Material Appearance Modeling
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Browsing Material Appearance Modeling by Subject "I.3.3 [Computer Graphics]"
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Item 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 RushmeierThe 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.Item Deep Dual Loss BRDF Parameter Estimation(The Eurographics Association, 2018) Boss, Mark; Groh, Fabian; Herholz, Sebastian; Lensch, Hendrik P. A.; Reinhard Klein and Holly RushmeierSurface 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.Item Diffraction Prediction in HDR Measurements(The Eurographics Association, 2017) Lucat, Antoine; Hegedus, R.; Pacanowski, Romain; Reinhard Klein and Holly RushmeierModern 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.Item Fresnel Equations Considered Harmful(The Eurographics Association, 2019) Hoffman, Naty; Klein, Reinhard and Rushmeier, HollyMicrofacet 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.Item Image-based Fitting of Procedural Yarn Models(The Eurographics Association, 2018) Saalfeld, Alina; Reibold, Florian; Dachsbacher, Carsten; Reinhard Klein and Holly RushmeierWhile 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.Item Image-based Remapping of Material Appearance(The Eurographics Association, 2017) Sztrajman, Alejandro; Krivánek, Jaroslav; Wilkie, Alexander; Weyrich, Tim; Reinhard Klein and Holly RushmeierDigital 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.Item Isotropic BRDF Measurements with Quantified Uncertainties(The Eurographics Association, 2016) Hegedus, R.; Lucat, A.; Redon, J.; Pacanowski, R.; Reinhard Klein and Holly RushmeierImage-based BRDF measurements on spherical material samples present a great opportunity to shorten significantly the acquisition time with respect to more traditional, non-multiplexed measurement methods for isotropic BRDFs. However, it has never been analyzed deeply, what measurement accuracy can be achieved in such a setup; what are the main contributing uncertainty factors and how do they relate to calibration procedures. In this paper we present a new set of isotropic BRDF measurements with their radiometric and geometric uncertainties acquired within such an imaging setup.We discuss the most prominent optical phenomena that a ect measurement accuracy and pave the way for more thorough uncertainty analysis in forthcoming image-based BRDF measurements. Our newly acquired data with their quantified uncertainties will be helpful for comparing the quality and accuracy of the di erent experimental setups and for designing other such image-based BRDF measurement devices.Item Linear Transport Theory and Applications to Rendering(The Eurographics Association, 2014) Jakob, Wenzel; Reinhard Klein and Holly RushmeierIn 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.Item The MAM2014 Sample Set(The Eurographics Association, 2014) Rushmeier, Holly; Reinhard Klein and Holly RushmeierModeling the material appearance of physical materials requires access to the materials. Sets of identical physical material models were prepared for distribution at the workshop on material appearance modeling 2014 (MAM2014). The sample set is intended to facilitate the comparison of measurements and models from different laboratories and psychophysical experiments comparing simulated and physical appearance.Item Rendering Transparent Materials with a Complex Refractive Index: Semi-conductor and Conductor Thin Layers(The Eurographics Association, 2019) Gerardin, Morgane; Holzschuch, Nicolas; Martinetto, Pauline; Klein, Reinhard and Rushmeier, HollyDuring physical simulation of light transport, we separate materials between conductors and dielectrics. The former have a complex refractive index and are treated as opaque, the latter a real one and are treated as transparent. However, thin layers with a complex refractive index can become transparent if their thickness is small compared to the extinction coeffcient. This happens with thin metallic layers, but also with many pigments that are semiconductors: their extinction coeffcient (the imaginary part of their refractive index) is close to zero for part of the visible spectrum. Spectral effects inside these thin layers (attenuation and interference) result in dramatic color changes.Item A Simple Diffuse Fluorescent BBRRDF Model(The Eurographics Association, 2018) Jung, Alisa; Hanika, Johannes; Marschner, Steve; Dachsbacher, Carsten; Reinhard Klein and Holly RushmeierFluorescence - 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.Item Towards a Principled Kernel Prediction for Spatially Varying BSSRDFs(The Eurographics Association, 2018) Elek, Oskar; Krivánek, Jaroslav; Reinhard Klein and Holly RushmeierWhile 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.Item What is the Reddening Effect and does it really exist?(The Eurographics Association, 2019) Clausen, Olaf; Marroquim, Ricardo; Fuhrmann, Arnulph; Weigand, Holger; Klein, Reinhard and Rushmeier, HollyThe simulation of light-matter interaction is a major challenge in computer graphics. Particularly challenging is the modelling of light-matter interaction of rough surfaces, which contain several different scales of roughness where many different scattering phenomena take place. There are still appearance critical phenomena that are weakly approximated or even not included at all by current BRDF models. One of these phenomena is the reddening effect, which describes a tilting of the reflectance spectra towards long wavelengths especially in the specular reflection. The observation that the reddening effect takes place on rough surfaces is new and the characteristics and source of the reddening effect have not been thoroughly researched and explained. Furthermore, it was not even clear whether the reddening really exists or the observed effect resulted from measurement errors. In this work we give a short introduction to the reddening effect and show that it is indeed a property of the material reflectance function, and does not originate from measurement errors or optical aberrations.