Material Appearance Modeling

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Appearance of Interfaced Lambertian Microfacets, using STD Distribution
(The Eurographics Association, 2017) Ribardière, M.; Meneveaux, D.; Bringier, B.; Simonot, L.; Reinhard Klein and Holly Rushmeier
This paper presents the use of Student’s T-Distribution (STD) with interfaced Lambertian (IL) microfacets. The resulting model increases the range of materials while providing a very accurate adjustment of appearance. STD has been recently proposed as a generalized distribution of microfacets which includes Beckmann and GGX widely used in computer graphics; IL corresponds to a physical representation of a Lambertian substrate covered with a flat Fresnel interface. We illustrate the appearance variations that can be observed, and discuss the advantages of using such a combination.
Bonn Appearance Benchmark
(The Eurographics Association, 2020) Merzbach, Sebastian; Klein, Reinhard; Klein, Reinhard and Rushmeier, Holly
There is a general shortage of standardized comparisons in the field of appearance modeling. We therefore introduce a benchmark for assessing the quality of reflectance models on a dataset of high quality material measurements obtained with a commercial appearance scanner. The dataset currently consists of 56 fabric materials which are measured as radiometrically calibrated HDR images together with a precise surface geometry. We pose a public challenge to attract further participation and spark new research. Participants evaluate their models on provided directional light and view sampling to recreate the appearance of a set of unseen images per material. The results are automatically evaluated under various image metrics and ranked in a public leaderboard. Our benchmark provides standardized testing and thus enables fair comparisons between related works. We also release baseline SVBRDF material fits.
BTF-based Material Representations: Current Challenges
(The Eurographics Association, 2013) Ruiters, R.; Klein, R.; Reinhard Klein and Holly Rushmeier
The development of Bidirectional Texture Functions (BTFs) has made it possible for a wide range of different materials to acquire their characteristic appearance from a real-world counterpart and reproduce it faithfully in a high-quality rendering, that is hard to distinguish from an actual photograph. However, they have not yet found wide-spread use in practical applications. In this paper, we discuss, from our point of view, the main reasons for this and which unanswered questions and challenges for future research in this area remain. We focus on three different aspects: How can BTFs be measured and represented more efficiently? How can they be edited intuitively? And finally, can we find a perceptual difference metric between materials?
Comparative Study of Layered Material Models
(The Eurographics Association, 2019) Bati, Mégane; Pacanowski, Romain; Barla, Pascal; Klein, Reinhard and Rushmeier, Holly
The accurate reproduction of layered materials is an important part of physically-based rendering applications. Since no exact analytical model exists for any configuration of layer stacks, available models make approximations. In this paper, we propose to evaluate them with a numerical approach: we simulate BRDFs and BTDFs for layered materials in order to compare existing models against a common reference. We show that: (1) no single model always outperforms the others and (2) significant differences remain between simulated and modeled materials. We analyse the reasons for these discrepancies and introduce immediate corrections.
Experimental Analysis of BSDF Models
(The Eurographics Association, 2017) Kurt, Murat; Reinhard Klein and Holly Rushmeier
The Bidirectional Scattering Distribution Function (BSDF) describes the appearance of an optically thin, translucent material by its interaction with light at a surface point. Various BSDF models have been proposed to represent BSDFs. In this paper, we experimentally analyze a few of BSDF models in terms of their accuracy to represent measured BSDFs, their required storage sizes and computation times. To make a fair comparison of BSDF models, we measured three samples of optically thin, translucent materials (hunter douglas, orange glass, structured glass) by using pgII gonio-photometer. Based on rendered images, required storage sizes and computation times, we compare the performance of the BSDF models. We show that datadriven BSDF models give a more accurate representation of measured BSDFs, while data-driven BSDF models require much more storage sizes and computation times.We also show that BSDF measurements from highly anisotropic translucent materials can not be expressed by an analytical BSDF model visually correctly.
Exploring Material Recognition for Estimating Reflectance and Illumination From a Single Image
(The Eurographics Association, 2016) Weinmann, Michael; Klein, Reinhard; Reinhard Klein and Holly Rushmeier
In this paper, we propose a novel approach for recovering illumination and reflectance from a single image. Our approach relies on the assumption that the surface geometry has already been reconstructed and a-priori knowledge in form of a database of digital material models is available. The first step of our technique consists in recognizing the respective material in the image using synthesized training data based on the given material database. Subsequently, the illumination conditions are estimated based on the recognized material and the surface geometry. Using this novel strategy we demonstrate that reflectance and illumination can be estimated reliably for several materials that are beyond simple Lambertian surface reflectance behavior because of exhibiting mesoscopic effects such as interreflections and shadows.
A Genetic Algorithm Based Heterogeneous Subsurface Scattering Representation
(The Eurographics Association, 2020) Kurt, Murat; Klein, Reinhard and Rushmeier, Holly
In this paper, we present a novel heterogeneous subsurface scattering (sss) representation, which is based on a combination of Singular Value Decomposition (SVD) and genetic optimization techniques. To find the best transformation that is applied to measured subsurface scattering data, we use a genetic optimization framework, which tries various transformations to the measured heterogeneous subsurface scattering data to find the fittest one. After we apply the best transformation, we compactly represent measured subsurface scattering data by separately applying the SVD per-color channel of the transformed profiles. In order to get a compact and accurate representation, we apply the SVD on the model errors, iteratively. We validate our approach on a range of optically thick, real-world translucent materials. It's shown that our genetic algorithm based heterogeneous subsurface scattering representation achieves greater visual accuracy than alternative techniques for the same level of compression.
Geometric Accuracy Analysis of Stationary BTF Gonioreflectometers
(The Eurographics Association, 2015) Havran, V.; Reinhard Klein and Holly Rushmeier
The accurate BTF data representation requires specialized measurement gantries, some of them designed as gonireflectometers. These consist of an illumination source and a camera mounted on two robotic arms, one degree of freedom possibly achieved by rotation stage which a measured sample is mounted on. While there are several variations of the gonioreflectometer gantry, the principle of all remains the same, positioning directly the illumination and detector on a hemispherical surface over a sample. We analyze the positioning error of such gonioreflectometers. The input parameters are the required spatial resolution of a BTF sample and the distance between the camera used as a detector and the BTF sample. Our analysis confirms that the requirements for mechatronic actuators for the positioning of the sample and arms are very high and near the limit of state of the art technology.
Grand Challenges: Material Models in the Automotive Industry
(The Eurographics Association, 2013) Schregle, R.; Denk, C.; Slusallek, P.; Glencross, M.; Reinhard Klein and Holly Rushmeier
Material reflectance definitions are core to high fidelity visual simulation of objects within a compelling 3D scene. In the automotive industry these are used across the entire business process: from conceptualisation of a new product range, through to the final sale. However, current state-of-the-art of material representations leave much to be desired for fast and practical deployment in the industry. Even after decades of research and development, there are no interoperable standards for material models to facilitate exchange between applications. A large discrepancy also exists between the quality of material models used (and indeed the quality at which they can be displayed) across the spectrum of use-cases within the industry. Focussing on the needs of the Automotive Industry, in this position paper, we summarise the main issues that limit the effective use of material models. Furthermore, we outline specific solutions we believe could be investigated in order to address this problem. This paper is the result of a review conducted in conjunction with several key players in the automotive field.
Image-based Remapping of Material Appearance
(The Eurographics Association, 2017) Sztrajman, Alejandro; Krivánek, Jaroslav; Wilkie, Alexander; Weyrich, Tim; Reinhard Klein and Holly Rushmeier
Digital 3D content creation requires the ability to exchange assets across multiple software applications. For many 3D asset types, standard formats and interchange conventions are available. For material definitions, however, inter-application exchange is still hampered by different software packages supporting different BRDF models. To make matters worse, even if nominally identical BRDF models are supported, these often differ in their implementation, due to optimisations and safeguards in individual renderers. To facilitate appearance-preserving translation between different BRDF models whose precise implementation is not known (arguably the standard case with commercial systems), we propose a robust translation scheme which leaves BRDF evaluation to the targeted rendering system, and which expresses BRDF similarity in image space. As we will show, even naïve applications of a nonlinear fit which uses such an image space residual metric work well in some cases; however, it does suffer from instabilities for certain material parameters. We propose strategies to mitigate these instabilities and perform reliable parameter remappings between differing BRDF definitions. We report on experiences with this remapping scheme, both with respect to robustness and visual differences of the fits.
Improving Spectral Upsampling with Fluorescence
(The Eurographics Association, 2020) König, Lars; Jung, Alisa; Dachsbacher, Carsten; Klein, Reinhard and Rushmeier, Holly
Modern photorealistic rendering simulates spectral behaviour of light. Since many assets are still created in different RGB color spaces, spectral upsampling of the RGB colors to a spectral representation is required to use them in a spectral renderer. Limiting the upsampled spectra to physically valid and natural, i.e. smooth, spectra results in a more realistic image, but decreases the size of the gamut of colors that can be recreated. In order to upsample wide gamut color spaces with colors outside the gamut of physically valid reflectance spectra, a previous approach added fluorescence to create accurate and physically valid representations. We extend this approach to increase the realism and accuarcy while considering memory and computation time.
Linear Transport Theory and Applications to Rendering
(The Eurographics Association, 2014) Jakob, Wenzel; Reinhard Klein and Holly Rushmeier
In this talk, I will give an overview of one-dimensional Linear Transport Theory, which concerns itself with the study of random scattering and absorption processes and the inference of large-scale behavior from simple local scattering models. Research over the last 75 years has led to a rich toolbox of solution techniques for these types of problems, including Monte Carlo, Diffusion Theory, H-functions, Discrete Ordinates, and the Adding-Doubling method. I will give an intuitive overview of each of these techniques and discuss advantages and disadvantages. Following this, I will discuss how this problem is relevant to rendering, where it leads to a flexible and efficient method for rendering general layered materials.
The Material Definition Language
(The Eurographics Association, 2015) Kettner, L.; Raab, M.; Seibert, D.; Jordan, J.; Keller, A.; Reinhard Klein and Holly Rushmeier
We introduce the physically-based Material Definition Language (MDL). Based on the principle of strictly separating material definition and rendering algorithms, each MDL material is applicable across different rendering paradigms ranging from realtime over interactive solutions to advanced light transport simulation.
Neural Appearance Synthesis and Transfer
(The Eurographics Association, 2019) Mazlov, Ilya; Merzbach, Sebastian; Trunz, Elena; Klein, Reinhard; Klein, Reinhard and Rushmeier, Holly
Appearance acquisition is a challenging problem. Existing approaches require expensive hardware and acquisition times are long. Alternative ''in-the-wild'' few-shot approaches provide a limited reconstruction quality. Furthermore, there is a fundamental tradeoff between spatial resolution and the physical sample dimensions that can be captured in one measurement. In this paper, we investigate how neural texture synthesis and neural style transfer approaches can be applied to generate new materials with high spatial resolution from high quality SVBRDF measurements. We perform our experiments on a new database of measured SVBRDFs.
On the Advancement of BTF Measurement on Site
(The Eurographics Association, 2018) Havran, Vlastimil; Hosek, Jan; Nemcova, Sarka; Cap, Jiri; Reinhard Klein and Holly Rushmeier
We present our progress to the on-site measurement of the spatially varying surface reflectance represented by bidirectional texture function (BTF). This requires a physical realization of a portable instrument that can be brought to the sample, outside the laboratory. We discuss our motivation, the main issues, and challenges for making such an instrument possible. We focus on the design of the mechanical parts that are required for an easy manipulation of the device on site and describe our experiences with the instrument in practice. The design uses a miniaturized rotary light stage. It allows for measurement of HDR images with the acquisition rate of 1000 HDR images per minute, where one HDR image consists of 4 individual exposures.
Practical Experiences with Using Autocollimator for Surface Reflectance Measurement
(The Eurographics Association, 2016) Havran, Vlastimil; Reinhard Klein and Holly Rushmeier
We present our experiences with using an autocollimator to set up the surface reflectance measurement for both BRDF and BTF. Assuming the measured material appearance is put on a locally flat surface, the autocollimator allows us to set the perpendicularity of the measured sample in stationary measurement setups. The principle works also vice versa, we can align the measurement setup against a stationary sample for on-site measurements. The autocollimator requires to use a collimated beam of light, a mirror, a beam splitter, and a detector. We describe the autocollimator principle, problems, and the issues involved when using an autocollimator for surface reflectance measurement setups.
Reducing Anisotropic BSDF Measurement to Common Practice
(The Eurographics Association, 2014) Ward, Greg; Kurt, Murat; Bonneel, Nicolas; Reinhard Klein and Holly Rushmeier
We address the problem of measuring and representing reflection and transmission for anisotropic materials without relying on mathematical models or a large sample database. By eliminating assumptions of material behavior, we arrive at a general method that works for any surface class, from metals to fabrics, fritted glazing, and prismatic films. To make data gathering practical, we introduce a robust analysis method that interpolates a sparse set of incident angle measurements to obtain a continuous function over the full 4-D domain. We then convert this interpolant to a standard representation tailored for efficient rendering and supported by a common library that facilitates data sharing. We conclude with some remaining challenges to making anisotropic BSDF measurements truly practical for rendering
A Simple Diffuse Fluorescent BBRRDF Model
(The Eurographics Association, 2018) Jung, Alisa; Hanika, Johannes; Marschner, Steve; Dachsbacher, Carsten; Reinhard Klein and Holly Rushmeier
Fluorescence - the effect of a photon being absorbed at one wavelength and re-emitted at another - is present in many common materials such as clothes and paper. Yet there has been little research in rendering or modeling fluorescent surfaces. We discuss the design decisions leading to a simple model for a diffuse fluorescent BBRRDF (bispectral bidirectional reflection and reradiation distribution function). In contrast to reradiation matrix based models our model is continuous in wavelength space. It can be parameterized by artificially designed spectra as well as by many publicly available physical measurements. It combines fluorescence and non-fluorescent reflectance, as most real-world materials are not purely fluorescent but also reflect some light without changing its wavelength. With its simple parameterization the BBRRDF is intended as a starting point for any physically based spectral rendering system aiming to simulate fluorescence. To that end we show how to continuously sample both incident and exitant wavelengths from our BBRRDF which makes it suitable for bidirectional transport, and we discuss energy and photon conservation in the context of fluorescence.
Spectral Rendering with the Bounded MESE and sRGB Data
(The Eurographics Association, 2019) Peters, Christoph; Merzbach, Sebastian; Hanika, Johannes; Dachsbacher, Carsten; Klein, Reinhard and Rushmeier, Holly
In a recent journal paper, we introduced a technique to represent reflectance spectra by an arbitrary number of Fourier coefficients. As a special case, we converted tristimulus data to three Fourier coefficients. After summarizing this work, we introduce the Fourier sRGB color space. It is defined in terms of Fourier coefficients but designed to behave similar to sRGB. Textures stored in Fourier sRGB support efficient spectral rendering but can be compressed with techniques designed for sRGB textures. Compression errors are similar to sRGB.
Statistical Characterization of Surface Reflectance
(The Eurographics Association, 2014) Havran, Vlastimil; Sbert, Mateu; Reinhard Klein and Holly Rushmeier
The classification of surface reflectance functions as diffuse, specular, and glossy has been introduced by Heckbert more than two decades ago. Many rendering algorithms are dependent on such a classification, as different kinds of light transport will be handled by specialized methods, for example caustics require specular bounce or refraction. Due to the increasing wealth of surface reflectance models including those based on measured data, it has not been possible to keep such a characterization simple. Each surface reflectance model is mostly handled separately, or alternatively, the rendering algorithm restricts itself to the use of some subset of reflectance models. We suggest a characterization for arbitrary surface reflectance representation by standard statistical tools, namely normalized variance known as Squared-Coefficient-of-Variation (SCV).We show by videos that there is even a weak perceptual correspondence with the proposed reflectance characterization, when we use monochromatic surface reflectance and the images are normalized so they have the unit albedo.

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