32-Issue 4

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https://diglib.eg.org/handle/10.2312/182

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An Area-Preserving Parametrization for Spherical Rectangles
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ureña, Carlos; Fajardo, Marcos; King, Alan; Nicolas Holzschuch and Szymon Rusinkiewicz
We present an area-preserving parametrization for spherical rectangles which is an analytical function with domain in the unit rectangle [0;1]2 and range in a region included in the unit-radius sphere. The parametrization preserves areas up to a constant factor and is thus very useful in the context of rendering as it allows to map random sample point sets in [0;1]2 onto the spherical rectangle. This allows for easily incorporating stratified, quasi-Monte Carlo or other sampling strategies in algorithms that compute scattering from planar rectangular emitters.
Exponential Soft Shadow Mapping
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Shen, Li; Feng, Jieqing; Yang, Baoguang; Nicolas Holzschuch and Szymon Rusinkiewicz
In this paper we present an image-based algorithm to render visually plausible anti-aliased soft shadows in real time. Our technique employs a new shadow pre-filtering method based on an extended exponential shadow mapping theory. The algorithm achieves faithful contact shadows by adopting an optimal approximation to exponential shadow reconstruction function. Benefiting from a novel overflow free summed area table tile grid data structure, numerical stability is guaranteed and error filtering response is avoided. By integrating an adaptive anisotropic filtering method, the proposed algorithm can produce high quality smooth shadows both in large penumbra areas and in high frequency sharp transitions, meanwhile guarantee cheap memory consumption and high performance.
Line-Sweep Ambient Obscurance
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Timonen, Ville; Nicolas Holzschuch and Szymon Rusinkiewicz
Screen-space ambient occlusion and obscurance have become established methods for rendering global illumi- nation effects in real-time applications. While they have seen a steady line of refinements, their computational complexity has remained largely unchanged and either undersampling artefacts or too high render times limit their scalability. In this paper we show how the fundamentally quadratic per-pixel complexity of previous work can be reduced to a linear complexity. We solve obscurance in discrete azimuthal directions by performing line sweeps across the depth buffer in each direction. Our method builds upon the insight that scene points along each line can be incrementally inserted into a data structure such that querying for the largest occluder among the visited samples along the line can be achieved at an amortized constant cost. The obscurance radius therefore has no impact on the execution time and our method produces accurate results with smooth occlusion gradients in a few milliseconds per frame on commodity hardware.
Photon Beam Diffusion: A Hybrid Monte Carlo Method for Subsurface Scattering
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Habel, Ralf; Christensen, Per H.; Jarosz, Wojciech; Nicolas Holzschuch and Szymon Rusinkiewicz
We present photon beam diffusion, an efficient numerical method for accurately rendering translucent materials. Our approach interprets incident light as a continuous beam of photons inside the material. Numerically integrating diffusion from such extended sources has long been assumed computationally prohibitive, leading to the ubiquitous single-depth dipole approximation and the recent analytic sum-of-Gaussians approach employed by Quantized Diffusion. In this paper, we show that numerical integration of the extended beam is not only feasible, but provides increased speed, flexibility, numerical stability, and ease of implementation, while retaining the benefits of previous approaches. We leverage the improved diffusion model, but propose an efficient and numerically stable Monte Carlo integration scheme that gives equivalent results using only 3-5 samples instead of 20-60 Gaussians as in previous work. Our method can account for finite and multi-layer materials, and additionally supports directional incident effects at surfaces. We also propose a novel diffuse exact single-scattering term which can be integrated in tandem with the multi-scattering approximation. Our numerical approach furthermore allows us to easily correct inaccuracies of the diffusion model and even combine it with more general Monte Carlo rendering algorithms. We provide practical details necessary for efficient implementation, and demonstrate the versatility of our technique by incorporating it on top of several rendering algorithms in both research and production rendering systems.
Probabilistic Visibility Evaluation for Direct Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Billen, Niels; Engelen, Björn; Lagae, Ares; Dutré, Philip; Nicolas Holzschuch and Szymon Rusinkiewicz
The efficient evaluation of visibility in a three-dimensional scene is a longstanding problem in computer graphics. Visibility evaluations come in many different forms: figuring out what object is visible in a pixel; determining whether a point is visible to a light source; or evaluating the mutual visibility between 2 surface points. This paper provides a new, experimental view on visibility, based on a probabilistic evaluation of the visibility function. Instead of checking the visibility against all possible intervening geometry, the visibility between 2 points is now evaluated by testing only a random subset of objects. The result is not a Boolean value that is either 0 or 1, but a numerical value that can even be negative. Because we use the visibility evaluation as part of the integrand in illumination computations, the probabilistic evaluation of visibility becomes part of the Monte Carlo procedure of estimating the illumination integral, and results in an unbiased computation of illumination values in the scene. Moreover, the number of intersections tests for any given ray is decreased, since only a random selection of geometric primitives is tested. Although probabilistic visibility is an experimental and new idea, we present a practical algorithm for direct illumination that uses the probabilistic nature of visibility evaluations.
Temporally Coherent Adaptive Sampling for Imperfect Shadow Maps
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Barák, Tomas; Bittner, Jiri; Havran, Vlastimil; Nicolas Holzschuch and Szymon Rusinkiewicz
We propose a new adaptive algorithm for determining virtual point lights (VPL) in the scope of real-time instant radiosity methods, which use a limited number of VPLs. The proposed method is based on Metropolis-Hastings sampling and exhibits better temporal coherence of VPLs, which is particularly important for real-time applications dealing with dynamic scenes. We evaluate the properties of the proposed method in the context of the algorithm based on imperfect shadow maps and compare it with the commonly used inverse transform method. The results indicate that the proposed technique can significantly reduce the temporal flickering artifacts even for scenes with complex materials and textures. Further, we propose a novel splatting scheme for imperfect shadow maps using hardware tessellation. This scheme significantly improves the rendering performance particularly for complex and deformable scenes. We thoroughly analyze the performance of the proposed techniques on test scenes with detailed materials, moving camera, and deforming geometry.

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Browsing 32-Issue 4 by Subject "I.3.7 [Computer Graphics]"

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