The gamuts of input and output colour imaging media

Transcription

1 In Proceedings of IS&T/SPIE Electronic Imging 1 The gmuts of input nd output colour imging medi án Morovic,* Pei Li Sun* nd Peter Morovic * Colour & Imging Institute, University of Dery, UK School of Informtion Systems, University of Est Angli, UK ABSTRACT The colour gmuts of colour imging medi re importnt prmeters in the reproduction of colour imges etween them nd their ssumed mgnitudes directly influence the degree to which colours re modified. In spite of this, the determintion of gmut oundries is often done in wy tht ignores some sic implictions tht follow from the definition of colour gmuts. This is prtly due to the fct tht some of these implictions re not understood nd prtly due to the fct tht if they re understood their mgnitude is underestimted. Hence, the pproch tht is tken in this pper is to first discuss the theoreticl implictions of wht colour gmuts re nd susequently to illustrte them y experimentl mens. Firstly, colour imging medi cn e divided into two ctegories which hve very different chrcteristics in terms of colour gmut. The determintion of gmuts of input colour imging medi, for exmple, introduces numer of prolems tht do not rise for output medi s it involves the determintion of the rnge cross which they cn cpture colour informtion. This results in significnt difficulty from prcticl point of view s it necessittes the vilility of stimuli from lrger gmut thn tht of the input gmut to e determined. As this ltter gmut is to e determined nd hence s yet unknown, the former gmut needs to e very lrge so s to e usle in generl. Secondly, the most crucil fctor tht is commonly ignored is tht viewing conditions re intrinsic to colour gmuts i.e. tht one cn only tlk out the colour gmut of set of stimuli if the corresponding viewing conditions re specified. The consequences of this will e shown y looking t the gmuts of commonly used output colour imging medi under rnge of viewing conditions with the im of estlishing how the resulting gmuts chnge in terms of colour ppernce. Amongst other things, this will show tht the gmuts of different medi cn e ffected to different degrees cross the rnge of illumintion levels used in this study. The mgnitude of chnges rnged from virtully no difference for some LCD displys up to six fold chnge for some projected medi. Hence it will e shown tht insted of medium hving single gmut, it hs multitude of them. Descriing colour reproduction medium using single gmut oundry inevitly leds to mismtches etween wht tht gmut oundry suggests nd how the gmut of the medium is seen under different conditions. While one solution to the prolem is to generte numer of gmut oundries for ech medium viewing condition comintion, this would result in n explosion of gmut oundry descriptors nd in some degree of inflexiility. Alterntively nd preferly this reltionship could e modelled nd this would result in the possiility of hving single reference gmut per medium which could then e modified to suit prticulr viewing conditions. Understnding the nture of colour gmuts is of significnt prcticl s well s theoreticl importnce s it cn often e the source of errors in cross medi colour reproduction pplictions. Keywords: colour imging medi, gmut clcultion, colour ppernce, viewing conditions 1. INTRODUCTION medi originl medium trnsformtion reproduction medium intent Figure 1. Elements of cross medi colour reproduction. 1 Before discussing the ctul topic of this pper the nture of colour gmuts it is useful to first hve look t context within which its understnding is of importnce. Arguly where this is most importnt is cross medi colour

2 reproduction, which will here e defined s the process y which colour informtion from n originl medium is trnsferred to reproduction medium so s to chieve predetermined reltionship etween them. 1 Such definition entils tht there re the following four elements tht constitute colour reproduction (Figure 1): colour informtion, colour imging medi, cross medi trnsformtion nd the intended reltionship etween originl nd reproduction (i.e. rendering intent). Of these elements, it is the imging medi tht need to e understood in more detil for the purposes of this pper s it is their gmuts tht we re concerned with here. A digitl colour imging medium here provides link etween digitl dt nd colour stimuli nd cn e of two principl types depending on whether colour stimuli re its inputs or outputs. Output colour imging medi (e.g. monitors, printers) produce colour stimuli on the sis of digitl dt sent to them wheres input medi (e.g. digitl cmers, scnners) produce digitl dt sed on sensing colour stimuli 2 (Figure 2). INPUTS colour stimuli input medium digitl cmer scnner OUTPUTS digitl dt digitl dt output medium colour stimuli monitor projector printer Figure 2. Types of digitl colour imging medi. 2 Considering the ove view, colour gmuts re involved in the first two elements of cross medi colour reproduction colour informtion nd colour imging medi wherey the colour gmut of the informtion (e.g. n imge, set of colours) of the medium in which tht informtion is present cn e considered. The presence of colour gmuts in these cross medi colour reproduction elements is in terms of them eing their properties nd it will e n im of this pper to suggest in wht wy this is the cse. Further, colour gmuts re lso present in the trnsformtion element, however, here they hve prmetric rther thn chrcteristic role i.e. they influence the nture of the trnsformtion rther thn eing its property. Hence the ccurte determintion of colour gmuts will directly influence how colour informtion is trnsformed in the process of communicting it etween originl nd reproduction medi. Now tht the cross medi colour reproduction context, in which colour gmuts will e considered here, hs een introduced, more systemic pproch to understnding wht colour gmut is cn e ttempted. The next section will im to nswer the questions of wht colour gmut is nd wht it is tht hs colour gmut. Section 3 will look t n exmple of clculting input gmuts nd Sections 4 nd 5 will then contin detils of work done for illustrting the chrcteristics of output gmuts. Finlly, some generl conclusions out the topic of this pper will e drwn. 2. DETERMINING GAMUT BOUNDARIES Figure 3. CAM97s2 Gmut of print mde with colour lser printer, illuminted y D5 with cd/m 2 luminnce of reference white nd seen ginst mid grey ckground y the CIE Stndrd Colorimetric Oserver (2 ) Terminology As with ny suject, there is rnge of possile interprettions of the sic terms used in cross medi reproduction, nd so s to void misunderstndings the definitions of key gmut relted terms will e given next: 3

3 Colour reproduction medium: medium for displying or cpturing colour informtion, e.g. CRT monitor, digitl cmer or scnner. Note, tht in the cse of printing, the colour reproduction medium is not the printer ut the comintion of printer, colornts nd sustrte. Colour gmut: rnge of colours chievle on given colour reproduction medium (or present in n imge on tht medium) under given set of viewing conditions it is volume in colour spce. Colour gmut oundry: surfce determined y colour gmut s extremes (e.g. Figure 3.). Note, tht the ove terminology is tht used y the CIE s Technicl Committee 8 3 on Gmut Mpping 4 nd hence represents the consensus of numer of reserchers investigting the present suject. Two importnt implictions of these definitions, which re often overlooked, will e discussed next Theoreticl implictions of definitions The most crucil fctor tht is commonly ignored is tht viewing conditions re intrinsic to colour gmuts i.e. one cn only tlk out the colour gmut of set of stimuli if the corresponding viewing conditions re specified. This follows directly from our generl understnding of colour which is seen s phenomenon tht rises when the following re present: oserver, stimulus nd viewing conditions (illumintion, geometry, surround, etc.). If this is the cse for individul colours it necessrily is lso the cse for colour gmuts which re simply their rnges. Hence it is essentil to specify the detils of ll three of these components when tlking out colour gmut nd it lso mens tht it is meningless to tlk out the colour gmut of set of stimuli in generl (e.g. the set of colours displyle on given CRT). Such set only hs colour gmut when seen y some specific oserver under some specific viewing conditions. Considering tht the colour gmut of colour reproduction medium is nothing ut the colour gmut of the set of stimuli it is cple of displying/cpturing, it is essentil to specify viewing conditions nd oserver detils so s to e le to tlk out such colour gmuts. For exmple, tlking out the colour gmut of printed imge is meningless s it could ssume numer of forms suject to viewing conditions nd oservers i.e., printed imge hs set of possile colour gmuts rther thn single one. Looking t such n imge in the drk gives zero volume gmut (to use n extreme exmple), different levels of illumintion result in different gmut volumes, illumintion chromticity chnges gmut shpe s well s volume nd viewing distnce nd flre in the environment mke difference too. Hence, the question Wht is the gmut of this print? cnnot e nswered nd should insted e rephrsed to Wht is the gmut of this print under viewing conditions X for oserver Y? This point in prticulr will e illustrted in Sections 4 nd 5 so s to show tht understnding colour gmuts in this wy is necessry rther thn dismissle s eing overly pedntic. In ddition to these fctors influencing colour gmuts, the colour spce in which they re computed nd visulised lso plys crucil nd well recognised role Implictions on chrcteristion nd profiling Both implictions of the definitions given ove hve importnt prcticl reverertions tht re hitully overlooked. Any system contining device chrcteristion must e interpreted s hving medium chrcteristion nd this must e determined for ech individul oserver viewing condition (O VC) comintion. It is not possile to chrcterise medium in generl. A prominent exmple of systems where chrcteristion is used re colour mngement systems like those using the ICC frmework. 5 If in this context device profile is determined for some medium then it needs to e ensured tht the medium is chrcterised under the O VC under which it will e used. Chrcterising CRT in drk, flre free environment nd then using the chrcteristion to predict ppernce under usul office conditions is invlid nd will result in sustntil error nd mistrust of the system used. This entils either the need for lrge numer of profiles for single medium or for the possiility to trnsform stndrd O VC profile to predict ppernce under rnge of specific O VCs. While it is not within the scope of the present pper to investigte the ltter nd more idel of these options, the experimentl dt presented here is ment to e motivtion for emrking on more systemic modelling of medium ppernce s function of O VC chnge Gmut oundry clcultion for given O VC Given prticulr O VCs, the gmut of set of colours, which could either e ment to represent colour reproduction medium or n imge on it, cn e descried using numer of methods. 6 9 In this study the Segment Mxim method for clculting gmut oundry descriptors (SMGBD) 9 ws used nd will e riefly introduced next. Using this method, the gmut oundry of set of colours is descried y mtrix contining the most extreme colours for ech segment of colour spce. Here the segmenttion ws crried out in terms of sphericl coordintes clculted from orthogonl CAM97s2 1 coordintes using the following formulæ:

4 r = [( E ) 2 + ( E ) 2 + ( E ) 2 ] 1/2 (1) α = tn 1 (( E )/( E )) (2) θ = tn 1 [( E )/(( E ) 2 + ( E ) 2 ) 1/2 ] (3) Here nd re the orthogonl coordintes corresponding to CAM97s2 s cylindricl C nd h coordintes, E is defined s point inside the gmut to e descried nd cn, for exmple, e otined y verging the coordintes of the points which will e used for clculting n SMGBD (here E=[5,,] in terms of ), r is the distnce of colour from E, α is n ngle hving rnge of 3 nd θ is the ngle in plne of constnt α hving rnge of 1 (Figure 4.). E r () E θ α () Figure 4. Overview of SMGBD in CAM97s2: () sphericl coordintes, () sphere segmented in terms of α nd θ (6 6 segments used for illustrtion; one segment highlighted). The SMGBD mtrix is clculted y first dividing colour spce into n n segments (n=16 ws used here) in terms of α nd θ (Figure 4.). Then, the CAM97s2 coordintes of ech of the given set of colours re trnsformed into sphericl coordintes using formulæ 1 3. From these, the colour with the lrgest r is stored for ech of the n n segments wherey it is not only r, tht is stored for given segment ut the corresponding sphericl ngles s well. If in the end there re segments in which there re no colours, vlues for them re linerly interpolted on the sis of the nerest SMGBD mtrix entries. The result of this method is i liner surfce descriing the set of colours on which it is sed without the constrint of convexity. Given such SMGBD, the gmut oundry for ny hue ngle cn then e otined using the Flexile Sequentil Line Gmut Boundry method 9 nd gmut volumes cn lso e clculted y summing up the volumes of tetrhedr formed y E nd sets of three points from the SMGBD forming tringles on the gmut oundry surfce. 3. DETERMINING INPUT GAMUTS Colour imging medi cn e divided into two ctegories which hve very different chrcteristics in terms of colour gmut. For medi tht cpture colour informtion (e.g. scnners, digitl cmers, etc.) nd cn hence e seen s input medi, the determintion of their colour gmuts involves the determintion of the rnge cross which they cn cpture vrition in colour informtion. This results in significnt difficulty from prcticl point of view s it necessittes the vilility of stimuli from lrger gmut thn tht of the input gmut to e determined. As this ltter gmut is to e determined nd hence s yet unknown, the former gmut needs to e very lrge so s to e usle in generl. A further issue rises when input medi re chrcterised sed on set of specific smples nd their colour gmut is estimted on the sis of the chrcteristion model. In this cse wht one gets is not necessrily the gmut of the medium ut tht of the intersection of the medium s gmut nd the gmut of the smples on the sis of which the medium ws chrcterised. (Note, however, tht if the medium gmut is suset of the chrcteristion set s gmut then wht one gets is the medium gmut.) A wy of determining the gmuts of input medi hs een proposed recently 2 nd is sed on simulting the responses of n input medium to given spectrl power distriutions. The gmut of n input medium is then determined on the sis of hving set of spectr tht cover the mjority of ll possile spectr, knowing medium s responses to them nd then determining oundry eyond which the medium does not produce vrition in its responses. Such n pproch consists in three principl prts: genertion of set of stimuli for determining the gmut oundries of input medi, modelling of the responses of input medi nd clcultion of gmut oundries of input medi. The first of these is chieved y clculting the gmut of ll possile surfce reflectnces in colour ppernce spce (e.g. CIELAB), then evenly smpling tht gmut long lines emnting from the centre of the gmut nd finlly clculting spectr tht correspond to these colours using metmer set recovery method. 11 Once these spectr re ville, the responses of n input medium cn e predicted nd

5 for ech of the evenly smpled lines. The gmut oundry is then found y finding tht smple long ech of these lines for which the cmer response is not significntly different from the response for the following smple long tht line, strting from the centre of colour spce. An illustrtion of using this technique cn e seen in Figure 5 which shows the colour gmut of n Agf Studiocm for two different f stop settings. The gmut lelled ll spectr represents the gmut of spectr predictle using the metmer set recovery method. L* * L* * ll spectr f stop: 5.6 f stop:11 Figure 5. CIELAB gmuts of input medi. 4. EXPERIMENTAL METHOD FOR DETERMINING OUTPUT GAMUTS Tle 1. Overview of medi used in experiment (use of these mnemonics in the text will e indicted y using itlics). Mnemonic Description Lser Prints mde on plin pper using HP Color Lseret 5 Therml Prints mde on glossy sustrte using Tektronix Phser 4 with cyn, mgent nd yellow colornts only CRT Apple 21" Studio Disply with white point clirted to D5 LCD TFT LCD of Acer Extens 367T noteook with white point set to D65 Projector Imges projected using Snyo PLC 55B TFT LCD Projector Output medi do not suffer from the difficulties fced when clculting the gmuts of input medi s their gmut cn e determined if their input cn e smpled in its entirety. And this certinly is not prolem for digitl devices tht necessrily hve finite nd known rnge of possile inputs. However, different issue rises in conjunction with output medi wherey some colour reproduction devices re medi while others re not. CRTs or other types of displys, for exmple, re directly colour imging medi s they themselves disply colour informtion. Printers on the other hnd re not colour imging medi rther, they cn give rise to rnge of printed medi often using rnge of sustrtes nd colornts. Hence, the question of printer s gmut cnnot e nswered even if viewing condition nd oserver detils re supplied. Insted, it is only prints tht cn hve colour gmuts. The dt gthered for the ske of illustrting how the theoreticl points mde in the Section 2 re mnifest in output medi re mesurements of colours from numer of medi tken under numer of viewing conditions. Oserver differences nd differences in the reltive spectrl power distriutions of the illumintion were not considered in this study (CIE

6 Stndrd Colorimetric Oserver (2 ) 12 ws used throughout) nd the ckground nd surround were lso kept constnt t reflectnce of % for ll setups. Tle I shows the mnemonics nd descriptions of the medi used in the experiment. Tle 2. CAM97s2 prmeters for medium, viewing technique nd viewing condition comintions. Medium Viewing technique Perfect diffuser (cd/m2) Bckground Luminnce of dpting field (% of dopted white) Surround conditions Lser Isolted 35, 2, 13, 55 Averge Therml Isolted 35, 2, 13, 55 Averge Therml Simultneous 35, 2, 13 Averge nd CRT 55 Dim CRT Isolted 35, 2, 13 Averge 55 Dim Drk LCD Isolted 35, 13 Averge Drk Projector Isolted 35, 2, 13, 55 Averge Drk These medi were mesured using Minolt CS telespectrordiometer (TSR) under rnge of levels of illumintion in terms of X L Y L Z 13 L where Y L is luminnce in cd/m 2. The Projector imges were projected from distnce of 2 m nd mesured from distnce of 3 m under two levels of illumintion resulting from hving the diffuse ceiling lights on nd off respectively in room with lckened windows. For the other medi 1 m mesuring distnce ws used under some of four levels of illumintion resulting in the following luminnces for the perfect diffuser t the loction where colours were mesured: 35, 2, 13 nd 55 cd/m 2. The different levels of illumintion were otined y illuminting the vrious medi using comintions of ceiling lights nd n OHP projector s well s using filters to vry the output of the ltter. Mesurements were tken orthogonl to the medi mesured (i.e. t ) nd the OHP projector ws positioned t 45 reltive to the surfce of the medi so s to void speculr reflection t the loction from which mesurements were tken. All illumintion used here ws diffuse. Note, tht while cre ws tken to minimise white point differences etween the medi considered here, this ws not completely possile. While hving set of dt with identicl white points for ll medi would e desirle, the present, less perfect, set exhiits the sme nture of gmut differences s would e shown y the more optiml set. To otin SMGBDs for the ove set of medi under these viewing nd mesuring conditions, set of 26 colours were generted nd mesured per medium viewing condition. These colours were determined y those device dependent coordinte comintions of the, 5 nd % levels eing on the gmut surfce in device dependent terms. CAM97s2 coordintes were then clculted from perfect diffuser normlised XYZ vlues otined from mesurements mde under numer of conditions which will e descried in the following section. Bsed on the 26 originl colour smples, which cover the surfce of the RGB gmut cue evenly, 3456 (= ) simulted gmut oundry colours were generted for the gmut oundry clcultion in CAM97s2 colour spce. A i liner interpoltion technique ws used for simulting colours within ech squre consisting of 2 2 originl smples. RGB distnces etween the interpolted colour nd the four originl smples were regrded s the weights for the interpoltion in spce. For ll clcultions the ckground (CAM97s2 Y ) hd luminnce which ws % of the luminnce of the dopted white. When colour ppernce ws clculted for medi viewed in isoltion, surround conditions were set to verge nd dopted white ws tken to e the medium white point (i.e. the sustrte for Therml nd Lser nd R=G=B=% for CRT, LCD nd Projector) s mesured under ctul viewing conditions. The luminnce of the dpting field (CAM97s2 L A ) ws set to 1/5 of the dopted white. When clculting ppernce for medi viewed simultneously the dopted white ws tken to e the medium white point with the higher luminnce (i.e. the Therml white point for the three highest illumintion levels nd the CRT white point for the 55 cd/m 2 level). The CAM97s2 surround conditions were set to verge for the three highest illumintion levels nd to dim for the lowest. For summry of CAM97s2 prmeter vlues see Tle 2.

7 5. EXPERIMENTAL RESULTS FOR OUTPUT GAMUTS The mesurements tken under the conditions descried in Section 3 cn yield numer of insights out the reltionship of the colour gmuts of vrious medi under rnge of conditions. As the im of this pper is primrily to look t gmut differences, it will first e the nture of the difference etween the gmuts of the CRT nd Therml medi under rnge of illumintion conditions tht will e looked t here. This will then e followed y look t how the gmuts of individul medi chnge with chnges of illumintion level Comprison of CRT nd Therml gmuts First of ll, we will focus on the reltionship etween the CRT gmuts nd the Therml gmuts s it is this reltionship etween displys nd prints tht is most often of interest to users of colour mngement systems. The difference etween memers of these two sets cn e considered from t lest two points of view. First, wht it would e if the two medi were viewed simultneously under different illumintion levels. Second, wht it would e if they were viewed seprtely under the sme or different viewing conditions. The first of these scenrios would result in single dopted white (i.e. the medium white point with higher luminnce) wherey in the second the dopted whites would e the white points of the individul medi. Note, tht in ll cses the medium white points re mesured telespectrordiometriclly under the ctul viewing conditions. CRT Therml Figure 6. CRT nd Therml gmuts under stndrd conditions. Before considering the rnge of possiilities tht rise under the viewing conditions nd points of view considered here, we will first look t wht gmut differences re suggested when ech of the two medi (CRT nd Therml) re mesured y conventionl mens (i.e. the wy they would e mesured if one would mke just single device profile for ech of them). To do this, the CRT ws mesured in the wy descried in Section 3 ut in drk room which resulted in very little environmentl flre nd the setting of surround conditions to drk in CAM97s2. The Therml printer ws mesured using GretgMceth Spectrolino spectrophotometer, XYZ vlues were clculted for stndrd illuminnt D5, surround conditions were set to verge nd the ckground (Y ) hd luminnce which ws % of the luminnce of the dopted white. The luminnce of the dpting field (L A ) ws set to 1/5 of the CRT white s luminnce for oth medi s the mesurements of the print did not result in luminnce dt. Clerly there re numer of wys for clculting single device profiles nd the present one is only ment to e n exmple. Wht is more importnt is tht it is single rther thn wht its detils re. The gmut differences resulting from this dt re shown in Figure 6 nd their gmut volumes were 512,935 nd 7,656 cuic CAM97s2 units for the Therml nd CRT medi respectively. Note tht ll plots in this pper re projections onto the plne while plots re intersections of gmut oundry nd the plne. 35 Therml CRT luminnce of perfect diffuser (cd/m2) gmut volume (x cuic units) Figure 7. Gmut volumes for CRT nd Therml medi viewed simultneously under viewing conditions resulting in different perfect diffuser luminnces.

8 Luminnce of perfect diffuser 35 cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m CRT Therml Figure 8. CRT nd Therml gmuts viewed simultneously (under rnge of illumintion levels; nd xes rnge from to nd xis from to ). We cn now turn to considering the dt collected under ctul viewing conditions y first looking t the simultneous scenrio. The gmut differences for the CRT Therml pir re then shown in Figure 8 nd the corresponding gmut volumes re shown in Figure 7. As pointed out when discussing the stndrd dt, the key point to note is tht the given pir of medi cn result in numer of gmut differences vrying in mgnitude nd nture. Wht these figures show most clerly nd wht would e the cse even if they did not exhiit some white point chromticity differences (which here most ffect the 55 cd/m 2 setup) is tht the CRT nd Therml medi chnge in opposite directions with chnges in the level of illumintion. While the Therml gmut decreses with decrese of illumintion level, the CRT gmut increses. For the levels looked t here this lso mens tht CRT to Therml gmut mpping would t one end of the rnge involve little compression nd could mke use of expnsion while t the other end there would e significnt need for compression nd virtully no room for expnsion. Viewing these medi under ny of the ctul conditions considered here would result in drmtic differences etween wht is seen to e the gmut difference nd wht gmut difference is suggested y the stndrd gmuts. The point, however, is not criticism of the stndrd gmuts ut tht single gmut representtion cnnot mtch the rnge of possile gmut differences etween set of medi. The gmuts clculted under ny of the ctul illumintion levels considered here would eqully dly represent the other ctul levels s would the stndrd gmuts. So fr the gmuts of the CRT nd Therml medi were considered from the point of view of simultneous viewing nd this represents cses where imges on these two medi re compred side y side. For exmple, if n imge on the CRT is to e reproduced on the Therml medium nd this reproduction is then held next to the CRT for comprison, then the two imges could e from gmuts which hve rnge of gmut differences s illustrted in Figures 7 nd 8. Next, we will look t the possile gmut differences etween these two medi when viewed seprtely. This corresponds to gmut comprisons etween the gmuts of sets of colours seen under different viewing conditions y single oserver t different times or y multiple oservers t either single or multiple times. Going ck to the cross medi reproduction exmple from the simultneous scenrio, in this seprte viewing scenrio it would correspond to it eing either only the CRT or only the Therml medium tht is present in n oservers field of view. For exmple, if one person views the CRT imge under one set of conditions (e.g. in grphics studio) nd nother person views the Therml imge under nother set (e.g. outdoors), then there re two gmuts corresponding to these two medi eing viewed under these two conditions nd hence their difference cn lso e considered. Given the four levels of illumintion used ove, there re sixteen (4 4) gmut difference possiilities if the two medi re viewed seprtely. The result of such comprison of gmuts is shown in Figures 9 to 11. From these figures we cn gin see the sme trend s exhiited in the dt otined from their simultneous viewing. The Therml gmut decreses with perfect diffuser luminnce while the CRT gmut exhiits n increse. It cn lso e seen tht

9 gmut differences re smller in generl under these seprte viewing conditions while still hving significnt vrition in the possile differences due to differences in illumintion level. Viewing the CRT medium under 55 cd/m 2 conditions nd the Therml under 35 cd/m 2 conditions results in the two medi hving very lrge overlp etween their gmuts. Viewing oth medi under 2 cd/m 2 conditions gives very similr gmut volumes with gmut volume difference of only 2,224 cuic CAM97s2 units. The lrgest difference in this rnge of illumintion levels results from hving oth medi under the 55 cd/m 2 condition where the gmut volume difference is 429,954 cuic CAM97s2 units. To put this figure into perspective we cn see tht it is two orders of mgnitude lrger thn the smllest difference nd lmost twice the gmut volume of the Therml medium under these conditions (i.e. 251,9). Hence the question of how lrge the difference is etween the CRT nd Therml gmuts cn oth e tht they re lmost the sme nd tht the CRT gmut is lmost three times s lrge s the Therml gmut. It ll depends on viewing conditions nd s cn e seen from this dt their influence is drmtic. Trying to descrie colour imging medi using single gmuts cnnot cover the wide rnge of perceived gmuts they cn exhiit. This will further e illustrted y seeing how the gmuts of individul medi seen in isoltion chnge with chnges in illumintion levels. 35 Therml CRT luminnce of perfect diffuser (cd/m 2 ) gmut volume (x cuic units) Figure 9. Gmut volumes for CRT nd Therml medi viewed seprtely under viewing conditions resulting in different perfect diffuser luminnces. Perfect diffuser luminnce for CRT viewing conditions 35 cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m CRT Therml Figure 1. CRT nd Therml gmuts viewed seprtely (under rnge of illumintion levels; xis rnges from to nd xis rnges from to ). 35 cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m 2 Perfect diffuser luminnce for Therml viewing conditions

10 Perfect diffuser luminnce for CRT viewing conditions 35 cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m CRT Therml Figure 11. CRT nd Therml gmuts viewed seprtely ( nd xes rnge from to ) Other Medium Gmuts To llow for esier inter comprison etween the other medi considered here, gmut oundries s well s gmut volumes re shown together in Figures 12 nd 13. The Therml nd CRT oundries, eing sed on the sme dt s used in the previous section, gin show tht these gmuts chnge in opposite directions with chnge of illumintion level nd tht the Therml medium is suject to fr less chnge thn the CRT medium. This is due to the ppernce of ll colours from the Therml medium eing determined solely y the intensity of illumintion wheres the ppernce of the CRT medium is the result of comintion of its constnt output nd vrying illumintion levels. Due to the surfce of the CRT eing glossy, different levels of illumintion result in chnges in flre in the environment nd hence different lightnesses for the medium s lck point. In other words chnges in illumintion levels primrily introduce scling to the Therml medium s colours in tristimulus terms, wheres they result primrily in n offset for the CRT, hence hving different impct on corresponding lightness rnges cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m 2 Perfect diffuser luminnce for Therml viewing conditions

11 As the Lser medium is similr to the Therml medium since oth of them re prints, the sme trend s seen for the Therml gmuts cn gin e seen here for the Lser gmuts. Overll it cn e seen tht the gmut does not chnge significntly with chnge of illumintion level. Moreover the lightness rnge stys virtully identicl nd chnges in chromtic rnge re due primrily to chnges in the redness greenness direction. The gmuts of the LCD medium cn e seen to e fr less dependent on viewing conditions thn the gmuts of the other medi considered so fr. This is proly due to the LCD medium s mtte surfce properties comined with it eing self luminous medium nd hence dependent primrily on its own output which is illumintion independent. The LCD s mtte surfce properties lso ccount for there not eing ny flre effects for this medium etween the two levels of illumintion. Finlly, the gmut of the Projector medium cn e seen to e very strongly illumintion dependent, s it chieves colours y hving the light it projects reflected from surfce. If there is other (chromtic) light reflected from tht surfce then we gin hve similr sitution s ws the cse with the CRT. The reson then for the effect here eing stronger is tht, t R=G=B=% input, illumintion of the CRT results in only first surfce reflection from glss in front of lck ckground, while tht sme input to the Projector results in reflection of the light source from the highly reflecting (white) surfce onto which the Projector projects Luminnce of perfect diffuser 35 cd/m 2 2 cd/m 2 13 cd/m 2 55 cd/m CRT Lser LCD Projector Therml Figure 12. Gmuts Lser, Therml, LCD, CRT nd Projector medi seen in isoltion under different levels of illumintion Luminnce of Perfect Diffuser (cd/m2) Lser Therml LCD CRT Projector gmut volume (x cuic units) Figure 13. Gmut volumes of Lser, Therml, LCD, CRT nd Projector medi Projector t cd/m

12 6. CONCLUSIONS Colour imging medi re ffected y chnges in illumintion s result of different wys of reproducing colours, including self luminnce nd reflection, nd vriety of possile surfce properties rnging from the gloss of glss to the mtteness of uncoted pper. These differences necessrily nd in most cses drmticlly influence the corresponding colour gmuts nd hence necessitte description of medium gmuts in viewing condition dependent wy. Descriing colour reproduction medium using single gmut oundry inevitly leds to mismtch etween wht tht gmut oundry suggests nd how the gmut of the medium is seen under different conditions. While one solution to the prolem is to generte numer of gmut oundries for ech medium viewing condition comintion, this would result in n explosion of gmut oundry descriptors even if one were to do it only for stndrdised O VCs nd it would lwys result in some degree of inflexiility. Alterntively nd preferly this reltionship could e modelled nd this would result in the possiility of hving single reference gmut per medium which could then e modified to suit prticulr viewing conditions. As cn e seen from Fig. 17 medium gmuts cn e ffected to different degrees rnging from virtully no difference s for the LCD medium, to sixfold chnge s for the Projector medium. The most importnt point to e tken from this pper is tht insted of medium hving single gmut, it hs multitude of them. ACKNOWLEDGEMENTS This pper is sed on n eponymous pper presented t the Colour Imge Science conference 14 nd hs een inspired y n exchnge of emils initited y Michel Stokes (Microsoft) nd lter lso involving Noy Ktoh (Sony) in Ferury We would like to thnk them oth for ringing up this useful topic. Thnks lso go to ulin Shw nd Dr. Stephen Hordley for their comments. REFERENCES 1.. Morovic, Colour Reproduction Pst, Present nd Future, Liro de Acts V Congreso Ncionl de Color (Proceedings of the 5th Ntionl Congress on Colour), pp. 9 15, Terrss, Spin, Morovic nd P. Morovic, Determining Colour Gmuts of Digitl Cmers nd Scnners, Color Reserch nd Appliction., vol. 26, 1. (sumitted for puliction) 3.. Morovic nd M. R. Luo, Developing Algorithms for Universl Colour Gmut Mpping, Colour Engineering: Vision nd Technology, McDonld L. W. (ed.), ohn Wiley & Sons, pp , / W. Kress nd M. Stevens, Derivtion of 3 Dimensionl Gmut Descriptors for Grphic Arts Output Devices, TAGA Proceedings, pp , G.. Brun nd M. D. Firchild, Techniques for Gmut Surfce Definition nd Visuliztion, Proceedings of 5 th IS&T/SID Color Imging Conference, pp , Morovic nd M. R. Luo, Gmut Mpping Algorithms Bsed on Psychophysicl Experiment, Proceedings of the 5 th IS&T/SID Color Imging Conference, pp , Morovic nd M. R. Luo, Clculting Medium nd Imge Gmut Boundries for Gmut Mpping, Color Reserch nd Appliction., vol. 25,. 1. C.. Li, M. R. Luo nd R. W. G. Hunt, The CAM97s2 Model, IS&T/SID 7th Color Imging Conference: Color Science, Systems nd Applictions, pp , G. D. Finlyson nd P. M. Morovic, Metmer Constrined Colour Correction, Proceedings of 7th IS&T/SID Color Imging Conference, p 26 31, CIE, CIE Puliction 15.2, Colorimetry, Second Edition, R. W. G. Hunt, Mesuring Colour, Second Edition, Ellis Horwood, Morovic nd P. L. Sun, How different re Colour Gmuts in Cross medi Colour Reproduction?, Colour Imge Science Conference, Dery, ,.