EGWR: Eurographics Workshop on Rendering

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Accelerating Path Tracing by Re-Using Paths
(The Eurographics Association, 2002) Bekaert, Philippe; Sbert, Mateu; Halton, John; P. Debevec and S. Gibson
This paper describes a new acceleration technique for rendering algorithms like path tracing, that use so called gathering random walks. Usually in path tracing, each traced path is used in order to compute a contribution to only a single point on the virtual screen. We propose to combine paths traced through nearby screen points in such a way that each path contributes to multiple screen points in a provably good way. Our approach is unbiased and is not restricted to diffuse light scattering. It complements previous image noise reduction techniques for Monte Carlo ray tracing. We observe speed-ups in the computation of indirect illumination of one order of magnitude.
Accelerating Ray Tracing using Constrained Tetrahedralizations
(The Eurographics Association and Blackwell Publishing Ltd, 2008) Lagae, Ares; Dutre, Philip
In this paper we introduce the constrained tetrahedralization as a new acceleration structure for ray tracing. A constrained tetrahedralization of a scene is a tetrahedralization that respects the faces of the scene geometry. The closest intersection of a ray with a scene is found by traversing this tetrahedralization along the ray, one tetrahedron at a time. We show that constrained tetrahedralizations are a viable alternative to current acceleration structures, and that they have a number of unique properties that set them apart from other acceleration structures: constrained tetrahedralizations are not hierarchical yet adaptive; the complexity of traversing them is a function of local geometric complexity rather than global geometric complexity; constrained tetrahedralizations support deforming geometry without any effort; and they have the potential to unify several data structures currently used in global illumination.
Accurate Fitting of Measured Reflectances Using a Shifted Gamma Micro-facet Distribution
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Bagher, Mohammad Mahdi; Soler, Cyril; Holzschuch, Nicolas; Fredo Durand and Diego Gutierrez
Material models are essential to the production of photo-realistic images. Measured BRDFs provide accurate representation with complex visual appearance, but have larger storage cost. Analytical BRDFs such as Cook- Torrance provide a compact representation but fail to represent the effects we observe with measured appearance. Accurately fitting an analytical BRDF to measured data remains a challenging problem. In this paper we introduce the SGD micro-facet distribution for Cook-Torrance BRDF. This distribution accurately models the behavior of most materials. As a consequence, we accurately represent all measured BRDFs using a single lobe. Our fitting procedure is stable and robust, and does not require manual tweaking of the parameters.
Acquisition and Rendering of Transparent and Refractive Objects
(The Eurographics Association, 2002) Matusik, Wojciech; Pfister, Hanspeter; Ziegler, Remo; Ngan, Addy; McMillan, Leonard; P. Debevec and S. Gibson
This paper introduces a new image-based approach to capturing and modeling highly specular, transparent, or translucent objects. We have built a system for automatically acquiring high quality graphical models of objects that are extremely difficult to scan with traditional 3D scanners. The system consists of turntables, a set of cameras and lights, and monitors to project colored backdrops. We use multi-background matting techniques to acquire alpha and environment mattes of the object from multiple viewpoints. Using the alpha mattes we reconstruct an approximate 3D shape of the object. We use the environment mattes to compute a high-resolution surface reflectance field. We also acquire a low-resolution surface reflectance field using the overhead array of lights. Both surface reflectance fields are used to relight the objects and to place them into arbitrary environments. Our system is the first to acquire and render transparent and translucent 3D objects, such as a glass of beer, from arbitrary viewpoints under novel illumination.
Adaptive Frameless Rendering
(The Eurographics Association, 2005) Dayal, Abhinav; Woolley, Cliff; Watson, Benjamin; Luebke, David; Kavita Bala and Philip Dutre
We propose an adaptive form of frameless rendering with the potential to dramatically increase rendering speed over conventional interactive rendering approaches. Without the rigid sampling patterns of framed renderers, sampling and reconstruction can adapt with very fine granularity to spatio-temporal color change. A sampler uses closed-loop feedback to guide sampling toward edges or motion in the image. Temporally deep buffers store all the samples created over a short time interval for use in reconstruction and as sampler feedback. GPU-based reconstruction responds both to sampling density and space-time color gradients. Where the displayed scene is static, spatial color change dominates and older samples are given significant weight in reconstruction, resulting in sharper and eventually antialiased images. Where the scene is dynamic, more recent samples are emphasized, resulting in less sharp but more up-to-date images. We also use sample reprojection to improve reconstruction and guide sampling toward occlusion edges, undersampled regions, and specular highlights. In simulation our frameless renderer requires an order of magnitude fewer samples than traditional rendering of similar visual quality (as measured by RMS error), while introducing overhead amounting to 15% of computation time.
Adaptive Numerical Cumulative Distribution Functions for Efficient Importance Sampling
(The Eurographics Association, 2005) Lawrence, Jason; Rusinkiewicz, Szymon; Ramamoorthi, Ravi; Kavita Bala and Philip Dutre
As image-based surface reflectance and illumination gain wider use in physically-based rendering systems, it is becoming more critical to provide representations that allow sampling light paths according to the distribution of energy in these high-dimensional measured functions. In this paper, we apply algorithms traditionally used for curve approximation to reduce the size of a multidimensional tabulated Cumulative Distribution Function (CDF) by one to three orders of magnitude without compromising its fidelity. These adaptive representations enable new algorithms for sampling environment maps according to the local orientation of the surface and for multiple importance sampling of image-based lighting and measured BRDFs.
Adaptive Visibility-Driven View Cell Construction
(The Eurographics Association, 2006) Mattausch, Oliver; Bittner, Jirí; Wimmer, Michael; Tomas Akenine-Moeller and Wolfgang Heidrich
We present a new method for the automatic partitioning of view space into a multi-level view cell hierarchy. We use a cost-based model in order to minimize the average rendering time. Unlike previous methods, our model takes into account the actual visibility in the scene, and the partition is not restricted to planes given by the scene geometry. We show that the resulting view cell hierarchy works for different types of scenes and gives lower average rendering time than previously used methods.
Adaptive Volumetric Shadow Maps
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Salvi, Marco; Vidimce, Kiril; Lauritzen, Andrew; Lefohn, Aaron
We introduce adaptive volumetric shadow maps (AVSM), a real-time shadow algorithm that supports high-quality shadowing from dynamic volumetric media such as hair and smoke. The key contribution of AVSM is the introduction of a streaming simplification algorithm that generates an accurate volumetric light attenuation function using a small fixed memory footprint. This compression strategy leads to high performance because the visibility data can remain in on-chip memory during simplification and can be efficiently sampled during rendering. We demonstrate that AVSM compression closely approximates the ground-truth correct solution and performs competitively to existing real-time rendering techniques while providing higher quality volumetric shadows.
Alias-Free Shadow Maps
(The Eurographics Association, 2004) Aila, Timo; Laine, Samuli; Alexander Keller and Henrik Wann Jensen
In this paper we abandon the regular structure of shadow maps. Instead, we transform the visible pixels P(x, y, z) from screen space to the image plane of a light source P0(x0, y0, z0). The (x0, y0) are then used as sampling points when the geometry is rasterized into the shadow map. This eliminates the resolution issues that have plagued shadow maps for decades, e.g., jagged shadow boundaries. Incorrect self-shadowing is also greatly reduced, and semi-transparent shadow casters and receivers can be supported. A hierarchical software implementation is outlined
All-focused light field rendering
(The Eurographics Association, 2004) Kubota, Akira; Takahashi, Keita; Aizawa, Kiyoharu; Chen, Tsuhan; Alexander Keller and Henrik Wann Jensen
We present a novel reconstruction method that can synthesize an all in-focus view from under-sampled light fields, significantly suppressing aliasing artifacts. The presented method consists of two steps; 1) rendering multiple views at a given view point by performing light field rendering with different focal plane depths; 2) iteratively reconstructing the all in-focus view by fusing the multiple views. We model the multiple views and the desired all in-focus view as a set of linear equations with a combination of textures at the focal depths. Aliasing artifacts can be modeled as spatially (shift) varying filters. We can solve this set of linear equations by using an iterative reconstruction approach. This method effectively integrates focused regions in each view into an all in-focus view without any local processing steps such as estimation of depth or segmentation of the focused regions.
All-Frequency Precomputed Radiance Transfer for Glossy Objects
(The Eurographics Association, 2004) Liu, Xinguo; Sloan, Peter-Pike; Shum, Heung-Yeung; Snyder, John; Alexander Keller and Henrik Wann Jensen
We introduce a method based on precomputed radiance transfer (PRT) that allows interactive rendering of glossy surfaces and includes shadowing effects from dynamic, "all-frequency" lighting. Specifically, source lighting is represented by a cube map at resolution nL
All-Frequency Relighting of Non-Diffuse Objects using Separable BRDF Approximation
(The Eurographics Association, 2004) Wang, Rui; Tran, John; Luebke, David; Alexander Keller and Henrik Wann Jensen
This paper presents a technique, based on pre-computed light transport and separable BRDF approximation, for interactive rendering of non-diffuse objects under all-frequency environment illumination. Existing techniques using spherical harmonics to represent environment maps and transport functions are limited to low-frequency light transport effects. Non-linear wavelet lighting approximation is able to capture all-frequency illumination and shadows for geometry relighting, but interactive rendering is currently limited to diffuse objects. Our work extends the wavelet-based approach to relighting of non-diffuse objects. We factorize the BRDF using separable decomposition and keep only a few low-order approximation terms, each consisting of a 2D light map paired with a 2D view map. We then pre-compute light transport matrices corresponding to each BRDF light map, and compress the data with a non-linear wavelet approximation. We use modern graphics hardware to accelerate precomputation. At run-time, a sparse light vector is multiplied by the sparse transport matrix at each vertex, and the results are further combined with texture lookups of the view direction into the BRDF view maps to produce view-dependent color. Using our technique, we demonstrate rendering of objects with several non-diffuse BRDFs under all-frequency, dynamic environment lighting at interactive rates.
Ambient Occlusion for Animated Characters
(The Eurographics Association, 2006) Kontkanen, Janne; Aila, Timo; Tomas Akenine-Moeller and Wolfgang Heidrich
We present a novel technique for approximating ambient occlusion of animated objects. Our method automatically determines the correspondence between animation parameters and per-vertex ambient occlusion using a set of reference poses as its input. Then, at runtime, the ambient occlusion is approximated by taking a dot product between the current animation parameters and static per-vertex coefficients. According to our results, both the computational and storage requirements are low enough for the technique to be directly applicable to computer games running on current graphics hardware. The resulting images are also significantly more realistic than the commonly used static ambient occlusion solutions.
Anaglyph Stereo Without Ghosting
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Sanftmann, Harald; Weiskopf, Daniel; Ravi Ramamoorthi and Erik Reinhard
Anaglyph stereo provides a low-budget solution to viewing stereoscopic images. However, it may suffer from ghosting and bad color reproduction. Here we address the first issue. We present a novel technique to perceptually calibrate an anaglyph stereoscopic system and to use the calibration to eliminate ghosting from the anaglyph image. We build a model based on luminance perception by the left and right eyes through the anaglyph glasses. We do not rely on power spectra of a monitor or on transmission spectra of anaglyph glasses, but show how the five parameters of our model can be captured with just a few measurements within a minute. We present how full color, half color, and gray anaglyphs can be rendered with our technique and compare them to the traditional method.
An Analysis of the In-Out BRDF Factorization for View-Dependent Relighting
(The Eurographics Association and Blackwell Publishing Ltd, 2008) Mahajan, Dhruv; Tseng, Yu-Ting; Ramamoorthi, Ravi
Interactive rendering with dynamic natural lighting and changing view is a long-standing goal in computer graphics. Recently, precomputation-based methods for all-frequency relighting have made substantial progress in this direction. Many of the most successful algorithms are based on a factorization of the BRDF into incident and outgoing directions, enabling each term to be precomputed independent of viewing direction, and re-combined at run-time. However, there has so far been no theoretical understanding of the accuracy of this factorization, nor the number of terms needed. In this paper, we conduct a theoretical and empirical analysis of the BRDF in-out factorization. For Phong BRDFs, we obtain analytic results, showing that the number of terms needed grows linearly with the Phong exponent, while the factors correspond closely to spherical harmonic basis functions. More generally, the number of terms is quadratic in the frequency content of the BRDF along the reflected or half-angle direction. This analysis gives clear practical guidance on the number of factors needed for a given material. Different objects in a scene can each be represented with the correct number of terms needed for that particular BRDF, enabling both accuracy and interactivity.
Analytic Tangent Irradiance Environment Maps for Anisotropic Surfaces
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Mehta, Soham Uday; Ramamoorthi, Ravi; Meyer, Mark; Hery, Christophe; Fredo Durand and Diego Gutierrez
Environment-mapped rendering of Lambertian isotropic surfaces is common, and a popular technique is to use a quadratic spherical harmonic expansion. This compact irradiance map representation is widely adopted in interactive applications like video games. However, many materials are anisotropic, and shading is determined by the local tangent direction, rather than the surface normal. Even for visualization and illustration, it is increasingly common to define a tangent vector field, and use anisotropic shading. In this paper, we extend spherical harmonic irradiance maps to anisotropic surfaces, replacing Lambertian reflectance with the diffuse term of the popular Kajiya-Kay model. We show that there is a direct analogy, with the surface normal replaced by the tangent. Our main contribution is an analytic formula for the diffuse Kajiya-Kay BRDF in terms of spherical harmonics; this derivation is more complicated than for the standard diffuse lobe. We show that the terms decay even more rapidly than for Lambertian reflectance, going as l??3, where l is the spherical harmonic order, and with only 6 terms (l
An Analytical Model for Skylight Polarisation
(The Eurographics Association, 2004) Wilkie, A.; Ulbricht, C.; Tobler, Robert F.; Zotti, G.; Purgathofer, W.; Alexander Keller and Henrik Wann Jensen
Under certain circumstances the polarisation state of the illumination can have a significant influence on the appearance of scenes; outdoor scenes with specular surfaces - such as water bodies or windows - under clear, blue skies are good examples of such environments. In cases like that it can be essential to use a polarising renderer if a true prediction of nature is intended, but so far no polarising skylight models have been presented. This paper presents a plausible analytical model for the polarisation of the light emitted from a clear sky. Our approach is based on a suitable combination of several components with well-known characteristics, and yields acceptable results in considerably less time than an exhaustive simulation of the underlying atmospheric scattering phenomena would require.
Animatable Facial Reflectance Fields
(The Eurographics Association, 2004) Hawkins, Tim; Wenger, Andreas; Tchou, Chris; Gardner, Andrew; Göransson, Fredrik; Debevec, Paul; Alexander Keller and Henrik Wann Jensen
We present a technique for creating an animatable image-based appearance model of a human face, able to capture appearance variation over changing facial expression, head pose, view direction, and lighting condition. Our capture process makes use of a specialized lighting apparatus designed to rapidly illuminate the subject sequentially from many different directions in just a few seconds. For each pose, the subject remains still while six video cameras capture their appearance under each of the directions of lighting. We repeat this process for approximately 60 different poses, capturing different expressions, visemes, head poses, and eye positions. The images for each of the poses and camera views are registered to each other semi-automatically with the help of fiducial markers. The result is a model which can be rendered realistically under any linear blend of the captured poses and under any desired lighting condition by warping, scaling, and blending data from the original images. Finally, we show how to drive the model with performance capture data, where the pose is not necessarily a linear combination of the original captured poses.
Anomalous Dispersion in Predictive Rendering
(The Eurographics Association and Blackwell Publishing Ltd, 2009) Weidlich, Andrea; Wilkie, Alexander
In coloured media, the index of refraction does not decrease monotonically with increasing wavelength, but behaves in a quite non-monotonical way. This behaviour is called anomalous dispersion and results from the fact that the absorption of a material influences its index of refraction.So far, this interesting fact has not been widely acknowledged by the graphics community. In this paper, we demonstrate how to calculate the correct refractive index for a material based on its absorption spectrum with the Kramers-Kronig relation, and we discuss for which types of objects this effect is relevant in practice.
Anti-aliasing and Continuity with Trapezoidal Shadow Maps
(The Eurographics Association, 2004) Martin, Tobias; Tan, Tiow-Seng; Alexander Keller and Henrik Wann Jensen
This paper proposes a new shadow map technique termed trapezoidal shadow maps to calculate high quality shadows in real-time applications. To address the resolution problem of the standard shadow map approach, our technique approximates the eye's frustum as seen from the light with a trapezoid to warp it onto a shadow map. Such a trapezoidal approximation, which may first seem straightforward, is carefully designed to achieve the goal of good shadow quality for objects from near to far, and to address the continuity problem that is found in all existing shadow map approaches. The continuity problem occurs mainly when the shadow map quality changes significantly from frame to frame due to the motion of the eye or the light. This results in flickering of shadows. On the whole, our proposed approach is simple to implement without using complex data structures and it maps well to graphics hardware as shown in our experiments with large virtual scenes of hundreds of thousands to over a million of triangles.

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