Proposal of a new concept for color‐appearance modeling

A new type of color‐appearance model (CAM) is proposed together with its concept and flow of formulations. The topics described are: (1) The existence of two kinds of color‐appearance models, CAMs previously used and CAMs newly proposed. (2) All the CAMs, previously developed and used, do not predict color‐appearance attribute of perceived lightness of object colors under any illuminations. They may be adequately called “the model for predicting color‐appearance match between object colors under different adapting conditions.” (3) Newly improved CAMs take the Helmholtz–Kohlrausch effect in the VCC method into account. They can determine object colors with the same Tone (equi‐perceived lightness, equi‐whiteness‐blackness, and equi‐perceived chroma) irrespective of hues under reference illuminant. The newly improved models can be named Integrated CAMs. Their applicable fields are described in detail. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 113–120, 2007     

Prediction of Munsell appearance scales using various color‐appearance models

The chromaticities of the Munsell Renotation Dataset were applied to eight color‐appearance models. Models used were: CIELAB, Hunt, Nayatani, RLAB, LLAB, CIECAM97s, ZLAB, and IPT. Models were used to predict three appearance correlates of lightness, chroma, and hue. Model output of these appearance correlates were evaluated for their uniformity, in light of the constant perceptual nature of the Munsell Renotation data. Some background is provided on the experimental derivation of the Renotation Data, including the specific tasks performed by observers to evaluate a sample hue leaf for chroma uniformity. No particular model excelled at all metrics. In general, as might be expected, models derived from the Munsell System performed well. However, this was not universally the case, and some results, such as hue spacing and linearity, show interesting similarities between all models regardless of their derivation. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 132–144, 2000     

Proposal of an opponent‐colors system based on color‐appearance and color‐vision studies

A new theoretical color order system is proposed on the basis of various studies on color appearance and color vision. It has three orthogonal opponent‐colors axes and an improved chromatic strength of each hue. The system has color attributes whiteness w, blackness bk, grayness gr, chroma C, and hue H. A method is given for determining Munsell notations of any colors on any equi‐hue planes in the system. A method is also given for determining grayness regions and grayness values on hue‐chroma planes in the system. It is concluded that colors with the same color attributes [w, gr, bk, C] but with different hues in the theoretical space have approximately the same perceived lightness, the same degree of vividness (“azayakasa” in Japanese), and also the same color tone. The tone concept, for example used in the Practical Color Coordinate System (PCCS), is clarified perceptually. The proposed system is a basic and latent color‐order system to PCCS. In addition, the concept of veiling grayness by a pure color with any hue is introduced. Further, relationships are clarified between generalized chroma c(gen) and grayness. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 135–150, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10234     

Predictions of Munsell values with the same perceived lightness at any specified chroma irrespective of hues—Determination of any tonal colors

Simple formulas are proposed for predicting the Munsell value of colors with the same tone (the same values for whiteness‐blackness, perceived lightness, and chroma irrespective of hue). The formulas can be used for any tone. In other words, the method can determine the Munsell value with the same perceived lightness at any specified chroma irrespective of hue. The chromatic strength (CS) function is only used for the derivations. The formulas are very simple, and can be used not only in the colorimetry but also in the color design field. The concept described in this study is that a common CS function can be used for transforming each of the three color attributes (hue, lightness, and chroma) from their uniform color space metric to their corresponding color appearance space attribute. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2011     

Comparing two‐step and one‐step chromatic adaptation transforms using the CAT16 model

Color‐appearance models, CIECAM02 and CAM16, usually include two one‐step chromatic adaptation transforms: a forward (one‐step) transform to convert data from a first illuminant to CIE illuminant E, plus a reverse (one‐step) transform to convert the results from CIE illuminant E to a second illuminant. In practice, however, one‐step chromatic adaptation models, that avoid the use of the intermediate CIE illuminant E, are also employed. Tests using the one‐step CAT16 model indicate failures of both the symmetry and transitivity properties, except in the case where the degree of adaptation D is equal to unity. The magnitude of these failures depends on the specific illuminants selected, and increases as the degree of adaptation decreases. From four possible two‐step CAT16 models, we have identified two that obey the symmetry and transitivity properties, one with slightly better predictions of the experimental corresponding‐color datasets available in the literature, and more consistent with the one‐step CAT16 model. The findings of this article confirm that, for incomplete adaptation, the use of the one‐step CAT is incorrect, and we propose that the use of a two‐step CAT16 model be mandatory for future applications.     

Transformation from Munsell space to Swedish NCS—Through Nayatani theoretical space

A relationship and a method of transformation are given on color attributes between Munsell, Nayatani Theoretical, and NCS color spaces. The purpose of the study is to clarify the conceptual differences in structure between the three spaces. The structural differences are explained by using the transformations on five color samples. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 151–157, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10235     

Relationships among chromatic tone,perceived lightness,and degree of vividness

The present study describes the usefulness and importance of chromatic tone concept on object colors. It is clarified that the concept of a tone category consists of the same perceived lightness and the same degree of vividness of chromatic object colors in the tone irrespective of hue. Prediction equations are given to color attributes on perceived lightness and degree of vividness. They clearly show different functions on metric lightness and metric chroma on the two color attributes. It is also clarified that the theoretical opponent‐colors system by the author (NT system) gives a basis for defining the tone concept, perceived lightness, and degree of vividness. The results of the present study are useful for understanding fundamental color notion “tone,” which is important both in the fields of colorimetry (fundamental color‐perception study) and color design (practical application). In addition, attributes of equivalent whiteness–blackness [W‐Bk]eq and equivalent chroma Ceq are proposed. © 2005 Wiley Periodicals, Inc. Col Res Appl, 30, 221–234, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20108     

Application of a color‐appearance model to vision through atmospheric haze

Environmental laws mandate the protection of visibility conditions in national parks and wilderness areas from atmospheric haze, which occurs due to the emissions of anthropogenic air pollutants. To calculate the improvement in visibility that results from the reduction of these air pollutants, it is necessary to quantify the relationship of haze to the color appearance of objects being viewed through it. To this end, a field study was conducted in the Great Smoky Mountains National Park in eastern Tennessee. Color appearance of objects was quantified by color matching with a special visual colorimeter. The Hunt94 color‐appearance model was used to compare these matches with simultaneous spectral measurements. In addition to the chromaticity coordinates and luminance of the object, the Hunt94 model requires a number of parameters such as the chromaticity of the adapting light and chromatic and brightness surround induction factors, which need to be determined for the outdoor viewing conditions. This article describes how this was accomplished. Although it accounts for differences in viewing conditions between the colorimeter and daylight conditions, the Hunt94 model does not account for the perceptual transparency effect of haze on outdoor color appearance. These results challenge the manner in which current air quality–visibility models are being utilized. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 112–120, 1999     

On attributes of achromatic and chromatic object‐color perceptions

Adapting luminance dependencies of various color attributes of object colors (lightness, brightness, whiteness‐blackness, whiteness‐blackness strength, chroma, and colorfulness) were clarified under white illumination with various adapting illuminances. The correlation between the perceptions of lightness and brightness and those of whiteness‐blackness and whiteness‐blackness strength is also clarified for achromatic object colors. The difference between the increase of brightness and that of whiteness‐blackness contrast (the effect studied by Stevens and Jameson—Hurvich) by raising their adapting illuminance is resolved without any contradiction. It is also shown that the nonlinear color‐appearance model developed by the author and his colleagues is able to explain the complex characteristics of all the above color attributes of object colors by making minor modifications to it. In addition, two kinds of classifications of various color attributes are given; one is based on the similarity of perception level, and the other on the degree of adapting illuminance dependency. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 318–332, 2000     

Colour‐appearance modeling using feedforward networks with Bayesian regularization method— Part I: Forward model

In this article, a method of predicting colour appearance (from colorimetric attributes to colour‐appearance attributes, i.e., forward model) using an artificial neural network is presented. The neural network model developed is a multilayer feedforward neural network model for predicting colour appearance (FNNCAM for short). The model was trained by LUTCHI colour‐appearance datasets. The Levenberg–Marquardt algorithm is incorporated into the back‐propagation procedure to accelerate the training of FNNCAM and the Bayesian regularization method is applied to the training of neural networks to improve generalization. The results of FNNCAM obtained are quite promising. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 424–434, 2000     

Assessing the application of an image color appearance model to basic self‐luminous scenes

Image color appearance models (Image CAMs) have been developed to predict the perception of complex scenes and are mainly used for image rendering and video reproduction applications. Among these Image CAMs, iCAM is an Image CAM that takes an image as the input and provides the perceptual attributes for each pixel. On the other hand, nonimaging CAMs are widely used and validated, but they always assume a simple test scene of a uniform flat stimulus, a quasi‐neutral background, and a surround. This study presents an evaluation of the performance of iCAM when applied to these simple self‐luminous scenes in predicting the influence of background luminance, background size, saturation, and stimulus size on stimulus brightness. The results show that iCAM is capable of predicting the effect of background luminance and some background size scenarios. However, for unrelated self‐luminous stimuli (dark background), the model predictions do not match the reference data. An evaluation of the effect of the filter kernel size and its relation to the physiological mechanism of image processing inside the visual system has been investigated. Furthermore, the impact of saturation and stimulus size on brightness seems to be underestimated by the model, because the Helmholtz‐Kohlrausch and stimulus size effects are not included. Hence, these findings call for an enhanced Image CAM.     

Semantic interpretation of color differences and color‐rendering indices

A series of visual experiments were carried out to rate the similarity of color appearance of two color stimuli on categorical and continuous semantic rating scales. Pairs of color stimuli included two copies of the same colored real or artificial object illuminated by a test light source and a reference light source. A formula was developed to predict a category of color similarity (e.g., “moderate” or “good”) from an instrumentally measured color difference. Given a numeric value of a color difference between the two members of a pair of colors, for example, 2.07, the formula is able to predict a category of color similarity, for example, “good.” Because color‐rendering indices are based on color differences, the formula could be applied to interpret the values of the new color‐rendering index (n‐CRI or CRI2012) in terms of such semantic categories. This semantic interpretation enables nonexpert users of light sources to understand the color‐rendering properties of light sources and the differences on the numeric scale of the color‐rendering index in terms of regular language. For example, a numeric value of 87 can be interpreted as “good.” © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 252–262, 2014; Published online 14 March 2013 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/col.21798     

Constant hue bands in boundary colors discovered using a new appearance model

Boundary colors are observed when light from a scene is dispersed by a prism or diffraction grating. We discovered that patterns with repeating black and white stripes can produce repeating bands of boundary colors with two hues. These hues are virtually constant as measured by chromaticity or CIELAB. We found seven cases of this kind using a new appearance model for boundary colors. The model correctly predicts that green and magenta bands recur as stripe widths and dispersion strength vary. The first green/magenta case in the sequence traces out an accurate ellipse in XYZ color space. Green and magenta bands are prominent in supernumerary rainbows and interference rings, and we explain why that might be the case. The explanation is based on an interesting property of the visible spectrum. In addition to the green/magenta cases, the other cases are orange/cyan, yellowish‐green/purple, and yellow/violet. The success of the boundary color appearance model implies that bands are perceived as if the wavelength responses of the cones were essentially independent, which contradicts the actual behavior of cones. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 135–146, 2015     

Media‐dependent color appearance modeling based on artificial neural networks

In this study, we tried to consider various color appearance factors and device characterization together by visual experiment to simplify the across‐media color appearance reproduction. Two media, CRT display (soft‐copy) and NCS color atlas (hard‐copy), were used in our study. A total of 506 sample pairs of RGB and HVC, which are the attributes of NCS color chips, were obtained according to psychophysical experiments by matching soft copy and hard copy by a panel of nine observers. In addition, a set of error back‐propagation neural networks was used to realize experimental data generalization. In order to get a more perfect generalizing effect, the whole samples were divided into four parts according to different hues and the conversion between HVC and R_HVCG_HVCB_HVC color space was implemented. The current results show that the displays on the CRT and the color chips can match well. In this way, a CRT‐dependent reproduction modeling based on neural networks was formed, which has strong practicability and can be applied in many aspects. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 218–228, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20209     

A hue rectangle and its color metrics in a modified opponent‐colors system

In the proposed modified opponent‐colors system, the hue regular rectangles show the chromatic coordinates of any chromatic colors better than hue circles. In the hue rectangles equihue and equichroma loci are shown together with equigrayness loci. In the color perception space of the modified opponent‐colors system, a city‐block metric must be used instead of a Euclidean one for distance. The reason for this is described in detail. The proposed color perception space constitutes a regular octahedron. © 2002 Wiley Periodicals, Inc. Col Res Appl, 27, 171–179, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10046     

Some modifications to Hering's opponent‐colors theory

Some modifications are made to the achromatic color perceptions in Hering's opponent‐colors theory. They are the introduction of the reference color Gray and the use of the orthogonal coordinate system. The modified opponent‐colors theory has a symmetrical structure for the three opponent‐colors axes, whiteness‐grayness‐blackness, redness‐grayness‐greenness, and yellowness‐grayness‐blueness, and it unifies the Hunt and the Stevens and Jameson–Hurvich effects. It is also noted that two kinds of color‐appearance spaces exist. One is the color‐appearance space derived from color perceptions of object colors (called the CPS color‐appearance space). The other is that modeled from their colorimetric values for predicting color perceptions (called the UCS color‐appearance space). The CPS color‐appearance space is mainly described in this article. Scaling of the CPS color‐appearance space and the existence of the reference color perception Gray are discussed in detail. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 290–304, 2001     

Evaluating the 1931 CIE color‐matching functions

The use of colorimetry within industry has grown extensively in the last few decades. Central to many of today's instruments is the CIE system, established in 1931. Many have questioned the validity of the assumptions made by Wright¹ and Guild,² some suggesting that the 1931 color‐matching functions are not the best representation of the human visual system's cone responses. A computational analysis was performed using metameric data to evaluate the CIE 1931 color‐matching functions as compared to with other responsivity functions. The underlying assumption was that an optimal set of responsivity functions would yield minimal color‐difference error between pairs of visually matched metamers. The difference of average color differences found in the six chosen sets of responsivity functions was small. The CIE 1931 2° color‐matching functions on average yielded the largest color difference, 4.56 ΔE. The best performance came from the CIE 1964 10° color‐matching functions, which yielded an average color difference of 4.02 ΔE. An optimization was then performed to derive a new set of color‐matching functions that were visually matched using metameric pairs of spectral data. If all pairs were to be optimized to globally minimize the average color difference, it is expected that this would produce an optimal set of responsivity functions. The optimum solution was to use a weighted combination of each set of responsivity functions. The optimized set, called the Shaw and Fairchild responsivity functions, was able to reduce the average color difference to 3.92 ΔE. In the final part of this study a computer‐based simulation of the color differences between the sets of responsivity functions was built. This simulation allowed a user to load a spectral radiance or a spectral reflectance data file and display the tristimulus match predicted by each of the seven sets of responsivity functions. © 2002 Wiley Periodicals, Inc. Col Res Appl, 27, 316–329, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10077     

Gray and grayness—its complexities in color appearance of surface colors

Complexities on the roles of reference color gray and grayness are reviewed. They are essential in color appearance, but gray is an implicit color. Although “grayness” is not explicitly used in visual color assessment of surface colors or color order systems, gray can be combined with any colors having six primary‐color components using the term “grayish,” for example, grayish red and grayish yellow. However, the existing region of grayness is limited in a part of color‐appearance space. Illuminance dependency of gray perception is also clarified. Existence of two kinds of psychometric quantities are suggested: one is the attribute of grayness based on its psychological amount in a grayish color under study, and the other is the attribute of brightness of the grayish color under a specified illuminance, psychophysical quantity. The Nayatani‐Theoretical color order system, which uses three opponent‐colors axes, can clarify the above complexities of gray and grayness. Its importance is the same as six primary colors, red–green, yellow–blue, and white–black. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 37–44, 2014     

Investigating color appearance in optical see‐through augmented reality

Color appearance for real objects has been studied over decades and it has been well modeled. However, in augmented reality (AR) environments, virtual content is added to a real background and a mixed appearance is perceived. In this research, we studied color appearance in AR and investigated the applicability of the CAM16 color appearance model, one of the most comprehensive current color appearance models, in an AR environment. Using a benchtop optical mixing apparatus as an AR simulator, objective measurements of mixed colors in AR were performed. Then a psychophysical color matching experiment was performed with combinations of mixed foreground and background colors. The results showed that CAM16 is not accurate in predicting the color appearance in AR environment; therefore, it was modified with the addition of chromatic simultaneous contrast, resulting in an improved fit to the AR experiment data. A second psychophysical color matching experiment was performed on a single display to compare the color perception in AR with color perception in real world from a single display.