MAM2018: Eurographics Workshop on Material Appearance Modeling

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Frontmatter: Eurographics 2018 Workshop on Material Appearance Modeling
(The Eurographics Association, 2018) Klein, Reinhard; Rushmeier, Holly; Reinhard Klein and Holly Rushmeier
A Method for Fitting Measured Car Paints to a Game Engine's Rendering Model
(The Eurographics Association, 2018) Kneiphof, Tom; Golla, Tim; Weinmann, Michael; Klein, Reinhard; Reinhard Klein and Holly Rushmeier
Car paints are visually complex materials that are of great importance for numerous real-time applications, including not only the game and movie industries but also virtual prototyping and design as well as advertisement. In addition to the creation of plausible materials by designers, more and more industrial effort is spent on capturing large databases of digitized materials. However, capturing complex reflectance characteristics of car paints involves the use of specialized, commercially available devices that come with predefined, standardized material formats. Using these digitized materials within other frameworks such as game engines is a challenging task due to the lacking compatibility of the involved rendering models. In this paper, we address these compatibility issues by fitting the available parameters of the game engine's material model to best match the appearance of the measured material.
Towards a Principled Kernel Prediction for Spatially Varying BSSRDFs
(The Eurographics Association, 2018) Elek, Oskar; Krivánek, Jaroslav; Reinhard Klein and Holly Rushmeier
While the modeling of sub-surface translucency using homogeneous BSSRDFs is an established industry standard, applying the same approach to heterogeneous materials is predominantly heuristical. We propose a more principled methodology for obtaining and evaluating a spatially varying BSSRDF, on the basis of the volumetric sub-surface structure of the simulated material. The key ideas enabling this are a simulation-data driven kernel for aggregating the spatially varying material parameters, and a structure-preserving decomposition of the sub-surface transport into a local and a global component. Our current results show significantly improved accuracy for planar materials with spatially varying scattering albedo, with added discussion about extending the approach for general geometries and full heterogeneity of the material parameters.
Image-based Fitting of Procedural Yarn Models
(The Eurographics Association, 2018) Saalfeld, Alina; Reibold, Florian; Dachsbacher, Carsten; Reinhard Klein and Holly Rushmeier
While common in real life, rendering fiber and cloth accurately is challenging. Recent fiber-based, procedural rendering approaches proved to be able to capture a great amount of details of real yarn. However, the current automatic method of fitting the model parameters is expensive and inaccessible as it relies on micro CT scans of the reference yarn. The alternative is to have an artist fit the parameters by hand, which is impractical because of the large number of parameters. We present a proof-of-concept for a purely image-based approach to fit the parameters of a procedural yarn model. Using gradient descent and pixel-based loss functions, we are able to extract a subset of the model parameters from rendered images with known parameters. The appearance of the fitted models is nearly indistinguishable from the reference images.
Perception of Car Shape Orientation and Anisotropy Alignment
(The Eurographics Association, 2018) Filip, Jiri; Kolafová, Martina; Reinhard Klein and Holly Rushmeier
The color designers are used to introduce customized product design, visually communicating the unique impression of a car. They always carefully observe harmony of color and body shape to obtain desired visual impression. This paper studies to what extent anisotropic appearance improves a visual impression of a car body beyond a standard isotropic one. To address this challenge, we ran several psychophysical studies identifying the proper alignment of an anisotropic axis over a car body. We have shown that subjects preferred an anisotropy axis orthogonal to car body orientation and that the majority of subjects found the anisotropic appearance more visually appealing than the isotropic one.
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.
Iso Photographic Rendering
(The Eurographics Association, 2018) Porral, Philippe; Lucas, Laurent; Muller, Thomas; Randrianandrasana, Joël; Reinhard Klein and Holly Rushmeier
In the field of computer graphics, the simulation of the visual appearance of materials requires an accurate computation of the light transport equation. Consequently, material models need to take into account various factors which may influence the spectral radiance perceived by the human eye. Though numerous relevant studies on the reflectance properties of materials have been conducted to date, environment maps used to simulate visual behaviors remain chiefly trichromatic. Whereas questions regarding the accurate characterization of natural lighting have been raised for some time, there are still no real sky environment maps that include both spectral radiance and polarization data. Under these conditions the simulations carried out are approximate and therefore insufficient for the industrial world where investment-sensitive decisions are often made based on these very calculations.
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.
Towards Practical Rendering of Fiber-Level Cloth Appearance Models
(The Eurographics Association, 2018) Alejandre, Adrian; Aliaga, Carlos; Marco, Julio; Jarabo, Adrian; Muñoz, Adolfo; Reinhard Klein and Holly Rushmeier
Accurate representation of realistic cloth appearance is of high importance in many industry fields such as entertainment and textile design. However, microstructure of fibers and their optical properties generate very complex lighting effects, often not reproduced by empirical and theoretical models. In contrast, data-driven appearance models obtained with simulations on explicit representations of fibers have proved to yield accurate cloth appearance, but resulting in discretized distribution functions that require costly precomputations, massive storage, and expensive evaluation in render time. Finding efficient representations for these models is therefore of key importance to find good trade-offs between accuracy and affordable computational costs. In this work we explore the use of different analytical models to represent these data-driven distributions, which arises as a promising middle-ground solution to this problem with benefits in both storage, computational cost, and affordable generation of new fiber appearance models. We base this analysis on our recent work where we provide highly detailed tabulations of different types of cloth fibers appearance. We analyze the spectral component of different fiber appearance functions, and observe that just ten levels of spherical harmonics are sufficient to represent the appearance many smooth fibers. We also propose a generic method to fit Gaussian Mixture Models to massively tabulated appearance functions, reducing storage costs from hundreds of MBs to a few KBs, and producing equivalent results 40 times faster. We finally analyze how interpolations in the space of fibers absorption can be exploited to generate novel fiber appearance functions without requiring costly brute-force precomputations.
ICL Multispectral Light Stage: Building a Versatile LED Sphere with Off-the-shelf Components
(The Eurographics Association, 2018) Kampouris, Christos; Ghosh, Abhijeet; Reinhard Klein and Holly Rushmeier
We describe the design and implementation of a versatile multispectral light stage (LED sphere) consisting of 168 RGB and color temperature controllable white (W+) lamps, respectively. The light stage is powered with two sets of off-the-shelf programmable MR16 LED lamps producing RGB and color temperature controllable white (2700K - 5700K) illumination. The design is heavily inspired by various USC-ICT light stages, particularly Light Stages 3, 5 and X. However, unlike a typical geodesic (subdivided icosahedron) dome structure, the structure of the LED sphere has been fabricated along spherical coordinates with latitude-longitude profiles for a simplified wiring and control layout of the LED lamps, and for simplifying polarization of incident illumination. These design decisions facilitate construction while providing a versatile solution for a variety of applications including reflectance capture, image-based lighting reproduction, and multiview facial geometry and appearance acquisition.
Towards Physically Based Material Appearance in the Thermal Infrared Spectrum: A Short Survey
(The Eurographics Association, 2018) Haraké, Laura; Burkard, Eva; Reinhard Klein and Holly Rushmeier
In the context of photorealistic rendering, global illumination mainly relies on material models from the visible spectrum, whereas thermal infrared signatures receive only little attention. This paper outlines physical principles for determining the temperature distributions within a 3D scene and with it the overall radiance reaching an IR sensor. Hitherto existing approaches for physically based rendering are surveyed. Supposing that those methods still can benefit from recent rendering concepts in visible spectrum, we point out possible transfers.
Deep Dual Loss BRDF Parameter Estimation
(The Eurographics Association, 2018) Boss, Mark; Groh, Fabian; Herholz, Sebastian; Lensch, Hendrik P. A.; Reinhard Klein and Holly Rushmeier
Surface parameter estimation is an essential field in computer games and movies. An exact representation of a real-world surface allows for a higher degree of realism. Capturing or artistically creating these materials is a time-consuming process. We propose a method which utilizes an encoder-decoder Convolutional Neural Network (CNN) to extract parameters for the Bidirectional Reflectance Distribution Function (BRDF) automatically from a sparse sample set. This is done by implementing a differentiable renderer, which allows for a loss backpropagation of rendered images. This photometric loss is essential because defining a numerical BRDF distance metric is difficult. A second loss is added, which compares the parameters maps directly. Therefore, the statistical properties of the BRDF model are learned, which reduces artifacts in the predicted parameters. This dual loss principal improves the result of the network significantly. Opposed to previous means this method retrieves information of the whole surface as spatially varying BRDF (SVBRDF) parameters with a sufficiently high resolution for intended real-world usage. The capture process for materials only requires five known light positions with a fixed camera position. This reduces the scanning time drastically, and a material sample can be obtained in seconds with an automated system.

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