Rendering - Experimental Ideas & Implementations 2017

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Ambient Dice
(The Eurographics Association, 2017) Iwanicki, Michal; Sloan, Peter-Pike; Matthias Zwicker and Pedro Sander
We present a family of basis functions designed to accurately and efficiently represent illumination signals on the unit sphere. The bases are built of locally supported functions, needing three to six basis functions in a given direction. This minimizes the number of memory transactions and bandwidth requirements needed for reconstruction. There are three variations of our basis. All are based on storing coefficients at the 12 vertices of an icosahedron. The first one stores the values directly, together with their directional derivatives and hybrid Bézier patches are used for interpolation. This allows one to achieve quality comparable to 3rd-5th order spherical harmonics while still requiring 27 coefficients for the reconstruction. The second variation encodes the signal in YCoCg space and uses a reduced quality, linear reconstruction for the chromaticity components - requiring only 15 coefficients while marginally reducing the quality. The third option exploits the partition of unity formed by cos2 and cos4 restricted to a hemisphere oriented along the directions of the icosahedron vertices. It uses 18 coefficients for the reconstruction, but trades the additional bandwidth requirements for simpler calculations. The quality of that version is still comparable to 3rd order spherical harmonics (SH). We name the basis Ambient Dice as a reference to both: the Ambient Cube basis - as ours is an extension of some of its properties - and the 20-sided dice commonly used in pen-and-paper role-playing games, which is an icosahedron.
Controlling and Sampling Visibility Information on the Image Plane
(The Eurographics Association, 2017) Lessig, Christian; Matthias Zwicker and Pedro Sander
Anti-aliasing on the image plane is a classic problems in computer graphics. While mip-mapping provides an efficient means to pre-filter texture information, no comparable technique exists for visibility. We address visibility-induced aliasing by exploiting that the Fourier transform of a discontinuity decays slowly only in the normal direction. Pre-filtering is thus only necessary in this direction and, after a coordinate transformation, the corresponding one dimensional problem can be solved analytically or tabulated. The resulting pre-filtered signal can be reconstructed exactly from pointwise samples and we derive corresponding sampling theorems that are tailored to the pre-filtering as well as a set of irregular sampling locations. We demonstrate our methodology for the classical Shannon-Nyquist setting but also for shift-invariant spaces where exact reconstruction kernels with significantly faster decay than the sinc-function are available. Our experimental results demonstrate that our pre-filtering is highly effective and that going beyond the Shannon-Nyquist setting reduces aliasing error further.
Eurographics Symposium on Rendering 2017 - Experimental Ideas and Implementations: Frontmatter
(Eurographics Association, 2017) Zwicker, Matthias; Sander, Pedro;
Extended Path Integral Formulation for Volumetric Transport
(The Eurographics Association, 2017) Hachisuka, Toshiya; Georgiev, Iliyan; Jarosz, Wojciech; Křivánek, Jaroslav; Nowrouzezahrai, Derek; Matthias Zwicker and Pedro Sander
We propose an extension of the path integral formulation amenable to the expression of volumetric light transport with photon beam estimates. Our main contribution is a generalization of Hachisuka et al.'s extended path space formulation [HPJ12] to light transport in participating media. Our formulation supports various point- and beam-based volumetric density estimators, unifying them with path integration in the spirit of the work by Kˇrivánek et al. [KGH 14]. One unique and useful property of our formulation is that it recasts beam-based density estimation as Monte Carlo path vertex sampling in a higher-dimensional space, rather than beam merging in a lower-dimensional space, which enables a practical algorithm for beam estimators with 3D-blur kernels. We thus establish a complementary theoretical foundation for the development of rendering algorithms using points, beams, and paths in participating media.
Frequency Based Radiance Cache for Rendering Animations
(The Eurographics Association, 2017) Dubouchet, Renaud Adrien; Belcour, Laurent; Nowrouzezahrai, Derek; Matthias Zwicker and Pedro Sander
We propose a method to render animation sequences with direct distant lighting that only shades a fraction of the total pixels. We leverage frequency-based analyses of light transport to determine shading and image sampling rates across an animation using a samples cache. To do so, we derive frequency bandwidths that account for the complexity of distant lights, visibility, BRDF, and temporal coherence during animation. We finaly apply a cross-bilateral filter when rendering our final images from sparse sets of shading points placed according to our frequency-based oracles (generally < 25% of the pixels, per frame).
Gradient-Domain Vertex Connection and Merging
(The Eurographics Association, 2017) Sun, Weilun; Sun, Xin; Carr, Nathan A.; Nowrouzezahrai, Derek; Ramamoorthi, Ravi; Matthias Zwicker and Pedro Sander
Recently, gradient-domain rendering techniques have shown great promise in reducing Monte Carlo noise and improving overall rendering efficiency. However, all existing gradient-domain methods are built exclusively on top of Monte Carlo integration or density estimation. While these methods can be effective, combining Monte Carlo integration and density estimation has been shown (in the primal domain) to more robustly handle a wider variety of light paths from arbitrarily complex scenes. We present gradient-domain vertex connection and merging (G-VCM), a new gradient-domain technique motivated by primal domain VCM. Our method enables robust gradient sampling in the presence of complex transport, such as specular-diffuse-specular paths, while retaining the denoising power and fast convergence of gradient-domain bidirectional path tracing. We show that G-VCM is able to handle a variety of scenes that exhibit slow convergence when rendered with previous gradient-domain methods.
k-d Tree Construction Designed for Motion Blur
(The Eurographics Association, 2017) Yang, Xin; Liu, Qi; Yin, Baocai; Zhang, Qiang; Zhou, Dongsheng; Wei, Xiaopeng; Matthias Zwicker and Pedro Sander
We present a k-d tree construction algorithm designed to accelerate rendering of scenes with motion blur, in application scenarios where a k-d tree is either required or desired. Our associated data structure focuses on capturing incoherent motion within the nodes of a k-d tree and improves both data structure quality and efficiency over previous methods. At build-time stage, we tracks primitives with motion that is significantly distinct from other primitives within the node, guarantee valid node references and the correctness of the data structure via primitive duplication heuristic and propagation rules. Our experiments with this hierarchy show artifact-free motion-blur rendering using a k-d tree, and demonstrate improvements against a traditional BVH with interpolation and a MSBVH structure designed to handle moving primitives, particularly in render time.
Local Quasi-Monte Carlo Exploration
(The Eurographics Association, 2017) Tessari, Lorenzo; Hanika, Johannes; Dachsbacher, Carsten; Matthias Zwicker and Pedro Sander
In physically-based image synthesis, the path space of light transport paths is usually explored by stochastic sampling. The two main families of algorithms are Monte Carlo/quasi-Monte Carlo sampling and Markov chain Monte Carlo. While the former is known for good uniform discovery of important regions, the latter facilitates efficient exploration of local effects. We introduce a hybrid sampling technique which uses quasi-Monte Carlo points to achieve good stratification in both stages: we use the Halton sequence to generate initial seed paths and rank-1 lattices for local exploration. This method avoids the hard problem of introducing QMC sequences into MCMC while still stratifying samples both globally and locally. We propose perturbation strategies that prefer dimensions close to the camera, facilitating efficient reuse of transport path suffixes. This framework provides maximum control of the sampling scheme by the programmer, which can be hard to achieve with Markov chain-based methods. We show that local QMC exploration can generate results on par with state of the art light transport sampling methods, while providing more uniform convergence, improving temporal consistency.
Material Design in Augmented Reality with In-Situ Visual Feedback
(The Eurographics Association, 2017) Shi, Weiqi; Wang, Zeyu; Sezgin, Metin; Dorsey, Julie; Rushmeier, Holly; Matthias Zwicker and Pedro Sander
Material design is the process by which artists or designers set the appearance properties of virtual surface to achieve a desired look. This process is often conducted in a virtual synthetic environment however, advances in computer vision tracking and interactive rendering now makes it possible to design materials in augmented reality (AR), rather than purely virtual synthetic, environments. However, how designing in an AR environment affects user behavior is unknown. To evaluate how work in a real environment influences the material design process, we propose a novel material design interface that allows designers to interact with a tangible object as they specify appearance properties. The setup gives designers the opportunity to view the real-time rendering of appearance properties through a virtual reality setup as they manipulate the object. Our setup uses a camera to capture the physical surroundings of the designer to create subtle but realistic reflection effects on the virtual view superimposed on the tangible object. The effects are based on the physical lighting conditions of the actual design space. We describe a user study that compares the efficacy of our method to that of a traditional 3D virtual synthetic material design system. Both subjective feedback and quantitative analysis from our study suggest that the in-situ experience provided by our setup allows the creation of higher quality material properties and supports the sense of interaction and immersion.
Single-Pass Stereoscopic GPU Ray Casting Using Re-Projection Layers
(The Eurographics Association, 2017) Buchacher, Arend; Erdt, Marius; Matthias Zwicker and Pedro Sander
Stereoscopic rendering of volume data for virtual reality applications is costly, as the computation complexity virtually doubles compared to common monoscopic rendering. This paper presents a single-pass stereoscopic GPU volume ray casting technique which significantly reduces the time needed to produce the second view. The approach builds upon previous work on ray segment re-projection techniques for non-parallel software ray casting that is initially inapplicable to GPU ray casting. Following the previous approach, ray casting is only executed for the left view. At the same time, ray segments are re-projected to layers of a texture array which leverages the constraints of the previous approach. In a subsequent compositing pass the layers are blended to produce the final image. Additionally, ways to determine an appropriate set of parameters are presented. Performance experiments show significant time savings on producing the second view over the naive two-pass approach achieving well over 60% speed-up in a typical virtual reality setup. The trade-off is an overhead of memory consumption that is proportional to the number of layers and image resolution and a marginal reduction in image quality. In qualitative experiments, average DSSIM values of less than 1% were recorded.
sLayer: a System for Multi-Layered Material Sculpting
(The Eurographics Association, 2017) Calabrese, Claudio; Fratarcangeli, Marco; Pellacini, Fabio; Matthias Zwicker and Pedro Sander
Many real world materials have a stratified structure, composed by the proximity and the interaction of multiple highly-detailed layers. Example of these materials are peeling paint, old tree bark and rusted metals. While digital sculpting is particularly well-suited to model these aged surfaces, the interaction between layers is not accounted for. We present a system for sculpting multi-layers materials where collision between layers are handled interactively while brushing meshes that scales up to the million of polygons necessary to model aged surfaces. We do so by observing that if the average mean edge length is maintained constant throughout the modeling session, we can use a single data structure, namely a uniform grid, to accelerate all the sculpting operations.We present a brush rasterization pipeline that uses this data structure for multi-layer editing.We also show that by adding a few interface tools for layer creation and selection, we can create detailed surface similar to real-world ones. To the best of our knowledge, our work is the first to show sculpting of highly-detailed, multi-layered materials in real-time.
Temporal Coherence for Metropolis Light Transport
(The Eurographics Association, 2017) Woestijne, Joran Van de; Frederickx, Roald; Billen, Niels; Dutré, Philip; Matthias Zwicker and Pedro Sander
Metropolis Light Transport is a powerful global illumination algorithm, yet it has some issues that make it less suitable for animation rendering. Due to the algorithm's local exploration of path space, difficult light paths can appear very late in the rendering process or might be missing from the final image altogether. This unpredictable convergence behaviour is especially troublesome when rendering animations, since these paths need to be rediscovered for every frame. An inability to rediscover difficult light paths across the entire animation causes unpleasant flickering artefacts. Our algorithm tackles this issue by introducing temporal mutations for Metropolis Light Transport, which perturb a light path from one point in time to another, potentially across many frames. This allows us to propagate difficult paths through the entire animation. Our technique supports multiple animation types, such as camera motion and object motion, and can robustly handle different shutter setups. The convergence speed of individual frames with motion blur is increased and convergence variations between frames are diminished, especially for specular transport and difficult indirect lighting.
VAO++: Practical Volumetric Ambient Occlusion for Games
(The Eurographics Association, 2017) Bokšanský, Jakub; Pospíšil, Adam; Bittner, Jiří; Matthias Zwicker and Pedro Sander
Ambient occlusion is one of the commonly used methods to increase visual fidelity in real-time rendering applications. We propose several extensions of the recently introduced volumetric ambient occlusion method. These extensions improve the properties of the methods with a particular focus on the quality vs performance tradeoff and wide applicability in contemporary games. We describe the implementation of the proposed algorithm and its extensions. We implemented the method as a camera effect within the Unity game engine. The results show that our implementation compares favorably with the standard ambient occlusion in Unity both in terms of quality and speed.

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