Achromatic perception in color image displays

The perception of achromatic colors is an important aspect of CRT color appearance. Achromatic colors are important for practical reasons such as image color balance and as fundamental components of chromatic adaptation and color constancy research. Data on achromatic colors is absolutely essential for applying CIE colorimetry to CRT-hardcopy matching. For example, the CIELAB formulas require the specification of the tristimulus values of white. Unfortunately, psychophysical data on the perception of achromatic colors, including white, in CRT-hardcopy matching situations is not readily available in the literature. The purpose of this research was to investigate factors that affect the perception of achromatic colors in CRT and hardcopy images viewed in a desktop environment. Four psychophysical experiments were performed in this research. In these experiments, three observers made judgements of achromatic colors on a CRT monitor and in hardcopy images in isolated and matching situations. The color of image balance and ambient illumination in the laboratory was varied between 2700K tungsten and 6000K daylight-fluorescent. The results show that chromatic adaptation was controlled almost totally by the CRT image. Adaptation to tungsten was found to be incomplete. When adaptation was incomplete, the chromaticities of achromatic color judgements fell into two categories similar to Bartleson\u27s type I and II, where type II show higher color constancy. Judgements were more likely to be of type II when surface color attributes were present in the CRT image and when the observers were instructed to consider the CRT colors in surface mode. When the images contained more illuminant attributes, the results resembled type I. Hardcopy always produced type II results

Evaluation and improvement of the workflow of digital imaging of fine art reproduction in museums

Fine arts refer to a broad spectrum of art formats, ie~painting, calligraphy, photography, architecture, and so forth. Fine art reproductions are to create surrogates of the original artwork that are able to faithfully deliver the aesthetics and feelings of the original. Traditionally, reproductions of fine art are made in the form of catalogs, postcards or books by museums, libraries, archives, and so on (hereafter called museums for simplicity). With the widespread adoption of digital archiving in museums, more and more artwork is reproduced to be viewed on a display. For example, artwork collections are made available through museum websites and Google Art Project for art lovers to view on their own displays. In the thesis, we study the fine art reproduction of paintings in the form of soft copy viewed on displays by answering four questions: (1) what is the impact of the viewing condition and original on image quality evaluation? (2) can image quality be improved by avoiding visual editing in current workflows of fine art reproduction? (3) can lightweight spectral imaging be used for fine art reproduction? and (4) what is the performance of spectral reproductions compared with reproductions by current workflows? We started with evaluating the perceived image quality of fine art reproduction created by representative museums in the United States under controlled and uncontrolled environments with and without the presence of the original artwork. The experimental results suggest that the image quality is highly correlated with the color accuracy of the reproduction only when the original is present and the reproduction is evaluated on a characterized display. We then examined the workflows to create these reproductions, and found that current workflows rely heavily on visual editing and retouching (global and local color adjustments on the digital reproduction) to improve the color accuracy of the reproduction. Visual editing and retouching can be both time-consuming and subjective in nature (depending on experts\u27 own experience and understanding of the artwork) lowering the efficiency of artwork digitization considerably. We therefore propose to improve the workflow of fine art reproduction by (1) automating the process of visual editing and retouching in current workflows based on RGB acquisition systems and by (2) recovering the spectral reflectance of the painting with off-the-shelf equipment under commonly available lighting conditions. Finally, we studied the perceived image quality of reproductions created by current three-channel (RGB) workflows with those by spectral imaging and those based on an exemplar-based method

Digital Color Imaging

This paper surveys current technology and research in the area of digital color imaging. In order to establish the background and lay down terminology, fundamental concepts of color perception and measurement are first presented us-ing vector-space notation and terminology. Present-day color recording and reproduction systems are reviewed along with the common mathematical models used for representing these devices. Algorithms for processing color images for display and communication are surveyed, and a forecast of research trends is attempted. An extensive bibliography is provided

Preferred lightness and chromatic image contrast reproduction

In this study, the image preference as a function of lightness and chromatic contrast of images produced on an ink-jet printer is examined. The purpose is to develop image manipulation rules, useful in the development of printer algorithms to produce images that are preferred by viewers over images that have been printed without application of these rules. Five images are used during the psychophysical experiment, two business graphics and three pictorial, processed in three different ways in RLAB color space, once having only the tightness contrast varied, then only the chromatic contrast, and finally both lightness and chromatic contrast varied. The results showed that for the graphics images seen without a CRT original used for comparison, the mean preference was an increase in lightness contrast, while with an original available for comparison, the mean preference indicated a decrease in both lightness and chromatic contrast. For pictorial images, in the first phase of the experiment the mean preference was an increase in both lightness and chromatic contrast, and after comparison, a decrease in lightness and simultaneous decrease in lightness and chromatic contrast are the most preferred

Modelling of colour appearance

A colour may have a different appearance under different viewing conditions. This causes many problems in the colour reproduction industry. Thus the importance of prediction of colour appearance has arisen. In this study, a mathematical model to predict colour appearance was developed based on the investigation of the changes of colour appearance under a wide range of media and viewing conditions. The media studied included large cut-sheet transparency films, 35mm projected slides, reflection samples and monitor colours. The viewing conditions varied were light source, luminance level and viewing background. Colour appearance was studied using the magnitude estimation technique. In general, colours appeared more colourful, lighter and brighter with an increase in luminance level. Background and flare light had considerable influence on colour appearance for cut-sheet transparency media. Simultaneous contrast effects occurred when a monitor colour was displayed against a chromatic surround. The monitor colour appeared lighter with a darker induction field. When a coloured area was enlarged, lightness tended to increase while colourfulness tended to decrease. Colour appearance was also affected by the closest neighbouring colour. In this case, the hue of the colour largely shifted towards the direction of the opponent hue of the induction colour. The data obtained were applied to test three colour spaces and two colour appearance models. For reflection media, the Hunt91 model performed the best. However it was not satisfactory when applied to transmissive media. Based on these results, the Hunt93 model was developed by modification of the Hunt91 model. The new model widens the application range of the Hunt91 and is reversible

The Effect of the Field of Adaptation on Chromatic Discrimination Ellipses

In this paper results of chromatic discrimination measurements obtained in two lighting scenarios are compared. Chromatic discrimination thresholds were measured in relation with systematically manipulated chromaticities seen under different visual angles. In both cases the Ellipse module of the Cambridge Colour Test (CCT) was obtained binocularly by normal colour-observers. Discrimination ellipses fitted to the measured thresholds were compared in terms of the adapting chromaticity and the visual angle of the adapting stimulus. Our findings show that changes in the reference chromaticity of the CCT have stronger effect on the elongation of the chromatic discrimination ellipses compared to the effect of the adapting chromaticity obtained in a light booth.Further comparison of discrimination thresholds estimated towards the confusion directions and the corresponding radii of the Ellipse test results suggests that the reliability of the Ellipse test results depends on the relation between the measurement directions and the confusion directions

Comparing color appearance models using pictorial images

Eight different color appearance models were tested using pictorial images. A psychophysical paired comparison experiment was performed where 30 color-normal observers judged reference and test images via successive-Ganzfeld haploscopic viewing such that each eye maintained constant chromatic adaptation and inter-ocular interactions were minimized. It was found that models based on von Kries had best performance, specifically CIELAB, HUNT, RLAB, and von Kries

Computational analysis and revision of the Nayatani et al. appearence model

A literature search showed Nayatani \u27s color appearance model to be inferior to Hunt\u27s model. The goal of this research was to determine what modifications could be made to improve Nayatani\u27s model. The coefficients\u27 (xi), (eta), and (zeta), in Nayatani\u27s model represent the chromaticity coordinates of the illuminant in the fundamental primary system. These coefficients were used in calculating the effective adapting level for each cone and act as normalizing factors i n the calculation of Nayatani color space. The CSAJ data set was used to determine a baseline for Nayatani\u27s model, and results from modifications were compared relative to Hunt. Nayatani\u27s Ml model set the coefficients\u27 equal to 1, and Illuminant A was used to normalize the estimates of the fundamental primaries. Nayatani\u27s results were compared to the CSAJ D65 reference data. The average AE*ab error was essentially equal for Munsell Value 3/, 5/, and 7/, while the total AE*ab error was reduced from 7.19 to 6.3. In addition, light colors shifted yellow and dark colors shifted blue for all three Value levels. This correctly predicts the Helson-Judd effect, and the results appear to match the Hunt model. The exponents of Nayatani\u27s nonlinear p functions were rounded to the hundredths place to test the relevance of four significant figures. The (delta)L*, (delta)a*, (delta)b* and (delta)E*ab errors were the identical to those in Nayatani\u27s standard model for all Value levels. Thus there is no justification for specifying exponents to four decimal places. Variations in the adapting luminance showed (delta)E*ab errors to be nonlinear at low luminance levels

The LLAB model for quantifying colour appearance

A reliable colour appearance model is desired by industry to achieve high colour fidelity between images produced using a range of different imaging devices. The aim of this study was to derive a reliable colour appearance model capable of predicting the change of perceived attributes of colour appearance under a wide range of media/viewing conditions. The research was divided into three parts: characterising imaging devices, conducting a psychophysical experiment, and developing a colour appearance model. Various imaging devices were characterised including a graphic art scanner, a Cromalin proofing system, an IRIS ink jet printer, and a Barco Calibrator. For the former three devices, each colour is described by four primaries: cyan (C), magenta (M), yellow (Y), and black (K). Three set of characterisation samples (120 and 31 black printer, and cube data sets) were produced and measured for deriving and testing the printing characterisation models. Four black printer algorithms (BPA), were derived. Each included both forward and reverse processes. A 2nd BPA printing model taking into account additivity failure, grey component replacement (GCR) algorithm gave the most accurate prediction to the characterisation data set than the other BPA models. The PLCC (Piecewise Linear interpolation assuming Constant Chromaticity coordinates) monitor model was also implemented to characterise the Barco monitor. The psychophysical experiment was conducted to compare Cromalin hardcopy images viewed in a viewing cabinet and softcopy images presented on a monitor under a wide range of illuminants (white points) including: D93, D65, D50 and A. Two scaling methods: category judgement and paired comparison, were employed by viewing a pair of images. Three classes of colour models were evaluated: uniform colour spaces, colour appearance models and chromatic adaptation transforms. Six images were selected and processed via each colour model. The results indicated that the BFD chromatic transform gave the most accurate predictions of the visual results. Finally, a colour appearance model, LLAB, was developed. It is a combination of the BFD chromatic transform and a modified version of CIELAB uniform colour space to fit the LUTCRI Colour Appearance Data previously accumulated. The form of the LLAB model is much simpler and its performance is more precise to fit experimental data than those of the other models

Evaluation and optimal design of spectral sensitivities for digital color imaging

The quality of an image captured by color imaging system primarily depends on three factors: sensor spectral sensitivity, illumination and scene. While illumination is very important to be known, the sensitivity characteristics is critical to the success of imaging applications, and is necessary to be optimally designed under practical constraints. The ultimate image quality is judged subjectively by human visual system. This dissertation addresses the evaluation and optimal design of spectral sensitivity functions for digital color imaging devices. Color imaging fundamentals and device characterization are discussed in the first place. For the evaluation of spectral sensitivity functions, this dissertation concentrates on the consideration of imaging noise characteristics. Both signal-independent and signal-dependent noises form an imaging noise model and noises will be propagated while signal is processed. A new colorimetric quality metric, unified measure of goodness (UMG), which addresses color accuracy and noise performance simultaneously, is introduced and compared with other available quality metrics. Through comparison, UMG is designated as a primary evaluation metric. On the optimal design of spectral sensitivity functions, three generic approaches, optimization through enumeration evaluation, optimization of parameterized functions, and optimization of additional channel, are analyzed in the case of the filter fabrication process is unknown. Otherwise a hierarchical design approach is introduced, which emphasizes the use of the primary metric but the initial optimization results are refined through the application of multiple secondary metrics. Finally the validity of UMG as a primary metric and the hierarchical approach are experimentally tested and verified