30-Issue 4

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https://diglib.eg.org/handle/10.2312/164

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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.
Coherent Out-of-Core Point-Based Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Kontkanen, Janne; Tabellion, Eric; Overbeck, Ryan S.; Ravi Ramamoorthi and Erik Reinhard
We describe a new technique for coherent out-of-core point-based global illumination and ambient occlusion. Point-based global illumination (PBGI) is used in production to render tremendously complex scenes, so in-core storage of point and octree data structures quickly becomes a problem. However, a simple out-of-core extension of a classical top-down octree building algorithm would be extremely inefficient due to large amount of I/O required. Our method extends previous PBGI algorithms with an out-of-core technique that uses minimal I/O and stores data on disk compactly and in coherent chunks for later access during shading. Using properties of a space-filling Z-curve, we are able to preprocess the data in two passes: an external 1D-sort and an octree construction pass.
Direct Ray Tracing of Phong Tessellation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Ogaki, Shinji; Tokuyoshi, Yusuke; Ravi Ramamoorthi and Erik Reinhard
There are two major ways of calculating ray and parametric surface intersections in rendering. The first is through the use of tessellated triangles, and the second is to use parametric surfaces together with numerical methods such as Newton's method. Both methods are computationally expensive and complicated to implement. In this paper, we focus on Phong Tessellation and introduce a simple direct ray tracing method for Phong Tessellation. Our method enables rendering smooth surfaces in a computationally inexpensive yet robust way.
Efficient Computation of Blue Noise Point Sets through Importance Sampling
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Kalantari, Nima Khademi; Sen, Pradeep; Ravi Ramamoorthi and Erik Reinhard
Dart-throwing can generate ideal Poisson-disk distributions with excellent blue noise properties, but is very computationally expensive if a maximal point set is desired. In this paper, we observe that the Poisson-disk sampling problem can be posed in terms of importance sampling by representing the available space to be sampled as a probability density function (pdf). This allows us to develop an efficient algorithm for the generation of maximal Poisson-disk distributions with quality similar to naïve dart-throwing but without rejection of samples. In our algorithm, we first position samples in one dimension based on its marginal cumulative distribution function (cdf). We then throw samples in the other dimension only in the regions which are available for sampling. After each 2D sample is placed, we update the cdf and data structures to keep track of the available regions. In addition to uniform sampling, our method is able to perform variable-density sampling with small modifications. Finally, we also propose a new min-conflict metric for variable-density sampling which results in better adaptation of samples to the underlying importance field.
Efficient Packing of Arbitrarily Shaped Charts for Automatic Texture Atlas Generation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Nöll, Tobias; Stricker, Didier; Ravi Ramamoorthi and Erik Reinhard
Texture atlases are commonly used as representations for mesh parameterizations in numerous applications including texture and normal mapping. Therefore, packing is an important post-processing step that tries to place and orient the single parameterizations in a way that the available space is used as efficiently as possible. However, since packing is NP hard, only heuristics can be used in practice to find near-optimal solutions. In this publication we introduce the new search space of modulo valid packings. The key idea thereby is to allow the texture charts to wrap around in the atlas. By utilizing this search space we propose a new algorithm that can be used in order to automatically pack texture atlases. In the evaluation section we show that our algorithm achieves solutions with a significantly higher packing efficiency when compared to the state of the art, especially for complex packing problems.
An Energy-Conserving Hair Reflectance Model
(The Eurographics Association and Blackwell Publishing Ltd., 2011) d'Eon, Eugene; Francois, Guillaume; Hill, Martin; Letteri, Joe; Aubry, Jean-Marie; Ravi Ramamoorthi and Erik Reinhard
We present a reflectance model for dielectric cylinders with rough surfaces such as human hair fibers. Our model is energy conserving and can evaluate arbitrarily many orders of internal reflection. Accounting for compression and contraction of specular cones produces a new longitudinal scattering function which is non-Gaussian and includes an off-specular peak. Accounting for roughness in the azimuthal direction leads to an integral across the hair fiber which is efficiently evaluated using a Gaussian quadrature. Solving cubic equations is avoided, caustics are included in the model in a consistent fashion, and more accurate colors are predicted by considering many internal pathways.
Guided Image Filtering for Interactive High-quality Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bauszat, Pablo; Eisemann, Martin; Magnor, Marcus; Ravi Ramamoorthi and Erik Reinhard
Interactive computation of global illumination is a major challenge in current computer graphics research. Global illumination heavily affects the visual quality of generated images. It is therefore a key attribute for the perception of photo-realistic images. Path tracing is able to simulate the physical behaviour of light using Monte Carlo techniques. However, the computational burden of this technique prohibits interactive rendering times on standard commodity hardware in high-quality. Trying to solve the Monte Carlo integration with fewer samples results in characteristic noisy images. Global illumination filtering methods take advantage of the fact that the integral for neighbouring pixels may be very similar. Averaging samples of similar characteristics in screen-space may approximate the correct integral, but may result in visible outliers. In this paper, we present a novel path tracing pipeline based on an edge-aware filtering method for the indirect illumination which produces visually more pleasing results without noticeable outliers. The key idea is not to filter the noisy path traced images but to use it as a guidance to filter a second image composed from characteristic scene attributes that do not contain noise by default. We show that our approach better approximates the Monte Carlo integral compared to previous methods. Since the computation is carried out completely in screen-space it is therefore applicable to fully dynamic scenes, arbitrary lighting and allows for high-quality path tracing at interactive frame rates on commodity hardware.
Importance Point Projection for GPU-based Final Gathering
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Maletz, David; Wang, Rui; Ravi Ramamoorthi and Erik Reinhard
We present a practical importance-driven method for GPU-based final gathering. We take as input a point cloud representing directly illuminated scene geometry; we then project and splat the points to microbuffers, which store each shading pixel's occluded radiance field. We select points for projection based on importance, defined as each point's estimated contribution to a shading pixel. For each selected point, we calculate its splat size adaptively based on its importance value. The main advantage of our method is that it's simple and fast, and provides the capability to incorporate additional importance factors such as glossy reflection paths. We also introduce an image-space adaptive sampling method, which combines adaptive image subdivision with joint bilateral upsampling to robustly preserve fine details. We have implemented our algorithm on the GPU, providing high-quality rendering for dynamic scenes at near interactive rates.
Improved Stochastic Progressive Photon Mapping with Metropolis Sampling
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Chen, Jiating; Wang, Bin; Yong, Jun-Hai; Ravi Ramamoorthi and Erik Reinhard
This paper presents an improvement to the stochastic progressive photon mapping (SPPM), a method for robustly simulating complex global illumination with distributed ray tracing effects. Normally, similar to photon mapping and other particle tracing algorithms, SPPM would become inefficient when the photons are poorly distributed. An inordinate amount of photons are required to reduce the error caused by noise and bias to acceptable levels. In order to optimize the distribution of photons, we propose an extension of SPPM with a Metropolis-Hastings algorithm, effectively exploiting local coherence among the light paths that contribute to the rendered image. A well-designed scalar contribution function is introduced as our Metropolis sampling strategy, targeting at specific parts of image areas with large error to improve the efficiency of the radiance estimator. Experimental results demonstrate that the new Metropolis sampling based approach maintains the robustness of the standard SPPM method, while significantly improving the rendering efficiency for a wide range of scenes with complex lighting.
Least Squares Vertex Baking
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Kavan, Ladislav; Bargteil, Adam W.; Sloan, Peter-Pike; Ravi Ramamoorthi and Erik Reinhard
We investigate the representation of signals defined on triangle meshes using linearly interpolated vertex attributes. Compared to texture mapping, storing data only at vertices yields significantly lower memory overhead and less expensive runtime reconstruction. However, standard approaches to determine vertex values such as point sampling or averaging triangle samples lead to suboptimal approximations. We discuss how an optimal solution can be efficiently calculated using continuous least-squares. In addition, we propose a regularization term that allows us to minimize gradient discontinuities and mach banding artifacts while staying close to the optimum. Our method has been integrated in a game production lighting tool and we present examples of representing signals such as ambient occlusion and precomputed radiance transfer in real game scenes, where vertex baking was used to free up resources for other game components.
ManyLoDs: Parallel Many-View Level-of-Detail Selection for Real-Time Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Holländer, Matthias; Ritschel, Tobias; Eisemann, Elmar; Boubekeur, Tamy; Ravi Ramamoorthi and Erik Reinhard
Level-of-Detail structures are a key component for scalable rendering. Built from raw 3D data, these structures are often defined as Bounding Volume Hierarchies, providing coarse-to-fine adaptive approximations that are well-adapted for many-view rasterization. Here, the total number of pixels in each view is usually low, while the cost of choosing the appropriate LoD for each view is high. This task represents a challenge for existing GPU algorithms. We propose ManyLoDs, a new GPU algorithm to efficiently compute many LoDs from a Bounding Volume Hierarchy in parallel by balancing the workload within and among LoDs. Our approach is not specific to a particular rendering technique, can be used on lazy representations such as polygon soups, and can handle dynamic scenes. We apply our method to various many-view rasterization applications, including Instant Radiosity, Point-Based Global Illumination, and reflection / refraction mapping. For each of these, we achieve real-time performance in complex scenes at high resolutions.
Optimizing Environment Maps for Material Depiction
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bousseau, Adrien; Chapoulie, Emmanuelle; Ramamoorthi, Ravi; Agrawala, Maneesh; Ravi Ramamoorthi and Erik Reinhard
We present an automated system for optimizing and synthesizing environment maps that enhance the appearance of materials in a scene. We first identify a set of lighting design principles for material depiction. Each principle specifies the distinctive visual features of a material and describes how environment maps can emphasize those features. We express these principles as linear or quadratic image quality metrics, and present a general optimization framework to solve for the environment map that maximizes these metrics. We accelerate metric evaluation using an approach dual to precomputed radiance transfer (PRT). In contrast to standard PRT that integrates light transport over the lighting domain to generate an image, we pre-integrate light transport over the image domain to optimize for lighting. Finally we present two techniques for transforming existing photographic environment maps to better emphasize materials. We demonstrate the effectiveness of our approach by generating environment maps that enhance the depiction of a variety of materials including glass, metal, plastic, marble and velvet.
Perception of Visual Artifacts in Image-Based Rendering of Façades
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Vangorp, Peter; Chaurasia, Gaurav; Laffont, Pierre-Yves; Fleming, Roland W.; Drettakis, George; Ravi Ramamoorthi and Erik Reinhard
Image-based rendering (IBR) techniques allow users to create interactive 3D visualizations of scenes by taking a few snapshots. However, despite substantial progress in the field, the main barrier to better quality and more efficient IBR visualizations are several types of common, visually objectionable artifacts. These occur when scene geometry is approximate or viewpoints differ from the original shots, leading to parallax distortions, blurring, ghosting and popping errors that detract from the appearance of the scene. We argue that a better understanding of the causes and perceptual impact of these artifacts is the key to improving IBR methods. In this study we present a series of psychophysical experiments in which we systematically map out the perception of artifacts in IBR visualizations of façades as a function of the most common causes. We separate artifacts into different classes and measure how they impact visual appearance as a function of the number of images available, the geometry of the scene and the viewpoint. The results reveal a number of counter-intuitive effects in the perception of artifacts. We summarize our results in terms of practical guidelines for improving existing and future IBR techniques.
Perceptual Global Illumination Cancellation in Complex Projection Environments
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Sheng, Yu; Cutler, Barbara; Chen, Chao; Nasman, Joshua; Ravi Ramamoorthi and Erik Reinhard
The unintentional scattering of light between neighboring surfaces in complex projection environments increases the brightness and decreases the contrast, disrupting the appearance of the desired imagery. To achieve satisfactory projection results, the inverse problem of global illumination must be solved to cancel this secondary scattering. In this paper, we propose a global illumination cancellation method that minimizes the perceptual difference between the desired imagery and the actual total illumination in the resulting physical environment. Using Gauss-Newton and active set methods, we design a fast solver for the bound constrained nonlinear least squares problem raised by the perceptual error metrics. Our solver is further accelerated with a CUDA implementation and multi-resolution method to achieve 1-2 fps for problems with approximately 3000 variables. We demonstrate the global illumination cancellation algorithm with our multi-projector system. Results show that our method preserves the color fidelity of the desired imagery significantly better than previous methods.
A Physically Plausible Model for Light Emission from Glowing Solid Objects
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Wilkie, Alexander; Weidlich, Andrea; Ravi Ramamoorthi and Erik Reinhard
The emissive properties of glowing solid objects appear to be something that the graphics community has not considered in depth before. While the volumetric emission of plasma, i.e. flames, has been discussed numerous times, and while the emission characteristics of entire luminaires can be handled via IESNA profiles, the exact appearance of glowing solid objects appears to have eluded detailed scrutiny so far. In this paper, we discuss the theoretical background to thermally induced light emission of objects, describe how one can handle this behaviour with very little effort in a physically based rendering system, and provide examples for the visual importance of handling this in a plausible fashion.
Preface and Table of Contents
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Ravi Ramamoorthi and Erik Reinhard
Progressive Expectation-Maximization for Hierarchical Volumetric Photon Mapping
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Jakob, Wenzel; Regg, Christian; Jarosz, Wojciech; Ravi Ramamoorthi and Erik Reinhard
State-of-the-art density estimation methods for rendering participating media rely on a dense photon representation of the radiance distribution within a scene. A critical bottleneck of such kernel-based approaches is the excessive number of photons that are required in practice to resolve fine illumination details, while controlling the amount of noise. In this paper, we propose a parametric density estimation technique that represents radiance using a hierarchical Gaussian mixture. We efficiently obtain the coefficients of this mixture using a progressive and accelerated form of the Expectation Maximization algorithm. After this step, we are able to create noise-free renderings of high-frequency illumination using only a few thousand Gaussian terms, where millions of photons are traditionally required. Temporal coherence is trivially supported within this framework, and the compact footprint is also useful in the context of real-time visualization. We demonstrate a hierarchical ray tracing-based implementation, as well as a fast splatting approach that can interactively render animated volume caustics.
A Ray Tracing Approach to Diffusion Curves
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bowers, John C.; Leahey, Jonathan; Wang, Rui; Ravi Ramamoorthi and Erik Reinhard
Diffusion curves [OBW*08] provide a flexible tool to create smooth-shaded images from curves defined with colors. The resulting image is typically computed by solving a Poisson equation that diffuses the curve colors to the interior of the image. In this paper we present a new method for solving diffusion curves by using ray tracing. Our approach is analogous to final gathering in global illumination, where the curves define source radiance whose visible contribution will be integrated at a shading pixel to produce a color using stochastic ray tracing. Compared to previous work, the main benefit of our method is that it provides artists with extended flexibility in achieving desired image effects. Specifically, we introduce generalized curve colors called shaders that allow for the seamless integration of diffusion curves with classic 2D graphics including vector graphics (e.g. gradient fills) and raster graphics (e.g. patterns and textures). We also introduce several extended curve attributes to customize the contribution of each curve. In addition, our method allows any pixel in the image to be independently evaluated, without having to solve the entire image globally (as required by a Poisson-based approach). Finally, we present a GPU-based implementation that generates solution images at interactive rates, enabling dynamic curve editing. Results show that our method can easily produce a variety of desirable image effects.
Silhouette-Aware Warping for Image-Based Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Chaurasia, Gaurav; Sorkine, Olga; Drettakis, George; Ravi Ramamoorthi and Erik Reinhard
Image-based rendering (IBR) techniques allow capture and display of 3D environments using photographs. Modern IBR pipelines reconstruct proxy geometry using multi-view stereo, reproject the photographs onto the proxy and blend them to create novel views. The success of these methods depends on accurate 3D proxies, which are difficult to obtain for complex objects such as trees and cars. Large number of input images do not improve reconstruction proportionally; surface extraction is challenging even from dense range scans for scenes containing such objects. Our approach does not depend on dense accurate geometric reconstruction; instead we compensate for sparse 3D information by variational image warping. In particular, we formulate silhouette-aware warps that preserve salient depth discontinuities. This improves the rendering of difficult foreground objects, even when deviating from view interpolation. We use a semi-automatic step to identify depth discontinuities and extract a sparse set of depth constraints used to guide the warp. Our framework is lightweight and results in good quality IBR for previously challenging environments.
Stone Weathering in a Photograph
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Xue, Su; Dorsey, Julie; Rushmeier, Holly; Ravi Ramamoorthi and Erik Reinhard
The appearance of weathering effects on stone is important for creating outdoor scenes in computer graphics. To achieve them, previous research has built upon physical simulation, which, while yielding a degree of realism, is computationally expensive and inapplicable to the situation when the object geometry is unknown. Also, physical simulation requires specific knowledge of the stone properties and environmental processes. In this paper, we present a simple visual simulation pipeline for creating weathering effects on stone within a single image. Two primary effects of stone weathering, i.e., smoothing and roughening, are considered. In addition, erosion on the object silhouette is treated. These challenging effects involve significant geometry changes, which are intractable for previous image-based editing techniques. The effectiveness of our technique is illustrated on a variety of scenes and types of stone. While it can be fully automatic, it also allows easy user interaction.

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