Appearance Changes due to Light Exposure

The fading of materials due to light exposure over time is a major contributor to the overall aged appearance of man-made objects. Although much attention has been devoted to the modeling of aging and weathering phenomena over the last decade, comparatively little attention has been paid to fading effects. In this dissertation, we present a theoretical framework for the physically-based simulation of time-dependent spectral changes induced by absorbed radiation. This framework relies on the general volumetric radiative transfer theory, and it employs a physicochemical approach to account for variations in the absorptive properties of colourants. Employing this framework, a layered fading model that can be readily integrated into existing rendering systems is developed using the Kubelka-Munk theory. We evaluate its correctness through comparisons of measured and simulated fading results. Challenges in the acquisition of reliable measurements are discussed. The performance characteristics of the proposed model are analysed, and techniques for improving the runtime cost are outlined. Finally, we demonstrate the effectiveness of this model through renderings depicting typical fading scenarios

The Appearance of Platelet-Polymer Composite Coatings: Microstructural Characterization, Hybrid Modeling, and Predictive Design.

The appearance of a platelet-containing polymer composite coating is governed by the microstructure and optical properties included scattering particles and platelets. Many models attempt to predict the coating's appearance, but do not utilize the complete 3D-microstructure, reducing their predictive utility. In this thesis, laser scanning confocal microscopy was used to measure the effect of platelet orientation on angle-dependent lightness, and quantify the spacing between platelets, from which a new microstructural property, the gap factor, was determined. The gap factor is a measure of the average gap size between platelets per unit material surface length. It ranged from 0 to 2 for the systems studied in this thesis. An increase in gap factor of about 0.1, keeping the orientation similar, reduced the near-specular lightness of the physical samples by more than 20%. A 3D hybrid-simulation was created using wave-optics to simulate the bidirectional-reflection-distribution-function (BRDF) for individual platelets. This was combined with ray-tracing to quantify the scattering behavior of a platelet array. This model more accurately predicted the lightness of a silver paint sample than an orientation-based microfacet-model, and was used to study how the surface roughness of the platelets influences lightness. The lightness at 15 degrees off-specular was about 130 when the root-mean square of the amplitude of the roughness, sigma(RMS), was much less than the wavelength of light. Lightness reduced to about 80 when sigma(RMS) was about equal to the wavelength of light. This effect of sigma(RMS) on lightness was found to be more significant with decreases in the roughness correlation length. The hybrid model was also used to study how width, thickness, and volume concentration of the platelets change the near-specular and backscattered lightness. The observed reduction in near-specular lightness with gap factor was verified. However, the resultant 2nd-order exponential decay was weaker than observed. This was attributed wave-scattering by faces and edges, behavior not included in the current model, but may be added in the future. This hybrid model can be used in the future to design unique microstructures to produce new and novel visual or functional effects using manufacturing techniques such as 3D-printing.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133404/1/cseubert_1.pd

Determinació del factor de cobertura en teixits de calada per processament de la imatge digital i espectroscòpia de reflexió

In this thesis we applied, in a complementary way, techniques of digital image processing and reflectance spectroscopy to perform cover factor (CF) measurements in monochrome openwork fabrics. To evaluate the results provided by both techniques, we used the average CF obtained by a group of 18 expert observers as a reference. IMAGE PROCESSING PART: We developed a new, automatic and objective method for measuring both total and partial CF, and the width of the threads, from the digital image of the fabric. Two algorithms were developed for total and partial CF determinations. The image processing technique used to measure the total CF was the segmentation of the holes from the fabric threads. Conventional methods of segmentation did not lead to similar results than those of the total visual CF. Therefore, we designed a new algorithm (MSSQ) to measure the total CF of fabrics, independently of their degree of coverage. All the CF obtained in 81 typified textile samples were correlated with the CF obtained visually. The best fit to the Stevens' psychophysical law was achieved with the method MSSQ. These results were also consistent with the construction parameters of fabrics. The determination of partial CF in the warp and weft directions was performed by filtering in the Fourier space the warp and weft threads of the sample. For the design of these filters we applied the Radon transform to the power spectrum of the image of the fabric, in order to determine the direction of the warp and weft harmonics. The values of partial CF were consistent with the parameters that characterize the threads and were logical, expected and confirmed the robustness of the method. The values of the diameters of the threads were also consistent with the parameters of the yarn manufacturing and with the manual measurement performed on the image of the same fabric. This method is easily implementable in a laboratory of textile analysis. REFLECTANCE PART: We modelled the reflectance of a flat layer of fabric arranged on a support of known reflectance using the Kubelka-Munk theory. For this purpose we assumed that the optical behaviour of a layer of fabric could be determined by the superposition of two ideal elements: a neutral density filter with no thickness and with different values of transmittance depending on both the direction of the beam (input or output) and whether the beam passes through the covered part of the yarns or through the holes, plus a uniform and homogeneous layer of a material that meets the assumptions of the theory of Kubelka-Munk. The final model contained CF as a parameter. For a set of different samples of fabrics, the integrated spectral reflectance of a stack of increasing number of layers was experimentally measured with a spectroradiometer, up to stabilize the measure. The experimental reflectances were introduced in the generalized model to calculate the values of the parameters involved, particularly the CF. We solved the resulting system of 101 ¿ (m - 1) equations using the least squares method, where m is the total number of stacked layers of fabric, with 103 unknowns. This yielded a couple of estimates of CF. We established correlations between the measured CF (obtained as the solution of these equations) and the visual CF and analyzed the effect of perturbations in the reflectance measurements to explain the discrepancies between these two estimates of CF. Even if we observed problems of precision in one of the estimates and inaccuracy in the other, arising from an unknown parameter in the model, the intervention of the CF in the described model has been proven.Postprint (published version

The algebraic specification of spatial data types with applications to constructive volume geometry.

Spatial objects are modelled as total functions, mapping a topological space of points to a topological algebra of data attributes. High-level operations on these spatial objects form algebras of spatial objects, which model spatial data types. This thesis presents a comprehensive account of the theory of spatial data types. The motivation behind the general theory is Constructive Volume Geometry (CVG). CVG is an algebraic framework for the specification, representation and manipulation of graphics objects in 3D. By using scalar fields as the basic building blocks, CVG gives an abstract representation of spatial objects, with the goal of unifying the many representations of objects used in 3D computer graphics today. The general theory developed in this thesis unifies discrete and continuous spatial data, and the many examples where such data is used - from computer graphics to hardware design. Such a theory is built from the algebraic and topological properties of spatial data types. We examine algebraic laws, approximation methods, and finiteness and computability for general spatial data types. We show how to apply the general theory to modelling (i) hardware and (ii) CVG. We pose the question "Which spatial objects can be represented in the algebraic framework developed for spatial data types?". To answer such a question, we analyse the expressive power of our algebraic framework. Applying our results to the CVG framework yields a new result: We show any CVG spatial object can be approximated by way of CVG terms, to arbitrary accuracy

Hardware accelerated volume texturing.

The emergence of volume graphics, a sub field in computer graphics, has been evident for the last 15 years. Growing from scientific visualization problems, volume graphics has established itself as an important field in general computer graphics. However, the general graphics fraternity still favour the established surface graphics techniques. This is due to well founded and established techniques and a complete pipeline through software onto display hardware. This enables real-time applications to be constructed with ease and used by a wide range of end users due to the readily available graphics hardware adopted by many computer manufacturers. Volume graphics has traditionally been restricted to high-end systems due to the complexity involved with rendering volume datasets. Either specialised graphics hardware or powerful computers were required to generate images, many of these not in real-time. Although there have been specialised hardware solutions to the volume rendering problem, the adoption of the volume dataset as a primitive relies on end-users with commodity hardware being able to display images at interactive rates. The recent emergence of programmable consumer level graphics hardware is now allowing these platforms to compute volume rendering at interactive rates. Most of the work in this field is directed towards scientific visualisation. The work in this thesis addresses the issues in providing real-time volume graphics techniques to the general graphics community using commodity graphics hardware. Real-time texturing of volumetric data is explored as an important set of techniques in delivering volume datasets as a general graphics primitive. The main contributions of this work are; The introduction of efficient acceleration techniques; Interactive display of amorphous phenomena modelled outside an object defined in a volume dataset; Interactive procedural texture synthesis for volume data; 2D texturing techniques and extensions for volume data in real-time; A flexible surface detail mapping algorithm that removes many previous restrictions Parts of this work have been presented at the 4th International Workshop on Volume Graphics and also published in Volume Graphics 2005