EGSR04: 15th Eurographics Symposium on Rendering

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https://diglib.eg.org/handle/10.2312/383

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A Self-Shadow Algorithm for Dynamic Hair using Density Clustering
(The Eurographics Association, 2004) Mertens, Tom; Kautz, Jan; Bekaert, Philippe; Reeth, Frank Van; Alexander Keller and Henrik Wann Jensen
Self-shadowing is an important factor in the appearance of hair and fur. In this paper we present a new rendering algorithm to accurately compute shadowed hair at interactive rates using graphics hardware. No constraint is imposed on the hair style, and its geometry can be dynamic. Similar to previously presented methods, a 1D visibility function is constructed for each line of sight of the light source view. Our approach differs from other work by treating the hair geometry as a 3D density field, which is sampled on the fly using simple rasterization. The rasterized fragments are clustered, effectively estimating the density of hair along a ray. Based hereon, the visibility function is constructed. We show that realistic selfshadowing of thousands of individual dynamic hair strands can be rendered at interactive rates using consumer graphics hardware.
Spherical Harmonic Gradients for Mid-Range Illumination
(The Eurographics Association, 2004) Annen, Thomas; Kautz, Jan; Durand, Frédo; Seidel, Hans-Peter; Alexander Keller and Henrik Wann Jensen
Spherical harmonics are often used for compact description of incident radiance in low-frequency but distant lighting environments. For interaction with nearby emitters, computing the incident radiance at the center of an object only is not sufficient. Previous techniques then require expensive sampling of the incident radiance field at many points distributed over the object. Our technique alleviates this costly requirement using a first-order Taylor expansion of the spherical-harmonic lighting coefficients around a point. We propose an interpolation scheme based on these gradients requiring far fewer samples (one is often sufficient). We show that the gradient of the incident-radiance spherical harmonics can be computed for little additional cost compared to the coefficients alone. We introduce a semi-analytical formula to calculate this gradient at run-time and describe how a simple vertex shader can interpolate the shading. The interpolated representation of the incident radiance can be used with any low-frequency light-transfer technique.
Real-time appearance preserving out-of-core rendering with shadows
(The Eurographics Association, 2004) Guthe, Michael; Borodin, Pavel; Balázs, Ákos; Klein, Reinhard; Alexander Keller and Henrik Wann Jensen
Despite recent advances in finding efficient LOD-representations for gigantic 3D objects, rendering of complex, gigabyte-sized models and environments is still a challenging task, especially under real-time constraints and high demands on the visual accuracy. The two general approaches are using either a polygon- or a point-based representation for the simplified geometry. With the polygon-based approaches high frame rates can be achieved by sacrificing the exact appearance and thus the image quality. Point-based approaches on the other hand preserve higher image quality at the cost of higher primitive counts and therefore lower frame rates. In this paper we present a new hybrid point-polygon LOD algorithm for real-time rendering of complex models and environments including shadows. While rendering different LODs, we preserve the appearance of an object by using a novel error measure for simplification which allows us to steer the LOD generation in such a way that the geometric as well as the appearance deviation is bounded in image space. Additionally, to enhance the perception of the models shadows should be used. We present a novel LOD selection and prefetching method for real-time rendering of hard shadows. In contrast to the only currently available method for out-of-core shadow generation, our approach entirely runs on a single CPU system.
Image-Based Stereoscopic Painterly Rendering
(The Eurographics Association, 2004) Stavrakis, E.; Gelautz, M.; Alexander Keller and Henrik Wann Jensen
We present a new image-based stereoscopic painterly algorithm that we use to automatically generate stereoscopic paintings. Our work is motivated by contemporary painters who have explored the aesthetic implications of painting stereo pairs of canvases. We base our method on two real images, acquired from spatially displaced cameras. We derive a depth map by utilizing computer vision depth-from-stereo techniques and use this information to plan and render stereo paintings. These paintings can be viewed stereoscopically, in which case the pictorial medium is perceptually extended by the viewer to better suggest the sense of distance.
Feature-Based Textures
(The Eurographics Association, 2004) Ramanarayanan, G.; Bala, K.; Walter, B.; Alexander Keller and Henrik Wann Jensen
This paper introduces feature-based textures, a new image representation that combines features and samples for high-quality texture mapping. Features identify boundaries within an image where samples change discontinuously. They can be extracted from vector graphics representations, or explicitly added to raster images to improve sharpness. Texture lookups are then interpolated from samples while respecting these boundaries. We present results from a software implementation of this technique demonstrating quality, efficiency and low memory overhead.
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.
Progressively-Refined Reflectance Functions from Natural Illumination
(The Eurographics Association, 2004) Matusik, Wojciech; Loper, Matthew; Pfister, Hanspeter; Alexander Keller and Henrik Wann Jensen
In this paper we present a simple, robust, and efficient algorithm for estimating reflectance fields (i.e., a description of the transport of light through a scene) for a fixed viewpoint using images of the scene under known natural illumination. Our algorithm treats the scene as a black-box linear system that transforms an input signal (the incident light) into an output signal (the reflected light). The algorithm is hierarchical - it progressively refines the approximation of the reflectance field with an increasing number of training samples until the required precision is reached. Our method relies on a new representation for reflectance fields. This representation is compact, can be progressively refined, and quickly computes the relighting of scenes with complex illumination. Our representation and the corresponding algorithm allow us to efficiently estimate the reflectance fields of scenes with specular, glossy, refractive, and diffuse elements. The method also handles soft and hard shadows, inter-reflections, caustics, and subsurface scattering. We verify our algorithm and representation using two measurement setups and several scenes, including an outdoor view of the city of Cambridge.
Rendering Procedural Terrain by Geometry Image Warping
(The Eurographics Association, 2004) Dachsbacher, Carsten; Stamminger, Marc; Alexander Keller and Henrik Wann Jensen
We describe an approach for rendering large terrains in real-time. A digital elevation map defines the rough shape of the terrain. During rendering, procedural geometric and texture detail is added by the graphics hardware. We show, how quad meshes can be generated quickly that have a locally varying resolution that is optimized for the inclusion of procedural detail.We obtain these distorted meshes by importance based warping of geometry images. The resulting quad mesh can then be rendered very efficiently by graphics hardware, which also adds all visible procedural detail using vertex and fragment programs.
Hemispherical Rasterization for Self-Shadowing of Dynamic Objects
(The Eurographics Association, 2004) Kautz, Jan; Lehtinen, Jaakko; Aila, Timo; Alexander Keller and Henrik Wann Jensen
We present a method for interactive rendering of dynamic models with self-shadows due to time-varying, lowfrequency lighting environments. In contrast to previous techniques, the method is not limited to static or preanimated models. Our main contribution is a hemispherical rasterizer, which rapidly computes visibility by rendering blocker geometry into a 2D occlusion mask with correct occluder fusion. The response of an object to the lighting is found by integrating the visibility function at each of the vertices against the spherical harmonic functions and the BRDF. This yields transfer coefficients that are then multiplied by the lighting coefficients to obtain the final, shadowed exitant radiance. No precomputation is necessary and memory requirements are modest. The method supports both diffuse and glossy BRDFs.
Estimating source spectra and spectral albedos from RGB data for rerendering
(The Eurographics Association, 2004) Koenderink, J. J.; Alexander Keller and Henrik Wann Jensen
I consider the problem of estimating material properties (the spectral albedo) on the basis of -object colors- (at worst only RGB data say). I show how to obtain a priori likely estimates for the white point, the spectral composition of the source, and the spectral albedos of the objects in a scene. I also show how to construct the general solutions. These general solutions are so broad as to render them practically useless. There are good reasons to disregard the larger part of the solution space, because very general considerations suggest that the specific solutions constructed with the methods discussed here are very likely to yield sensible and useful results in practice. From a principled perspective it is desirable to be able to construct the full solution space though. Since the results are in the scene, rather than the image domain, they are suitable for rerendering purposes.
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.
PointWorks: Abstraction and Rendering of Sparsely Scanned Outdoor Environments
(The Eurographics Association, 2004) Xu, Hui; Gossett, Nathan; Chen, Baoquan; Alexander Keller and Henrik Wann Jensen
This paper describes a system, dubbed PointWorks, for rendering three-dimensionally digitized outdoor environments in non-photorealistic rendering styles. The challenge in rendering scanned outdoor environments is accommodating their inaccuracy, incompleteness, and large size to deliver a smooth animation without suggesting the underlying data deficiency. The key method discussed in this paper is employing artistic drawing techniques to illustrate features of varying importance and accuracy. We employ a point-based representation of the scanned environment and operate directly on point-based models for abstraction and rendering. We develop a framework for producing mainly two artistic styles: painterly and profile lines. Strategies have also been employed to leverage modern graphics hardware for achieving interactive rendering of large scenes.
Rendering Forest Scenes in Real-Time
(The Eurographics Association, 2004) Decaudin, Philippe; Neyret, Fabrice; Alexander Keller and Henrik Wann Jensen
Forests are crucial for scene realism in applications such as light simulators. This paper proposes a new representation allowing for the real-time rendering of realistic forests covering an arbitrary terrain. It lets us produce dense forests corresponding to continuous non-repetitive fields made of thousands of trees with full parallax. Our representation draws on volumetric textures and aperiodic tiling: the forest consists of a set of edgecompatible prisms containing forest samples which are aperiodically mapped onto the ground. The representation allows for quality rendering, thanks to appropriate 3D non-linearfiltering. It relies on LODs and on a GPUfriendly structure to achieve real-time performance. Dynamic lighting and shadowing are beyond the scope of this paper. On the other hand, we require no advanced graphics feature except 3D textures and decent fill and vertex transform rates. However we can take advantage of vertex shaders so that the slicing of the volumetric texture is entirely done on the GPU.
A Self-Reconfigurable Camera Array
(The Eurographics Association, 2004) Zhang, Cha; Chen, Tsuhan; Alexander Keller and Henrik Wann Jensen
This paper presents a self-reconfigurable camera array system that captures video sequences from an array of mobile cameras, renders novel views on the fly and reconfigures the camera positions to achieve better rendering quality. The system is composed of 48 cameras mounted on mobile platforms. The contribution of this paper is twofold. First, we propose an efficient algorithm that is capable of rendering high-quality novel views from the captured images. The algorithm reconstructs a view-dependent multi-resolution 2D mesh model of the scene geometry on the fly and uses it for rendering. The algorithm combines region of interest (ROI) identification, JPEG image decompression, lens distortion correction, scene geometry reconstruction and novel view synthesis seamlessly on a single Intel Xeon 2.4 GHz processor, which is capable of generating novel views at 4-10 frames per second (fps). Second, we present a view-dependent adaptive capturing scheme that moves the cameras in order to show even better rendering results. Such camera reconfiguration naturally leads to a nonuniform arrangement of the cameras on the camera plane, which is both view-dependent and scene-dependent.
Practical Rendering of Multiple Scattering Effects in Participating Media
(The Eurographics Association, 2004) Premoze, Simon; Ashikhmin, Michael; Tessendorf, Jerry; Ramamoorthi, Ravi; Nayar, Shree; Alexander Keller and Henrik Wann Jensen
Volumetric light transport effects are significant for many materials like skin, smoke, clouds, snow or water. In particular, one must consider the multiple scattering of light within the volume. While it is possible to simulate such media using volumetric Monte Carlo or finite element techniques, those methods are very computationally expensive. On the other hand, simple analytic models have so far been limited to homogeneous and/or optically dense media and cannot be easily extended to include strongly directional effects and visibility in spatially varying volumes. We present a practical method for rendering volumetric effects that include multiple scattering. We show an expression for the point spread function that captures blurring of radiance due to multiple scattering. We develop a general framework for incorporating this point spread function, while considering inhomogeneous media - this framework could also be used with other analytic multiple scattering models.
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.
An Efficient Hybrid Shadow Rendering Algorithm
(The Eurographics Association, 2004) Chan, Eric; Durand, Fredo; Alexander Keller and Henrik Wann Jensen
We present a hybrid algorithm for rendering hard shadows accurately and efficiently. Our method combines the strengths of shadow maps and shadow volumes. We first use a shadow map to identify the pixels in the image that lie near shadow discontinuities. Then, we perform the shadow-volume computation only at these pixels to ensure accurate shadow edges. This approach simultaneously avoids the edge aliasing artifacts of standard shadow maps and avoids the high fillrate consumption of standard shadow volumes. The algorithm relies on a hardware mechanism for rapidly rejecting non-silhouette pixels during rasterization. Since current graphics hardware does not directly provide this mechanism, we simulate it using available features related to occlusion culling and show that dedicated hardware support requires minimal changes to existing technology.
Sketch Interpretation and Refinement Using Statistical Models
(The Eurographics Association, 2004) Simhon, Saul; Dudek, Gregory; Alexander Keller and Henrik Wann Jensen
We present a system for generating 2D illustrations from hand drawn outlines consisting of only curve strokes. A user can draw a coarse sketch and the system would automatically augment the shape, thickness, color and surrounding texture of the curves making up the sketch. The styles for these refinements are learned from examples whose semantics have been pre-classified. There can be several styles applicable on a curve and the system automatically identifies which one to use and how to use it based on a curve's shape and its context in the illustration. Our approach is based on a Hierarchical Hidden Markov Model. We present a two level hierarchy in which the refinement process is applied at: the curve level and the scene level.
Lattice-Boltzmann Lighting
(The Eurographics Association, 2004) Geist, Robert; Rasche, Karl; Westall, James; Schalkoff, Robert; Alexander Keller and Henrik Wann Jensen
A new technique for lighting participating media is suggested. The technique is based on the lattice-Boltzmann method, which is gaining popularity as alternative to finite-element methods for flow computations, due to its ease of implementation and ability to handle complex boundary conditions. A relatively simple, grid-based photon transport model is postulated and then shown to describe, in the limit, a diffusion process. An application to lighting clouds is provided, where cloud densities are generated by combining two well-established techniques. Performance of the new lighting technique is not real-time, but the technique is highly parallel and does offer an ability to easily represent complex scattering events. Sample renderings are included.
Combining Higher-Order Wavelets and Discontinuity Meshing: a Compact Representation for Radiosity
(The Eurographics Association, 2004) Holzschuch, N.; Alonso, L.; Alexander Keller and Henrik Wann Jensen
The radiosity method is used for global illumination simulation in diffuse scenes, or as an intermediate step in other methods. Radiosity computations using Higher-Order wavelets achieve a compact representation of the illumination on many parts of the scene, but are more expensive near discontinuities, such as shadow boundaries. Other methods use a mesh, based on the set of discontinuities of the illumination function. The complexity of this set of discontinuities has so far proven prohibitive for large scenes, mostly because of the difficulty to robustly manage a geometrically complex set of triangles. In this paper, we present a method for computing radiosity that uses higher-order wavelet functions as a basis, and introduces discontinuities only when they simplify the resulting mesh. The result is displayed directly, without post-processing.

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