Spectrally Based Material Color Equivalency: Modeling and Manipulation

A spectrally based normalization methodology (Wpt normalization) for linearly transforming cone excitations or sensor values (sensor excitations) to a representation that preserves the perceptive concepts of lightness, chroma and hue is proposed resulting in a color space with the axes labeled W , p, t. Wpt (pronounced “Waypoint ) has been demonstrated to be an effective material color equivalency space that provides the basis for defining Material Adjustment Transforms that predict the changes in sensor excitations of material spectral reflectance colors due to variations in observer or illuminant. This is contrasted with Chromatic Adaptation Transforms that predict color appearance as defined by corresponding color experiments. Material color equivalency as provided by Wpt and Wpt normalization forms the underlying foundation of this doctoral research. A perceptually uniform material color equivalency space (“Waypoint Lab or WLab) was developed that represents a non-linear transformation of Wpt coordinates, and Euclidean WLab distances were found to not be statistically different from ∆E⋆94 and ∆E00 color differences. Sets of Wpt coordinates for variations in reflectance, illumination, or observers were used to form the basis of defining Wpt shift manifolds. WLab distances of corresponding points within or between these manifolds were utilized to define metrics for color inconstancy, metamerism, observer rendering, illuminant rendering, and differences in observing conditions. Spectral estimation and manipulation strategies are presented that preserve various aspects of “Wpt shift potential as represented by changes in Wpt shift manifolds. Two methods were explored for estimating Wpt normalization matrices based upon direct utilization of sensor excitations, and the use of a Wpt based Material Adjustment Transform to convert Cone Fundamentals to ”XYZ-like Color Matching Functions was investigated and contrasted with other methods such as direct regression and prediction of a common color matching primaries. Finally, linear relationships between Wpt and spectral reflectances were utilized to develop approaches for spectral estimation and spectral manipulation within a general spectral reflectance manipulation framework – thus providing the ability to define and achieve “spectrally preferred color rendering objectives. The presented methods of spectral estimation, spectral manipulation, and material adjustment where utilized to: define spectral reflectances for Munsell colors that minimize Wpt shift potential; manipulate spectral reflectances of actual printed characterization data sets to achieve colorimetry of reference printing conditions; and lastly to demonstrate the spectral estimation and manipulation of spectral reflectances using images and spectrally based profiles within an iccMAX color management workflow

Object-Color Description Under Varying Illumination

Given a fixed and uniform illumination, metameric objects appear as the same color. However, when the illumination is altered, two metameric reflecting objects under the first illuminant may no longer produce the same color signal under the second. This situation is called metamer mismatching. Metamer mismatching poses several challenges for the camera and display industries as well as color-based computer vision technology.In light of metamer mismatching, the present study criticizes the conventional approaches to color description when the illuminant alters, and then lays a foundation to robustly describe object colors under varying illumination conditions. Later, the degree of metamer mismatching is used as a measure of the quality of lights. We demonstrate that although the common color spaces such as CIELAB and related spaces in the literature may work well for a fixed illuminant, they can lead to poor results when the illuminant is changed. In view of these problems, new descriptors for hue, lightness and chroma are presented that are based on properties of a Gaussian-like spectrum metameric to the given color tristimulus coordinates. Experiments show that the new Gaussian-based appearance descriptors correlate with different descriptors as well as the CIECAM02 appearance model does on average. Furthermore, the Gaussian-based descriptors are significantly more stable than the descriptors defined in the CIECAM02 appearance model.Afterwards, the problem of predicting how the color signal arising in response to light reflected from the surface of an object is likely to change when the lighting alters is investigated. A new method, called the Gaussian Metamer (GM) method is proposed for predicting what a color signal observed from a surface under a first light is likely to be when the same surface is lit instead by a second light. Due to metamer mismatching, there is not a unique answer for this problem. Our approach is to use one of the possible metamers that is likely to do well on average. The results outperform other state-of-the-art prediction methods

Determinants of colour constancy

Colour constancy describes the ability of our visual system to keep colour percepts stable through illumination changes. This is an outstanding feat given that in the retinal image surface and illuminant properties are conflated. Still, in our everyday lives we are able attribute stable colour-labels to objects to make communication economic and efficient. Past research shows colour constancy to be imperfect, compensating for 40% and 80% of the illumination change. While different constancy determinants are suggested, no carefully controlled study shows perfect constancy. The first study presented here addresses the issue of imperfect constancy by investigating colour constancy in a cue rich environment, using a task that resembles our everyday experience with colours. Participants were asked to recall the colour of unique personal objects in natural environment under four chromatic illuminations. This approach yielded perfect colour constancy. The second study investigated the relation between illumination discrimination and chromatic detection. Recent studies using an illumination discrimination paradigm suggest that colour constancy is optimized for bluish daylight illuminations. Because it is not clear if illumination discrimination is directly related to colour constancy or is instead explained by sensitivity to changes in chromaticity of different hues, thresholds for illumination discrimination and chromatic detection for the same 12 illumination hues were compared. While the reported blue bias could be replicated, thresholds for illumination discrimination and chromatic detection were highly related, indicating that lower sensibility towards bluish hues is not exclusive to illumination discrimination. Accompanying the second study, the third study investigated the distribution of colour constancy for 40 chromatic illuminations of different hue using achromatic adjustments and colour naming. These measurements were compared to several determinants of colour constancy, including the daylight locus, colour categories, illumination discrimination, chromatic detection, relational colour constancy and metameric mismatching. In accordance with the observations in study 2, achromatic adjustments revealed a bias towards bluish daylight illumination. This blue bias and naming consistency explained most of the variance in achromatic adjustments, while illumination discrimination was not directly related to colour constancy. The fourth study examined colour memory biases. Past research shows that colours of objects are remembered as being more saturated than they are perceived. These works often used natural objects that exist in a variety of colour and hue, such as grass or bananas. The approach presented here directly compared perceived and memorized colours for unique objects, used also in the first study, and confirmed the previous findings that on average, objects were remembered more saturated than they were perceived.Farbkonstanz beschreibt die Fähigkeit unseres visuellen Systems Farbeindrücke unter Beleuchtungsänderungen beständig zu halten. Dies ist eine außergewöhnliche Leistung, wenn man in Betracht zieht, dass in dem Lichtsignal welches das Auge erreicht Eigenschaften der Beleuchtung und der Oberflächen konfundiert sind. Trotz dieser Problematik sind wir in unserem alltäglichen Leben in der Lage Objekten stabile Farbnamen zuzuordnen, und damit unsere Kommunikation effizient und ökonomisch zu gestalten. Bisherige Studien zur Farbkonstanz berichten jedoch, dass Farbkonstanz nicht perfekt ist, Beleuchtungswechsel wurden nur zwischen 40-80% kompensiert. Während unterschiedliche Determinanten der Farbkonstanz vorgeschlagen wurden, konnte bisher keine sorgfältig kontrollierte Studie perfekte Farbkonstanz zeigen. In der ersten Studie dieser Arbeit wurde dieser Aspekt untersucht, indem Farbkonstanz in einer hinweisreichen Umgebung unter Verwendung einer Aufgabe, die möglichst präzise unserer alltäglichen Erfahrung im Umgang mit Farben wiederspiegelt, gemessen wurde. Die Versuchsteilnehmer wurden aufgefordert die Farbe eines spezifischen persönlichen Gegenstandes unter vier farbigen Beleuchtungen aus dem Gedächtnis abzurufen. Unter Verwendung dieses Ansatzes konnte perfekte Farbkonstanz erreicht werden. Die zweite Studie untersuchte die Beziehung zwischen Beleuchtungs-Diskrimination und chromatischer Detektion. Die Ergebnisse von kürzlich veröffentlichten Forschungsarbeiten, welche ein Beleuchtungs-Diskriminations-Paradigma verwendeten, zeigen das diese Diskrimination in Richtung bläulicher Beleuchtung verzerrt ist. Daraus wurde geschlossen, das Farbkonstanz für bläuliche Tageslicht-Beleuchtungen optimiert ist . Da es aber nicht klar ist, ob Beleuchtungs-Diskrimination in direkter Beziehung zur Farbkonstanz steht, oder aber vielmehr auf die Sensitivität für chromatische Veränderungen zurückführen ist, wurden Wahrnehmungsschwellen für Beleuchtungs-Diskrimination und chromatische Detektion für die selben 12 Beleuchtungsfarben gemessen und verglichen. Während die bereits berichtete Verzerrung in Richtung der bläulichen Tageslichtbeleuchtung repliziert werden konnte, wurde ebenfalls eine hoher Zusammenhang zwischen chromatischer Detektion und Beleuchtungs-Diskrimination gefunden, welcher darauf hinweist, dass die Verzerrung in Richtung bläulicher Farben keine exklusive Eigenschaft der Beleuchtung-Diskrimination ist. Anknüpfend an die zweite Studie wurde in der dritten Studie die Verteilung von Farbkonstanz über 40 chromatische Beleuchtungen anhand von achromatischen Einstellungen und Farbbenennung untersucht. Farbkonstanz wurde auf ihren Zusammenhang zu mehreren Determinanten der Farbkonstanz überprüft, unter anderem mit Tageslichtvariationen, Farbkategorien, Beleuchtungs-Diskrimination, relationaler Farbkonstanz und metameric mismatching. In Übereinstimmung mit der zweiten Studie wurde auch für achromatische Einstellungen eine Verzerrung in Richtung bläulicher Tageslichtbeleuchtungen gefunden. Diese Verzerrung und der Konsensus der Beleuchtungsbenennung erklärten den Großteil der Varianz der achromatischen Einstellungen, während Beleuchtungs-Diskrimination nicht in direkter Verbindung zur Farbkonstanz stand. In der vierten Studie wurden Verzerrungen des Farbgedächtnisses untersucht. Frühere Studien berichten, dass Objektfarben häufig gesättigter erinnert werden als sie tatsächlich wahrgenommen werden. In diesen Studien wurden häufig natürliche Objekte verwendet, die in einer Vielzahl an Farbtönen und Sättigungen existieren, wie beispielsweise Gras oder Bananen. In dem hier präsentierten Ansatz wurden Farbwahlen aus dem Gedächtnis mit Farbwahlen der direkten Objektwahrnehmung für persönliche, spezifische Objekte, die auch schon in der ersten Studie verwendet wurden, verglichen. Die Ergebnisse der vorherigen Studien konnten für diese Objekte repliziert werden: Im Durchschnitt wurden Objektfarben gesättigter erinnert als das Objekt im direkten Vergleich wahrgenommen wurde

Realistic visualisation of cultural heritage objects

This research investigation used digital photography in a hemispherical dome, enabling a set of 64 photographic images of an object to be captured in perfect pixel register, with each image illuminated from a different direction. This representation turns out to be much richer than a single 2D image, because it contains information at each point about both the 3D shape of the surface (gradient and local curvature) and the directionality of reflectance (gloss and specularity). Thereby it enables not only interactive visualisation through viewer software, giving the illusion of 3D, but also the reconstruction of an actual 3D surface and highly realistic rendering of a wide range of materials. The following seven outcomes of the research are claimed as novel and therefore as representing contributions to knowledge in the field: A method for determining the geometry of an illumination dome; An adaptive method for finding surface normals by bounded regression; Generating 3D surfaces from photometric stereo; Relationship between surface normals and specular angles; Modelling surface specularity by a modified Lorentzian function; Determining the optimal wavelengths of colour laser scanners; Characterising colour devices by synthetic reflectance spectra

Prototype software for colorant formulation using Gamblin conservation colors

When selecting pigments from a large set for restorative inpainting, it can often be challenging to create a mixture that will provide an exact match to the original artwork under a range of viewing and illumination conditions. In this research, a prototype computer program was developed that will aid the user by providing a color match and paint recipe that exhibits minimal metamerism when compared to the original artwork. The Gamblin Conservation Colors, a set of 43 colorants specially formulated for inpainting, were characterized in terms of their optical properties, absorption and scattering, according to Kubelka-Munk turbid media theory. Formulations were made using traditional spectrophotometric measurements and image-based measurements. The multispectral imaging system consisted of a trichromatic CFA camera coupled with two absorption filters; spectral reflectance data for each pixel location was estimated with a transformation based on calibration target images. Three targets were used for testing formulation accuracy: a target consisting of mixtures of Gamblin Conservation Colors, and two oil paintings. Pigment selection was reasonably successful, and good predictions resulted from both measurement techniques, but for more complex tasks such as pigment identification, a more rigorous colorant characterization approach may be needed. Predictions from image-based measurements were generally less accurate, and improvements in the camera model would likely remedy this. It is expected that this software will be of assistance to conservators by simplifying the process of selecting from a large set of available pigments, as well as reducing the possibility of damage to painted surfaces in cases where direct measurements are impractical. The open source nature of the software provides the opportunity for changes and addition of features in the future

Colour Communication Within Different Languages

For computational methods aiming to reproduce colour names that are meaningful to speakers of different languages, the mapping between perceptual and linguistic aspects of colour is a problem of central information processing. This thesis advances the field of computational colour communication within different languages in five main directions. First, we show that web-based experimental methodologies offer considerable advantages in obtaining a large number of colour naming responses in British and American English, Greek, Russian, Thai and Turkish. We continue with the application of machine learning methods to discover criteria in linguistic, behavioural and geometric features of colour names that distinguish classes of colours. We show that primary colour terms do not form a coherent class, whilst achromatic and basic classes do. We then propose and evaluate a computational model trained by human responses in the online experiment to automate the assignment of colour names in different languages across the full three-dimensional colour gamut. Fourth, we determine for the first time the location of colour names within a physiologically-based cone excitation space through an unconstrained colour naming experiment using a calibrated monitor under controlled viewing conditions. We show a good correspondence between online and offline datasets; and confirm the validity of both experimental methodologies for estimating colour naming functions in laboratory and real-world monitor settings. Finally, we present a novel information theoretic measure, called dispensability, for colour categories that predicts a gradual scale of basicness across languages from both web- and laboratory- based unconstrained colour naming datasets. As a result, this thesis contributes experimental and computational methodologies towards the development of multilingual colour communication schemes

Infant colour perception

We have learnt a lot about the development and maturation of colour vision in infants, but know comparatively little about the development and structure of colour perception once trichromacy has been established. This thesis aims to characterize infant colour perception in the first year of life over several areas which have previously been well documented in adults; colour categorisation, colour preference, perception of illusions, and infant sensitivity to natural scene statistics. Paper 1 (Skelton, Catchpole, Abbott, Bosten, and Franklin, 2017) systematically mapped infant colour categories revealing that infant colour categories are related to the retinogeniculate pathways of colour vision. Paper 1 also illustrates and quantifies the correspondence between infant colour categories and commonalities found across the worlds’ colour lexicon. Paper 2 shows that infants look for longer at the colours which adults prefer, and provides evidence for an influence of low-level mechanisms on infant response to colour and adult preference. Paper 3 (under review) reopens the question perception of the Munker-White illusion in infants aged 4 to 8 months, through the replication of a previous study and the use of an alternative method, with discussion of the reasons for studying illusions in infancy. Finally, paper 4 provides the first substantive evidence that infants aged 4 months are sensitive to natural scene statistics, as evidenced though the presence of a blue-yellow discrimination bias commonly associated with the distribution of chromaticities found in natural scenes. Paper 4 also has critical implications for the use of psychophysical methods in infants and adults. The findings inform our understanding of infant colour perception and general perceptual processes by outlining some of the structure around which infants’ colour perception is built. The early structure of infants’ perception of colour is shown to often have commonalities with adult colour perception

Accurate Colour Reproduction of Human Face using 3D Printing Technology

The colour of the face is one of the most significant factors in appearance and perception of an individual. With the rapid development of colour 3D printing technology and 3D imaging acquisition techniques, it is possible to achieve skin colour reproduction with the application of colour management. However, due to the complicated skin structure with uneven and non-uniform surface, it is challenging to obtain accurate skin colour appearance and reproduce it faithfully using 3D colour printers. The aim of this study was to improve the colour reproduction accuracy of the human face using 3D printing technology. A workflow of 3D colour image reproduction was developed, including 3D colour image acquisition, 3D model manipulation, colour management, colour 3D printing, postprocessing and colour reproduction evaluation. Most importantly, the colour characterisation methods for the 3D imaging system and the colour 3D printer were comprehensively investigated for achieving higher accuracy