The digital reproduction of materials has developed greatly over the past decades. The improved interactive rendering technology available nowadays enables broad digital visualization applications like gaming, cinema and film production, advertising, and online shopping. These recent advances in digital technologies are also playing an important role in the improvement of some industrial processes such as computer-aided design and manufacturing, virtual prototyping, and scientific visualization and simulation. Currently, many rendering software packages provide impressive images and often even claim photorealism. However, producing realistic appearance images is very challenging taking into account the high sensitivity of the human visual system. The visual appearance of products is still an important aspect to take into account even for the digital simulation of materials, since the appearance of these simulated products on the screen is still a critical parameter in the purchase decision of customers. During the last years different efforts have been carried out by industrial manufacturers in different applications, such as textile, cosmetic, automotive, etc., to provide attractive visual effects and new visual impressions of their products using, for instance, innovative effect pigments, also called goniochromatic pigments. The digital rendering of these pigments is a very active hot topic since this type of coatings changes considerably its visual attributes such as color and texture with the illumination/viewing geometry. Achieving accurate simulation of these materials demands an extra effort due to the physical complexity of their surfaces. Special BRDFs (bidirectional reflectance distribution functions) reflectance models are needed to characterize their visual appearance. This complex appearance is produced due to the presence of special effect pigments containing metallic, interference, or pearlescent pigments, which are responsible for the strong dependence of the color of these coatings on viewing and illumination directions. These pigments also exhibit visually complex texture effects such as sparkle and graininess. Under bright direct illumination conditions, such as sunlight, the flakes create a sparkling effect, while under diffuse illumination such as a cloudy sky, effect coatings create a salt and pepper appearance or a light/dark irregular pattern, which is usually referred to as graininess or coarseness. Two main issues limit the digital reproduction of effect pigments. The first issue is related to the current display technologies. The quality of the displays is an essential component toward accurate color reproduction of materials. Previous studies have evaluated the validity of available display technologies for the visualization and digital reproduction of effect pigments, which are usually not enough for the reproduction of such a wide variety of colors due to their limited color gamut. The second limitation is more related to the current rendering software. The color accuracy of their images is often not sufficient for the reproduction of colors and effects produced by these materials. The available rendering software provides impressive images that serve the needs for applications such as the cinema and games industries, but when it comes to more critical applications such as automotive design, the color accuracy of their rendered images is not accurate enough, especially for such complex materials such as effect pigments. The first issue is addressed in this thesis by, evaluating the performance of the new Quantum dots (QDs) display technology for the reproduction of effect pigments. For further improving the display capability, a new solution is given by developing a multi-primary display model based on the QDs technology (addressed in the first research article of this thesis in chapter 1). The proposed multi-primary display model provides an expanded color gamut, which guarantees a better reproduction of effect pigments. In a first step, the emission spectral radiance curves of the three RGB channels of a commercial QD display were fitted to a four-parameter function. From this modeling, it is possible to gain new theoretical color primaries by selecting new spectral peaks (cyan, yellow, magenta, and/or additional RGB primaries) and imposing colorimetric conditions for the resulting white of this proposed theoretical multi-primary display. Proper characterization to assess the performance of the display was conducted to know if the basic “gain-offset-gamma” (GOG) model can be used for direct and inverse color reproduction (from RGB to CIE-XYZ, and vice versa). The GOG model was found to well characterize this display. The spatial uniformity of the display was also evaluated in luminance and color chromaticity terms. Finally, with the primaries modeling and color characterization based on the GOG model, a 5-primary model (RGBYC) was tested. The evaluation of this theoretical RGBYC display model confirms the gamut enlargement, which can also improve goniochromatic color reproduction. In the second place, and focusing on the second issue, a big portion of the work of this thesis was dedicated to the development of a new 3D rendering tool for improved and accurate visualization of the complete appearance of effect coatings, including metallic effects, sparkle, and iridescence (addressed in the second and third research articles of this thesis in chapters 2 and 3). This task was carried on by firstly building a specific rendering framework for this purpose, using a multi-spectral and physically based rendering approach, and secondly, by validating the performance of this rendering framework through psychophysical tests. Spectral reflectance measurements and sparkle indices from a commercially available multi-angle spectrophotometer (BYK-mac i) were used together with a physically based approach, such as flake-based reflectance models, to efficiently implement the appearance reproduction from a small number of bidirectional measurement geometries. With this rendering framework, a virtual representation of a set of effect coating samples is reproduced on an iPad display, by simulating how these samples would be viewed inside a Byko-spectra effect light booth. Therefore, for this purpose, an accurate virtual representation of the Byko light booth was built using a physically based representation of global illumination. The rendering framework also accounts for the colorimetric specifications of the rendering display (iPad5) by applying the recent device-specific MDCIM model. The appearance fidelity of the rendering was validated through psychophysical methods. For this task, observers were asked to evaluate the most important visual attributes that directly affect the appearance of effect coatings, i.e., color, the angular dependence of color (color flop), and visual texture (sparkle and graininess). Observers were asked to directly compare the rendered samples with the real samples inside the Byko-spectra effect light booth. The visual validation was performed in three different steps. In the first study, the accuracy of rendering the color of solid samples is evaluated. In a second step, the accuracy of rendering the color flop of effect coatings is validated by conducting two separate visual tests, by using flat and curved samples respectively. In the third and last step, the digital reproduction of both color and texture of metallic samples is tested, by including texture effects in the rendering by using a sparkle visualization model. The parameters of the sparkle visualization model were optimized based on sparkle measurement data from the BYK-mac i instrument using a matrix-adjustment model. Results from the visual evaluations prove the high color accuracy of the developed rendering tool. In the first test, the visual acceptability of the rendering was 80%. This percentage is much better than what was found in a previous investigation using the default sRGB color encoding space. Results of the second study show an improved accuracy when curved samples were used (acceptability of 93% vs 80%). The final visual test shows high visual acceptability of the rendering at 90%. In conclusion, this thesis provides a method for accurate digital simulation of effect coatings, by developing a multispectral and physically based rendering approach on a simple iPad tablet computer. The research developed in this thesis comes with many advances in the scientific and industrial levels, with a great contribution to the development of innovative tools for digitization of materials, as needed in today’s society. / This thesis was carried out under the financial assistance of the Spanish Ministry of Economy and Competitiveness through the pre-doctoral fellowship FPIBES-2016-077325, and the research projects DPI2015-65814-R and RTI2018-096000-B-I00.

Age-related maculopathy (ARM) is a central retinal disease with unclear pathogenesis. It is the major cause of permanent vision loss in adults over 50 years and is increasing in prevalence and incidence, faster than the aging population would suggest. Early in the disease process (early ARM) there is little or no vision loss and there are only slight retinal changes with abnormal deposits within Bruch's membrane. As the disease progresses (late ARM or age-related macular degeneration, AMD) vision loss may be quite severe due to atrophy (dry AMD) or the development of chorioretinal neovascularisation (CNV, wet AMD). It is hard to predict from conventional eye examinations and clinical vision tests which cases will progress to the severe, dry or wet forms of the disease. Moreover, most of the conventional clinical tests are based upon subjective vision measures. Objective tests which detect ARM earlier would be a useful aid to diagnosis and to monitoring progression. The multifocal electroretinogram (mfERG) is a relatively new clinical tool which enables the recording of electrical potentials from multiple, small areas of the central retina and thus assesses function from specific retinal locations. It is therefore useful in detecting focal retinal diseases such as hereditary or acquired maculopathies or in monitoring retinal laser or surgical treatment effects. There is cone and rod impairment in ARM and histopathological and psychophysical evidence for a preferential vulnerability of rods compared to cones. This research project investigated if an objective tool such as the mfERG could detect early ARM,its progression and the treatment effects of multiple photodynamic therapies (PDT) on retinal function in late ARM, prior to a battery of subjective vision measures. For comparison purposes a subjective assessment of central retinal function was performed using high and low contrast distance visual acuities (VA), near VA, low luminance VA (SKILL cards), contrast sensitivity (Pelli-Robson, P-R), saturated and desaturated Panel D-15 (sat Panel D-15, desat Panel D-15) and central visual fields (Humphrey 10-2, mean sensitivity, MS and mean defects, MD). As an objective assessment of central retinal function the cone- and rod-mediated multifocal electroretinograms were recorded. Subjective and objective tests of retinal function were compared in early ARM and an age-matched control group (chapter 3). Seventeen eyes of seventeen subjects with early ARM and twenty control subjects with normal vision were measured. For the cone-mediated mfERG responses conventional averaging methods were used and results were correlated with subjective vision tests. The conventional cone-mediated mfERG failed to distinguish between the early ARM and control subjects whereas subjective vision measures such as HC- and LC-VA, desat Panel D-15, MS, P-R were significantly reduced in the ARM group. However, there were significant correlations between the cone-mediated mfERG and the desat Panel D-15 results in the ARM group. This suggests that the mfERG measures similar retinal processes that detect colour vision deficiency under desaturated conditions. There was no significant correlation between cone-mediated mfERG measures and funduscopic changes. The conclusion from this study was that the subjective vision tests detected early ARM better than the objective cone-mediated mfERG. Thus the aim of detecting early ARM objectively was not met by the cone-mediated mfERG suggesting the need to develop other objective tests such as a rod-mediated mfERG. Whether the preferential rod vulnerability others have reported in early ARM could be detected by the rod-mediated mfERG was determined in the next study (chapter 4). A protocol for recording rod-mediated mfERG responses was developed by determining the optimal testing luminance to reduce the effect of stray light and elicit maximal rod-mediated responses. Sixteen of the seventeen ARM subjects and seventeen control subjects from the previous study were tested. For analysis, a customized computer template fitting method was developed in MATLAB (Mathworks, Natick, MA, USA). This method has been shown to be useful for low signal-to-noise ratio responses that characterize the rod-mediated mfERG. Significantly delayed rod-mediated mfERG responses were found whereas cone-mediated mfERG responses were within the normal range. This suggested that the effect of ARM on the rod system could be detected objectively with the rod-mediated mfERG before changes in the cone-mediated mfERG. Which of the tests best detected progression of vision loss was investigated in chapter 5. Visual function of 26 (13 ARM and 13 control subjects) of the original 37 subjects (17 ARM and 20 control subjects) had cone- and rod-mediated mfERG and the subjective vision measures repeated after one year. The main purpose was to determine which of the tests best detected progression of vision loss. The mfERG results were analysed by using both averaged and local responses and by using the computer template fitting procedure. On average no significant worsening of either objective or subjective function measures was evident after one year. These results reinforce the slow progression of the disease. With a longer follow-up period progression of ARM may translate into measurable changes in the mfERG and the other visual function tests. The effect of multiple photodynamic therapies (PDT) on cone- and rod-mediated function was assessed with the mfERG in the last study (chapter 6). The cumulative treatment effects of PDT in five subjects with late ARM were determined. Having demonstrated that the rod-mediated mfERG was applicable in early ARM, this study also aimed to investigate how useful it was in late ARM where there is substantially greater rod loss. Cone- and rod-mediated mfERGs, visual acuities, contrast sensitivities and central visual fields were investigated a week before treatment began and then one month after each PDT treatment. The subjects received three treatments each over an average period of five and a half months. In some subjects there were significant transient reductions in cone- and rod-mediated amplitudes possibly reflecting alterations in choroidal hypoperfusion dynamics one month after treatment. Further, b-wave component of the mfERG became increasingly misshapen after each PDT treatment suggesting an ischemic insult mainly targeting post-receptoral sites. However, objective and subjective function was stabilized after multiple PDT treatments in most of the subjects. This pilot study of five cases showed that there was no additional damage to cone- and rod-mediated outer retinal function after three PDT treatments. One of the novel findings of this research was that the rod-mediated function measured with the mfERG was impaired in early ARM. This finding supports histopathological and psychophysical evidence of rod vulnerability in early ARM. The results of these studies also suggest that early ARM affects different aspects of visual function which is reflected by different outcomes from objective and subjective vision tests. A model (chapter 7) based upon the results was developed proposing a hypoxic insult with a preferential alteration of post-receptoral sites in early ARM. The cone-mediated mfERG documented the retinal damage and possible treatment effects on outer retinal function of the multiple PDTs which did not further deteriorate. Thus, this technique might assist in the development of optimal treatment modalities for ARM, especially in retreatment regimes. Greater variability was found for the rod-mediated mfERG and its clinical use in PDT treatment regimes still needs to be investigated. In conclusion, this research has provided a better understanding of the disease process and treatment effects in ARM and might contribute to improvements in diagnosis and treatment of ARM.