Application Domains
New Software and Platforms
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## Section: New Results

### Illumination Simulation and Materials

#### A Physically-Based Reflectance Model Combining Reflection and Diffraction

Participant : Nicolas Holzschuch.

Figure 7. Surface micro-geometry contributes to its visible aspect (material reflectance). Two physical phenomena are acting together: reflection on micro-facets and diffraction. Our reflectance model combines them, with the proper energy repartition between them. The importance of diffraction depends on the roughness of the material. Even when it it relatively small, as for green-metallic-paint2 , it has a significant impact of the aspect of the material. Our model explains even a very difficult material like alum-bronze (middle row) as a single material.

#### A Robust and Flexible Real-Time Sparkle Effect

Participant : Beibei Wang.

We present a fast and practical procedural sparkle effect for snow and other sparkly surfaces which we integrated into a recent video game. Following from previous work, we generate the sparkle glints by intersecting a jittered 3D grid of sparkle seed points with the rendered surface. By their very nature, the sparkle effect consists of high frequencies which must be dealt with carefully to ensure an anti-aliased and noise free result (See Figure 8). We identify a number of sources of aliasing and provide effective techniques to construct a signal that has an appropriate frequency content ready for sampling at pixels at both foreground and background ranges of the scene. This enables artists to push down the sparkle size to the order of 1 pixel and achieve a solid result free from noisy flickering or other aliasing problems, with only a few intuitive tweakable inputs to manage [9].

Figure 8. Two scenes rendered with our sparkle effect

#### Capturing Spatially Varying Anisotropic Reflectance Parameters using Fourier Analysis

Participants : Nicolas Holzschuch, Alban Fichet.

Reflectance parameters condition the appearance of objects in photorealistic rendering. Practical acquisition of reflectance parameters is still a difficult problem. Even more so for spatially varying or anisotropic materials, which increase the number of samples required. We present an algorithm for acquisition of spatially varying anisotropic materials, sampling only a small number of directions. Our algorithm uses Fourier analysis to extract the material parameters from a sub-sampled signal. We are able to extract diffuse and specular reflectance, direction of anisotropy, surface normal and reflectance parameters from as little as 20 sample directions (See Figure 9). Our system makes no assumption about the stationarity or regularity of the materials, and can recover anisotropic effects at the pixel level. This work has been published at Graphics Interface 2016 [6].

Figure 9. Our acquisition pipeline: first, we place a material sample on our acquisition platform, and acquire photographs with varying incoming light direction. In a second step, we extract anisotropic direction, shading normal, albedo and reflectance parameters from these photographs and store them in texture maps. We later use these texture maps to render new views of the material.

#### Estimating Local Beckmann Roughness for Complex BSDFs

Participant : Nicolas Holzschuch.

Figure 10. Indirect lighting (exposure in b and c increased for printouts) on three test scenes rendered with different materials: (a) multilayer coated plastic material, (b) measured materials on a ring, (c) CTD material on a car. The insets show difference to reference in CIE'76 $\Delta E$. Top: single Gaussian, bottom: our local Gaussian approximation. We can render both analytic (a, c) and measured materials (b) more robustly because the local Gaussian approximation facilitates more even exploration of path space.

#### MIC based PBGI

Participant : Beibei Wang.

#### Point-Based Light Transport for Participating Media with Refractive Boundaries

Participants : Beibei Wang, Jean-Dominique Gascuel, Nicolas Holzschuch.

Illumination effects in translucent materials are a combination of several physical phenomena: absorption and scattering inside the material, refraction at its surface. Because refraction can focus light deep inside the material, where it will be scattered, practical illumination simulation inside translucent materials is difficult. In this paper, we present an a Point-Based Global Illumination method for light transport on translucent materials with refractive boundaries. We start by placing volume light samples inside the translucent material and organising them into a spatial hierarchy. At rendering, we gather light from these samples for each camera ray. We compute separately the samples contributions to single, double and multiple scattering, and add them (See Figure 11). Our approach provides high-quality results, comparable to the state of the art, with significant speed-ups (from 9$×$ to 60$×$ depending on scene c omplexity) and a much smaller memory footprint [10], [12].

Figure 11. Our algorithm (a), compared with Bi-Directional Path Tracing (BDPT) (b), Photon Mapping with Beam-Radiance Estimate (BRE) (c) and Unified Points, Beams and Paths (UPBP) (d) (e). Our algorithm is up to 60 times faster than UPBP, with similar quality. Material: olive oil, $\alpha =0.0042,0.4535,0.0995$; $\ell =9.7087,11.6279,2.7397$. For this material with low albedo $\alpha$ and large mean-free-path $\ell$, low-order scattering effects dominate.