Abstract
This paper proposes a lightweight bidirectional scattering distribution function (BSDF) model for layered materials with anisotropic reflection and refraction properties. In our method, each layer of the materials can be described by a microfacet BSDF using an anisotropic normal distribution function (NDF). Furthermore, the NDFs of layers can be defined on tangent vector fields, which differ from layer to layer. Our method is based on a previous study in which isotropic BSDFs are approximated by projecting them onto base planes. However, the adequateness of this previous work has not been well investigated for anisotropic BSDFs. In this paper, we demonstrate that the projection is also applicable to anisotropic BSDFs and that the BSDFs are approximated by elliptical distributions using covariance matrices.
Highlights
In the last several decades, the visual quality of computer graphics has been improved significantly due to the long-standing efforts of both the research and industrial communities
While accurate representation [1, 2] and sampling of light transport paths [3] for layered materials have been proposed in the context of offline rendering, light transport in layered materials is usually approximated using analytic models in real-time rendering
The von Mises–Fisher (vMF) distributions cannot capture the heavy tails of directional distributions, which is often required for modeling metallic materials
Summary
In the last several decades, the visual quality of computer graphics has been improved significantly due to the long-standing efforts of both the research and industrial communities. Success in reflectance modeling has enabled representation of a surprisingly wide variety of real-world materials in computer graphics Among such materials, those comprising of thin layers of different material. We replace this scalar variance with a 2 × 2 covariance matrix to define the anisotropy of the distribution This extension is non-trivial because changes in the shapes of directional distributions have not been well investigated for rough boundary surfaces between layers with anisotropic scattering properties. We derive the distribution shapes for entire directional distribution by considering the coordinate transform between those for NDFs and projected directional distributions We implement this extended atomic operator for anisotropic reflections/refractions on a real-time rendering system [9] by following a publicly available implementation of the previous method [10]. The experimental results demonstrate that our extension synthesizes almost identical appearances to those obtained by offline Monte Carlo path tracing while its computational overhead from the previous method is as small as only 2.5%
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