35-Issue 4

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https://diglib.eg.org/handle/10.2312/15440

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Adaptive Image-Space Sampling for Gaze-Contingent Real-time Rendering
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Stengel, Michael; Grogorick, Steve; Eisemann, Martin; Magnor, Marcus; Elmar Eisemann and Eugene Fiume
With ever-increasing display resolution for wide field-of-view displays-such as head-mounted displays or 8k projectors- shading has become the major computational cost in rasterization. To reduce computational effort, we propose an algorithm that only shades visible features of the image while cost-effectively interpolating the remaining features without affecting perceived quality. In contrast to previous approaches we do not only simulate acuity falloff but also introduce a sampling scheme that incorporates multiple aspects of the human visual system: acuity, eye motion, contrast (stemming from geometry, material or lighting properties), and brightness adaptation. Our sampling scheme is incorporated into a deferred shading pipeline to shade the image's perceptually relevant fragments while a pull-push algorithm interpolates the radiance for the rest of the image. Our approach does not impose any restrictions on the performed shading. We conduct a number of psycho-visual experiments to validate scene- and task-independence of our approach. The number of fragments that need to be shaded is reduced by 50 % to 80 %. Our algorithm scales favorably with increasing resolution and field-of-view, rendering it well-suited for head-mounted displays and wide-field-of-view projection.
Parallel Multiple-Bounce Irradiance Caching
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Jones, Nathaniel L.; Reinhart, Christoph F.; Elmar Eisemann and Eugene Fiume
Building designers rely on predictive rendering techniques to design naturally and artificially lit environments. However, despite decades of work on the correctness of global illumination rendering techniques, our ability to accurately predict light levels in buildings-and to do so in a short time frame as part of an iterative design process-remains limited. In this paper, we present a novel approach to parallelizing construction of an irradiance cache over multiple-bounce paths. Relevant points for irradiance calculation based on one or multiple cameras are located by tracing rays through multiple-bounce paths. Irradiance values are then saved to a cache in reverse bounce order so that the irradiance calculation at each bounce samples from previously calculated values. We show by comparison to high-dynamic range photography of a moderately complex space that our method can predict luminance distribution as accurately as RADIANCE, the most widely validated tool used today for architectural predictive rendering of daylit spaces, and that it is faster by an order of magnitude.
Perceptually Motivated BRDF Comparison using Single Image
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Havran, Vlastimil; Filip, Jiri; Myszkowski, Karol; Elmar Eisemann and Eugene Fiume
Surface reflectance of real-world materials is now widely represented by the bidirectional reflectance distribution function (BRDF) and also by spatially varying representations such as SVBRDF and the bidirectional texture function (BTF). The raw surface reflectance measurements are typically compressed or fitted by analytical models, that always introduce a certain loss of accuracy. For its evaluation we need a distance function between a reference surface reflectance and its approximate version. Although some of the past techniques tried to reflect the perceptual sensitivity of human vision, they have neither optimized illumination and viewing conditions nor surface shape. In this paper, we suggest a new image-based methodology for comparing different anisotropic BRDFs. We use optimization techniques to generate a novel surface which has extensive coverage of incoming and outgoing light directions, while preserving its features and frequencies that are important for material appearance judgments. A single rendered image of such a surface along with simultaneously optimized lighting and viewing directions leads to the computation of a meaningful BRDF difference, by means of standard image difference predictors. A psychophysical experiments revealed that our surface provides richer information on material properties than the standard surfaces often used in computer graphics, e.g., sphere or blob.
A Phenomenological Model for Throughfall Rendering in Real-time
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Weber, Yoann; Jolivet, Vincent; Gilet, Guillaume; Nanko, Kazuki; Ghazanfarpour, Djamchid; Elmar Eisemann and Eugene Fiume
This paper aims at rendering interactive visual effects inherent to complex interactions between trees and rain in real-time in order to increase the realism of natural rainy scenes. Such a complex phenomenon involves a great number of physical processes influenced by various interlinked factors and its rendering represents a thorough challenge in Computer Graphics.We approach this problem by introducing an original method to render drops dripping from leaves after interception of raindrops by foliage. Our method introduces a new hydrological model representing interactions between rain and foliage through a phenomenological approach. Our model reduces the complexity of the phenomenon by representing multiple dripping drops with a new fully functional form evaluated per-pixel on-the-fly and providing improved control over density and physical properties. Furthermore, an efficient real-time rendering scheme, taking full advantage of latest GPU hardware capabilities, allows the rendering of a large number of dripping drops even for complex scenes.
Product Importance Sampling for Light Transport Path Guiding
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Herholz, Sebastian; Elek, Oskar; Vorba, Jiří; Lensch, Hendrik; Křivánek, Jaroslav; Elmar Eisemann and Eugene Fiume
The efficiency of Monte Carlo algorithms for light transport simulation is directly related to their ability to importance-sample the product of the illumination and reflectance in the rendering equation. Since the optimal sampling strategy would require knowledge about the transport solution itself, importance sampling most often follows only one of the known factors - BRDF or an approximation of the incident illumination. To address this issue, we propose to represent the illumination and the reflectance factors by the Gaussian mixture model (GMM), which we fit by using a combination of weighted expectation maximization and non-linear optimization methods. The GMM representation then allows us to obtain the resulting product distribution for importance sampling on-the-fly at each scene point. For its efficient evaluation and sampling we preform an up-front adaptive decimation of both factor mixtures. In comparison to state-of-the-art sampling methods, we show that our product importance sampling can lead to significantly better convergence in scenes with complex illumination and reflectance.
Solid Angle Sampling of Disk and Cylinder Lights
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Gamito, Manuel N.; Elmar Eisemann and Eugene Fiume
A new unbiased sampling approach is presented, which allows the direct illumination from disk and cylinder light sources to be sampled with a uniform probability distribution within their solid angles, as seen from each illuminated point. This approach applies to any form of global illumination path tracing algorithm (forward or bidirectional), where the direct illumination integral from light sources needs to be estimated. We show that taking samples based on the solid angle of these two light sources leads to improved estimates and reduced variance of the Monte Carlo integral for direct illumination. This work follows from previously known unbiased methods for the solid angle sampling of triangular and rectangular light sources and extends the class of lights that can be rendered with these improved sampling algorithms.
Eurographics Symposium on Rendering 2016: Frontmatter
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Elmar Eisemann; Eugene Fiume
Efficient Ray Tracing Through Aspheric Lenses and Imperfect Bokeh Synthesis
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Joo, Hyuntae; Kwon, Soonhyeon; Lee, Sangmin; Eisemann, Elmar; Lee, Sungkil; Elmar Eisemann and Eugene Fiume
We present an efficient ray-tracing technique to render bokeh effects produced by parametric aspheric lenses. Contrary to conventional spherical lenses, aspheric lenses do generally not permit a simple closed-form solution of ray-surface intersections. We propose a numerical root-finding approach, which uses tight proxy surfaces to ensure a good initialization and convergence behavior. Additionally, we simulate mechanical imperfections resulting from the lens fabrication via a texture-based approach. Fractional Fourier transform and spectral dispersion add additional realism to the synthesized bokeh effect. Our approach is well-suited for execution on graphics processing units (GPUs) and we demonstrate complex defocus-blur and lens-flare effects.
Improving the Dwivedi Sampling Scheme
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Meng, Johannes; Hanika, Johannes; Dachsbacher, Carsten; Elmar Eisemann and Eugene Fiume
Despite recent advances in Monte Carlo rendering techniques, dense, high-albedo participating media such as wax or skin still remain a difficult problem. In such media, random walks tend to become very long, but may still lead to a large contribution to the image. The Dwivedi sampling scheme, which is based on zero variance random walks, biases the sampling probability distributions to exit the medium as quickly as possible. This can reduce variance considerably under the assumption of a locally homogeneous medium with constant phase function. Prior work uses the normal at the Point of Entry as the bias direction. We demonstrate that this technique can fail in common scenarios such as thin geometry with a strong backlight. We propose two new biasing strategies, Closest Point and Incident Illumination biasing, and show that these techniques can speed up convergence by up to an order of magnitude. Additionally, we propose a heuristic approach for combining biased and classical sampling techniques using Multiple Importance Sampling.
Sparse High-degree Polynomials for Wide-angle Lenses
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Schrade, Emanuel; Hanika, Johannes; Dachsbacher, Carsten; Elmar Eisemann and Eugene Fiume
Rendering with accurate camera models greatly increases realism and improves the match of synthetic imagery to real-life footage. Photographic lenses can be simulated by ray tracing, but the performance depends on the complexity of the lens system, and some operations required for modern algorithms, such as deterministic connections, can be difficult to achieve. We generalise the approach of polynomial optics, i.e. expressing the light field transformation from the sensor to the outer pupil using a polynomial, to work with extreme wide angle (fisheye) lenses and aspherical elements. We also show how sparse polynomials can be constructed from the large space of high-degree terms (we tested up to degree 15). We achieve this using a variant of orthogonal matching pursuit instead of a Taylor series when computing the polynomials. We show two applications: photorealistic rendering using Monte Carlo methods, where we introduce a new aperture sampling technique that is suitable for light tracing, and an interactive preview method suitable for rendering with deep images.
Fast Filtering of Reflection Probes
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Manson, Josiah; Sloan, Peter-Pike; Elmar Eisemann and Eugene Fiume
Game and movie studios are switching to physically based rendering en masse, but physically accurate filter convolution is difficult to do quickly enough to update reflection probes in real-time. Cubemap filtering has also become a bottleneck in the content processing pipeline. We have developed a two-pass filtering algorithm that is specialized for isotropic reflection kernels, is several times faster than existing algorithms, and produces superior results. The first pass uses a quadratic b-spline recurrence that is modified for cubemaps. The second pass uses lookup tables to determine optimal sampling in terms of placement, mipmap level, and coefficients. Filtering a full 128 2 cubemap on an NVIDIA GeForce GTX 980 takes between 160 μs and 730 μs with out method, depending on the desired quality.
Nonlinearly Weighted First-order Regression for Denoising Monte Carlo Renderings
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Bitterli, Benedikt; Rousselle, Fabrice; Moon, Bochang; Iglesias-Guitian, Jose A.; Adler, David; Mitchell, Kenny; Jarosz, Wojciech; Novak, Jan; Elmar Eisemann and Eugene Fiume
We address the problem of denoising Monte Carlo renderings by studying existing approaches and proposing a new algorithm that yields state-of-the-art performance on a wide range of scenes. We analyze existing approaches from a theoretical and empirical point of view, relating the strengths and limitations of their corresponding components with an emphasis on production requirements. The observations of our analysis instruct the design of our new filter that offers high-quality results and stable performance. A key observation of our analysis is that using auxiliary buffers (normal, albedo, etc.) to compute the regression weights greatly improves the robustness of zero-order models, but can be detrimental to first-order models. Consequently, our filter performs a first-order regression leveraging a rich set of auxiliary buffers only when fitting the data, and, unlike recent works, considers the pixel color alone when computing the regression weights. We further improve the quality of our output by using a collaborative denoising scheme. Lastly, we introduce a general mean squared error estimator, which can handle the collaborative nature of our filter and its nonlinear weights, to automatically set the bandwidth of our regression kernel.
Line Sampling for Direct Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Billen, Niels; Dutré, Philip; Elmar Eisemann and Eugene Fiume
Computing direct illumination efficiently is still a problem of major significance in computer graphics. The evaluation involves an integral over the surface areas of the light sources in the scene. Because this integral typically features many discontinuities, introduced by the visibility term and complex material functions, Monte Carlo integration is one of the only general techniques that can be used to compute the integral. In this paper, we propose to evaluate the direct illumination using line samples instead of point samples. A direct consequence of line sampling is that the two-dimensional integral over the area of the light source is reduced to a one-dimensional integral. We exploit this dimensional reduction by relying on the property that commonly used sampling patterns, such as stratified sampling and low-discrepancy sequences, converge faster when the dimension of the integration domain is reduced. We show that, while line sampling is generally more computationally intensive than point sampling, the variance of a line sample is smaller than that of a point sample, resulting in a higher order of convergence.
Forward Light Cuts: A Scalable Approach to Real-Time Global Illumination
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Laurent, Gilles; Delalandre, Cyril; Rivière, Grégoire de La; Boubekeur, Tamy; Elmar Eisemann and Eugene Fiume
We present Forward Light Cuts, a novel approach to real-time global illumination using forward rendering techniques. We focus on unshadowed diffuse interactions for the first indirect light bounce in the context of large models such as the complex scenes usually encountered in CAD application scenarios. Our approach efficiently generates and uses a multiscale radiance cache by exploiting the geometry-specific stages of the graphics pipeline, namely the tessellator unit and the geometry shader. To do so, we assimilate virtual point lights to the scene's triangles and design a stochastic decimation process chained with a partitioning strategy that accounts for both close-by strong light reflections, and distant regions from which numerous virtual point lights collectively contribute strongly to the end pixel. Our probabilistic solution is supported by a mathematical analysis and a number of experiments covering a wide range of application scenarios. As a result, our algorithm requires no precomputation of any kind, is compatible with dynamic view points, lighting condition, geometry and materials, and scales to tens of millions of polygons on current graphics hardware.

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