29-Issue 4

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https://diglib.eg.org/handle/10.2312/155

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Two Methods for Fast Ray-Cast Ambient Occlusion
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Laine, Samuli; Karras, Tero
Ambient occlusion has proven to be a useful tool for producing realistic images, both in offline rendering and interactive applications. In production rendering, ambient occlusion is typically computed by casting a large number of short shadow rays from each visible point, yielding unparalleled quality but long rendering times. Interactive applications typically use screen-space approximations which are fast but suffer from systematic errors due to missing information behind the nearest depth layer.In this paper, we present two efficient methods for calculating ambient occlusion so that the results match those produced by a ray tracer. The first method is targeted for rasterization-based engines, and it leverages the GPU graphics pipeline for finding occlusion relations between scene triangles and the visible points. The second method is a drop-in replacement for ambient occlusion computation in offline renderers, allowing the querying of ambient occlusion for any point in the scene. Both methods are based on the principle of simultaneously computing the result of all shadow rays for a single receiver point.
A Dynamic Noise Primitive for Coherent Stylization
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Benard, P.; Lagae, A.; Vangorp, P.; Lefebvre, S.; Drettakis, G.; Thollot, J.
We present a new solution for temporal coherence in non-photorealistic rendering (NPR) of animations. Given the conflicting goals of preserving the 2D aspect of the style and the 3D scene motion, any such solution is a tradeoff. We observe that primitive-based methods in NPR can be seen as texture-based methods when using large numbers of primitives, leading to our key insight, namely that this process is similar to sparse convolution noise in procedural texturing. Consequently, we present a new primitive for NPR based on Gabor noise, that preserves the 2D aspect of noise, conveys the 3D motion of the scene, and is temporally continuous. We can thus use standard techniques from procedural texturing to create various styles, which we show for interactive NPR applications. We also present a user study to evaluate this and existing solutions, and to provide more insight in the trade-off implied by temporal coherence. The results of the study indicate that maintaining coherent motion is important, but also that our new solution provides a good compromise between the 2D aspect of the style and 3D motion.
Patch-based Texture Interpolation
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Ruiters, Roland; Schnabel, Ruwen; Klein, Reinhard
In this paper, we present a novel exemplar-based technique for the interpolation between two textures that combines patch-based and statistical approaches. Motivated by the notion of texture as a largely local phenomenon, we warp and blend small image neighborhoods prior to patch-based texture synthesis. In addition, interpolating and enforcing characteristic image statistics faithfully handles high frequency detail. We are able to create both intermediate textures as well as continuous transitions. In contrast to previous techniques computing a global morphing transformation on the entire input exemplar images, our localized and patch-based approach allows us to successfully interpolate between textures with considerable differences in feature topology for which no smooth global warping field exists.
Compressive estimation for signal integration in rendering
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Sen, Pradeep; Darabi, Soheil
In rendering applications, we are often faced with the problem of computing the integral of an unknown function. Typical approaches used to estimate these integrals are often based on Monte Carlo methods that slowly converge to the correct answer after many point samples have been taken. In this work, we study this problem under the framework of compressed sensing and reach the conclusion that if the signal is sparse in a transform domain, we can evaluate the integral accurately using a small set of point samples without requiring the lengthy iterations of Monte Carlo approaches. We demonstrate the usefulness of our framework by proposing novel algorithms to address two problems in computer graphics: image antialiasing and motion blur. We show that we can use our framework to generate good results with fewer samples than is possible with traditional approaches.
An Optimizing Compiler for Automatic Shader Bounding
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Clarberg, Petrik; Toth, Robert; Hasselgren, Jon; Akenine-Moeller, Tomas
Fragment-Parallel Composite and Filter
(The Eurographics Association and Blackwell Publishing Ltd, 2010)
A Layered Particle-Based Fluid Model for Real-Time Rendering of Water
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Bagar, Florian; Scherzer, Daniel; Wimmer, Michael
We present a physically based real-time water simulation and rendering method that brings volumetric foam to the real-time domain, significantly increasing the realism of dynamic fluids. We do this by combining a particle-based fluid model that is capable of accounting for the formation of foam with a layered rendering approach that is able to account for the volumetric properties of water and foam. Foam formation is simulated through Weber number thresholding. For rendering, we approximate the resulting water and foam volumes by storing their respective boundary surfaces in depth maps. This allows us to calculate the attenuation of light rays that pass through these volumes very efficiently. We also introduce an adaptive curvature flow filter that produces consistent fluid surfaces from particles independent of the viewing distance.
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.
SafeGI: Type Checking to Improve Correctness in Rendering System Implementation
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Ou, Jiawei; Pellacini, Fabio
Historically, rendering system development has been mainly focused on improving the numerical accuracy of the rendering algorithms and their runtime efficiency. In this paper, we propose a method to improve the correctness not of the algorithms themselves, but of their implementation. Specifically, we show that by combining static type checking and generic programming, rendering system and shader development can take advantage of compile-time checking to perform dimensional analysis, i.e. to enforce the correctness of physical dimensions and units in light transport, and geometric space analysis, i.e. to ensure that geometric computations respect the spaces in which points, vectors and normals were defined. We demonstrate our methods by implementing a CPU path tracer and a GPU renderer which previews direct illumination. While we build on prior work to develop our implementations, the main contribution of our work is to show that dimensional analysis and geometric space checking can be successfully integrated into the development of rendering systems and shaders.
Interactive, Multiresolution Image-Space Rendering for Dynamic Area Lighting
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Nichols, Greg; Penmatsa, Rajeev; Wyman, Chris
Area lights add tremendous realism, but rendering them interactively proves challenging. Integrating visibility is costly, even with current shadowing techniques, and existing methods frequently ignore illumination variations at unoccluded points due to changing radiance over the light s surface. We extend recent image-space work that reduces costs by gathering illumination in a multiresolution fashion, rendering varying frequencies at corresponding resolutions. To compute visibility, we eschew shadow maps and instead rely on a coarse screen-space voxelization, which effectively provides a cheap layered depth image for binary visibility queries via ray marching. Our technique requires no precomputation and runs at interactive rates, allowing scenes with large area lights, including dynamic content such as video screens.
Interactive Rendering of Non-Constant, Refractive Media Using the Ray Equations of Gradient-Index Optics
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Cao, Chen; Ren, Zhong; Guo, Baining; Zhou, Kun
Existing algorithms can efficiently render refractive objects of constant refractive index. For a medium with a continuously varying index of refraction, most algorithms use the ray equation of geometric optics to compute piecewise-linear approximations of the non-linear rays. By assuming a constant refractive index within each tracing step, these methods often need a large number of small steps to generate satisfactory images. In this paper, we present a new approach for tracing non-constant, refractive media based on the ray equations of gradient-index optics. We show that in a medium of constant index gradient, the ray equation has a closed-form solution, and the intersection point between a ray and the medium boundaries can be efficiently computed using the bisection method. For general non-constant media, we model the refractive index as a piecewise-linear function and render the refraction by tracing the tetrahedron-based representation of the media. Our algorithm can be easily combined with existing rendering algorithms such as photon mapping to generate complex refractive caustics at interactive frame rates. We also derive analytic ray formulations for tracing mirages - a special gradient-index optical phenomenon.
Interactive Editing of Lighting and Materials using a Bivariate BRDF Representation
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Sitthi-Amorn, Pitchaya; Romeiro, Fabiano; Zickler, Todd; Lawrence, Jason
We present a new Precomputed Radiance Transfer (PRT) algorithm based on a two dimensional representation of isotropic BRDFs. Our approach involves precomputing matrices that allow quickly mapping environment lighting, which is represented in the global coordinate system, and the surface BRDFs, which are represented in a bivariate domain, to the local hemisphere at a surface location where the reflection integral is evaluated. When the lighting and BRDFs are represented in a wavelet basis, these rotation matrices are sparse and can be efficiently stored and combined with pre-computed visibility at run-time. Compared to prior techniques that also precompute wavelet rotation matrices, our method allows full control over the lighting and materials due to the way the BRDF is represented. Furthermore, this bivariate parameterization preserves sharp specular peaks and grazing effects that are attenuated in conventional parameterizations. We demonstrate a prototype rendering system that achieves real-time framerates while lighting and materials are edited.
Invisible Seams
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Ray, Nicolas; Nivoliers, Vincent; Lefebvre, Sylvain; Levy, Bruno
Surface materials are commonly described by attributes stored in textures (for instance, color, normal, or displacement). Interpolation during texture lookup provides a continuous value field everywhere on the surface, except at the chart boundaries where visible discontinuities appear. We propose a solution to make these seams invisible, while still outputting a standard texture atlas. Our method relies on recent advances in quad remeshing using global parameterization to produce a set of texture coordinates aligning texel grids across chart boundaries. This property makes it possible to ensure that the interpolated value fields on both sides of a chart boundary precisely match, making all seams invisible. However, this requirement on the uv coordinates needs to be complemented by a set of constraints on the colors stored in the texels. We propose an algorithm solving for all the necessary constraints between texel values, including through different magnification modes (nearest, bilinear, biquadratic and bicubic), and across facets using different texture resolutions. In the typical case of bilinear magnification and uniform resolution, none of the texels appearing on the surface are constrained. Our approach also ensures perfect continuity across several MIP-mapping levels.
On the Effective Dimension of Light Transport
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Lessig, Christian; Fiume, Eugene
Light transport is often characterized within a high-dimensional space although practitioners have long known that it commonly behaves as a much lower-dimensional phenomenon. We study the effective dimension of light transport over a neighborhood on the scene manifold and show that under plausible assumptions the dimensionality is characterized by the spectrum of the spatio-spectral concentration problem. This allows us to improve existing estimates for the dimension in computer graphics using a more insightful derivation and for the first time we obtain optimal representations. The relevance of our results for existing rendering applications is discussed.
Fragment-Parallel Composite and Filter
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Patney, Anjul; Tzeng, Stanley; Owens, John D.
We present a strategy for parallelizing the composite and filter operations suitable for an order-independent rendering pipeline implemented on a modern graphics processor. Conventionally, this task is parallelized across pixels/subpixels, but serialized along individual depth layers. However, our technique extends the domain of parallelization to individual fragments (samples), avoiding a serial dependence on the number of depth layers, which can be a constraint for scenes with high depth complexity. As a result, our technique scales with the number of fragments and can sustain a consistent and predictable throughput in scenes with both low and high depth complexity, including those with a high variability of depth complexity within a single frame. We demonstrate composite/filter performance in excess of 50M fragments/sec for scenes with more than 1500 semi-transparent layers.
On Floating-Point Normal Vectors
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Meyer, Quirin; Suessmuth, Jochen; Sussner, Gerd; Stamminger, Marc; Greiner, Guenther
In this paper we analyze normal vector representations. We derive the error of the most widely used representation, namely 3D floating-point normal vectors. Based on this analysis, we show that, in theory, the discretization error inherent to single precision floating-point normals can be achieved by 250.2 uniformly distributed normals, addressable by 51 bits. We review common sphere parameterizations and show that octahedron normal vectors perform best: they are fast and stable to compute, have a controllable error, and require only 1 bit more than the theoretical optimal discretization with the same error.
A New Ward BRDF Model with Bounded Albedo
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Geisler-Moroder, David; Duer, Arne
Due to its realistic appearance, computational convenience, and efficient Monte Carlo sampling, Ward s anisotropic BRDF is widely used in computer graphics for modeling specular reflection. Incorporating the criticism that the Ward and the Ward-Duer model do not meet energy balance at grazing angles, we propose a modified BRDF that is energy conserving and preserves Helmholtz reciprocity. The new BRDF is computationally cheap to evaluate, admits efficient importance sampling, and thus sustains the main benefits of the Ward model. We show that the proposed BRDF is better suited for fitting measured reflectance data of a linoleum floor used in a real-world building than the Ward and the Ward-Duer model.
Grammar-based Encoding of Facades
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Haegler, Simon; Wonka, Peter; Arisona, Stefan Mueller; Van Gool, Luc; Mueller, Pascal
In this paper we propose a real-time rendering approach for procedural cities. Our first contribution is a new lightweight grammar representation that compactly encodes facade structures and allows fast per-pixel access. We call this grammar F-shade. Our second contribution is a prototype rendering system that renders an urban model from the compact representation directly on the GPU. Our suggested approach explores an interesting connection from procedural modeling to real-time rendering. Evaluating procedural descriptions at render time uses less memory than the generation of intermediate geometry. This enables us to render large urban models directly from GPU memory.
BendyLights: Artistic Control of Direct Illumination by Curving Light Rays
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Kerr, William B.; Pellacini, Fabio; Denning, Jonathan D.
In computer cinematography, artists routinely use non-physical lighting models to achieve desired appearances. This paper presents BendyLights, a non-physical lighting model where light travels nonlinearly along splines, allowing artists to control light direction and shadow position at different points in the scene independently. Since the light deformation is smoothly defined at all world-space positions, the resulting non-physical lighting effects remain spatially consistent, avoiding the frequent incongruences of many non-physical models. BendyLights are controlled simply by reshaping splines, using familiar interfaces, and require very few parameters. BendyLight control points can be keyframed to support animated lighting effects. We demonstrate BendyLights both in a realtime rendering system for editing and a production renderer for final rendering, where we show that BendyLights can also be used with global illumination.
An Image-Based Approach for Stochastic Volumetric and Procedural Details
(The Eurographics Association and Blackwell Publishing Ltd, 2010) Gilet, G.; Dischler, J-M.
Noisy volumetric details like clouds, grounds, plaster, bark, roughcast, etc. are frequently encountered in nature and bring an important contribution to the realism of outdoor scenes. We introduce a new interactive approach, easing the creation of procedural representations of stochastic volumetric details by using a single example photograph. Instead of attempting to reconstruct an accurate geometric representation from the photograph, we use a stochastic multi-scale approach that fits parameters of a multi-layered noise-based 3D deformation model, using a multi-resolution filter banks error metric. Once computed, visually similar details can be applied to arbitrary objects with a high degree of visual realism, since lighting and parallax effects are naturally taken into account. Our approach is inspired by image-based techniques. In practice, the user supplies a photograph of an object covered by noisy details, provides a corresponding coarse approximation of the shape of this object as well as an estimated lighting condition (generally a light source direction). Our system then determines the corresponding noise-based representation as well as some diffuse, ambient, specular and semi-transparency reflectance parameters. The resulting details are fully procedural and, as such, have the advantage of extreme compactness, while they can be infinitely extended without repetition in order to cover huge surfaces.

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