Material Appearance Modeling

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Estimating Homogeneous Data-driven BRDF Parameters from a Reflectance Map under Known Natural Lighting
(The Eurographics Association, 2019) Cooper, Victoria L.; Bieron, James C.; Peers, Pieter; Klein, Reinhard and Rushmeier, Holly
In this paper we demonstrate robust estimation of the model parameters of a fully-linear data-driven BRDF model from a reflectance map under known natural lighting. To regularize the estimation of the model parameters, we leverage the reflectance similarities within a material class. We approximate the space of homogeneous BRDFs using a Gaussian mixture model, and assign a material class to each Gaussian in the mixture model. Next, we compute a linear solution per material class. Finally, we select the best candidate as the final estimate. We demonstrate the efficacy and robustness of our method using the MERL BRDF database under a variety of natural lighting conditions.
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.
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.
On Visual Attractiveness of Anisotropic Effect Coatings
(The Eurographics Association, 2019) Filip, Jiří; Kolafová, Martina; Klein, Reinhard and Rushmeier, Holly
With the global trend in customer preference towards achromatic car colors, color designers in coating industry strive to create novel design-critical appearances based on novel effect pigments. At microscopic scale, the pigment particles allow to create specific optical effects like sparkle under directed lighting along with a specific texture in diffuse lighting, while at a macroscopic scale they create the appearance of angle-dependent color and a strong luminance contrast. Although individual particles in effect coatings exhibit anisotropic behavior, the majority of effect coatings exhibit isotropic appearance at a macroscopic scale due to a random orientation of the particles which can be explained with the manufacturing process of the coating. This paper demonstrates an visual appearances achievable by using anisotropic effect coatings based on magnetic pigments. In a psychophysical study, we assessed visual attractiveness of these coatings on a car-like shape for different viewing angles.
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.
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.

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