AUTOMATIC WHITE BALANCE FOR DIGITAL STILL CAMERA

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1 AUTOMATI WHITE ALANE FOR DIGITAL STILL AMERA Tzn-Sheng hiou ( 邱贊生 ), hiou-shnn Fuh ( 傅楸善 ), nd Vsh hikne Detment of omute Science nd Infomtion Engineeing, Ntionl Tiwn Univesity, Tiei, Tiwn. E-mil: fuh@csie.ntu.edu.tw ASTRAT Thee e mny key fctos t hve get effect uon e qulity of digitl cmes. White lnce is one of e keys to e qulity of digitl cmes. Tditionl white lnce meods e oweless in some exteme conditions. In ode to get moe ecise esult we divided e olem of djusting e white lnce in ee ts - Detecting white oint, Judging white lnce, nd Adjusting white lnce. In e esent study we oosed new white lnce och to esectively solve ese min olems of white lnce. The exeimentl esults show t e oosed meod hs e est esults in o visul nd evlutive esults. The comlexity of e oosed system is ccetle. The oosed meod cn e esily lied to e digitl cmes to get good effects. Keywods: white lnce, colo lnce. 1. INTRODUTION Due to e continuing decline in e cost e digitl cmes e ecoming incesingly oul. Affodility is needed fo e continued exnsion of e digitl cme mket, ut e qulity of digitl colo imge is lso e imotnt considetion fo e consume. Thee e mny key fctos t hve get effect uon e qulity of digitl cme. White lnce is one of e keys to e qulity of digitl cme. An imge is comosed of nume of ixels eesenting e ctued scene. Ech ixel hs esective comonent vlues eesenting e intensity of incident light of ticul colo t is detected y colo senso. The colo of n oject ecoded y colo senso tyiclly vies wi light souces t illuminte e scene. This is due to e colo temetue diffeence of e light souces. olo temetue is e solute temetue t which we het stndd lck ody (metl) to mke it dite e sme light s cetin light souce. When white oject is illuminted wi low colo temetue light souce, e ctued imge will show e oject s hving slightly eddish colo. Similly, if e white oject is illuminted wi high colo temetue light souce, e oject in e ctued imge will hve luish colo. The humn vision my not e le to distinguish e colo diffeence cused y vious light souces due to e colo constncy of humn eye [1]. olos emin constnt ough ecognition desite ey e viewed unde diffeent light souces. In summy, e mechnism emloyed in digitl cme to comenste e colo diffeence cused y vious light souces is white lnce. This is e min investigtion.. AKGROUND The tditionl meods used to djust e white lnce utomticlly e mentioned elow, Gy Wold Meod This meod tkes n imge nd scles its ed, geen, nd lue colo comonents to mke e gy wold ssumtion [] hold. Advnced Gy Wold Meod This meod is oximtely e sme s e gy wold meod in ddition to filteing out ose ixels hving highly chomtic colo. Fuzzy Rules Meod In is meod e imge is conveted fom e RG colo sce to e Y colo sce [3] nd seized e colo s chcteistics in e Y colo sce fo e white lnce djustment. The fuzzy ules sed on exeimentl esults e descied elow: 1. The veges of nd fo ech segment will e weighted wi smll vlues unde e conditions of high-end nd low-end luminnce in ode to void eing stuted nd cololess.. The veges of nd fo ech segment e weighted less fo dk colos n ight colos. 3. When lge oject o ckgound occuies moe n one segment, its colo will dominte t segment. The tio of to will e simil mong djcent segments. The given weighting fo ose segments hving unifom chomtic colo is smll in ode to void ove-comenstion on e colo of e ictue. If e tio of to of e segment is oximtely etween 1.5 to 0.5, e oility of eing white egion inceses, e given weighting is e lgest. Pefect Reflecto Meod This meod exloits e efect eflecto ssumtion [4] to coect e imge. It loctes e efeence white oint y finding e ixel wi e getest luminnce vlue nd efoms e white lnce djustment ccoding to e efeence white oint. Pecentile Pefect Reflecto Meod

2 This meod sttes five ecent of comonent vlues in e imge hving e highest ed, geen, nd lue vlues. These vlues e selected fom e given ecentile of e histogm of t imge to comute e glol white oint nd en efom e white lnce djustment ccoding to e glol white oint. 3. THE NEW WHITE ALANE METHOD y using e dvntges of ove mention meods we hve oosed new white lnce meod. The new white lnce meod consists of ee comonents, s shown in Fig.1. Stt White oint detecting unit White lnce judging unit White lnce djusting unit End Fig.1: The comonents lock digm of e new white lnce hd Fig. 1 The comonent lock digm of e new white lnce meod. 3.1 White Point Detecting Unit In e White Point Detecting unit, fist we found out ough nd ecise efeence white oints. Fo detecting ough white oint, e escied eshold must e found out fo t, we ut e GetgMce olohecke unde two light souces, one eesents e extemely high colo temetue (Dylight) nd e oe eesents e extemely low colo temetues (Hoizon). Second, we used smll window inside ech colo tch in e ottom of e GetgMce olohecke nd clculted e vege of ose ixels wiin e windows to get e RG vlues of e chomtic colo tches. Thid, we conveted e chomtic colo tches fom e RG colo sce to e Y colo sce. Finlly, we clculted ose chomticity vlues ( + ) of e chomtic colo tches in e ove two imges nd icked u e lgest vlue s e escied eshold ( H, 60 in ou exeiments). Then ll ixels in e imge e conveted fom RG colo sce to e Y colo sce nd used e condition in which e chomticity vlue of ixel is equl to o less n escied eshold ( H ) to judge whee o not is ixel hs e highly chomtic colo. It is fomulted s shown in Eqution 1.1, + H whee + is defined s e chomticity vlue of ixel; nd H is defined s e escied eshold. Using is eqution we cn esily detect highly chomtic colos nd emove it. To get ough efeence oint, we took e vege of ll ixels fom which we emoved e highly chomtic ixels. w, fo detecting ecise efeence oint we use dvnce glol oint detecting meod. We oseve y exeiments t, ixel t seems to e e most oite fo white oint hve two common oeties: fist, ixels hve e eltively high luminnce vlue nd second, ixels hve e highe ed, geen, nd lue comonent vlues. We hve mde use of e fist oety to uild e elted white oint detecting meod. w we ty to tke dvntge of e second oety to uild new white oint detecting meod. In ode to get ixels t hve e highe ed, geen, nd lue comonent vlues fom n imge, we must detemine fist e suitle eshold fo ech colo comonent of e imge. Suitle eshold fo ech comonent is geneted fo n imge sed on ixels t e in given ecentile of e histogm of ech colo comonent of e imge [5]. This mens t e ed comonent eshold ( R ) is defined s e eightie ecentile of e ed comonent of e imge, e geen comonent eshold ( G ) is defined s e eightie ecentile of e geen comonent of e imge nd e lue comonent eshold ( ) is defined s e eightie ecentile of e lue comonent of e imge. To get ech comonent eshold of e imge, we ick u ose ixels whose ed, geen, nd lue vlues e esectively highe n e elted comonent eshold ( R, G, nd ) s e efeence white oint. It is fomulted s shown in Eqution 1., R R G G whee R, G, nd e e esective comonent eshold. Genelly, it woks vey well to look fo e efe- ence white oint ut ee is ossiility t e efeence white oint detected y is meod my e e yellow nd cyn. To distinguish e ixels fo oite white oint fom e simil ixels of yellow nd cyn, we convet e imge fom e RG colo sce to e Y colo sce nd seize e colo chcteistics in e Y colo sce. The ixels t seem to e e most oite fo white hve two useful chcteistics in e Y colo sce. Fist, e solute vlue of is lowe n escied eshold nd e solute vlue of is lowe n escied eshold. It is fomulted s shown in Eqution 1.3 A A (1.1) (1.) (1.3)

3 whee A is defined s e escied eshold fo e solute vlue of nd A is defined s e escied eshold fo e solute vlue of. Second, e tio of to flls into escied nge. It is fomulted s shown in Eqution 1.4, Rl R (1.4) u whee R l is defined s e lowe ound fo e escied nge, nd R u is defined s e ue ound fo e escied nge. Howeve, to decide ose oe esholds ( A, A, Rl, nd R ), we use e sme exeiment u efomed in e ough efeence white oint meod t is, we tke imges of e GetgMce olohecke in e extemely high nd extemely low colo temetues nd ose solute vlues of nd of e chomtic colo tches fo e ove-mentioned imges, e clculted nd en we ick u e lgest vlue s e eshold fo e solute vlues of ( A, 45 in ou exeiments) nd ( A, 45 in ou exeiments), en we clculte ose tios of to of e chomtic colo tches in e ove two imges nd ick u e minimum s e lowe ound fo e escied nge ( R, -1.5 in l ou exeiments) nd ick u e mximum s e ue ound fo e escied nge ( R, in ou exeiments). u W hite oint detecting unit Detem ine ech colo com onent eshold of n im ge ( R, G, nd ). Red e RG vlues of ixel nd com ute its elted Y vlues. + H A, A Rl Ru Fig. 1.1 The flow cht of e white oint detecting unit. If ixel stisfies e ove conditions en we gin confim t is ixel is e most oite fo white oint nd us we ick it u s nume of ecise efeence white oint. Figue 1.1 shows e oetions efomed in e white oint detecting unit. Fist, clculte ech comonent eshold used in e lte oetion ( R, nd T he ixel is counted to clculte e ough efeence w hite oint ( R, G, ). R R G G T he ixel is counted to clculte e ecise efeence white oint ( R, G, ). A e ll ixels W hite lnce judging unit ). Second, ed e RG vlues of ixel nd convet em fom e RG colo sce to e Y colo sce. Thid, judge whee o not ixel hs e qulifiction fo e ough efeence white oint. The vlues ( R, ) eesent e vege of e ough efeence white ixels. Fou, judge whee o not ixel hs e qulifiction fo e ecise efeence white oint. The vlues ( R, ) eesent e vege of e ecise efeence white ixels. Finlly, judge whee ll ixels of e imge e ed. If ll ixels of e imge e ed, we cn go into e next unit nd ose elted efeence white oint dt e lso tnsfeed into e next unit. Oewise, we ed e next ixel of e imge nd efom e itetive stes. 3. The White lnce Judging Unit The function of e white lnce judging unit lies in judging whee o not to efom e white lnce djustment nd selecting suitle efeence white oint fom e ough efeence white oint nd e ecise efeence white oint fo e white lnce djusting unit (see Figue 1.). Fist, we ick u e efeence white oint dt t e otined fom e white oint detecting unit. Then we clculte R vlue, which is e tio ough of e ough efeence white ixels to ll ixels of n imge nd R vlue, which is e tio of e ecise ecise efeence white ixels to ll ixels of n imge. Second, judge whee o not is equl to o lge n P ough Rough vlue, defined s escied ootion (0. in ou exeiments). The ough efeence white ixels eesent ose ixels whose colos e not highly chomtic colos, if R is highe en ough ee is ossiility t e mjo otions of e imge e not occuied y highly chomtic colo nd on e conty, if R is lowe en ee is ough high ossiility t e mjo otions of e imge e occuied y highly chomtic colo. Thid, judge whee o not R is equl to o lge ecise n vlue which is defined s e escied P ough ootion (0.05 in ou exeiments). The ecise white ixels eesent ixels t e e most oite fo white, if is high en ee exists R ecise ecise enough ecise efeence white ixels in e imge, on e conty, if R is low en ee exist insufficient ecise efeence white ixels in e imge.

4 Fou, set e djusting modes, M nd stnds fo diffeent djusting modes. If e fist condition ( Rough P ough ) is stisfied en ee is ossiility t e mjo otions of e imge e not occuied y highly chomtic colo nd if e second condition ( R ecise P ecise ) is stisfied en ee exist enough ecise efeence white ixels in e imge. In cses whee e fist ( Rough P ough ) nd second ( R ecise P ecise ) conditions e stisfied, M is set to, in is sitution use ecise efeence white oint s e efeence white oint. In e cses whee, e fist condition ( Rough P ough ) is stisfied nd e second condition ( R ecise P ecise ) is not stisfied, M is set to 1, in is sitution choose e mino fom e ough efeence white oint nd e ecise efeence white oint s e efeence white oint. White lnce judging unit M is set to. White oint detecting unit omute ough nd ecise efeence white ixels tes ( R ough nd R ecise ). Rough P ough R R ecise Pecise ecise P ecise Fig.1. The flow cht of e white lnce judging unit. In cses whee e fist condition ( Rough P ough ) is not stisfied nd e second condition ( R ) is stisfied, ecise P ecise M is set to, is mens t we use ecise efeence white oint s e efeence white oint. In cses whee e fist condition ( Rough P ough ) is not stisfied nd lso e second condition ( R ecise P ecise ) is not stisfied, M is set to 0, is mens t we will efom noing fo e white lnce djustment. We used is mode ecuse in white lnce meods ee e cetin situtions t which it is not necessy to djust white lnce. If we djust white lnce cudely en we get stnge esults nd mostly it hens when e mjo otion of e imge is occuied y e highly chomtic colos. M is set to 1. White lnce djusting unit Fig. 1.: The flow cht of e white lnce judging unit. M is set to The White lnce Adjusting Unit The flow cht of e white lnce djusting unit is shown in Fig.1.3. It consists of two stes, one is comuting e scle fcto of ech colo comonent in n imge nd e oe is scling e comonent vlues ccoding to its esective scle fcto. To comute e scle fcto of ech colo comonent in n imge, in e white lnce judging unit we hve set e diffeent djusting mode vlue to indicte diffeent djusting situtions so it ecomes esy to get e elted scle fctos of e imge ccoding to e djusting mode vlue. Fist, we check whee o not e geen vlue of e efeence white oint is highe n escied eshold. In cses whee e geen vlue of e efeence white oint is lowe n escied eshold, e imge is dk, so e mening of efoming e white lnce djustment deceses. Second we clculte e scle fcto of ech colo comonent ccoding to e ough efeence white oint nd e scle fcto of ech colo comonent ccoding to e ecise efeence white oint. The ough efeence white oint defined s ( R, ), whee, R, nd e esective comonent vlues fo ech colo comonent nd Y is e luminnce vlue of e ough efeence white oint. It is fomulted s shown in Eqution 1.5. Y = 0.99* R * G * The scle fctos of ed, geen, nd lue comonent vlues e defined s R gin, Ggin,. Thus individully, we gin cn esily comute e scle fcto of ech colo comonent ccoding to e ough efeence white oint s shown in Eqution 1.6. R = Y / R. G gin gin gin = Y / G = Y / On e oe side, e ecise efeence white oint of n imge is defined s ( R, G, ) whee, R, G nd e esective comonent vlues fo ech colo comonent. The luminnce of e ecise efeence white oint is defined s Y. It is fomulted s shown in Eqution 1.7. Y = 0.99* R * G * We could esily comute e scle fcto of ech colo comonent ccoding to e ecise efeence white oint s shown in Eqution 1.8. R G gin gin gin = Y = Y = Y / R / / G Those scle fctos t we clculted ove my hve e exteme vlues (ult-high o ult-low). y using ese scle fctos if we cudely scle e comonent vlues en (1.5) (1.6) (1.7) (1.8)

we get stnge esults. To ovecome is olem, we cn use sigmoid function to elx e exteme scle fcto towd n ccetle vlue. It is fomulted s shown in Eqution 1.9. (1.9) Y = 1.05*(1 + tnh( X 1.5)) + 0.

5 we get stnge esults. To ovecome is olem, we cn use sigmoid function to elx e exteme scle fcto towd n ccetle vlue. It is fomulted s shown in Eqution 1.9. (1.9) Y = 1.05*(1 + tnh( X 1.5)) whee, X is e oiginl scle fcto nd Y is e djusted scle fcto. Second, select scle fctos of e imge ccoding to e djusting mode vlue. When M is equl to 0, e white lnce djustment is stoed. When M is equl to 1, we select e mino etween ( R gin Ggin, gin, ) nd ( R gin, Ggin, ) s e ctul gin scle fctos. When M is equl to, we select R, ) s e ctul scle fctos. Thid, scle ( gin gin gin ech colo comonent ccoding to its esective scle fcto. The test imge of GetgMce olohecke in diffeent light souces nd coesonding esults fte lying new white lnce meod. (See Figue.) Dylight Oiginl imge tken in Dy-light Imge, fte lying New White lnce Meod: R R =0.04, M =1) ( oug =1, ec White lnce judging unit omute scle fctos ccoding to e ough efeence white oint (Rgin, G gin, gin) ool White Oiginl imge tken in ool White Imge, fte lying New White lnce Meod: (R ough =0.99, R ecise =0.11, M =) omute scle fctos ccoding to ecise efeence white oint (Rgin, G gin, gin). White lnce djusting unit M 0 1 M Sto Use (Rgin, Ggin, gin) s scle fctos of e imge. Scle e comonent vlues ccoding to its esective scle fcto. Select e mino mong (Rgin, Ggin, gin) nd (Rgin, G gin, gin) s scle fctos of e imge. IN Oiginl imge tken in IN Imge, fte lying New White lnce Meod: (R ough =0.96, R ecise =0.03, M =1) Finish Fig.1.3 The flow cht of e white lnce djusting unit. 4. EXPERIMENTAL RESULTS We e inteested in e esults of distinct white lnce oches nd e diffeences etween em, so we tke ose test imges, unde e diffeent light souces nd simulted em wi gy wold meod, dvnced gy wold meod, fuzzy ules meod, efect eflecto meod, ecentile efect eflecto meod, e function of utomtic lnce in PhotoImct nd e new white lnce meod. The ssocited light souces used in ou exeiment wi its colo temetue e listed elow in Tle 1. Light Souce Dylight ool IN Hoizon White olo Tem. (Kelvin) Tle 1 olo temetue of diffeent light souces. Hoizon Oiginl imge tken in Hoizon ool White Oiginl imge tken in ool White Imge, fte lying New White lnce Meod: (R ough =0.99, R ecise =0.11, M =) Imge, fte lying New White lnce Meod: (R ough =0.96, R ecise =0.16, M =)

6 Fig. Visul esults in e diffeent light souces. As we e inteested in e esults of distinct white lnce oches nd e diffeence etween em so hee we tke ose test imges nd simulte em wi gy wold meod (GWM), dvnced gy wold meod (AGWM), fuzzy ules meod (FRM), efect eflecto meod (PRM), ecentile efect eflecto meod (PPRM), e function of utomtic lnce in PhotoImct (UPI), nd e new white lnce meod (NWM). Visul esults of two test imges (oiginl imge nd imge fte lying new white lnce meod) e shown in Figue. In ddition to visul veifiction we lso wnt to quntify e esult of diffeent white lnce oches. Unfotuntely, ee does not exist oe quntittive tool fo white lnce. Hee we oose simle evlutive meod. We use smll window ( 11X11 ixel lock) inside ech colo tch in e ottom of e GetgMce olohecke nd clculte e vege of ose ixels wiin e windows to get e RG vlues of e chomtic colo tches nd en we convet e chomtic colo tches fom e RG colo sce to e Y colo sce. We clculte e sum of ose chomticity vlues ( + ) of e chomtic colo tches in e imge nd use it s e evlution citeion fo white lnce. Tle shows e evlutive esults of ese seven meods fo e test imges shown in Figue. Tle. The evlutive esults of seven meods fo e test imges. Evlutive vlues Test imges Oiginl Mnul GWM AGWM FRM PRM PPRM UPI NWM Dylight ool White IN Hoizon ool White' Fom e exeimentl esults we find t e tditionl white lnce meods cn tilly solve e olem of djusting e white lnce nd it is hd to get ette esult y uely lying e tditionl white lnce meods to digitl cmes. Howeve, in ou esent study we hve used e dvntges of tditionl white lnce meods, such s e white lnce meod sed on e Gy Wold ssumtion t offes us e infomtion out e light cst ove e whole imge, while e white lnce meod sed on e Pefect Reflecto ssumtion ovides us ough meodology to find out e efeence white oint fom n imge. Moeove, e colo chcteistics in e Y colo sce hel us to ecisely look fo e efeence white oint fom n imge. In ddition to detecting e efeence white oint fom n imge nd djusting white lnce ccodingly, we must del wi e olem t whee o not e white lnce djustment is elly equied. If we ignoe is olem nd cudely djust white lnce, we will destoy e consistency of colos in e imge nd get stnge esults on cetin situtions. In is e e oosed och not only mixes e dvntges of e tditionl meods to fom new white oint detecting meod ut lso cefully coes wi e olem of whee o not we must efom e white lnce djustment. The simultion esult shows t e oosed meod hs e est esults in o visul esults nd ojective evlution 6. AKNOWLEDGEMENT This esech ws suoted y e Ntionl Science ouncil of Tiwn, R.O.., unde Gnts NS SP-4 nd NS 91-1-E , y e EeRise ootion, EeVision ootion, Tekom Technologies, Jeilin Technology, IA, nd Delt Electonics. 7. REFERENES [1] R.. Gonzlez nd R. E. Woods, Digitl Imge Pocessing, Addison Wesley, Reding, MA, 199. [] M. Fedo, Aoches to olo lncing, PSYH1/EE36couse oject, Detment of Psychology, Stnfod Univesity, [3] Y.. heng, W. H. hn, nd Y.Q. hen, Automtic White lnce fo Digitl Still me, IEEE Tnsctions on onsume Electonics, Volume 41, , [4] J. hing nd F. Al-Tukit, olo lncing Exeiments wi e HP-Photo Smt-30 Digitl me, PSYH1/EE36 couse oject, Detment of Psychology, Stnfod Univesity, [5] A. V. Dug nd O. Rshkovskiy, Glol White Point Detection nd White lnce fo olo Imges, U.S. Ptent #606997, ONLUSION

Week 8. Topic 2 Properties of Logarithms

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