineering frm Natinal Chung-Hsing University in 1999, respectively. During , he was a research assistant in Advanced Electrical Cntrl Lab (AECL), Natinal Chung Hsing University, fr studying mbile rbt cntrl systems using Kalman filter and fuzzy cntrl thery. Since 21, he has been a prject leader in Taiwan Wintek Crp. Currently; he is wrking tward his Ph.D. degree at Department f Electrical Engineering, Natinal Chung Hsing University. His current interests include neural netwrks, fuzzy cntrl and their applicatins t display mdules and image prcessing Ching-Chih Tsai received the Diplmat in Electrical Engineering frm Natinal Taipei Institute f Technlgy, Taipei, Taiwan, ROC, the MS in Cntrl Engineering frm Natinal Chia-Tung University, Hsinchu, Taiwan, ROC and the PhD in Electrical Engineering frm Nrthwestern University, Evanstn, IL,USA, in 1981, 1986 and 1991, respectively. Currently, he is a Prfessr in the Department f Electrical Engineering, Natinal Chung-Hsing University, Taichung, Taiwan, ROC. Frm 21 t 23, he served as the Chair at Taipei chapter, IEEE cntrl systems sciety, and, frm 23 t 25, he served as the Directr at Center fr Research Develpment and Engineering Technlgy, Cllege f Engineering, Natinal Chung-Hsing University. In 26, he served as the Chair at Taipei chapter, IEEE Rbtics and Autmatin sciety, and the Directr at Center fr Advanced Industry Technlgy and Precisin, Natinal Chung Hsing University. Since 27, he has been the President, Taichung Chapter, the Chinese Institute f Engineers.