MAM2019: Eurographics Workshop on Material Appearance Modeling

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https://diglib.eg.org/handle/10.2312/2632807

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Browsing MAM2019: Eurographics Workshop on Material Appearance Modeling by Subject "I.3.3 [Computer Graphics]"

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    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.
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    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, Holly
    During 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.
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    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, Holly
    The 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.