Rendering - Experimental Ideas & Implementations

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Practical Rendering of Thin Layered Materials with Extended Microfacet Normal Distributions
(The Eurographics Association, 2015) Guo, Jie; Qian, Jinghui; Pan, Jingui; Jaakko Lehtinen and Derek Nowrouzezahrai
We propose a practical reflectance model for rendering thin transparent layers with different sides varying in roughness and levels of gloss. To capture the effect of subsurface reflection, previous methods rely on importance sampling for each light-surface interaction. This soon becomes a computationally demanding task since a recursive sampling scheme is required to handle multiple internal reflections. In this paper, we first provide a comprehensive analysis of the relationship between the directional distribution of scattered light and the roughness of each layer boundary using joint spherical warping. Based on the analysis, we generalize the traditional microfacet theory for layered materials and introduce the extended normal distribution function (ENDF) to accurately model the behavior of subsurface reflection. With the ENDF, the number of sampling processes can be reduced to only once for each bounce of subsurface reflection. We demonstrate that our BSDF model based on the ENDF is easy to be implemented on top of Monte Carlo sampling based offline renderers and it incurs little computational overhead. Moreover, it can be also efficiently used in real-time applications with the help of GPU acceleration.
Practical Shading of Height Fields and Meshes using Spherical Harmonic Exponentiation
(The Eurographics Association, 2015) Giraud, Aude; Nowrouzezahrai, Derek; Jaakko Lehtinen and Derek Nowrouzezahrai
Interactively computing smooth shading effects from environmental lighting, such as soft shadows and glossy reflections, is a challenge in scenes with dynamic objects. We present a method to efficiently approximate these effects in scenes comprising animating objects and dynamic height fields, additionally allowing interactive manipulation of view and lighting. Our method extends spherical harmonic (SH) exponentiation approaches to support environmental shadowing from both dynamic blockers and dynamic height field geometry. We also derive analytic expressions for the view-evaluated BRDF, directly in the log-SH space, in order to support diffuse-to-glossy shadowed reflections while avoiding expensive basis-space product operations. We illustrate interactive rendering results using a hybrid, multi-resolution screen- and object-space visibility-marching algorithm that decouples geometric complexity from shading complexity.
Interactive Global Illumination Effects Using Deterministically Directed Layered Depth Maps
(The Eurographics Association, 2015) Aalund, Frederik Peter; Frisvad, Jeppe Revall; Bærentzen, Jakob Andreas; Jaakko Lehtinen and Derek Nowrouzezahrai
A layered depth map is an extension of the well-known depth map used in rasterization. Multiple layered depth maps can be used as a coarse scene representation. We develop two global illumination methods which use said scene representation. The first is an interactive ambient occlusion method. The second is an interactive singlebounce indirect lighting method based on photon differentials. All of this is implemented in a rasterization-based pipeline.
Color Clipping and Over-exposure Correction
(The Eurographics Association, 2015) Abebe, Mekides Assefa; Pouli, Tania; Kervec, Jonathan; Larabi, Chaker; Jaakko Lehtinen and Derek Nowrouzezahrai
Limitations of the camera or extreme contrast in scenes can lead to clipped areas in captured images. Irrespective of the cause, color clipping and over-exposure lead to loss of texture and detail, impacting the color appearance and visual quality of the image. We propose a new over-exposure and clipping correction method, which relies on the existing correlation between RGB channels of color images to recover clipped information. Using a novel region grouping approach, clipped regions are coherently treated both spatially and temporally. To reconstruct over-exposed areas where all channels are clipped we employ a brightness profile reshaping scheme, which aims to preserve the appearance of highlights, while boosting local brightness. Our method is evaluated using objective metrics as well as a subjective study based on an ITU standardized protocol, showing that our correction leads to improved results compared to previous related techniques. We explore several potential applications of our method, including extending to video as well as using it as a preprocessing step prior to inverse tone mapping.
Stereo from Shading
(The Eurographics Association, 2015) Chapiro, Alexandre; O'Sullivan, Carol; Jarosz, Wojciech; Gross, Markus; Smolic, Aljoscha; Jaakko Lehtinen and Derek Nowrouzezahrai
We present a new method for creating and enhancing the stereoscopic 3D (S3D) sensation without using the parallax disparity between an image pair. S3D relies on a combination of cues to generate a feeling of depth, but only a few of these cues can easily be modified within a rendering pipeline without significantly changing the content. We explore one such cue-shading stereopsis-which to date has not been exploited for 3D rendering. By changing only the shading of objects between the left and right eye renders, we generate a noticeable increase in perceived depth. This effect can be used to create depth when applied to flat images, and to enhance depth when applied to shallow depth S3D images. Our method modifies the shading normals of objects or materials, such that it can be flexibly and selectively applied in complex scenes with arbitrary numbers and types of lights and indirect illumination. Our results show examples of rendered stills and video, as well as live action footage.
MBVH Child Node Sorting for Fast Occlusion Test
(The Eurographics Association, 2015) Ogaki, Shinji; Derouet-Jourdan, Alexandre; Jaakko Lehtinen and Derek Nowrouzezahrai
Optimal BVH layout differs among ray types. To accelerate shadow rays, the use of a specialized traversal order, optionally with an additional data structure has been proposed. In this paper we show how sorting child nodes of MBVH (Multi Bounding Volume Hierarchy) improves the performance of occlusion test without changing the topology of the data structure.We introduce a cost metric suitable for MBVH which takes into account the distribution of representative rays, and prove that the cost can be minimized by sorting child nodes based on a very simple criterion. Our method is very easy to implement and requires only small amounts of storage and preprocessing time for sorting. We also demonstrate how rendering performance can be improved by up to 10% in conjunction with various algorithms.
Gradient-Domain Bidirectional Path Tracing
(The Eurographics Association, 2015) Manzi, Marco; Kettunen, Markus; Aittala, Miika; Lehtinen, Jaakko; Durand, Frédo; Zwicker, Matthias; Jaakko Lehtinen and Derek Nowrouzezahrai
Gradient-domain path tracing has recently been introduced as an efficient realistic image synthesis algorithm. This paper introduces a bidirectional gradient-domain sampler that outperforms traditional bidirectional path tracing often by a factor of two to five in terms of squared error at equal render time. It also improves over unidirectional gradient-domain path tracing in challenging visibility conditions, similarly to how conventional bidirectional path tracing improves over its unidirectional counterpart. Our algorithm leverages a novel multiple importance sampling technique and an efficient implementation of a high-quality shift mapping suitable for bidirectional path tracing. We demonstrate the versatility of our approach in several challenging light transport scenarios.
Filtering Environment Illumination for Interactive Physically-Based Rendering in Mixed Reality
(The Eurographics Association, 2015) Mehta, Soham Uday; Kim, Kihwan; Pajak, Dawid; Pulli, Kari; Kautz, Jan; Ramamoorthi, Ravi; Jaakko Lehtinen and Derek Nowrouzezahrai
Physically correct rendering of environment illumination has been a long-standing challenge in interactive graphics, since Monte-Carlo ray-tracing requires thousands of rays per pixel. We propose accurate filtering of a noisy Monte-Carlo image using Fourier analysis. Our novel analysis extends previous works by showing that the shape of illumination spectra is not always a line or wedge, as in previous approximations, but rather an ellipsoid. Our primary contribution is an axis-aligned filtering scheme that preserves the frequency content of the illumination. We also propose a novel application of our technique to mixed reality scenes, in which virtual objects are inserted into a real video stream so as to become indistinguishable from the real objects. The virtual objects must be shaded with the real lighting conditions, and the mutual illumination between real and virtual objects must also be determined. For this, we demonstrate a novel two-mode path tracing approach that allows ray-tracing a scene with image-based real geometry and mesh-based virtual geometry. Finally, we are able to de-noise a sparsely sampled image and render physically correct mixed reality scenes at over 5 fps on the GPU.
High Performance Non-linear Motion Blur
(The Eurographics Association, 2015) Guertin, Jean-Philippe; Nowrouzezahrai, Derek; Jaakko Lehtinen and Derek Nowrouzezahrai
Motion blur is becoming more common in interactive applications such as games and previsualization tools. Here, a common strategy is to approximate motion blur with an image-space post-process, and many recent approaches demonstrate very efficient and high-quality results [Sou13,GMN14]. Unfortunately, all such approaches assume underlying linear motion, and so they cannot approximate non-linear motion blur effects without significant visual artifacts.We present a new motion blur post-process that correctly treats the case of non-linear motion (in addition to linear motion) using an efficient curve-sampling scatter approach. We simulate plausible non-linear motion blur in 4ms at 1920 1080 and our approach has many desirable properties: its cost is independent of geometric complexity, it robustly estimates blurring extents to avoid typical over- and under-blurring artifacts, it supports unlimited motion magnitudes, and it is less noisy than existing techniques.
GPU-based Out-of-Core HLBVH Construction
(The Eurographics Association, 2015) Zeidan, Mahmoud; Nazmy, Taymoor; Aref, Mostafa; Jaakko Lehtinen and Derek Nowrouzezahrai
Recently the GPU has been used extensively in building indexing structures for moderately complex scenes that fit inside the GPU core. However, only few methods have been developed for constructing indexing structures for massive models larger than GPU memory. In this paper, we present an out-of-core HLBVH algorithm, a new method for constructing spatial hierarchies suitable for massive models that cannot fit into GPU device memory. A key insight of our method is how to bring and process out-of-core data blocks that do not fit into available device memory. Results show that our approach can compete with HLBVH hierarchy builder for large models on CPU. We also demonstrate the value of our algorithms in a GPU-based out-of-core path tracer that brings tree nodes and geometry into GPU core as needed, and efficiently achieve complex global illumination effects for models up to hundred million triangles.
Eurographics Symposium on Rendering - Experimental Ideas and Implementations
(Eurographics Association, 2015) Lehtinen, Jaakko; Nowrouzezahrai, Derek; -
Efficient Visibility Heuristics for kd-trees Using the RTSAH
(The Eurographics Association, 2015) Moulin, Matthias; Billen, Niels; Dutré, Philip; Jaakko Lehtinen and Derek Nowrouzezahrai
Acceleration data structures such as kd-trees aim at reducing the per-ray cost which is crucial for rendering performance. The de-facto standard for constructing kd-trees, the Surface Area Heuristic (SAH), does not take ray termination into account and instead assumes rays never hit a geometric primitive. The Ray Termination Surface Area Heuristic (RTSAH) is a cost metric originally used for determining the traversal order of the voxels for occlusion rays that takes ray termination into account. We adapt this RTSAH to building kd-trees that aim at reducing the per-ray cost of rays. Our build procedure has the same overall computational complexity and considers the same finite set of splitting planes as the SAH. By taking ray termination into account, we favor cutting off child voxels which are not or hardly visible to each other. This results in fundamentally different and more qualitative kd-trees compared to the SAH.
MatCap Decomposition for Dynamic Appearance Manipulation
(The Eurographics Association, 2015) Zubiaga, Carlos Jorge; Muñoz, Adolfo; Belcour, Laurent; Bosch, Carles; Barla, Pascal; Jaakko Lehtinen and Derek Nowrouzezahrai
In sculpting software, MatCaps (a shorthand for "Material Capture") are often used by artists as a simple and efficient way to design appearance. Similar to LitSpheres, they convey material appearance into a single image of a sphere, which can be easily transferred to an individual 3D object. Their main purpose is to capture plausible material appearance without having to specify lighting and material separately. However, this also restricts their usability, since material or lighting cannot later be modified independently. Manipulations as simple as rotating lighting with respect to the view are not possible. In this paper, we show how to decompose a MatCap into a new representation that permits dynamic appearance manipulation. We consider that the material of the depicted sphere act as a filter in the image, and we introduce an algorithm that estimates a few relevant filter parameters interactively.We show that these parameters are sufficient to convert the input MatCap into our new representation, which affords real-time appearance manipulations through simple image re-filtering operations. This includes lighting rotations, the painting of additional reflections, material variations, selective color changes and silhouette effects that mimic Fresnel or asperity scattering.
Apex Point Map for Constant-Time Bounding Plane Approximation
(The Eurographics Association, 2015) Laine, Samuli; Karras, Tero; Jaakko Lehtinen and Derek Nowrouzezahrai
We introduce apex point map, a simple data structure for constructing conservative bounds for rigid objects. The data structure is distilled from a dense k-DOP, and can be queried in constant time to determine a tight bounding plane with any given normal vector. Both precalculation and lookup can be implemented very efficiently on current GPUs. Applications include, e.g., finding tight world-space bounds for transformed meshes, determining perobject shadow map extents, more accurate view frustum culling, and collision detection.
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.

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