Color classification using color vision models

According to recent physiological research results, there are many individual differences already at the detection level of our color vision system. It is not completely clear yet, how the other levels of color vision system compensate the detection differences. Instead of detecting and analyzing colors exactly in the same way, we all just have learned to classify colors in a certain way, which seems to lead almost always to the same result independent of the individual differences in the color vision system. In this article, we experiment with four models developed for replicating certain properties of human color vision. We examine the color classification abilities of these models and show the differences and similarities in their behavior. © 2010 Wiley Periodicals, Inc. Col Res Appl, 2010.     

Color reproduction of authenticable luminescent backlit transmissive color images

We propose a method for creating authenticable color images under UV excitation backlight by printing an invisible luminescent white emissive ink layer on the verso side of a transmissive substrate and a classical cmy image on the recto side of the substrate. In order to obtain a backlit image whose colors are as close as possible to the original image colors, we map the input image sRGB color gamut into the gamut formed by the emissive white source attenuated by the classical ink halftones. The relationship between surface coverages of the classical cmy ink halftones and the resulting backlit colors is obtained by accounting for the transmission of the emissive white through the paper substrate and through the classical cmy ink halftones. The transmittance of the classical ink halftones is modeled by a new halftone absorbance prediction model. The lightness range of the luminescent backlit color gamut is expanded by printing a black and white UV‐absorbing instance of the original color image in superposition with the luminescent white emissive layer and in registration with the cmy image printed on the recto side. Luminescent backlit color images provide a high anticounterfeiting security, as they combine a verso printed invisible luminescent ink layer, a verso printed black and white instance of the original image and a matching recto printed cmy instance of the original image. The resulting luminescent backlit image colors are close to the original image colors only when observed in transmission mode under UV light. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 331–340, 2014     

Lighting source classification applied in color images to contrast enhancement

A novel methodology is proposed to identify the distribution of the intensity of the illuminant across a real-world image; with this information, the image can be classified as a homogeneous or nonhomogeneous lighting source. A criterion is proposed for deciding whether the illumination in the tested image is homogeneous or nonhomogeneous. This criterion is based on well-established methods that are dedicated to improving color characteristics and are commonly used to determine the color of the lighting source during a captured scene. The classification of homogeneous or nonhomogeneous illumination in real-world images is implemented in the BSD500 database, and the results are compared with a manual classification of images—proposed under subjective criteria—by the authors; in this way, one can analyze how the lighting source affects the image. This procedure provides a classification methodology for real-world image databases found in the scientific literature.     

Chroma variations and perceived quality of color images of natural scenes

Transformations of natural images in the perceptually uniform CIELUV color space have been investigated with respect to perceptual image quality. To this end, digitized color images of four natural scenes were described on the basis of their color point distributions in the CIELUV color space. A new set of images was created by varying the chroma value of each pixel while the lightness and hue angle were kept constant. The chroma was changed in two different ways: (1) through the addition or subtraction of the same amount of chroma to or from the chroma value of each pixel; (2) through multiplication of the chroma value of each pixel by a constant. In three experiments, subjects judged the perceptual quality, colorfulness, and naturalness of the images on a ten-point numerical category scale. The results indicate that colorfulness is the main perceptual attribute underlying image quality when chroma varies. Colorfulness itself was found to depend on both the average chroma and its variability. In general, the subjects preferred slightly more colorful images to the original ones. The perceptual quality of the images was found to be closely related to the naturalness of the images. © 1997 John Wiley & Sons, Inc. Col Res Appl, 22, 96–110, 1997     

Heuristic analysis influence of saliency in the color diversity of natural images

The estimation of chromatic diversity of natural images is commonly quantified through the computation of the number of discernible colors and has received much attention because of the different implications it has. However, the relationship between that number and the number of colors that really attracts the attention from an observer is still not clear and has been given little attention. New concepts about salient discernible colors‐the salient chromatic diversity of images‐ and remarkable salient colors‐connected colors in the same salient image area‐are introduced as opposed to the classical number of discernible object colors, which is usually evaluated for the global image without differentiating between probable attended and non‐attended image regions. We have used different well‐known saliency models to locate the salient regions in the scenes and have heuristically studied the extent to which those models preserve the chromatic diversity of natural images. Based on a bottom‐up approach, a reduction of around 40%‐55% in the number of discernible colors were obtained, and not all saliency algorithms preserved a uniform sampling of the original color gamut. Thus, our results suggests that particularly the graph‐based visual saliency model got good low dissimilarity values in comparison with other approaches that put emphasis solely on color as the main low‐level feature. Furthermore, we have introduced a quantification scheme of the average number of remarkable salient colors appearing in the images, and have proved how the heuristic‐based analysis of salient image areas can be used to create segmented images automatically according to their salient chromatic diversity.     

Color corrected single scale retinex based haze removal and color correction for underwater images

Underwater images are affected by scattering of dust particles leading to hazy images. The difference in absorption of light at different wavelength leads to colorcast. Restoring underwater images from haze and colorcast degradation is a challenging task. This article proposes color corrected single scale retinex algorithm for the restoration of hazy and colorcast underwater images. Further, the illumination of the underwater image is enhanced through illumination enhancer. In order to correct colorcast, the underwater images intensity shifters are proposed. Blur, natural image quality evaluator, blind/referenceless image spatial quality evaluator, entropy, peak signal to noise ratio are the metrics used to evaluate the performance of the proposed restoration technique and compared the output with the existing techniques. Compared to the existing techniques the proposed algorithm prominently removes haze and colorcast present in underwater images. The study excludes retinex-with-reset models from consideration.     

Color signal coding: Color vision and color television

In this article the similarities in color coding between color vision and color television are discussed. The similarities go beyond the trichromacy shared by both systems. Color television transforms the initial color information before broadcasting in a manner similar to the opponent-color transformation in the retina. The coded broadcast is decoded in the television receiver.     

彩色涤纶短纤维色差的控制

介绍了用于评定色差的CIELAB和CMC色差公式,并对2个公式在彩色涤纶短纤维色差控制中的实际运用做了对比实验。从对比结果和数据分析来看,CIELAB色差公式存在一定的局限性,CMC色差公式测试结果与目测具有更好的视觉一致性,完全可以在纺织行业色差控制上替代CIELAB色差公式并推广使用。     

Color space of the coloroid color system

The Coloroid color system is a Hungarian standard color order system. This article describes the color space of the Coloroid system and its relationship to the CIE colorimetric space and the spaces associated with the Munsell and DIN color order systems. The Coloroid system is presented as a compromise of principles of uniformity in regard to color difference and color harmony, as well as ease of mapping into the CIE colorimetric space.     

Color order systems and architectural color design

In its first part, this article traces some outlines of the history of color order systems which bear upon the question of aesthetic color combinations (syntactic aspect). The second part defines the position of color in the field of architectural design (psychological, contextual, and structural aspect). The function and usefulness of color order systems in the process of architectural color planning and realization (pragmatic aspect) are discussed in the third and last part.     

Color difference predicted by color component differences

A relationship between assessed color differences and assessed components of colors is presented. The perceptual difference between colors j and k is converted to a lightness difference of two Munsell grays, V_A and V_B, and d_jk = |V_A ? V_B|. Scaled values of principal hue components of a color ξ _α(H|V/C), α = R, Y, G, and B, are read from charts based on assessments of observers. Previous charts (Color Research and Application, 1999, 24, 266–279) are enlarged and extended. A linear combination of ΔV = |V_j ? V_k| and Δξ _α = |ξ _α(H_j|V_j/C_j) ? ξ _α(H_k|V_k /C_k)|, d?_jk, is the best to predict d_jk. The root‐mean‐squares of (d_jk ? d?_jk) is 0.34 V, about one third of the lightness difference from V to V + 1 on the Munsell Value Scale.     

Color preference affected by mode of color appearance

Most color preference research focuses on colors in an object color mode. In our daily life, however, colors are perceived not only as an object color mode but also as other modes, such as unnatural object color and light source color modes. To explore the effect of the color appearance mode on color preference, we examined the relationship between color preference and the mode of color appearance. Thirty‐three color chips were chosen from the Munsell notation varying in hues and chromas. The color chips were presented in different color appearance modes by changing the subject's room illuminance and the color chip room illuminance. The experimental results showed that the brightest and most saturated colors were preferred. It was found that the subject preferred color in a light source color mode and unnatural object color mode to color in an object color mode. Moreover, we found that hue had a small effect on color preference in the light source color mode. We also investigated the relationship between color preference and the perceived color attributes (perceived chromaticness, whiteness, and blackness). In a supplementary experiment, elementary color naming was conducted. The results showed that the perceived chromaticness, perceived whiteness, and perceived blackness play a role for the determination of color preference for different color appearance modes. We, consequently, suggest that color preference is dominated not only by color attributes but also by the mode of color appearance. © 2009 Wiley Periodicals, Inc. Col Res Appl, 2010     

Color discrimination subject to illuminant and color transition

Due to technological influence on the production process there are variations of the chromaticity coordinates that may cause visible differences of the perceived color of LEDs of the same production batch. To minimize the negative impact of such variations for the user the manufacturers carry out colorimetric measurements of LEDs and sort them into tolerance groups of similar chromaticity coordinates. The corresponding groups are defined by their respective center point surrounded by a quadrangular tolerance limit whereas the tolerance limits are based on tolerance ellipses gained in the 1940s by MacAdam. However, due to another scientific target of MacAdam's experiments, applying his ellipses as a base for grouping the chromaticity coordinates has to be questioned. With a view toward a method for grouping according to chromaticity coordinates for white LEDs with phosphor conversion, the results of research with test subjects regarding their discrimination and magnitude perception of chromatic differences are presented. In this research three chromaticities with the correlated color temperatures of 2700, 4000, and 6500 K and variations of color transitions are investigated, which match realistic luminaire designs for practical applications. The empirically determined thresholds are expanded according to the transformation to different chromatic adaptation conditions as per von Kries that are then compared to the results by MacAdam and the influence on adaptation is being discussed. A new concept for the calculation of tolerance limits for LED binning is presented. © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 457–467, 2016     

Interrelation of the natural color system and the munsell color order system

The CIE tristimulus values of the aim points of the Natural Color System (NCS) were converted to Munsell notations using a computer program. The values so converted were those from the four elementary hues R, Y, G, and B, and the intervening hues for which full pages of colors exist in the NCS Colour Atlas. The resulting notations were plotted on Munsell Value-Chroma and Hue-Chroma charts and analyzed for several features of interest, for example the locations of the points with 100% chromaticness and the relative spacing of the hue circles. Analytical equations are presented relating Munsell Chroma and NCS chromaticness, and Munsell Value and NCS blackness, for both achromatic and chromatic samples. Such analytical relationships could not be derived between the hues of the two systems, since a relatively wide range of hue in either system corresponds to constant hue in the other. This range appears to include both systematic and random components that must be removed before analytical relations between the two hues can be derived.     

Color emotions for multi‐colored images

We investigate the emotional response to colors in ordinary multicolored images. In psychophysical experiments, using both category scaling and interval scaling, observers are asked to judge images using three emotion factors: activity, weight, and heat. The color emotion metric was originally developed for single colors, and later extended to include pairs of colors. The same metric was recently used in image retrieval. The results show that people in general perceive color emotions for multi‐colored images in similar ways, and that observer judgments correlate with the recently proposed method used in image retrieval. The intended usage is in retrieval systems publicly available on the Internet, where both the user and the viewing environment is unknown, which requires novel ways of conducting the psychophysical experiments. © 2010 Wiley Periodicals, Inc. Col Res Appl, 36, 210–221, 2011;     

Color appearance and color connotation models for unrelated colors

The purpose of this research is to investigate the color appearance and color connotation of unrelated colors. To investigate color appearance (i.e., brightness, colorfulness, and hue) for unrelated colors, 22 observers have answered their color appearance for 50 unrelated color stimuli using the magnitude estimation method. Perceptual data obtained by the experiment is compared with the color attributes data estimated by unrelated‐color appearance models, CAM97u and CAM02u. It is found that both models perform reasonably well but the performance of CAM02u is better than that of CAM97u. For investigating color connotation for unrelated colors, 32 observers have judged their color connotation for the 50 unrelated color stimuli using the 10 color connotation scales (i.e., “Warm – Cool,” “Heavy – Light,” “Modern – Classical,” “Clean – Dirty,” “Active – Passive,” “Hard – Soft,” Tense – Relaxed,” “Fresh – Stale,” “Masculine – feminine,” and “like – Dislike”), and semantic differential method is used for measurement. It is found that the color connotation models developed for related colors perform poorly for unrelated colors. Experimental results indicate that brightness attribute is confusing to estimate and does not affect color connotation significantly for unrelated colors. Based on the psychophysical data, new models for “Warm‐Cool”, “Heavy‐Light”, “Active‐Passive” and “Hard‐Soft” were proposed using CAM02u hue, brightness, and colorfulness. Color connotations for unrelated colors are classified into three categories, which “Color solidity,” “Color heat,” and “Color purity.” © 2013 Wiley Periodicals, Inc. Col Res Appl, 40, 40–49, 2015     

Color and color mixture: Scalar and vector fundamentals

This article explores the consequences of the Wyszecki hypothesis, that every color stimulus (radiometric function) comprises two parts, a fundamental and a residual. (Wyszecki's terms were fundamental color stimulus and metameric black.) The fundamental alone is processed by the visual system and evokes the color sensation. The residual is not processed and is without effect on color sensations. Metamers, color stimuli evoking the identical color sensation but with different radiometric functions, have the same fundamental but different residuals. Matrix R, an orthogonal projector, resolves any radiometric function into its fundamental and residual. Unlimited numbers of metamers may be generated by adding other residuals to the same fundamental. The color-matching equation has historically been written with tristimulus values as coefficients, but so written, the equation balances only psychologically. When the stinuli of the color-matching equation are replaced by the fundamentals processed by the visual system (after the residuals are ejected by matrix-R operations), the equation balances psychologically, physically, and mathematically. Wyszecki's fundamental has two representations, the scalar fundamental as conceived by Wyszecki and the vector fundamental as developed in this article. Vector fundamentals define a fixed, invariant, fundamental color space governed by Euclidean geometry. The properties of this space are explored. The article develops relationships between color-matching functions, fundamentals, and orthonormal color functions. Two new specifications for color stimuli or sensations are introduced, tricolor values and tricolor coordinates. The article assumes knowledge of ordinary algebra and geometry, and procedures for computation of tristimulus values.     

Color mixing and color separation of pigments with concentration prediction

In this study, we propose a color mixing and color separation method for opaque surface made of the pigments dispersed in filling materials. The method is based on Kubelka–Munk model. Eleven different pigments with seven different concentrations have been used as training sets. The amount of concentration of each pigment in the mixture is estimated from the training sets by using the least‐square pseudo‐inverse calculation. The result depends on the number and type of pigments selected for calculation. At most we can select all pigments. The combinations resulted with negative concentrations or unusual high concentrations are discarded from the list of candidate combination. The optimal pigment's set and its concentrations are estimated by minimizing the reflectance difference of given reflectance and predicted reflectance. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 461–469, 2008     

Color of natural tooth crown in Japanese people

The aim of this study was to investigate the color of the natural maxillary incisor tooth from Japanese people of all age groups. These results were compared with the Trubyte Bioblend shade guide. The subjects of this study were in the age range of 13–84, 42 male and 45 female making 87 people in total. Areas with 1.0‐mm diameter at five sites were measured along the tooth axis for L*,a*,b*, according to CIELAB color spaces using a Spectroradiometric Color Computer. At the incisal site, two significant positive correlations were found between age and a* (r = 0.376, p < 0.001), and b* (r = 0.483, p < 0.001). At the center site, a significant negative correlation (r = −0.418, p < 0.001) was found between age and L*, but positive correlation (r = 0.497, p < 0.001) was found between age and b*. At the cervical site, a significant negative correlation (r = −0.326, p < 0.01) was found between age and L*, but positive correlation (r = 0.702, p < 0.001) was found between age and b*. Near the root, particularly, the values of a* were greater than those suggested by the Trubyte Bioblend shade guide. In conclusion, as the Trubyte Bioblend shade guide does not match the natural tooth color in red‐green chromaticity near the root, it is significant for us in dentistry to develop new shade guides that match the Japanese people based on the data collected. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 43–48, 2000