Optimal spectra for double object‐colour solids

Optimal colours for human vision occur on the boundary of a three‐dimensional object‐colour solid, and result from optimal reflectance spectra that take on only the values 0 and 1, with at most two transitions between those values. Different illuminants lead to different solids. If there are two illuminants and a single sensing device, then we can construct a six‐dimensional double object‐colour solid by concatenating colour signals from both illuminants. Colours on the boundary of a double‐object solid, and the spectra that generate them, can also be called optimal. This article shows that, while optimal spectra for double solids take on only the values 0 and 1, there is no maximum number of transitions between those values: given a device, we can always construct two illuminants such that the resulting double object‐colour solid has an optimal reflection spectrum with as many transitions as desired.     

A zonohedral approach to optimal colours

This article demonstrates that the CIE XYZ colour solid is a zonoid. An approximating zonohedral colour solid is constructed explicitly from a set of generating vectors, which are integrals of colour‐matching functions over narrow intervals of the visible spectrum. The zonohedral approach yields an intuitive, constructive proof of the Optimal Colour Theorem: the reflectance function of an optimal colour takes on only the values 0 or 1, with at most two transition wavelengths. In addition, zonohedral techniques can simplify computations: for example, optimal colours can be found without calculating transition wavelengths. Finally, zonohedra provide a simple, unified approach to colour space and eliminate much of the confusion arising from chromaticity diagrams. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013     

Individual differences in human colour vision as derived from stiles & Burch 10° colour matching functions

Individual differences between the 49 Stiles & Burch observers have been analyzed using the object‐colour space put forth recently (J of Vision 2009;9:1–23). A set of rectangular reflectance spectra has been used as a common frame of reference for representing object colours for all the observers. Being metameric to one of these rectangular reflectance spectra, every reflectance spectrum can be geometrically represented as a point in the three‐dimentional space. The interindividual differences reveal themselves in that, for various observers, the same reflectance spectrum maps to different points in this space. It has been found that on average such differences do not exceed the differences in object‐colour appearance induced by an illumination shift from daylight to the fluorescent daylight simulator F1. Such small individual variations have been accounted for by the fact that the cone spectral tuning curves have a special form that mitigates the individual differences in cone spectral positioning. © 2012 Wiley Periodicals, Inc. Col Res Appl, 2013     

On the colours dichromats see

Dichromatic colour vision is commonly believed to be a reduced form of trichromatic colour vision (referred to as the reductionist principle). In particular, the colour palette of the dichromats is believed to be a part of the colour palette of the trichromats. As the light‐colour palette differs from the object‐colour palette, the dichromatic colour palettes have been derived separately for light‐colours and object‐colours in this report. As to light‐colours, the results are in line with the widely accepted view that the dichromatic colour palettes contain only two hues. However, the dichromatic object‐colour palettes have proved to contain the same six component colours which constitute the trichromatic object‐colour palette (yellow, blue, red, green, black and white). Moreover, all the binary and tertiary combinations of the six component colours present in the trichromatic object‐colour palette also occur in the dichromatic object‐colour palettes. Yet, only five of the six component colours are experienced by dichromats as unitary (unique) object‐colours. The green unitary colour is absent in the dichromatic object‐colour palettes. The difference between the dichromatic and trichromatic object‐colour palettes arises from the fact that not every combination of the component‐colour magnitudes occurs in the dichromatic object‐colour palettes. For instance, in the dichromatic object‐colour palettes there is no colour with the strong green component colour. Furthermore, each achromatic (black or white) component colour of a particular magnitude is combined with the only combination of the chromatic components. In other words, the achromatic component colours are bound with the chromatic component combinations in dichromats. © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 112–124, 2014     

Comparison of real colour gamuts using a new reflectance database

A large set of data, comprising the spectral reflectances of real surface colours, has been accumulated. The data comprise 16 groups with different materials and include 85,879 measured spectra. From these data, CIELAB colorimetric coordinates were calculated under CIE illuminant D50 and the CIE 1931 standard colorimetric (2°) observer. Several published colour gamuts including those developed by Pointer and ISO reference colour gamut [ISO Graphic Technology Standard 12640‐3:2007] were compared using the present data set. It was found that the Pointer gamut is smaller than the new real data in most of the colour regions. The results also showed that the ISO reference colour gamut is larger than the new real accumulated data in most regions. The present finding indicates that there is a need to derive a new colour gamut based on the newly accumulated data for common applications. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 442–451, 2014     

Asymmetric colour matching: Memory matching versus simultaneous matching

We have compared corresponding pairs obtained by simultaneous matching (haploscopic matching) and by memory matching (after 10 min) using 34 reference tests selected from the Munsell Atlas (glossy), belonging to the four main hues 5Y, 5G, 5PB, and 5RP. These colours lie very close to the F₁ and F₂ axes in the SVF space, where we have analyzed our results. Illuminants D65 and A were used as reference and matching illuminants, respectively. Our results show for both kinds of matching a tendency to select more colourful colours than the original ones, with significant differences between matching and test colours, whereas hue does not seem to follow a definite pattern. This behavior is similar to that found in colour‐matching experiments without illuminant changes. The analogy does not hold for lightness, which in the present experiment does not seem to follow a clear pattern. The best matching colours lie along the red‐green axis and the worst matching colours along the blue‐yellow axis. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 458–468, 2001     

Colour memory for various sky,skin, and plant colours: Effect of the image context

In memory‐matching techniques, the remembered colour might differ from the original colour even if the viewing situation is the same. Our aim was to point out whether these so‐called memory shifts are significant in the everyday situations of viewing photos depicting sky, skin, or plant, or viewing standalone uniform colour patches of sky, skin, or plant colours. In many cases, significant memory shifts have been found. Considering only one type of object (sky or skin or plant), memory shifts turned out to be systematic in the sense that they were directed toward specific intervals of hue, chroma, and lightness. This tendency was more explicit for photos than for standalone colour patches. A method to quantify prototypical colours and their tolerance bounds was suggested. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 278–289, 2001     

Mathematical approach for predicting non‐negative tristimulus values using the CAT02 chromatic adaptation transform

It has been reported that for certain colour samples, the chromatic adaptation transform CAT02 imbedded in the CIECAM02 colour appearance model predicts corresponding colours with negative tristimulus values (TSVs), which can cause problems in certain applications. To overcome this problem, a mathematical approach is proposed for modifying CAT02. This approach combines a non‐negativity constraint for the TSVs of corresponding colours with the minimization of the colour differences between those values for the corresponding colours obtained by visual observations and the TSVs of the corresponding colours predicted by the model, which is a constrained non‐linear optimization problem. By solving the non‐linear optimization problem, a new matrix is found. The performance of the CAT02 transform with various matrices including the original CAT02 matrix, and the new matrix are tested using visual datasets and the optimum colours. Test results show that the CAT02 with the new matrix predicted corresponding colours without negative TSVs for all optimum colours and the colour matching functions of the two CIE standard observers under the test illuminants considered. However, the accuracy with the new matrix for predicting the visual data is approximately 1 CIELAB colour difference unit worse compared with the original CAT02. This indicates that accuracy has to be sacrificed to achieve the non‐negativity constraint for the TSVs of the corresponding colours. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2011     

Colour management in digital textile printing

On the basis of simulated prints, a customer can decide on the colour appropriateness for a selected pattern. This article presents the possibilities and correct procedure for colour management application in the field of digital printing onto a textile substrate. The aim of the research was to establish whether long‐term and expensive pre‐ and after‐treatments can be avoided with the help of an appropriate development of printer profiles. The latter would be conducted through a print simulation on paper printed with a Canon laser or ink‐jet printer. The results have confirmed that a print simulation on paper can be conducted with both printers, while a Canon ink‐jet printer offers better results. In addition, by calculating the colour inconstancy index CMCCON02, the illuminant influence on the colour change of substrates under five different illuminations was determined. The results showed that simulated colours should be compared with original colours under daylight illuminants (D50, D55 and D75), while indoor illuminants A and F2 are not appropriate for a comparison, especially in the case of simulation with a Canon ink‐jet printer (CMCCON02 index >5).     

Influences of lighting time course and background on categorical colour constancy with RGB‐LED light sources

Some previous studies have investigated the influence of the lighting time course and viewing background on the colour constancy using two‐dimensional flat stimuli simulated on a monitor. In the present study, we investigated the categorical colour constancy in real scenes by manipulating (a) the lighting time course, that is, adaptation period to the illuminant (brief adaptation or complete adaptation) and (b) the background structure of a stimulus (a uniform gray background with an approximately 25% spectral reflectance or a multicolour background consisting of the Macbeth ColorChecker and some fruit models). The neutral (u′ = 0.1994, v′ = 0.4671), red (u′ = 0.2433, v′ = 0.4622), green (u′ = 0.1525, v′ = 0.4697), blue (u′ = 0.2049, v′ = 0.4198), and yellow (u′ = 0.1892, v′ = 0.5112) illuminants were produced by an RGB‐LED lamp. For each chromatic illumination condition, subjects categorized 240 surfaces with Munsell Value 5/ in four viewing conditions with different combinations of the lighting time course and the background structure. A total of seven subjects participated in experiments with red and green illuminants and five subjects with blue and yellow illuminants. The results showed that the constancy index was the lowest (0.66) in the brief adaptation and gray background condition and the highest (0.74) in the complete adaptation and multicolour background condition. The results suggest that increasing the adaptation period alone or adding chromatic cues in the background with a brief adaptation can help to improve the colour constancy, and a time‐taking reference to surrounding coloured objects with the long presentation of the illuminant may also contribute to obtaining colour constancy.     

An integrated spectral imaging system for producing color images of static and moving objects

An integrated spectral imaging system constructed by synchronizing a programmable light source, a high‐speed monochrome camera, and a display device is proposed to produce tristimulus images of static and moving objects effectively in real time onto the display. This system is called the CIE‐XYZ display. Active spectral illuminants, containing both the device characteristics of camera and display, are projected onto object surfaces as time sequence. The images are captured synchronously by the camera and quickly transmitted to the display device in the RGB signal form so that the accurate tristimulus images are displayed. First, we describe the principle of the CIE‐XYZ tristimulus display. The theoretically optimal illuminants contain negative parts in the spectral curves. Second, we design practical illuminants with all positive spectral curves. The color images in our system are composed of the time sequence of RGB component images. Then, the synthesized color images on the display contain color artifacts when objects move fast. An image processing algorithm for correcting the motion color artifact is proposed based on optical flow estimation using a graphics processing unit. The comprehensive performance of the proposed system and algorithms is examined using both static and moving objects. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 329–340, 2015     

Modelling of chromatic contrast for retrieval of wallpaper images

Colour remains one of the key factors in presenting an object and, consequently, has been widely applied in retrieval of images based on their visual contents. However, a colour appearance changes with the change of viewing surroundings, the phenomenon that has not been paid attention yet while performing colour‐based image retrieval. To comprehend this effect, in this article, a chromatic contrast model, CAMcc, is developed for the application of retrieval of colour intensive images, cementing the gap that most of existing colour models lack to fill by taking simultaneous colour contrast into account. Subsequently, the model is applied to the retrieval task on a collection of museum wallpapers of colour‐rich images. In comparison with current popular colour models including CIECAM02, HSI and RGB, with respect to both foreground and background colours, CAMcc appears to outperform the others with retrieved results being closer to query images. In addition, CAMcc focuses more on foreground colours, especially by maintaining the balance between both foreground and background colours, while the rest of existing models take on dominant colours that are perceived the most, usually background tones. Significantly, the contribution of the investigation lies in not only the improvement of the accuracy of colour‐based image retrieval but also the development of colour contrast model that warrants an important place in colour and computer vision theory, leading to deciphering the insight of this age‐old topic of chromatic contrast in colour science. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 361–373, 2015     

A note about the abnormal hue angle change in CIELAB space

Liu et al. [Color Res Appl 1995;20:245–250] compared the CIELAB hue angles under CIE illuminants D65 and A for quantifying the color appearance changes for gem materials. They found that CIELAB hue angle for some gem materials under illuminant D65 was larger than under A, which is contrary to the perceived blue and purple appearances under daylight and incandescent light sources, respectively. They called this phenomenon as the abnormal hue angle change in the CIELAB space for the gem materials. In this article, we note the proper way to quantify the appearance changes is to use chromatic adaptation transforms (CATs), since we are only concerning the color change of the illumination. At the same time, it is found that the chromatic adaptations in the sharper sensor space and in the cone fundamental space provide different results, the ones related to the latter being in better agreement with current blue‐to‐purple color appearance change. © 2012 Wiley Periodicals, Inc. Col Res Appl, 38, 322–327, 2013     

Museum lighting: Optimizing the illuminant

Museums must choose illumination that will optimize observers' aesthetic experiences without compromising preservation of the artworks. Typical lighting is 3000 K at 200 lux. We examined the effects of color temperature by comparing the same “artwork” under several different illuminants (in side‐by‐side miniature museum rooms); for each painting, observers rated the preference for seeing it under one of the two illuminants; this was repeated for all pairings of illuminants. Our observers preferred higher color temperatures at about 3600 K. Between viewing trials observers' visual systems were adapted to a specific illuminant. This had no effect on preferred illumination of an artwork. To explore a wider range of artworks, we developed a set of numerical scales that could be applied directly to individual pictures; these were viewed, one at a time, in a single simulated room illuminated by one of a series of illuminants. Again, observers preferred a color temperature of about 3600 K. In both studies there was a small interaction between illuminant and color gamut of a given painting. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 121–127, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10231     

Generation of virtual illuminants for a balanced colorimetric match

Colorimetric matching is employed under a series of virtual illuminants to produce more controllable and equalised colour‐difference results under multiple illuminants. A method based on weighted principal component analysis of different artificial lights and the weighted mean is used to create a series of virtual illuminants. The weights were selected in a manner to create the virtual illuminants that benefit from the impact of a given illuminant, while the effects of other light sources on the formation of desired illuminants were also considered. The generated virtual illuminants were implemented in a colorimetric colour‐matching trial. Using this method, by choosing suitable weights for different lighting conditions, more balanced colour‐difference values were presented in the data set than was expected. The advantages of the suggested method were evaluated by the matching of a collection of 135 woollen samples under different virtual illuminants.     

Testing LED lighting for colour discrimination and colour rendering

Light‐emitting diode (LED) technology offers the possibility of obtaining white light, despite narrow‐band spectra. In order to characterize the colour discrimination efficiency of various LED clusters, we designed a classification test, composed of 32 caps equally distributed along the hue circle at about 3 ΔE* _ab‐unit intervals. Forty normal colour observers were screened under four different LED test light sources adjusted for best colour rendering, and under one control incandescent light of the same colour temperature. We used commercially available red, green, blue, and/or amber LED clusters. These yielded a poor colour rendering index (CRI). They also induced a significantly higher number of erroneous arrangements than did the control light. Errors are located around greenish‐blue and purplish‐red shades, parallel to the yellow‐axis direction, whereas when the distribution of light covers the full spectrum, the LED clusters achieve satisfactory colour discrimination efficiency. With respect to the lights we tested, the colour discrimination is correlated with the CIE CRIs as well as with a CRI based on our sample colours. We stress the fact that increasing the chroma of samples by lighting does not necessarily imply an improvement of colour discrimination. © 2008 Wiley Periodicals, Inc. Col Res Appl, 34, 8–17, 2009.     

Illuminant metamerism potentiality of metameric pairs

Indices for describing the degree of metamerism are based on either the deviation of the spectra of a metameric pair or the colour difference of the pair under test conditions. The magnitude of illuminant metamerism is commonly evaluated by measuring the colour difference under the test illuminant. The calculated colour differences absolutely vary with the selected test illuminants, so the measured (special) index of metamerism could be considered as a test‐illuminant‐dependent value. The spectral‐based indices of metamerism act as a single‐number value, but most of them are criticised for their poor correlation with visual assessment. In this paper, a general metric is developed for evaluating the upper limit of the degree of illuminant metamerism. The suggested approach combines the advantages of general and special indices, avoiding their drawbacks at the same time. The performance of the formula is analysed in a number of numerical experiments, as well as by practical testing.     

Colour and tolerance of preferred skin colours on digital photographic images

Skin‐tone has been an active research subject in photographic colour reproduction. There is a consistent conclusion that preferred skin colours are different from actual skin colours. However, preferred skin colours found from different studies are somewhat different. To have a solid understanding of skin colour preference of digital photographic images, psychophysical experiments were conducted to determine a preferred skin colour region and to study inter‐observer variation and tolerance of preferred skin colours. In the first experiment, a preferred skin colour region is searched on the entire skin colour region. A set of nine predetermined colour centers uniformly sampled within the skin colour ellipse in CIELAB a*b* diagram is used to morph skin colours of test images. Preferred skin colour centers are found through the experiment. In a second experiment, a twice denser sampling of nine skin colour centers around the preferred skin colour center determined in the first experiment are generated to repeat the experiment using a different set of test images and judged by a different panel of observers. The results from both experiments are compared and final preferred skin colour centers are obtained. Variations and hue and chroma tolerances of the observer skin colour preference are also analysed. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013     

Lightness and hue contrast effects in surface (fabric) colours

Simultaneous contrast effects on lightness and hue in surface colours were investigated. Test colours, surrounded by induction colours, were matched by colours surrounded by neutral gray. The matching colours were selected from a series of samples that varied in either lightness or hue respectively. The lightness experiments were carried out by a panel of 20 observers on 135 test/induction colour combinations. The hue experiments were conducted on 51 test/induction colour combinations by a panel of eight observers. The lightness of the test colour was found to decrease linearly with the lightness of the induction colour, regardless of the hue of the induction colour. The magnitude of the lightness contrast effect in fabric colours was found to be about one‐quarter of that found in CRT display colours in a previous study. The hue contrast effect found in this study followed the opponent‐colour theory. Two distinctly different regions could be identified when the hue difference was plotted against hue‐angle difference between the induction colour and the test colour. The slope of the line in the region where the hue of the induction colour is close to the test colour was much larger than the slope in the other region, indicating that the hue contrast effect was more obvious when the induction colour was close to the test colour. © 2006 Wiley Periodicals, Inc. Col Res Appl, 32, 55–64, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20285     

Psychophysical models of consumer expectations and colour harmony in the context of juice packaging

The aim of this study was to develop psychophysical models that predict the influence of pack colours on consumers' psychological responses of fruit juices, such as visually perceived expectations of freshness, quality, liking, and colour harmony. Two existing colour harmony models derived from experiments involving only uniform colour plaques were tested using the juice packaging experimental data. Both models failed to predict the visual results obtained. Nevertheless, two parameters relevant to chromatic difference and hue difference were somewhat associated with the visual results. This suggested that, among all colour harmony principles for uniform colours, only the equal‐hue and the equal‐chroma principles can be adopted to describe colour harmony of packaging used for juice. This has the implication that the principles of colour harmony may vary according to the context in which the colours are used. A new colour harmony model was developed for juice packaging, and a predictive model of freshness was derived. Both models adopted CIELAB colour attributes of the package colour and the fruit image colour to predict viewers' responses. Expected liking and juice quality can be predicted using the colour harmony model while expected freshness can be predicted using the predictive model of freshness. © 2013 Wiley Periodicals, Inc. Col Res Appl, 40, 157–168, 2015