Material Appearance Modeling

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An Adaptive Metric for BRDF Appearance Matching
(The Eurographics Association, 2020) Bieron, James; Peers, Pieter; Klein, Reinhard and Rushmeier, Holly
Image-based BRDF matching is a special case of inverse rendering, where the parameters of a BRDF model are optimized based on a photograph of a homogeneous material under natural lighting. Using a perceptual image metric, directly optimizing the difference between a rendering and a reference image can provide a close visual match between the model and reference material. However, perceptual image metrics rely on image-features and thus require full resolution renderings that can be costly to produce especially when embedded in a non-linear search procedure for the optimal BRDF parameters. Using a pixel-based metric, such as the squared difference, can approximate the image error from a small subset of pixels. Unfortunately, pixel-based metrics are often a poor approximation of human perception of the material's appearance. We show that comparable quality results to a perceptual metric can be obtained using an adaptive pixel-based metric that is optimized based on the appearance similarity of the material. As the core of our adaptive metric is pixel-based, our method is amendable to imagesubsampling, thereby greatly reducing the computational cost.
ALTA: A BRDF Analysis Library
(The Eurographics Association, 2014) Belcour, Laurent; Barla, Pascal; Pacanowski, Romain; Reinhard Klein and Holly Rushmeier
In this document, we introduce ALTA, a cross platform generic open-source library for Bidirectional Reflectance Distribution Function (BRDF) analysis. Among others, ALTA permits to estimate BRDF models parameters from measured data, to perform statistical analysis and also to export BRDF data models in a wide variety of formats.
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?
Can we Grasp the Color of Translucent Objects?
(The Eurographics Association, 2024) Gigilashvili, Davit; Chowdhury, Dipayan; Hardeberg, Jon Yngve; Hardeberg, Jon Yngve; Rushmeier, Holly
While colorimetry is traditionally measuring point colors, there is an increasing need to quantify colors of 3D objects in real-world scenes. 3D objects, especially translucent ones, exhibit high spatio-temporal variation in color. This raises multiple questions on how to measure color of 3D translucent objects, how to describe their color appearance, and how to quantify color differences among them. Or are these ill-posed problems in the first place? We discuss the first steps on this topic and suggest the future directions for color and appearance research.
Challenges in Appearance Capture and Predictive Modeling of Textile Materials
(The Eurographics Association, 2017) Castillo, Carlos; Aliaga, Carlos; López-Moreno, Jorge; Reinhard Klein and Holly Rushmeier
The appearance of cloth is the result of complex light interactions within the structures present in textile materials, particularly challenging due to their multi-scale nature. In addition to the inherent complexity of cloth rendering, there is a lack of connection between computer graphics techniques and manufacturing processes followed in industry. We discuss existing techniques and pose questions about which are the right paths to follow for a better synergy between CG and textile research, including (but not restricted to): defining a standard set of properties required to predict the appearance of cloth to be manufactured; developing both acquisition techniques reliable and suitable for industrial processes and other frameworks more focused on inexpensive capturing (e.g. based on single pictures, Pantone labels); finding material representations that are robust in absence of several low-level parameters; creating a standard for color depth depending on the dye type and dying technique; developing a standard to account for post-process steps (washing, chemical treatments, etc) on the mechanical and optical properties of the textiles.
The Challenges of Relighting from Multi-View Observations
(The Eurographics Association, 2024) Siersleben, Daniel; Ortiz-Cayon, Rodrigo; Istenic, Klemen; Tomoto, Yusuke; Schaub-Meyer, Simone; Hardeberg, Jon Yngve; Rushmeier, Holly
High-quality geometry reconstruction from multi-view images with subsequent appearance decomposition into the physical shading components could enable a seamless integration of neural reconstructions into the modern rendering workflow. While 3D reconstruction techniques have steadily improved, the task of inverse rendering by decomposing an appearance into lighting effects and material properties remains fundamentally ill-posed and highly ambiguous. We show that current state-of-the-art inverse rendering approaches fail to accurately recover material properties, significantly impacting relighting quality. Furthermore, we demonstrate that existing evaluation methods, which rely on image-based metrics, do not adequately capture the reconstruction quality in novel lighting conditions. Our findings illustrate the dependence of current systems on simplified setups with predefined illumination, which are necessary to reliably disentangle light and material contributions and to ultimately achieve convincing relighting.
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.
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.
Diffraction Prediction in HDR Measurements
(The Eurographics Association, 2017) Lucat, Antoine; Hegedus, R.; Pacanowski, Romain; Reinhard Klein and Holly Rushmeier
Modern imaging techniques have proved to be very efficient to recover a scene with high dynamic range values. However, this high dynamic range can introduce star-burst patterns around highlights arising from the diffraction of the camera aperture. The spatial extent of this effect can be very wide and alters pixels values, which, in a measurement context, are not reliable anymore. To address this problem, we introduce a novel algorithm that predicts, from a closed-form PSF, where the diffraction will affect the pixels of an HDR image, making it possible to discard them from the measurement. Our results gives better results than common deconvolution techniques and the uncertainty values (convolution kernel and noise) of the algorithm output are recovered.
DOME II: A Parallelized BTF Acquisition System
(The Eurographics Association, 2013) Schwartz, C.; Sarlette, R.; Weinmann, M.; Klein, R.; Reinhard Klein and Holly Rushmeier
Bidirectional Texture Functions (BTFs) provide a realistic depiction of the appearance of many real-world materials as they contain the spatially varying light scattering behavior of the material surface. Since editing of existing BTF data is still in its early stages, materials have to be measured from real-world samples. In contrast to the related Spatially Varying BRDFs (SVBRDFs), the reflectance information encoded in a BTF also includes nonlocal scattering effects and therefore does not obey energy conservation or reciprocity. While this higher degree of freedom also contributes to an increased realism, it inadvertently calls for an extensive measurement of reflectance samples, as many regularization approaches from BRDF measurement do not apply. In this paper, we present an automated, parallelized, robust, fast and transportable setup for the acquisition of BTFs from flat samples as well as 3D objects using camera and light arrays: the DOME II. In contrast to previous camera array approaches, the present setup, which is comprised of high-quality industry grade components, overcomes several issues regarding stability, reliability and precision. It achieves a well balanced state-of-the-art acquisition performance in terms of speed and quality at reasonable costs.
The Effects of Digital Cameras Optics and Electronics for Material Acquisition
(The Eurographics Association, 2017) Holzschuch, Nicolas; Pacanowski, Romain; Reinhard Klein and Holly Rushmeier
For material acquisition, we use digital cameras and process the pictures. We usually treat the cameras as perfect pinhole cameras, with each pixel providing a point sample of the incoming signal. In this paper, we study the impact of camera optical and electronic systems. Optical system effects are modelled by the Modulation Transfer Function (MTF). Electronic System effects are modelled by the Pixel Response Function (PRF). The former is convolved with the incoming signal, the latter is multiplied with it. We provide a model for both effects, and study their impact on the measured signal. For high frequency incoming signals, the convolution results in a significant decrease in measured intensity, especially at grazing angles. We show this model explains the strange behaviour observed in the MERL BRDF database at grazing angles.
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.
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.
Fresnel Equations Considered Harmful
(The Eurographics Association, 2019) Hoffman, Naty; Klein, Reinhard and Rushmeier, Holly
Microfacet shading models in film and game production have long used a simple approximation to the Fresnel equations, published by Schlick in 1994. Recently a growing number of film studios have transitioned to using the full Fresnel reflectance equations in lieu of Schlick's approximation. This transition has been facilitated by Gulbrandsen's 2014 parameterization which uses reflectance and edge tint instead of eta and kappa. Our recent investigations have found some unexpected drawbacks to this approach. In this presentation, we will show that in the context of RGB rendering (still by far the most common modality in film production), the Fresnel equations are actually less physically principled than Schlick's approximation. In addition, they are less robust in practice and less amenable to authoring. Most surprisingly, as commonly used the Fresnel equations result in less accurate matches to measured materials, compared to Schlick's approximation. The presentation primarily discusses metal reflectance, since our investigations so far have focused on metals.
From Color to Appearance in the Real World
(The Eurographics Association, 2013) Ellens, M.; Mueller, G.; Lamy, F.; Reinhard Klein and Holly Rushmeier
X-Rite's declared ambition is to create a digital ecosystem for appearance; a daunting challenge that has many dimensions and has not been accomplished so far. In this short paper we sketch a path from color to full appearance which aims at solving this challenge.
Frontmatter: Eurographics 2015 Workshop on Material Appearance Modeling
(The Eurographics Association, 2015) Klein, Reinhard; Rushmeier, Holly; -
Frontmatter: Eurographics 2018 Workshop on Material Appearance Modeling
(The Eurographics Association, 2018) Klein, Reinhard; Rushmeier, Holly; Reinhard Klein and Holly Rushmeier

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