Rendering - Experimental Ideas & Implementations 2016

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SR Experimental Ideas and Implementations 2016: Frontmatter
(Eurographics Association, 2016) Elmar Eisemann; Eugene Fiume
Fast Shadow Map Rendering for Many-Lights Settings
(The Eurographics Association, 2016) Selgrad, Kai; Müller, Jonas; Reintges, Christian; Stamminger, Marc; Elmar Eisemann and Eugene Fiume
In this paper we present a method to efficiently cull large parts of a scene prior to shadow map computations for many-lights settings. Our method is agnostic to how the light sources are generated and thus works with any method of light distribution. Our approach is based on previous work in culling for ray traversal to speed up area light sampling. Applied to shadow mapping our method works for high- and low-resolution shadow maps and, in contrast to previous work on many-lights rendering, does neither entail scene approximations nor imposes limits on light range, while still providing significant gains in performance. In contrast to standard culling methods shadow map rendering itself is sped up by a factor of 1.5 to 8.6 while the speedup of shadow map rendering, lookup and shading together ranges from 1.1 to 4.2.
Single-shot Layered Reflectance Separation Using a Polarized Light Field Camera
(The Eurographics Association, 2016) Kim, Jaewon; Izadi, Shahram; Ghosh, Abhijeet; Elmar Eisemann and Eugene Fiume
We present a novel computational photography technique for single shot separation of diffuse/specular reflectance as well as novel angular domain separation of layered reflectance. Our solution consists of a two-way polarized light field (TPLF) camera which simultaneously captures two orthogonal states of polarization. A single photograph of a subject acquired with the TPLF camera under polarized illumination then enables standard separation of diffuse (depolarizing) and polarization preserving specular reflectance using light field sampling. We further demonstrate that the acquired data also enables novel angular separation of layered reflectance including separation of specular reflectance and single scattering in the polarization preserving component, and separation of shallow scattering from deep scattering in the depolarizing component. We apply our approach for efficient acquisition of facial reflectance including diffuse and specular normal maps, and novel separation of photometric normals into layered reflectance normals for layered facial renderings. We demonstrate our proposed single shot layered reflectance separation to be comparable to an existing multi-shot technique that relies on structured lighting while achieving separation results under a variety of illumination conditions.
Subdivision Next-Event Estimation for Path-Traced Subsurface Scattering
(The Eurographics Association, 2016) Koerner, David; Novak, Jan; Kutz, Peter; Habel, Ralf; Jarosz, Wojciech; Elmar Eisemann and Eugene Fiume
We present subdivision next-event estimation (SNEE) for unbiased Monte Carlo simulation of subsurface scattering. Our technique is designed to sample high frequency illumination through geometrically complex interfaces with highly directional scattering lobes enclosing a scattering medium. This case is difficult to sample and a common source of image noise. We explore the idea of exploiting the degree of freedom given by path vertices within the interior medium to find two-bounce connections to the light that satisfy the law of refraction. SNEE first finds a surface point by tracing a probe ray and then performs a subdivision step to place an intermediate vertex within the medium according to the incoming light at the chosen surface point. Our subdivision construction ensures that the path will connect to the light while obeying Fermat's principle of least time. We discuss the details of our technique and demonstrate the benefits of integrating SNEE into a forward path tracer.
A Robust and Flexible Real-Time Sparkle Effect
(The Eurographics Association, 2016) Wang, Beibei; Bowles, How; Elmar Eisemann and Eugene Fiume
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. 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.
Point-Based Light Transport for Participating Media with Refractive Boundaries
(The Eurographics Association, 2016) Wang, Beibei; Gascuel, Jean-Dominique; Holzschuch, Nicolas; Elmar Eisemann and Eugene Fiume
Illumination e ects 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 di cult. 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. Our approach provides high-quality results, comparable to the state of the art, with significant speed-ups (from 9x to 60x depending on scene complexity) and a much smaller memory footprint.
Shape Depiction for Transparent Objects with Bucketed k-Buffer
(The Eurographics Association, 2016) Murray, David; Baril, Jerome; Granier, Xavier; Elmar Eisemann and Eugene Fiume
Shading techniques are useful to deliver a better understanding of object shapes. When transparent objects are involved, depicting the shape characteristics of each surface is even more relevant. In this paper, we propose a method for rendering transparent scenes or objects using classical tools for shape depiction in real time. Our method provides an efficient way to compute screen space curvature on transparent objects by using a novel screen space representation of a scene derived from Order Independent Transparency techniques. Moreover, we propose a customizable stylization that modulates the transparency per fragment, according to its curvature and its depth, which can be adapted for various kinds of applications.
4D-rasterization for Fast Soft Shadow Rendering
(The Eurographics Association, 2016) Wang, Lili; Zhao, Qi; Meng, Chunlei; Popescu, Voicu; Elmar Eisemann and Eugene Fiume
This paper describes an algorithm for rendering soft shadows efficiently by generalizing conventional triangle projection and rasterization from 2D to 4D. The rectangular area light source is modeled with a point light source that translates with two degrees of freedom. This generalizes the projection of triangles and of output image samples, as seen from the light, to the locus of projections as the light translates. The generalized projections are rasterized to determine a conservative set of sample/ triangle pairs, which are then examined to derive light occlusion masks for each sample. The algorithm is exact in the sense that each element of the occlusion mask of a sample is computed accurately by considering all potentially blocking triangles. The algorithm does not require any type of precomputation so it supports fully dynamic scenes. We have tested our algorithm on several scenes to render complex soft shadows accurately at interactive rates.
Deep Partitioned Shadow Volumes Using Stackless and Hybrid Traversals
(The Eurographics Association, 2016) Mora, Frédéric; Gerhards, Julien; Aveneau, Lilian; Ghazanfarpour, Djamchid; Elmar Eisemann and Eugene Fiume
Computing accurate hard shadows is a difficult problem in interactive rendering. Previous methods rely either on Shadow Maps or Shadow Volumes. Recently Partitioned Shadow Volumes (PSV) has been introduced. It revisits the old Shadow Volumes Binary Tree Space Partitioning algorithm, leading to a practicable and efficient technique. In this article, we analyze the PSV query algorithm and identify two main drawbacks: First, it uses a stack which is not GPU friendly; its size must be small enough to reduce the register pressure, but large enough to avoid stack overflow. Second, PSV struggles with configurations involving significant depth complexity, especially for lit points. We solve these problems by adding a depth information to the PSV data structure, and by designing a stackless query. In addition, we show how to combine the former PSV query with our stackless solution, leading to a hybrid technique taking advantage of both. This eliminates any risk of stack overflow, and our experiments demonstrate that these improvements accelerate the rendering time up to a factor of 3.
A General Micro-flake Model for Predicting the Appearance of Car Paint
(The Eurographics Association, 2016) Ergun, Serkan; Önel, Sermet; Ozturk, Aydin; Elmar Eisemann and Eugene Fiume
We present an approximate model for predicting the appearance of car paint from its paint composition. Representing the appearance of car paint is not trivial because of its layered structure which is composed of anisotropic scattering media. The Radiative Transfer Equation (RTE) is commonly used to represent the multiple scattering for the underlying structures. A number of techniques including the Monte Carlo approach, the discrete ordinates, the adding-doubling method, the Eddington approximation, as well as the 2-stream and diffusion approximations have been proposed so far to improve visualization accuracy. Each of these techniques hold advantages over the others when their appropriate conditions are met. The adding-doubling method, in particular, is recognized to be computationally simple and accurate. Jakob et al. [JAM 10] has generalized the RTE for anisotropic scattering structures and proposed to use a micro-flake model based on double-sided specularly reflecting flakes. They also developed an anisotropic diffusion approximation to solve the corresponding RTE. In this paper, considering the translucent micro-flakes we proposed to use a modified version of the model which was developed by Jakob et al. We utilized the adding-doubling method instead of the diffusion-approximation for the new micro-flake model. The proposed approach also provided a good ground for data compression used in the evaluation of RTE. Empirical comparisons have been made to assess the accuracy and computational efficiency of the proposed model. Based on the sample data, we showed that our model provides visually satisfactory results for the appearance of multi-layered car paint.
Bi-Directional Polarised Light Transport
(The Eurographics Association, 2016) Mojzík, Michal; Skřivan, Tomáš; Wilkie, Alexander; Křivánek, Jaroslav; Elmar Eisemann and Eugene Fiume
While there has been considerable applied research in computer graphics on polarisation rendering, no principled investigation of how the inclusion of polarisation information affects the mathematical formalisms that are used to describe light transport algorithms has been conducted so far. Simple uni-directional rendering techniques do not necessarily require such considerations: but for modern bi-directional light transport simulation algorithms, an in-depth solution is needed. In this paper, we first define the transport equation for polarised light based on the Stokes Vector formalism. We then define a notion of polarised visual importance, and we show that it can be conveniently represented by a 4 4 matrix, similar to the Mueller matrices used to represent polarised surface reflectance. Based on this representation, we then define the adjoint transport equation for polarised importance. Additionally, we write down the path integral formulation for polarised light, and point out its salient differences from the usual formulation for light intensities. Based on the above formulations, we extend some recently proposed advanced light transport simulation algorithms to support polarised light, both in surface and volumetric transport. In doing that, we point out optimisation strategies that can be used to minimise the overhead incurred by including polarisation support into such algorithms.
Local Shape Editing at the Compositing Stage
(The Eurographics Association, 2016) Zubiaga, Carlos Jorge; Guennebaud, Gael; Vergne, Romain; Barla, Pascal; Elmar Eisemann and Eugene Fiume
Modern compositing software permit to linearly recombine different 3D rendered outputs (e.g., diffuse and reflection shading) in post-process, providing for simple but interactive appearance manipulations. Renderers also routinely provide auxiliary buffers (e.g., normals, positions) that may be used to add local light sources or depth-of-field effects at the compositing stage. These methods are attractive both in product design and movie production, as they allow designers and technical directors to test different ideas without having to re-render an entire 3D scene. We extend this approach to the editing of local shape: users modify the rendered normal buffer, and our system automatically modifies diffuse and reflection buffers to provide a plausible result. Our method is based on the reconstruction of a pair of diffuse and reflection prefiltered environment maps for each distinct object/material appearing in the image. We seamlessly combine the reconstructed buffers in a recompositing pipeline that works in real-time on the GPU using arbitrarily modified normals.
Additional Progress Towards the Unification of Microfacet and Microflake Theories
(The Eurographics Association, 2016) Dupuy, Jonathan; Heitz, Eric; d'Eon, Eugene; Elmar Eisemann and Eugene Fiume
We study the links between microfacet and microflake theories from the perspective of linear transport theory. In doing so, we gain additional insights, find several simplifications and touch upon important open questions as well as possible paths forward in extending the unification of surface and volume scattering models. First, we introduce a semi-infinite homogeneous exponential-free-path medium that (a) produces exactly the same light transport as the Smith microsurface scattering model and the inhomogeneous Smith medium that was recently introduced by Heitz et al, and (b) allows us to rederive all the Smith masking and shadowing functions in a simple way. Second, we investigate in detail what new aspects of linear transport theory enable a volume to act like a rough surface. We show that this is mostly due to the use of non-symmetric distributions of normals and explore how the violation of this symmetry impacts light transport within the microflake volume without breaking global reciprocity. Finally, we argue that the surface profiles that would be consistent with very rough Smith microsurfaces have geometrically implausible shapes. To overcome this, we discuss an extension of Smith theory in the volume setting that includes NDFs on the entire sphere in order to produce a single unified reflectance model capable of describing everything from a smooth flat mirror all the way to a semi-infinite isotropically scattering medium with both low and high roughness regimes in between.
Node Culling Multi-Hit BVH Traversal
(The Eurographics Association, 2016) Gribble, Christiaan; Elmar Eisemann and Eugene Fiume
We introduce node culling multi-hit BVH traversal to enable faster multi-hit ray tracing in a bounding volume hierarchy (BVH). Existing, widely used ray tracing engines expose API features that enable implementation of multi-hit traversal without modifying their underlying-and highly optimized-BVH construction and traversal routines; however, this approach requires naive multi-hit traversal to guarantee correctness. We evaluate two low-overhead, minimally invasive, and flexible API mechanisms that enable node culling implementation entirely with user-level code, thereby leveraging existing BVH construction and traversal routines. Results show that node culling offers potentially significant improvement in multi-hit performance in a BVH for cases in which users request fewer-than-all hits.

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