EGSR14: 25th Eurographics Symposium on Rendering

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https://diglib.eg.org/handle/10.2312/14331

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Preface and Table of Contents
(The Eurographics Association and Blackwell Publishing Ltd., 2014) Wojciech Jarosz and Pieter Peers
Semi-Automated Video Morphing
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Liao, Jing; Lima, Rodolfo S.; Nehab, Diego; Hoppe, Hugues; Sander, Pedro V.; Wojciech Jarosz and Pieter Peers
We explore creating smooth transitions between videos of different scenes. As in traditional image morphing, good spatial correspondence is crucial to prevent ghosting, especially at silhouettes. Video morphing presents added challenges. Because motions are often unsynchronized, temporal alignment is also necessary. Applying morphing to individual frames leads to discontinuities, so temporal coherence must be considered. Our approach is to optimize a full spatiotemporal mapping between the two videos. We reduce tedious interactions by letting the optimization derive the fine-scale map given only sparse user-specified constraints. For robustness, the optimization objective examines structural similarity of the video content. We demonstrate the approach on a variety of videos, obtaining results using few explicit correspondences.
Probabilistic Visibility Evaluation using Geometry Proxies
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Billen, Niels; Lagae, Ares; Dutré, Philip; Wojciech Jarosz and Pieter Peers
Evaluating the visibility between two points is a fundamental problem for ray-tracing and path-tracing algorithms. Ideally, visibility computations are organized such that a minimum number of geometric primitives need to be checked for each ray. Replacing complex geometric shapes by a simpler set of primitives is one strategy to control the amount of intersection calculations. However, approximating the original geometry introduces inaccuracies in e.g. shadow regions when shadow rays are intersected with the approximate geometry. This paper presents a theoretical framework for probabilistic visibility evaluation. When intersecting a shadow ray with the scene, we randomly select the original geometry, the approximated geometry, or one of several correction terms, to be tested. Not all shadow rays will therefore intersect the original geometry, but our method is able to produce unbiased images that converge to the correct solution. Although probabilistic visibility evaluation is an experimental idea, we show several example scenes that highlight the potential for future improvements.
Error Analysis of Estimators that use Combinations of Stochastic Sampling Strategies for Direct Illumination
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Subr, Kartic; Nowrouzezahrai, Derek; Jarosz, Wojciech; Kautz, Jan; Mitchell, Kenny; Wojciech Jarosz and Pieter Peers
We present a theoretical analysis of error of combinations of Monte Carlo estimators used in image synthesis. Importance sampling and multiple importance sampling are popular variance-reduction strategies. Unfortunately, neither strategy improves the rate of convergence of Monte Carlo integration. Jittered sampling (a type of stratified sampling), on the other hand is known to improve the convergence rate. Most rendering software optimistically combine importance sampling with jittered sampling, hoping to achieve both. We derive the exact error of the combination of multiple importance sampling with jittered sampling. In addition, we demonstrate a further benefit of introducing negative correlations (antithetic sampling) between estimates to the convergence rate. As with importance sampling, antithetic sampling is known to reduce error for certain classes of integrands without affecting the convergence rate. In this paper, our analysis and experiments reveal that importance and antithetic sampling, if used judiciously and in conjunction with jittered sampling, may improve convergence rates. We show the impact of such combinations of strategies on the convergence rate of estimators for direct illumination.
Color Me Noisy: Example-based Rendering of Hand-colored Animations with Temporal Noise Control
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Fi er, Jakub; Lukác, Michal; Jamri ka, Ondrej; Cadik, Martin; Gingold, Yotam; Asente, Paul; Sykora, Daniel; Wojciech Jarosz and Pieter Peers
We present an example-based approach to rendering hand-colored animations which delivers visual richness comparable to real artwork while enabling control over the amount of perceived temporal noise. This is important both for artistic purposes and viewing comfort, but is tedious or even intractable to achieve manually. We analyse typical features of real hand-colored animations and propose an algorithm that tries to mimic them using only static examples of drawing media. We apply the algorithm to various animations using different drawing media and compare the quality of synthetic results with real artwork. To verify our method perceptually, we conducted experiments confirming that our method delivers distinguishable noise levels and reduces eye strain. Finally, we demonstrate the capabilities of our method to mask imperfections such as shower-door artifacts.
Vectorising Bitmaps into Semi-Transparent Gradient Layers
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Richardt, Christian; Lopez-Moreno, Jorge; Bousseau, Adrien; Agrawala, Maneesh; Drettakis, George; Wojciech Jarosz and Pieter Peers
We present an interactive approach for decompositing bitmap drawings and studio photographs into opaque and semi-transparent vector layers. Semi-transparent layers are especially challenging to extract, since they require the inversion of the non-linear compositing equation. We make this problem tractable by exploiting the parametric nature of vector gradients, jointly separating and vectorising semi-transparent regions. Specifically, we constrain the foreground colours to vary according to linear or radial parametric gradients, restricting the number of unknowns and allowing our system to efficiently solve for an editable semi-transparent foreground. We propose a progressive workflow, where the user successively selects a semi-transparent or opaque region in the bitmap, which our algorithm separates as a foreground vector gradient and a background bitmap layer. The user can choose to decompose the background further or vectorise it as an opaque layer. The resulting layered vector representation allows a variety of edits, such as modifying the shape of highlights, adding texture to an object or changing its diffuse colour.
Importance Sampling Microfacet-Based BSDFs using the Distribution of Visible Normals
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Heitz, Eric; d'Eon, Eugene; Wojciech Jarosz and Pieter Peers
We present a new approach to microfacet-based BSDF importance sampling. Previously proposed sampling schemes for popular analytic BSDFs typically begin by choosing a microfacet normal at random in a way that is independent of direction of incident light. To sample the full BSDF using these normals requires arbitrarily large sample weights leading to possible fireflies. Additionally, at grazing angles nearly half of the sampled normals face away from the incident ray and must be rejected, making the sampling scheme inefficient. Instead, we show how to use the distribution of visible normals directly to generate samples, where normals are weighted by their projection factor toward the incident direction. In this way, no backfacing normals are sampled and the sample weights contain only the shadowing factor of outgoing rays (and additionally a Fresnel term for conductors). Arbitrarily large sample weights are avoided and variance is reduced. Since the BSDF depends on the microsurface model, we describe our sampling algorithm for two models: the V-cavity and the Smith models. We demonstrate results for both isotropic and anisotropic rough conductors and dielectrics with Beckmann and GGX distributions.
AutoStyle: Automatic Style Transfer from Image Collections to Users' Images
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Liu, Yiming; Cohen, Michael; Uyttendaele, Matt; Rusinkiewicz, Szymon; Wojciech Jarosz and Pieter Peers
Stylizing photos, to give them an antique or artistic look, has become popular in recent years. The available stylization filters, however, are usually created manually by artists, resulting in a narrow set of choices. Moreover, it can be difficult for the user to select a desired filter, since the filters' names often do not convey their functions. We investigate an approach to photo filtering in which the user provides one or more keywords, and the desired style is defined by the set of images returned by searching the web for those keywords. Our method clusters the returned images, allows the user to select a cluster, then stylizes the user's photos by transferring vignetting, color, and local contrast from that cluster. This approach vastly expands the range of available styles, and gives each filter a meaningful name by default. We demonstrate that our method is able to robustly transfer a wide range of styles from image collections to users' photos.
Spectral Ray Differentials
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Elek, Oskar; Bauszat, Pablo; Ritschel, Tobias; Magnor, Marcus; Seidel, Hans-Peter; Wojciech Jarosz and Pieter Peers
Light refracted by a dispersive interface leads to beautifully colored patterns that can be rendered faithfully with spectral Monte-Carlo methods. Regrettably, results often suffer from chromatic noise or banding, requiring high sampling rates and large amounts of memory compared to renderers operating in some trichromatic color space. Addressing this issue, we introduce spectral ray differentials, which describe the change of light direction with respect to changes in the spectrum. In analogy with the classic ray and photon differentials, this information can be used for filtering in the spectral domain. Effectiveness of our approach is demonstrated by filtering for offline spectral light and path tracing as well as for an interactive GPU photon mapper based on splatting. Our results show considerably less chromatic noise and spatial aliasing while retaining good visual similarity to reference solutions with negligible overhead in the order of milliseconds.
User-Assisted Video Stabilization
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Bai, Jiamin; Agarwala, Aseem; Agrawala, Maneesh; Ramamoorthi, Ravi; Wojciech Jarosz and Pieter Peers
We present a user-assisted video stabilization algorithm that is able to stabilize challenging videos when stateof- the-art automatic algorithms fail to generate a satisfactory result. Current methods do not give the user any control over the look of the final result. Users either have to accept the stabilized result as is, or discard it should the stabilization fail to generate a smooth output. Our system introduces two new modes of interaction that allow the user to improve the unsatisfactory stabilized video. First, we cluster tracks and visualize them on the warped video. The user ensures that appropriate tracks are selected by clicking on track clusters to include or exclude them. Second, the user can directly specify how regions in the output video should look by drawing quadrilaterals to select and deform parts of the frame. These user-provided deformations reduce undesirable distortions in the video. Our algorithm then computes a stabilized video using the user-selected tracks, while respecting the usermodified regions. The process of interactively removing user-identified artifacts can sometimes introduce new ones, though in most cases there is a net improvement.We demonstrate the effectiveness of our system with a variety of challenging hand held videos.
Interactive Parameter Retrieval for Two-Tone Procedural Textures
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Gieseke, Lena; Koch, Sebastian; Hahn, Jens Uwe; Fuchs, Martin; Wojciech Jarosz and Pieter Peers
The choice of parameters for procedural textures to achieve a desired appearance poses a challenging problem even for experienced artists. We propose a method to automatically determine such parameters to reproduce the appearance of input images. Addressing two-tone textures, we separate the estimation of color and structure information and interpret the problem as image retrieval task from the space of procedural outputs. Applying a perceptually motivated image metric based on a texture descriptor enables us to precompute a comprehensive collection of possible parameter sets and yet achieve interactive retrieval performance. Our method supports a large variety of procedural texture models with a unified approach.
Bilinear Accelerated Filter Approximation
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Manson, Josiah; Schaefer, Scott; Wojciech Jarosz and Pieter Peers
Our method approximates exact texture filtering for arbitrary scales and translations of an image while taking into account the performance characteristics of modern GPUs. Our algorithm is fast because it accesses textures with a high degree of spatial locality. Using bilinear samples guarantees that the texels we read are in a regular pattern and that we use a hardware accelerated path. We control the texel weights by manipulating the u; v parameters of each sample and the blend factor between the samples. Our method is similar in quality to Cardinality-Constrained Texture Filtering [MS13] but runs two times faster.
C-LOD: Context-aware Material Level-of-Detail applied to Mobile Graphics
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Koulieris, George Alex; Drettakis, George; Cunningham, Douglas; Mania, Katerina; Wojciech Jarosz and Pieter Peers
Attention-based Level-Of-Detail (LOD) managers downgrade the quality of areas that are expected to go unnoticed by an observer to economize on computational resources. The perceptibility of lowered visual fidelity is determined by the accuracy of the attention model that assigns quality levels. Most previous attention based LOD managers do not take into account saliency provoked by context, failing to provide consistently accurate attention predictions. In this work, we extend a recent high level saliency model with four additional components yielding more accurate predictions: an object-intrinsic factor accounting for canonical form of objects, an object-context factor for contextual isolation of objects, a feature uniqueness term that accounts for the number of salient features in an image, and a temporal context that generates recurring fixations for objects inconsistent with the context. We conduct a perceptual experiment to acquire the weighting factors to initialize our model. We design C-LOD, a LOD manager that maintains a constant frame rate on mobile devices by dynamically re-adjusting material quality on secondary visual features of non-attended objects. In a proof of concept study we establish that by incorporating C-LOD, complex effects such as parallax occlusion mapping usually omitted in mobile devices can now be employed, without overloading GPU capability and, at the same time, conserving battery power.
A Physically-Based BSDF for Modeling the Appearance of Paper
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Papas, Marios; Mesa, Krystle de; Jensen, Henrik Wann; Wojciech Jarosz and Pieter Peers
We present a novel appearance model for paper. Based on our appearance measurements for matte and glossy paper, we find that paper exhibits a combination of subsurface scattering, specular reflection, retroreflection, and surface sheen. Classic microfacet and simple diffuse reflection models cannot simulate the double-sided appearance of a thin layer. Our novel BSDF model matches our measurements for paper and accounts for both reflection and transmission properties. At the core of the BSDF model is a method for converting a multi-layer subsurface scattering model (BSSRDF) into a BSDF, which allows us to retain physically-based absorption and scattering parameters obtained from the measurements. We also introduce a method for computing the amount of light available for subsurface scattering due to transmission through a rough dielectric surface. Our final model accounts for multiple scattering, single scattering, and surface reflection and is capable of rendering paper with varying levels of roughness and glossiness on both sides.
Layered Reconstruction for Defocus and Motion Blur
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Munkberg, Jacob; Vaidyanathan, Karthik; Hasselgren, Jon; Clarberg, Petrik; Akenine-Möller, Tomas; Wojciech Jarosz and Pieter Peers
Light field reconstruction algorithms can substantially decrease the noise in stochastically rendered images. Recent algorithms for defocus blur alone are both fast and accurate. However, motion blur is a considerably more complex type of camera effect, and as a consequence, current algorithms are either slow or too imprecise to use in high quality rendering. We extend previous work on real-time light field reconstruction for defocus blur to handle the case of simultaneous defocus and motion blur. By carefully introducing a few approximations, we derive a very efficient sheared reconstruction filter, which produces high quality images even for a low number of input samples. Our algorithm is temporally robust, and is about two orders of magnitude faster than previous work, making it suitable for both real-time rendering and as a post-processing pass for offline rendering.
Hero Wavelength Spectral Sampling
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Wilkie, Alexander; Nawaz, Sehera; Droske, Marc; Weidlich, Andrea; Hanika, Johannes; Wojciech Jarosz and Pieter Peers
We present a spectral rendering technique that offers a compelling set of advantages over existing approaches. The key idea is to propagate energy along paths for a small, constant number of changing wavelengths. The first of these, the hero wavelength, is randomly sampled for each path, and all directional sampling is solely based on it. The additional wavelengths are placed at equal distances from the hero wavelength, so that all path wavelengths together always evenly cover the visible range. A related technique, spectral multiple importance sampling, was already introduced a few years ago. We propose a simplified and optimised version of this approach which is easier to implement, has good performance characteristics, and is actually more powerful than the original method. Our proposed method is also superior to techniques which use a static spectral representation, as it does not suffer from any inherent representation bias. We demonstrate the performance of our method in several application areas that are of critical importance for production work, such as fidelity of colour reproduction, sub-surface scattering, dispersion and volumetric effects. We also discuss how to couple our proposed approach with several technologies that are important in current production systems, such as photon maps, bidirectional path tracing, environment maps, and participating media.

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