Testing the Possibility of Average-Color Perception

from Multi-Colored PatternsIchiro KURIKI

Human and Information Science Laboratory, NTT Communication Science Laboratories,3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

(Received February 18, 2004; Accepted May 10, 2004)

To test the possibility that an average color is perceived from a multi-colored pattern, the appearance of colored mosaicpatterns as a whole was measured by the method of adjustment. As the range of color distribution in a mosaic patternbecame larger, subjects’ matches deviated more from the photometric averages of each pattern and the subjectsperceived higher brightness from mosaic patterns. Since the saliency of element colors was different in an equiluminantmosaic pattern, two possible hypotheses for selecting a color which represents a mosaic pattern was examined by usingbrightness match results for each element color. The results show that the mosaic element with the highest-saturationstrongly influences the choice of a color to represent a mosaic pattern.

Key words: average color, multi color, mosaic pattern, brightness

1. Introduction

If you were to enlarge a digital image of a natural scene,even just a small part of an object whose color appears to beuniform, you would find that the area consists of pixels ofvarious colors and brightness. Indeed, almost none of theobjects around us have completely uniform color andbrightness at any point on it. However, there is essentiallyno direct approach to studying the color appearance of amulti-colored patch itself.

A few studies of the mechanisms of color-discriminationand pattern-segmentation mechanisms have used randomlyarranged multiple-color patterns.1–3) The purpose of usingmultiple colors was to restrict the variety of colors indirections that selectively stimulate the channels in themultiple color-selective mechanisms, including the oppo-nent-color system. The multi-colored stimuli were presentedin mosaic patterns, which were tiled with a large number ofsmall segments, and each of the segments was sized to beclearly visible to the subject.

In studies of color constancy, the average color of a scenehas been suggested as a cue to the illuminant chromaticity ofthe scene. For example, the gray world hypothesis4) suggeststhat the average color of the scene is equal to gray, and apsychophysical study has shown that a multi-coloredsurround and uniform gray surround have an equal effecton the appearance of a test color chip at the center.5) In fact,for natural scenes, assuming the average color to be grayworks effectively as a first approximation to cancel changesin object colors from illuminant color changes. In addition,some studies have shown that equating achromatic pointsexplains the perceptual shifts in color under changes inilluminant color.6,7) These studies may suggest that theachromatic point is important, not only for the visual systembut also for image-processing techniques, because it acts as arepresentative color of multi-colored scenes to estimateilluminant color in the real world. On the other hand, anotherstudy has shown that, compared with a uniform graysurround, a multi-colored surround works to improve colorconstancy.8) And another study has demonstrated based onphotometric data from natural objects that the average color

of the world is not gray.9) However, no study has directlyconfirmed whether the human visual system is able toaverage colors across a scene.

As is often experienced, the human visual system is ableto summarize a multi-colored object with a color term,which is known as categorization. For example, if you lookat tree foliage, you will say that the leaves are green, and atthe same time, you might be able to tell that each leaf has aslightly different shade of green. In most cases, colors in acertain range of variations can be summarized with a colorterm when the range falls within a category.10,11)

The primary purpose of this study is to investigate howthe color appearance of a multi-colored pattern is summa-rized as a representative color.

2. Methods

2.1 StimuliThe visual stimuli consisted of isoluminant color ele-

ments, each of which subtended 0:15� 0:15 deg. The size ofa mosaic pattern for the test stimulus and a uniform patternfor the matching stimulus subtended 3:0� 3:0 deg. The sizeof each pattern was set to cover the foveal area (approx-imately 2 deg in diameter) and fall within the approximatesize of macular pigments (approximately 5 deg in diameter).The size of each element was selected so that the color ofeach element would be clearly visible for all subjects and thenumber of color elements that fell on the fovea would bemore than 50.

The stimulus was generated by visual stimulus generator(VSG 2/5, Cambridge Research Systems) controlled by aPC (OptiPlex GX200, DELL), and presented on a CRTscreen (GDM-F520, SONY).

The subject watched the screen from 60 cm away using achin rest. The background of the screen was filled with grayat 25 cd/m2 with the chromaticity of equal-energy white.The luminance of the test and matching stimuli were 30 cd/m2, and the stimuli were placed at the left and right sides ofa dot at the center of the screen. The test and matchingstimuli were separated by 1 deg.

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# 2004 The Optical Society of Japan

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2.2 Mosaic colorsFor each mosaic pattern, the parameters were the center

color and distribution radius. Colors for the elements of amosaic pattern were selected to distribute around one of thecenter colors. There were five center colors, one of whichwas equal-energy white. The other four center chromaticitieswere determined to be located at the distance of 0.05 unit inEuclidian distance in the CIE u0v0 coordinates (�E�u0v0)from the equal-energy white (Fig. 1).

There were eight satellite colors for each center color andthey were located from the center color at 0 (controlcondition), 0.01, 0.02 and 0.03 units in �E�u0v0. Colors forthe mosaic elements were selected from nine colors: eightsatellite colors and one center color. Figure 2 show twotypical samples of mosaic patterns with radii of 0.01, 0.02,and 0.03 units in �E�u0v0 for green and gray center colors.The arrangement of element colors in a mosaic pattern wasrandomized between trials. The colors for 400 (20� 20)mosaic elements were randomly selected from the ninecolors.

The subjects perceived reddish colors in the elements in amosaic pattern with greenish center color, which is shown inthe top row of Fig. 2. If each of the element color werepresented alone against a dark background, no rednesswould be perceived from the element colors in the top row(See Fig. 1). This implies that the deviations of each elementcolor from the photometric-mean chromaticity of the mosaiccan be perceived with such a simple pattern. It may alsoimply that this kind of simple mosaic pattern alreadycontains sufficient cues to evoke a percept like colorconstancy.

2.3 Procedures2.3.1 Color matching between mosaic and uniform pat-

ternsIn color matching sessions, the subjects were asked to

determine a color which might represent the average color ofthe multi-colored (mosaic) pattern, which is hereafterreferred to as a representative color of a mosaic pattern.The subject reported the appearance of the representativecolor by adjusting the color of a uniform pattern presentedon the same screen. Locations of these patterns wererandomly alternated between the left and the right sides ofthe dot at the center of the screen to reduce the effect oflocation on both the screen and retina. The stimuli werecontinuously presented until the subject was able to make asatisfactory match, and a uniform screen with the back-ground color was presented for more than 5 s during inter-trial intervals to maintain chromatic adaptation to the equal-energy white. The subject viewed the stimulus monocularlyand was allowed to move the eyes to view each patterndirectly. The subject could adjust not only chromaticity butalso luminance (if the subject felt necessary). In one session,matches were conducted for five center-color conditions andfour distribution-radius conditions in a random order. Eachcondition was repeated five times.

The subjects were allowed to adjust luminance to matchbrightness, but they were not satisfied with luminanceadjustments. One subject complained that the increment ofluminance did not reduce the subjective difference inbrightness. This subject also claimed that any modificationof luminance made the matches worse. As will be shownlater (Fig. 9), the subjective difference in brightness wasconsistent, but small. It was very difficult, even for theauthor, to adjust luminance to a satisfactory point ofbrightness match. In a preliminary experiment with elaborateluminance adjustments, the deviation of luminance from thatfor a mosaic pattern was less than 10% (3.0 cd/m2). Butwhen the mosaic and uniform patterns were compareddirectly, the brightness difference was remarkable. There-fore, all subjects were asked to concentrate on matching hueand saturation. The brightness difference was measured byusing a forced choice task, which will be introduced in thenext section.

2.3.2 Forced-choice brightness judgments for mosaicpatterns

The matching of brightness (luminance) was very diffi-cult, but the perception of brightness difference betweenmosaic and uniform patterns was very consistent. Therefore,subjective differences in brightness were measured by usingtwo-alternative forced choice (two-AFC) between mosaicand uniform patterns. A mosaic and a uniform pattern waspresented at the left or right of a fixation dot for 1 s. Theuniform pattern had the same chromaticity as the centerchromaticity of the mosaic pattern, so as to reduce thedifficulty in brightness comparison judgments arising fromcolor differences between the stimuli. Subjects were asked tomaintain fixation on the dot at the center of screen and reportwhich of the two appeared brighter. The luminance of theuniform pattern was increased or decreased by a factor of1.05 in the staircase procedure according to the subject’sresponse. The luminance of the mosaic pattern was keptconstant at 30 cd/m2. The probability distribution for eachluminance condition was calculated after the session and fit

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Fig. 1. Panel (a) shows the CIE u0v0 chromaticities of centercolors. Panels (b)-(c) shows colors used in mosaic patterns. Thedotted circles represent radii from each center color. Colors on thecircle and its center were used in a mosaic pattern.

250 OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI

by the cumulative Gaussian function using the psignifittoolbox on MATLAB.12) There were five center-colorconditions per session, and the stimuli were presented witha randomly interleaved staircase procedure. Different dis-tribution radius conditions were presented in differentsessions. The results will be shown as luminance ratiosbetween mosaic and uniform patterns when they appearsubjectively equal in brightness, which corresponds to 50%probability in the psychometric function. Hereafter, theluminance ratio will be referred to as the brightness-to-luminance ratio for mosaic pattern (mosaic B/L).

During the presentation of a mosaic pattern, chromaticaberration of the ocular media might possibly produceluminance edges on the retina, and this could cause anartifact that enhances the apparent brightness of mosaicpatterns. Therefore, flicker-null adjustments were made toconfirm the extent of the luminance artifact. A mosaicpattern and a uniform pattern were alternately presented witha frequency of 20Hz at the same location: the center of thescreen. The subject adjusted the luminance of the uniformpattern so that the luminance flicker would be invisible. Theresults of this flicker photometry (not shown) confirmed that

all mosaic patterns matched their center colors at theluminance ratio around 1.0, and the standard deviation of thesettings was approximately 0.02 in the luminance ratio.

2.3.3 Brightness matching for element colorsThroughout the matching experiment and two-AFC

experiment on brightness perception, it became clear thatthe elements of the mosaics appeared to have differentbrightness, and this yielded differences in the saliencybetween mosaic elements. This difference in the saliencymay affect the perceptual weight for each element color atthe determination of a representative color for a mosaicpattern. Therefore, it is necessary to assess its effect on thedetermination of a representative color. For this assessment,the brightness of each mosaic-element color was measured.The results were then used to estimate the brightness ofmosaic patterns based on two assumptions, which will bedescribed in detail later and assessed in the analysis section(§4).

Brightness perception from a colored light can not bepredicted from its colorimetric characteristics. In general,the brightness increases as the saturation of the color

Fig. 2. Samples of mosaic patterns. The upper panels show mosaic patterns with a greenish center color ðu0; v0Þ ¼ð0:175; 0:508Þ and the lower panels show mosaics with a center color of equal-energy white ðu0; v0Þ ¼ ð0:210; 0:473Þ,respectively. In each row, the three panels show patterns with different distribution radii; from left �E�u0v0 ¼ 0:01, 0.02and 0.03.

OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI 251

increases.13,14) However, precise brightness judgments haveto be made for each color by subjective matches. Eventhough the brightness perception is not predictable fromcolorimetric values, such as tristimulus values or spectrum,the brightness perception does not generally jump abruptlyin color space. From such continuity of the brightnessperception in the very local area of the color space, it ispossible to estimate the brightness of a designated color byaveraging subjective judgment results at nearby points in thecolor space.

The subjects were asked to match brightness between twouniform patterns, each of which subtended 3:0� 3:0 deg.The patternes were separated by 1.0 deg and were presentedat the both sides of the center dot. One of the uniformpatterns was presented with a color used as the mosaicelement, and the other color chip was presented with thechromaticity of equal-energy white. The luminance of themosaic-element color was fixed to 30 cd/m2, whereas that ofthe color chip with the chromaticity of equal-energy whitewas adjustable. The stimulus was presented until the subjectreached a satisfactory point of brightness match. The resultsof the adjustment will be shown in the relative luminancethat provides with subjectively equal brightness, which willbe referred to as the brightness-to-luminance ratio formosaic elements (element B/L). It should be recalled thatthe mosaic B/L is determined by 2AFC with respect to thecenter-color of mosaic, and the element B/L is determinedby the method of adjustment with respect to the equal-energy white. There were five center-colors and nine satellitecolors for each center color, and they were presented inrandom order. The subject made five matches for each colorand the average was used in the analysis.

2.4 SubjectsTwo naı̈ve subjects and the author participated in the

experiments. All of them had normal or corrected-to-normalvisual acuity and all of them had normal color vision, astested by Ishihara pseudo-isochromatic plates.

3. Results

3.1 Matching representative colorsFigures 3, 4, 5, and 6 show the results of color matching

for all subjects. Different panels show results for differentsubjects and different distribution radii. Each filled symbolshows the result of one trial. The distribution of matchedcolors tends to deviate from the center color as thedistribution radius becomes larger. Though not preciselyshown, the physical average color of a mosaic pattern, whichwas calculated by averaging tristimulus values of theelement colors, is almost the same as the center color. Thedeviation of the physical average color from the center colorwas smaller than the size of the symbols in Fig. 1. However,the matching result was fairly close to the element with thehighest saturation.

The matching for the condition with the center color ofequal-energy white shows no clear trends on the distributionof the matched colors, which distributed almost randomlyaround the center color (i.e., equal-energy white). This

condition was different from the other four center-colorconditions in that the element colors fell within differentcategories of color. This probably prevented the subject fromchoosing a certain color among the element colors. It alsoimplies that the subject matched the representative color bytaking each element color into account, and that there is noexplicit appearance of a representative color from a mosaicpattern.

Figure 7 shows the relationships between the deviationsof matched chromaticity from the center color and thedistribution radii of the mosaic element colors. Thehorizontal axis represents the distribution radius of mosaicpatterns, and the vertical axis represents the averageddeviation of representative colors (Figs. 3–6) from centercolors for each mosaic pattern in the �E�u0v0 scale. Ingeneral, the deviations of matched color from the centercolor become larger as the distribution radius of the elementcolors become larger.

3.2 Two AFC brightness judgments for mosaic patternsFigure 8 shows typical results and the psychometric

functions to three distribution radius conditions for onecenter color condition for one subject. The mosaic B/L isdefined using this point of 50% probability, which is theluminance of uniform pattern of a center-color with respectto the luminance of mosaic pattern of the same center color(i.e., 30 cd/m2). The deviations of mosaic B/L from unityincrease monotonically with the distribution radius.

Figure 9 shows the results of brightness judgments for themosaic pattern with each center color condition, in the scaleof mosaic B/L. Error bars show the estimated standard errorto fit the data with a cumulative Gaussian function. Differentpanel show the results for different subject, and differentsymbol show the results for different distribution radius.Figure 9 clearly shows that apparent brightness of themosaic pattern becomes higher, as the distribution radiusbecomes larger. The amount of increment in mosaic B/L isabout the same as the magnitude of the error bar, whichmeans the effect is not dramatically large, but the differenceis clearly visible to the subject. This might be because thecomparison stimulus was set to a uniform pattern with thechromaticity of the center color of the mosaic pattern toreduce the difficulty of brightness judgments.

4. Analysis by Brightness Estimation

From the above two experiments, several consistent trendswere found among all subjects. In the color matching results,element colors with the nearly highest saturation in themosaic pattern seemed to be selected as a representativecolor for each mosaic pattern. In addition, the two-AFCbrightness-judgment experiment showed that the brightnessdifference between mosaic and uniform patterns of the sameluminance was not so large, but was very obvious. Also, thebrightness difference increased almost monotonically withthe distribution radius for the satellite-colors for the mosaicelement. As it was mentioned in the methods (§2.3.3), therewere clear differences in the saliencies among the elementsof a mosaic pattern. It is quite possible that the visual system

252 OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI

would assign higher weight to a more salient element uponthe determination of a representative color, and the saliencymight be primarily based on the brightness of each element.Therefore, investigating the effect of brightness differenceamong the determination of overall brightness perceptionfrom a mosaic pattern might be useful in inferring thealgorithm of the visual system for the determination of arepresentative colors. The following two assumptions on thisalgorithm will be examined.

There are two possible hypotheses: (1) The subject’svisual system chose the element color with the highestsaturation, and this was the determinant factor for thebrightness perception. (2) The subject’s visual systemcalculated the weighted sum of the brightness for eachelement color, whose weights were decided by the numberof each element color. These possibilities are referred to as

the highest-saturation assumption and the weighted-sumassumption, hereafter. The highest-saturation assumptionwill be realized as a physiological system if the visualsystem applies the winner-take-all algorithm based on thesaliency scale (in the present case, brightness) as adeterminant factor. The weighted-sum assumption can berealized as a physiological system by a receptive field, whichsimply receives excitatory inputs from a very large area inthe visual field. One of the simplest ways to calculate asummary-color of an area is to take the average of the area,and this receptive field works in that way.

Derivations of the luminance ratio for equal brightness formosaic patterns (mosaic B/L) based on these two assump-tions would result in different ways. The mosaic-B/Lpredicted with highest-saturation assumption would belarger than one, because color of highest saturation generally

Fig. 3. Matched results for the three subjects when the distribution radius = 0.0 in �E�u0v0. Open symbols show thecenter colors and filled symbols represent matched colors in each trial.

Fig. 4. Matched results for the three subjects when the distribution radius = 0.01 in �E�u0v0. Meanings of the symbolsare the same as in Fig. 2. Circles with dashed lines centered at each center color represent the radius of the color for eachmosaic element. The precise chromaticities are shown in Fig. 1.

OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI 253

Fig. 6. Matched results for the three subjects when the distribution radius = 0.03 in �E�u0v0. Meanings of the symbolsand circles are the same as in Fig. 3.

Fig. 5. Matched results for the three subjects when the distribution radius = 0.02 in �E�u0v0. Meanings of the symbolsand circles are the same as in Fig. 3.

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254 OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI

shows higher B/L than the center color. The B/L for someelements could be smaller than 1.0 and the rest of themcould be larger than 1.0, and the estimated mosaic B/L fromweighted-sum assumption could be closer to 1.0 than thatfrom the highest-saturation assumption. If there weredramatic increases in B/L across the center colors, weight-ed-sum assumption would predict a value larger than 1.0.The weights for each element color were fixed to 1/9because the frequency of emergence for each element colorwas determined randomly without any constraint, and thesame condition was repeated at least five times.

In order to examine these two possibilities, the mosaic B/L for each mosaic-pattern was estimated from the B/L formosaic elements, including center colors (element B/L),collected by the method of adjustment described in §2.3.3.The chromaticities of the center colors are shown as opensymbols in Fig. 10. The element B/L at an arbitrary pointwas estimated by the interpolation of these data. Theinterpolation was conducted by finding three nearest pointsin the data, and the element B/Ls were weighted with theinverse of the distance of each element from the chroma-ticity to be estimated. When any of the distances from threenearest data-points exceeded 0.03 the interpolation was not

conducted. Contour maps in the Fig. 10 show the results ofinterpolation for each subject.

Figure 11 shows the relationships between estimatedmosaic-B/L and the measured mosaic B/L for two naı̈vesubjects. The vertical axis shows the estimated mosaic B/Lrelative to the B/L for the center colors of each mosaic, afterthe two assumptions. The horizontal axis shows the subject’sjudgment, which is the same as the result in Fig. 9. Panels(a) and (b) show the estimates based on the highest-

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Fig. 9. Variations of matched relative luminance between differ-ent center colors. The vertical axis represents mosaic B/L. Thehorizontal axis represents the difference in center colors, where W,Or, GY, B and P stand for equal-energy white, orange, greenish,blue and purple center-colors, respectively (See Fig. 1 and text forthe details of chromaticities).

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Fig. 10. The interpolated maps of element B/L from subjectively matched results. In this figure, the step ofinterpolation was 0.001 in the u0v0 coordinates (See text for the details of interpolation procedure).

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Fig. 11. Comparisons between the estimated results and measuredresults. The horizontal axis represents the relative luminancebetween the center-color and mosaic pattern, measured by two-AFC (Figs. 8 and 9; measured mosaic B/L). The vertical axisrepresents ratios between estimated mosaic B/L. Note that mosaicB/L represents relative luminance to equate brightness between amosaic pattern and a uniform pattern with the center color of themosaic. Different symbols show different distribution radii. Thefitted line indicates positive correlation between fitted lines andestimated brightness. Panels (a) and (b) show estimates based onthe highest-saturation assumption, and panels (c) and (d) showestimates based on the weighted-sum assumption (See text fordetails).

OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI 255

saturation assumption, and panels (c) and (d) show theestimates based on the weighted-sum assumption.

Dotted lines show the linear fittings to the results ofoverall conditions. The formulae and correlation coefficientsshow the parameters of the fitted lines. Fitted lines in panels(a) and (b) show positive correlation coefficients: 0.1794 forsubject HC and 0.4352 for subject KS. The fitted lines inpanels (c) and (d) show zero or negative correlations. Theresults in Fig. 11 does not strongly support the highest-saturation assumption, but may support that the highest-saturation assumption predicts the mosaic B/L better thanthe weighted-mean assumption.

5. Discussions

5.1 General discussionsColor appearance of the mosaic pattern as a whole was

measured by matching the color with a uniform pattern, inpurpose of testing whether average color perception exists inhuman color vision. The result showed that the matchedcolor deviated from the physical average color as the mosaiccolor distribution radius became larger. The matches wereset near the color of highest saturation within the mosaicelements when the colors in a mosaic pattern roughly fellwithin a color category. However, when the colors in amosaic pattern distributed among various hues (when thecenter color was equal-energy white), the subject’s decisiondid not show a systematic trend. All subjects reportedsubjective difference in brightness between the mosaic andthe uniform patterns of the same luminance. The amount ofapparent brightness increase was about the same as theestimated standard deviation of the psychometric function ortwice of that. According to the analysis using the luminanceratio for the equivalent subjective brightness (mosaic andelement B/Ls), the color of highest saturation within themosaic elements could be a determinant factor of theappearance of the mosaic pattern as a whole.

5.2 Analysis based on brightness estimatesThe analysis in §4 using brightness matches for each

mosaic elements was not completely successful. As shownin Figs. 11(a) and (b), the correlation coefficients betweenestimations and subjective judgments had positive value, butwere far from 1.0.

However, the analysis in §4 was partly successful in thatthe possibility of weighted-sum assumption was clearlyrejected (Figs. 11(c) and (d)). The subjects were not able tomake brightness matches while adjusting the luminance ofrepresentative colors. The subjects felt representative-colormatches were satisfactory even though they did not changethe luminance. As shown in Fig. 9, the difference inbrightness was consistently present for all subjects betweenthe mosaic and uniform patterns of the same center color.However, the standard deviation of brightness judgmentswas very large and almost equal to the subjective differencein brightness between mosaic and uniform patterns.

Figures 8 and 9 show some considerable magnitude ofvariability in the brightness judgments. This variability mayexist because there was variability in the brightness among

mosaic-element colors. If the weighted sum algorithm weresuccessful in taking some average among mosaic elements,the resulting value would not have varied trial-to-trial. Sincethe subjects were able to discriminate the differences inbrightness among element colors, the performance inbrightness discrimination around the level of mosaicelements was sufficiently high. Therefore, if the weighted-sum algorithm was the primary factor in the brightnessjudgments, the variability would have to be much smaller.Conversely, if the subjects judged brightness by using arepresentative color after selecting one from the elementcolors, which could also vary moment to moment thevariability in the brightness evaluation could exist. Also, thevisual system might have assigned higher weight for thebrighter elements on the selection, because the overallevaluation of brightness from mosaic patterns showedconsistent increments (Figs. 8 and 9). Therefore, theweighted-sum assumption, which assigns equal weight (inthe present case, 1/9) to the brightness of each elementcolor, can not account for the results in the present study.

In summary, the analysis in §4 did not strongly supportthat the highest-saturation assumption to be true, but itshowed that this assumption explains the results in thepresent study better than weighted-sum assumption.

5.3 Is average chromaticity useless for human colorvision?

The colors that the subjects matched as a representativecolor of the mosaic pattern were not the same as thephotometric averages. The present experiment was conduct-ed only with elements of a single size, and the mosaicpattern subtended only 3.0 deg. As a physiological system toestimate the illuminant information, it might be possible toassume one with large receptive field to calculate theaverage of the color in a certain area. However, we are ableto perceive the color of illuminant in a small box,illuminated with a light source different from the one inthe room. Color constancy holds for illuminated environ-ments which subtends only a small part of the visual field.This means that the size of the illuminated area is notnecessarily the whole nor nearly equal to the whole of thevisual field. Therefore, it might be possible that the systemfor color constancy that adjusts appearance of objects underan illuminant does not use the average color of the scene.

The present study does not support the possibility that thehuman visual system has some explicit representation ofaveraged color from multi-colored patterns. However, thisdoes not simply mean that the human visual system does notuse such statistical characteristics. Human subjects candiscern color appearance of an object in an image of a scenetaken under colored illuminant (i.e., color constancy). Asimilar percept could be achieved even with the mosaicpattern used in the present study. The top row of Fig. 2shows mosaic patterns with greenish center color, but noneof mosaic elements would appear reddish if they were shownseparately in a dark background (See Fig. 1). However,redness could be perceived to the subject from some mosaicelements. The photometric implication of this phenomenonis that human subjects are able to identify the deviation of

256 OPTICAL REVIEW Vol. 11, No. 4 (2004) I. KURIKI

color from photometric average color of a part of the scene.In addition, mere colors within the image appear to becentered on the approximate color of the illuminant: thecolor of illuminant in the image does not appear colorless,and the colors obviously have some common offset fromgray. Therefore, the human visual system might somehowreduce the average color of the scene in an implicit way. It ispossible that the subjects in the present study were not ableto report such implicit representation of average color by themethod of adjustment. Some different approach to study thecharacteristics of such internal representations of the scenemight be necessary in order to explore the mechanisms ofcolor perception in complex scenes in the real world.

6. Conclusion

The human visual system does not have explicit percept ofaverage color from multi-colored patterns. The primaryfactor that determines the color appearance and brightness ofmosaic pattern as a whole is the highest chromaticity

element in it, when the majority of mosaic elements fallwithin a color category.

References

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