Weighted LabPQR Interim Connection Space Based on Human Color Vision for Spectral Color Reproduction

A weighted LabPQR interim connection space, based on human color vision, is proposed for retaining more visual color information. A new weight function proposed in our paper is connected with color-matching function and then further weighted the PQR dimensions of LabPQR compared with the other two weight functions and nonweight function. The results indicated that weighting obviously improved the colorimetric representing accuracy and robustness compared with nonweighting, and the new weight function outperformed the other two weight functions. The weighted LabPQR of the new weight function is most suitable for spectral color reproduction

Interim connection space based on colorimetric values for spectral image compression and reproduction

A new interim and connection space (ICS) and its reconstruction method are proposed. The proposed ICS,tD65A, consists of six colorimetric values or two sets of tristimulus values under CIE illuminant D65 and A respectively. In addition, a new spectral decomposition based on the tD65A ICS and the Wiener Estimation matrix MW was introduced for an improved spectral reconstruction. Accompanying the tD65A ICS, m important basis vectors for the metameric black space based on the new spectral decomposition, and a mapping matrix MP,k via a polynomial model of order k, were trained so that both the spectral and colorimetric accuracies for the reconstructed reflectance can be further enhanced. The proposed ICS and its reconstruction method can ensure exact colorimetric matches under two (real rather than synthetic) illuminants D65 and A, which is an advantage compared with other ICSs. The performance of the proposed method was tested and compared with five other ICSs using the NCS dataset and three spectral images respectively, using RMSE and GFC to measure the spectral accuracy, and using CIEDE2000 colour differences to measure the colorimetric accuracy under three types of illuminants (continuous, fluorescent, and LED). Performance test results showed the proposed methods outperform other ICSs in terms of both spectral accuracy and colorimetric measures (RMSE, GFC, and CIEDE2000 colour difference). Therefore, it is expected the proposed ICS and its reconstruction method can play an important role in spectral image compression and reproduction applications

Printing Beyond Color: Spectral and Specular Reproduction

For accurate printing (reproduction), two important appearance attributes to consider are color and gloss. These attributes are related to two topics focused on in this dissertation: spectral reproduction and specular (gloss) printing. In the conventional printing workflow known as the metameric printing workflow, which we use mostly nowadays, high-quality prints -- in terms of colorimetric accuracy -- can be achieved only under a predefined illuminant (i.e. an illuminant that the printing pipeline is adjusted to; e.g. daylight). While this printing workflow is useful and sufficient for many everyday purposes, in some special cases, such as artwork (e.g. painting) reproduction, security printing, accurate industrial color communication and so on, in which accurate reproduction of an original image under a variety of illumination conditions (e.g. daylight, tungsten light, museum light, etc.) is required, metameric reproduction may produce satisfactory results only with luck. Therefore, in these cases, another printing workflow, known as spectral printing pipeline must be used, with the ideal aim of illuminant-invariant match between the original image and the reproduction. In this workflow, the reproduction of spectral raw data (i.e. reflectances in the visible wavelength range), rather than reproduction of colorimetric values (colors) alone (under a predefined illuminant) is taken into account. Due to the limitations of printing systems extant, the reproduction of all reflectances is not possible even with multi-channel (multi-colorant) printers. Therefore, practical strategies are required in order to map non-reproducible reflectances into reproducible spectra and to choose appropriate combinations of printer colorants for the reproduction of the mapped reflectances. For this purpose, an approach called Spatio-Spectral Gamut Mapping and Separation, SSGMS, was proposed, which results in almost artifact-free spectral reproduction under a set of various illuminants. The quality control stage is usually the last stage in any printing pipeline. Nowadays, the quality of the printout is usually controlled only in terms of colorimetric accuracy and common printing artifacts. However, some gloss-related artifacts, such as gloss-differential (inconsistent gloss appearance across an image, caused mostly by variations in deposited ink area coverage on different spots), are ignored, because no strategy to avoid them exists. In order to avoid such gloss-related artifacts and to control the glossiness of the printout locally, three printing strategies were proposed. In general, for perceptually accurate reproduction of color and gloss appearance attributes, understanding the relationship between measured values and perceived magnitudes of these attributes is essential. There has been much research into reproduction of colors within perceptually meaningful color spaces, but little research from the gloss perspective has been carried out. Most of these studies are based on simulated display-based images (mostly with neutral colors) and do not take real objects into account. In this dissertation, three psychophysical experiments were conducted in order to investigate the relationship between measured gloss values (objective quantities) and perceived gloss magnitudes (subjective quantities) using real colored samples printed by the aforementioned proposed printing strategies. These experiments revealed that the relationship mentioned can be explained by a Power function according to Stevens' Power Law, considering almost the entire gloss range. Another psychophysical experiment was also conducted in order to investigate the interrelation between perceived surface gloss and texture, using 2.5D samples printed in two different texture types and with various gloss levels and texture elevations. According to the results of this experiment, different macroscopic texture types and levels (in terms of texture elevation) were found to influence the perceived surface gloss level slightly. No noticeable influence of surface gloss on the perceived texture level was observed, indicating texture constancy regardless of the gloss level printed. The SSGMS approach proposed for the spectral reproduction, the three printing strategies presented for gloss printing, and the results of the psychophysical experiments conducted on gloss printing and appearance can be used to improve the overall print quality in terms of color and gloss reproduction

Spectral Gamut Mapping and Gamut Concavity

A spectral gamut-mapping algorithm is introduced that works well for printers with a large number of inks. It finds the best mapping onto the convex hull of the printer spectral gamut while preserving color defined in CIE XYZ as much as possible. The technique employs a non-negative least-square fit. Since the gamut-mapping algorithm depends on the common assumption that the gamut is convex, an experimental study of the degree of gamut concavity is conducted. It finds that there is a significant amount of concavity, and that that the degree does not appear to change much as the number of inks is increased. Finally, the performance of the gamut-mapping algorithm and gamut coverage in spectral space is compared for 3-, 4-, 5- and 6-ink printers using both synthetic ink models and real ink data

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

Spectral printing of paintings using a seven-color digital press

The human visual system is trichromatic and therefore reduces higher dimensional spectral data to three dimensions. Two stimuli with different spectral power curve shapes can result in the same cone response and therefore match each other. Color reproduction systems take advantage of this effect and match color by creating the same cone response as the original but with different colorants. ICC color management transforms all colors into a three-dimensional reference color space, which is independent from any input or output devices. This concept works well for a single defined observer and illumination conditions, but in practice, it is not possible to control viewing conditions leading to severe color mismatches, particularly for paintings. Paintings pose unique challenges because of the large variety of available colorants resulting in a very large color gamut and considerable spectral variability. This research explored spectral color reproduction using a seven-color electrophotographic printing process, the HP Indigo 7000. Because of the restriction to seven inks from the 12 basic inks supplied with the press, the research identified both the optimal seven inks and a set of eight artist paints which can be spectrally reproduced. The set of inks was Cyan, Magenta, Yellow, Black, Reflex Blue, Violet and Orange. The eight paints were Cadmium Red Medium, Cadmium Orange, Cadmium Yellow Light, Dioxazine Purple, Phthalo Blue Green Shade, Ultramarine Blue, Quinacridone Crimson and Carbon Black. The selection was based on both theoretical and experimental analyses. The final testing was computational indicating the possibility of both spectral and colorimetric color reproduction of paintings

Image segmentation and pigment mapping of cultural heritage based on spectral imaging

The goal of the work reported in this dissertation is to develop methods for image segmentation and pigment mapping of paintings based on spectral imaging. To reach this goal it is necessary to achieve sufficient spectral and colorimetric accuracies of both the spectral imaging system and pigment mapping. The output is a series of spatial distributions of pigments (or pigment maps) composing a painting. With these pigment maps, the change of the color appearance of the painting can be simulated when the optical properties of one or more pigments are altered. These pigment maps will also be beneficial for enriching the historical knowledge of the painting and aiding conservators in determining the best course for retouching damaged areas of the painting when metamerism is a factor. First, a new spectral reconstruction algorithm was developed based on Wyszecki’s hypothesis and the matrix R theory developed by Cohen and Kappauf. The method achieved both high spectral and colorimetric accuracies for a certain combination of illuminant and observer. The method was successfully tested with a practical spectral imaging system that included a traditional color-filter-array camera coupled with two optimized filters, developed in the Munsell Color Science Laboratory. The spectral imaging system was used to image test paintings, and the method was used to retrieve spectral reflectance factors for these paintings. Next, pigment mapping methods were brought forth, and these methods were based on Kubelka-Munk (K-M) turbid media theory that can predict spectral reflectance factor for a specimen from the optical properties of the specimen’s constituent pigments. The K-M theory has achieved practical success for opaque materials by reduction in mathematical complexity and elimination of controlling thickness. The use of the general K-M theory for the translucent samples was extensively studied, including determination of optical properties of pigments as functions of film thickness, and prediction of spectral reflectance factor of a specimen by selecting the right pigment combination. After that, an investigation was carried out to evaluate the impact of opacity and layer configuration of a specimen on pigment mapping. The conclusions were drawn from the comparisons of prediction accuracies of pigment mapping between opaque and translucent assumption, and between single and bi-layer assumptions. Finally, spectral imaging and pigment mapping were applied to three paintings. Large images were first partitioned into several small images, and each small image was segmented into different clusters based on either an unsupervised or supervised classification method. For each cluster, pigment mapping was done pixel-wise with a limited number of pigments, or with a limited number of pixels and then extended to other pixels based on a similarity calculation. For the masterpiece The Starry Night, these pigment maps can provide historical knowledge about the painting, aid conservators for inpainting damaged areas, and digitally rejuvenate the original color appearance of the painting (e.g. when the lead white was not noticeably darkened)

Modeling and Halftoning for Multichannel Printers: A Spectral Approach

Printing has been has been the major communication medium for many centuries. In the last twenty years, multichannel printing has brought new opportunities and challenges. Beside of extended colour gamut of the multichannel printer, the opportunity was presented to use a multichannel printer for ‘spectral printing’. The aim of spectral printing is typically the same as for colour printing; that is, to match input signal with printing specific ink combinations. In order to control printers so that the combination or mixture of inks results in specific colour or spectra requires a spectral reflectance printer model that estimates reflectance spectra from nominal dot coverage. The printer models have one of the key roles in accurate communication of colour to the printed media. Accordingly, this has been one of the most active research areas in printing. The research direction was toward improvement of the model accuracy, model simplicity and toward minimal resources used by the model in terms of computational power and usage of material. The contribution of the work included in the thesis is also directed toward improvement of the printer models but for the multichannel printing. The thesis is focused primarily on improving existing spectral printer models and developing a new model. In addition, the aim was to develop and implement a multichannel halftoning method which should provide with high image quality. Therefore, the research goals of the thesis were: maximal accuracy of printer models, optimal resource usage and maximal image quality of halftoning and whole spectral reproduction system. Maximal colour accuracy of a model but with the least resources used is achieved by optimizing printer model calibration process. First, estimation of the physical and optical dot gain is performed with newly proposed method and model. Second, a custom training target is estimated using the proposed new method. These two proposed methods and one proposed model were at the same time the means of optimal resource usage, both in computational time and material. The third goal was satisfied with newly proposed halftoning method for multichannel printing. This method also satisfies the goal of optimal computational time but with maintaining high image quality. When applied in spectral reproduction workflow, this halftoning reduces noise induced in an inversion of the printer model. Finally, a case study was conducted on the practical use of multichannel printers and spectral reproduction workflow. In addition to a gamut comparison in colour space, it is shown that otherwise limited reach of spectral printing could potentially be used to simulate spectra and colour of textile fabrics