In this paper we present a parametric model for automatic color naming where each color category is modeled as a fuzzy set with a parametric membership function. The parameters of the functions are estimated in a fitting process using data derived from psychophysical experiments. The name assignments obtained by the model agree with previous psychophysical experiments, and therefore the high-level color-naming information provided can be useful for different computer vision applications where the use of a parametric model will introduce interesting advantages in terms of implementation costs, data representation, model analysis, and model updating.
© 2008 Optical Society of America
Color is a very important visual cue in human perception. Among the various visual tasks performed by humans that involve color, color naming is one of the most common. However, the perceptual mechanisms that rule this process are still not completely known . Color naming has been studied from very different points of view. The anthropological study of Berlin and Kay  was a starting point that stimulated much research about the topic in the subsequent decades. They studied color naming in different languages and stated the existence of universal color categories. They also defined the set of 11 basic categories that have the most evolved languages. These are white, black, red, green, yellow, blue, brown, purple, pink, orange, and gray. Since then, several studies have confirmed and extended their results [3, 4, 5, 6].
In computer vision, color has been numerically represented in different color spaces that, unfortunately, do not easily derive information about how color is named by humans. Hence a computational model of color naming would be very useful for several tasks such as segmentation, retrieval, tracking, or human–machine interaction. Although some models based on a pure tessellation of a color space have been proposed [7, 8, 9], the most accepted framework has been to consider color naming as a fuzzy process; that is, any color stimulus has a membership value between 0 and 1 to each color category. Kay and McDaniel  were the first to propose a theoretical fuzzy model for color naming. Later, some approaches from the computer vision field adopted this point of view. Lammens  developed a fuzzy computational model where the membership to the color categories was modeled by a variant of the Gaussian function that was fitted to Berlin and Kay’s data. In recent years, more complex and complete models have been proposed. Mojsilovic  defined a perceptual metric derived from color-naming experiments and proposed a model that also takes into account other perceptual issues such as color constancy and spatial averaging. Seaborn et al.  have developed a fuzzy model based on the application of the fuzzy k-means algorithm to the data obtained from the psychophysical experiments of Sturges and Whitfield . More recently, van den Broek et al.  have proposed a categorization method based on psychophysical data and the Euclidean distance. Apart from Lammens’ model, the rest are nonparametric models.
In this paper we present a fuzzy color-naming model based on the use of parametric membership functions whose advantages are discussed later in Section 2. The main goal of this model is to provide high-level color descriptors containing color-naming information useful for several computer vision applications [16, 17, 18, 19].
The paper is organized as follows. In Section 2, we explain the fuzzy framework and present our parametric approach. Next, in Section 3, we detail the process followed to estimate the parameters of the model. Section 4 is devoted to discussing the results obtained and, finally, in Section 5, we present the conclusions of this work.
2. PARAMETRIC MODEL
The essential contribution of this paper is to take a further step toward building computational engines to automate the color categorization task. As similarly done in previous works, such as Mojsilovic in  or Seaborn et al. in , we present the color-naming task as a decision problem formulated in the frame of the fuzzy-set theory . Whereas in the first work a nearest neighbor classifier is used, in the second one a fuzzy k-means algorithm is used. The essential difference of our proposal relies on the definition of a parametric model; that is, we propose a set of tuneable parameters that analytically define the shape of the fuzzy sets representing each color category. Parametric models have been previously used to model color information , and the suitability of such an approach can be summed up in the following points:
Inclusion of prior knowledge. Prior knowledge about the structure of the data allows us to choose the best model on each case. However, this could turn into a disadvantage if a nonappropriate function for the model is selected.
Compact categories. Each category is completely defined by a few parameters, and training data do not need to be stored after an initial fitting process. This implies lower memory usage and less computation time when the model is applied.
Meaningful parameters. Each parameter has a meaning in terms of the characterization of the data, which allows us to modify and improve the model by just adjusting the parameters.
Easy analysis. As a consequence of the previous point, the model can be analyzed and compared by studying the values of its parameters.
Considering the perceptual spaces derived from previous works and from psychophysical data, we have fitted color membership using a triple-sigmoid function [see Eq. (11)] for the eight basic chromatic categories (Red, Orange, Brown, Yellow, Green, Blue, Purple, and Pink). To this end, we have worked on the CIELab color space, since it is a quasi-perceptually-uniform color space where a good correlation between the Euclidean distance between color pairs and the perceived color dissimilarity can be observed. It is likely that other spaces could be suitable whenever one of the dimensions correlates with color lightness and the other two with chromaticity components. In this paper, we will denote any color point in such a space as , where I is the lightness and and are the chromaticity components of the color point.
Ideally, color memberships should be modeled by three-dimensional functions, i.e., functions defined by , but unfortunately it is not easy to infer precisely the way in which color-naming data are distributed in the color space, and hence finding parametric functions that fit these data is a very complicated task. For this reason, in our proposal the three-dimensional color space has been sliced into a set of levels along the lightness axis (see Fig. 1 ), obtaining a set of chromaticity planes over which membership functions have been modeled by two-dimensional functions. Therefore, any specific chromatic category will be defined by a set of functions, each one depending on a lightness component, as is expressed later in Eq. (12). Achromatic categories (Black, Gray, and White) will be given as the complementary function of the chromatic ones but weighted by the membership function of each one of the three achromatic categories. To go into the details of the proposed approach, we will first give the basis of the fuzzy framework, and afterward we will pose the considerations on the function shapes for the chromatic categories. Finally, the complementary achromatic categories will be derived.
2A. Fuzzy Color Naming
A fuzzy set is a set whose elements have a degree of membership. In a more formal way, a fuzzy set A is defined by a crisp set X, called the universal set, and a membership function, , which maps elements of the universal set into the [0, 1] interval, that is, .
Fuzzy sets are a good tool to represent imprecise concepts expressed in natural language. In color naming, we can consider that any color category, , is a fuzzy set with a membership function, , which assigns, to any color sample s represented in a certain color space, i.e., our universal set, a membership value within the [0,1] interval. This value represents the certainty we have that s belongs to category , which is associated with the linguistic term .
In our context of color categorization with a fixed number of categories, we need to impose the constraint that, for a given sample s, the sum of its memberships to the n categories must be the unity
Hence, for any given color sample s, it will be possible to compute a color descriptor, , such as
The information contained in such a descriptor can be used by a decision function, , to assign the color name of the stimulus s. The most easy decision rule we can derive is to choose the maximum from :
In our case the categories considered are the basic categories proposed by Berlin and Kay, that is, , and the set of categories is
2B. Chromatic Categories
According to the fuzzy framework defined previously, any function we select to model color categories must map values to the [0,1] interval, i.e., . In addition, the observation of the membership values of psychophysical data obtained from a color-naming experiment  made us hypothesize about a set of necessary properties that membership functions for the chromatic categories should fulfill:
• Triangular basis. Chromatic categories present a plateau, or area with no confusion about the color name, with a triangular shape and a principal vertex shared by all the categories.
• Different slopes. For a given chromatic category, the slope of naming certainty toward the neighboring categories can be different on each side of the category (e.g., transition from blue to green can be different from that from blue to purple).
• Central notch. The transition from a chromatic category to the central achromatic one has the form of a notch around the principal vertex.
In Fig. 2 we show a scheme of the preceding conditions on a chromaticity diagram where the samples of the color-naming experiment have been plotted.
After considering different membership functions [23, 24, 25] that fulfilled some of the previous properties, we have defined a new variant of them, the triple sigmoid with elliptical center (TSE), as a two-dimensional function, . The definition of the TSE starts from the one-dimensional sigmoid function:3a ].
This can be extended to a two-dimensional sigmoid function, , as3b].
By adding a translation, , and a rotation, α, to the previous equation, the function can adopt a wide set of shapes. In order to represent the formulation in a compact matrix form, we will use homogeneous coordinates . Let us redefine p to be a point in the plane expressed in homogeneous coordinates as , and let us denote the vectors and . We define as a function oriented in axis x with rotation α with respect to axis y, and as a function oriented in axis y with rotation α with respect to axis x:
By multiplying and , we define the double-sigmoid (DS) function, which fulfills the first two properties proposed before:4 .
To obtain the central notch shape needed to fulfill the third proposed property, let us define the elliptic-sigmoid (ES) function by including the ellipse equation in the sigmoid formula:5 .
Finally, by multiplying the DS by the ES (with a positive ), we define the TSE as
Hence, once we have the analytic form of the chosen function, the membership function for a chromatic category is given by
By fitting the parameters of the functions, it is possible to obtain the variation of the chromatic categories through the lightness levels. By doing this for all the categories, it will be possible to obtain membership maps; that is, for a given lightness level we have a membership value to each category for any color point of the level. Notice that since some categories exist only at certain lightness levels (e.g., brown is defined only for low lightness values and yellow only for high values), on each lightness level not all the categories will have memberships different from zero for any point of the level. Figure 7 shows an example of the membership map provided by the TSE functions for a given lightness level, in which there exist six chromatic categories. The other two chromatic categories in this example would have zero membership for any point of the level.
2C. Achromatic Categories
The three achromatic categories (Black, Gray, and White) are first considered as a unique category at each chromaticity plane. To ensure that the unity-sum constraint is fulfilled (i.e., the sum of all memberships must be one), a global achromatic membership, , is computed for each level as8 shows a scheme of this division along the lightness axis.
Hence, the membership of the three achromatic categories on a given chromaticity plane is computed by weighting the global achromatic membership [Eq. (13)] with the corresponding membership in the lightness dimension [Eqs. (14, 15, 16)]:4)]. In this way we can assure that the unity-sum constraint is fulfilled on each specific chromaticity plane,
3. FUZZY-SETS ESTIMATION
Once we have defined the membership functions of the model, the next step is to fit their parameters. To this end, we need a set of psychophysical data, D, composed of a set of samples from the color space and their membership values to the 11 categories,
Such data will be the knowledge basis for a fitting process to estimate the model parameters, taking into account our unity-sum constraint given in Eq. (18). In this case, the model will be estimated for the CIELab space, since it is a standard space with interesting properties. However, any other color space with a lightness dimension and two chromatic dimensions would be suitable for this purpose.
3A. Learning Set
The data set for the fitting process must be perceptually significant; that is, the judgements should be coherent with results from psychophysical color-naming experiments and the samples should cover all the color space. At present, there are no color-naming experiments providing fuzzy judgements. We proposed a fuzzy methodology for that purpose in , but the sampling of the color space is not large enough to fit the presented model.
Thus, to build a wide learning set, we have used the color-naming map proposed by Seaborn et al. in . This color map has been built by making some considerations on the consensus areas of the Munsell color space provided by the psychophysical data from the experiments of Sturges and Whitfield . Using such data and the fuzzy k-means algorithm, this method allows us to derive the memberships of any point in the Munsell space to the 11 basic color categories.
In this way, we have obtained the memberships of a wide sample set, and afterward we have converted this color sampling set to their corresponding CIELab representation. Our data set was initially composed of the 1269 samples of the Munsell Book of Color . Their reflectances and CIELab coordinates, calculated by using the CIE D65 illuminant, are available at the Web site of the University of Joensuu in Finland . In order to avoid problems in the fitting process due to the reduced number of achromatic and low-chroma samples, the set was completed with 18 achromatic samples (from to at steps of 0.5), 320 low-chroma samples (for values from 2 to 9, hue at steps of 2.5, and ), and 10 samples with , and (hues 5YR, 7.5YR, 10YR, 2.5Y, 5Y, 7.5Y, 10Y, 2.5GY, 5GY, and 7.5GY). The CIELab coordinates of these additional samples were computed with the Munsell Conversion software (Version 6.5.10). Therefore, the total number of samples of our learning set is 1617. Hence, with such a data set we accomplish the perceptual significance required for our learning set. First, by using Seaborn’s method, we include the results of the psychophysical experiment of Sturges and Whitfield, and, in addition, it covers an area of the color space that suffices for our purpose.
3B. Parameter Estimation
Before starting with the fitting process, the number of chromaticity planes and the values that define the lightness levels [see Eq. (12)] must be set. These values depend on the learning set used and must be chosen while taking into account the distribution of the samples from the learning set. In our case, the number of planes that delivered best results was found to be 6, and the values that define the levels were selected by choosing some local minima in the histogram of samples along the lightness axis. Figure 9 shows the samples' histogram and the values selected. However, if a more extensive learning set were available, a higher number of levels would possibly deliver better results.
For each chromaticity plane, the global goal of the fitting process is finding an estimation of the parameters, , that minimizes the mean squared error between the memberships from the learning set and the values provided by the model:
The previous minimization is subject to the unity-sum constraint:
Assumption 1: All the chromatic categories in a chromaticity plane share the same ES function, which models the membership transition to the achromatic categories. This means that all the chromatic categories share the set of estimated parameters for ES:
The second assumption refers to the membership transition between adjacent chromatic categories:
Assumption 2: Each pair of neighboring categories, and , share the parameters of slope and angle of the DS function, which define their boundary:
These assumptions considerably reduce the number of parameters to be estimated. Hence, for each chromaticity plane, we must estimate 2 parameters for the translation, , 4 for the ES function, , and a maximum of for the DS functions, since the other two parameters of can be obtained from the neighboring category [Eq. (23)].
Hence, following the two previous assumptions, the parameters of the chromatic categories at each chromaticity plane, , with , are estimated in two steps:
1. According to assumption 1, we estimate the parameters of a unique ES function, and , for each chromaticity plane by minimizing:4).
2. Considering assumption 2 allows us to estimate the rest of the parameters, , of each color category by minimizing the following expression for each pair of neighboring categories, and :
Once all the parameters of the chromatic categories have been estimated for all the chromaticity planes, the parameters used to differentiate among the three achromatic categories, are estimated by minimizing the expression4)].
All the minimizations to estimate the parameters are performed by using the simplex search method proposed in . After the fitting process, we obtain the parameters that completely define our color-naming model and that are presented and discussed in the next section.
4. RESULTS AND DISCUSSION
The essential result of this work is the set of parameters of the color-naming model that are summarized in Table 1 .
The evaluation of the fitting process is done in terms of two measures. The first one is the mean absolute error between the learning set memberships and the memberships obtained from the parametric membership functions:
The value of is a measure of the accuracy of the model fitting to the learning data set, and in our case the value obtained was . This measure was also computed for a test data set of 3149 samples. To build the test data set, the Munsell space was sampled at hues 1.25, 3.75, 6.25, and 8.75; values from 2.5 to 9.5 at steps of ; and chromas from 1 to the maximum available with a step of . As in the case of the learning set, the memberships of the test set that were considered the ground truth were computed with Seaborn’s algorithm. The corresponding CIELab values to apply our parametric functions were computed with the Munsell Conversion software. The value of obtained was 0.0218, which confirms the accuracy of the fitting that allows the model to provide membership values with very low error even for samples that were not used in the fitting process.
The second measure evaluates the degree of fulfillment of the unity-sum constraint. Considering as error the difference between the unity and the sum of all the memberships at a point, , the measure proposed is
To compute this measure, we have sampled each one of the six chromaticity planes with values from to 80 at steps of 0.5 units on both the a and b axes, which means that . The value obtained for indicates that the model provides a great fulfillment of that constraint, making the model consistent with the proposed framework.
Hence, for any point of the CIELab space we can compute the membership to all the categories and, at each chromaticity plane, these values can be plotted to generate a membership map. In Fig. 10 we show the membership maps of the six chromaticity planes considered, with the membership surfaces labeled with their corresponding color terms.
In many previous works on color naming, results have been evaluated in terms of the categorization of the Munsell space [2, 11, 13, 30]. To be able to compare our results to the previous ones, we will also categorize the Munsell space by applying the maximum criteria [Eq. (3)] as a decision rule to assign a color name to each chip of the Munsell data set.
To evaluate the plausibility of the model with psychophysical data, we compare our categorization to the results reported in two works of reference: the study of Berlin and Kay  and the experiments of Sturges and Whitfield . Figure 11 shows the boundaries found by Berlin and Kay in their work, superimposed on our categorization. Samples inside these boundaries assigned with a different name by our model are marked with a cross. As can be seen, there are a total of 17 samples out of 210 inside Berlin and Kay’s boundaries with a different name. The errors are concentrated on certain boundaries, namely, green-blue, blue-purple, purple-pink, and purple-red.
The comparison to Sturges and Whitfield’s results is presented in Fig. 12 . In Sturges and Whitfield’s experiment the samples labeled with the same name by all the subjects defined the consensus areas for each category. Among these samples, the fastest-named sample for each category was its focus. These areas are superimposed over our categorization to show that all the consensus and focal samples from Sturges and Whitfield’s experiment are assigned the same name by our model.
The analysis done to our TSE model (TSEM) was also performed on some previous categorizations. These are obtained by Lammens’s Gaussian model (LGM) , an English speaker presented by MacLaury (MES) in , Seaborn’s fuzzy k-means model (SFKM) , and our previous TS model (TSM) . The results are summarized in Table 2 .
As can be seen in the table, the results of our TSEM equal the previous best of Seaborn’s nonparametric model but add the advantages of having a parametric model that have been previously discussed in Section 2. Notice that although the learning process of both models was based on data derived from Sturges’s results, they are the most consistent with Berlin and Kay’s experiments and are also better than the results of the English speaker’s categorization, which shows the variability of the problem, since any individual subject’s judgements will normally differ from those of a color-naming experiment.
In this paper we have proposed a parametric fuzzy model for color naming based on the definition of the TSE as a membership function. The use of a parametric model introduces several advantages with respect to previous nonparametric approaches. These advantages, which have been discussed in Section 2, include a reduction in the implementation costs in terms of memory and computation time; a compact data representation; and simplicity for model analysis, since each parameter has a meaning in terms of the characterization of the data and, consequently, the model can be easily updated by just tuning some of the parameters.
The model has been conceived for any color space with two chromatic dimensions and a lightness dimension, but in the present work the parameters have been estimated for the CIELab space. The estimation process includes some constraints to assure the fulfillment of our imposed constraint that the memberships sum for any point must be one. The result is the set of parameters that defines a model that achieves a low fitting error to both the learning and test data sets and also fulfills the unity-sum constraint. The evaluation of the model when compared to previous results from the color-naming experiments of Berlin and Kay, and Sturges and Whitfield demonstrates that our model is plausible with these psychophysical data.
Hence, the memberships to the 11 basic color categories can be obtained for any point in the CIELab space to provide a color-naming descriptor with meaningful information about how humans name colors. The results are promising and have many applications to different computer vision tasks, such as image description, indexing, and segmentation, among others, where inclusion of this high-level information might improve their performance. The proposed representation of color information could also be used as a more perceptual measure of similarity for color, instead of the Euclidean distance in color spaces.
However, it must be pointed out that the model has been fitted to data derived from psychophysical experiments where a homogeneous color area is shown to an observer who has been adapted to the scene illuminant, and therefore the name assignment has been done under ideal conditions where influences from neither the illuminant nor the surround of the observed area have any effect on the naming process. In practice, the color-name assignment is a content-dependent task, and therefore perceptual considerations about the surround influence must be taken into account. The model we have proposed is assumed to work on perceived images, that is, images where the effects of perceptual adaptation to the illuminant and to the surround have been previously considered in a preprocessing step. Hence, the application of a color constancy algorithm can provide images under a canonical illuminant, thus simulating an adaptation process to the illuminant [31, 32, 33]. On the other hand, induction operators take into account the influence of the color surrounds in the final color representations as proposed in [34, 35].
Another fact that must be considered is that since Sturges and Whitfield’s experiments were done with physical color samples, the data used to fit the model reduce the space that occupy some categories (e.g., red) due to the limitations in the production of some colors with pigments. Hence, if the model is applied to other kinds of stimuli, e.g., lights, some errors could appear. This problem has already been detected in previous works .
One limitation of the model is the reduced vocabulary of color names that are considered. However, this vocabulary could be easily extended by using the fuzzy information provided by the model. Hence, compound nouns could be used for samples with a membership of 0.5 to two categories (e.g., samples with memberships 0.5 to green and 0.5 to blue could be named as blue-green), or the “-ish” suffix could be used on samples with a high membership to a category and up to a certain membership to another (e.g., samples with memberships 0.7 to green and 0.3 to blue could be named as bluish green). Nonetheless, the 11 basic categories considered will normally be enough for most of the applications the model can have, as psychophysical experiments have demonstrated that humans tend to use basic terms more frequently, more consistently, and faster than nonbasic color terms [3, 4].
It would also be interesting to obtain a wider set of data from a fuzzy psychophysical experiment covering an area of the color space as wide as possible and thus avoiding undersampling problems. With these psychophysical data, the proposed model could be improved on several points. First, it would be desirable to relax or even eliminate the first assumption done in the fitting process to allow for the membership transition from chromatic categories to the achromatic center to be different for each category. Second, the division of the color space into different lightness levels should be removed. Observation of the membership maps of the TSE model (Fig. 10) allows us to detect some tendencies in the evolution of the boundaries between color categories across lightness levels. Hence, the parameters of the membership functions could be interpolated along the levels defined in the current model to obtain the parameters of the membership functions for any given value of lightness.
However, to do this, the estimation of some parameters should be improved. We have noticed that for some categories, the β parameters do not vary across lightness, as could be expected. Intuitively, we could think that the values of β should be lower for high and low lightness, where colors are more easily confused, and therefore the transition from one color to another should be smoother and higher for intermediate lightness levels, where there is less uncertainty. However, some factors cause the evolution of β values to not always be as expected. The consensus areas of Sturges and Whitfield’s experiment (areas with no confusion between subjects) are assumed to have membership 1. Intuitively, we could think that these consensus areas should be larger in the intermediate lightness levels than in the extremes. However, this is not the case, and the extension of these areas at different lightness is much more similar than we could expect. Moreover, the color solid provided by the CIELab space is wider in the central levels of lightness than in the extremes. Hence, the consensus areas of Sturges and Whitfield are more spread out in the central areas than in the lower and higher levels of the lightness axis. This causes the membership transitions between regions of membership 1 to be smoother in the central lightness levels than in the low and high lightness levels. In addition, the slicing of the color space into different levels can also contribute to distortion of the boundaries, since all the samples on each level are collapsed on a chromaticity plane where memberships are modeled with our TSE functions. To solve this, we are doing new psychophysical experiments focused on the areas around boundaries in order to estimate better the parameters that define the transitions between categories.
Nonetheless, the final goal should be to define three-dimensional membership functions to model color categories. If a larger fuzzy data set were available, the membership distributions in the whole color space could be analyzed to define the properties that three-dimensional functions should fulfill to accurately model color categories in a way similar to what we did in this work for the two-dimensional functions. Unfortunately, this seems not to be an easy task.
This work has been partially supported by projects TIN2004-02970, TIN2007-64577, and Consolider-Ingenio 2010 (CSD2007-00018) of the Spanish Ministry of Education and Science (MEC) and European Community (EC) grant IST-045547 for the VIDI-video project. Robert Benavente is funded by the “Juan de la Cierva” program (JCI-2007-627) of the Spanish MEC. The authors would also like to acknowledge Agata Lapedriza for her suggestions during this work.
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