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.
Automatic Cinemagraph Portraits
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Bai, Jiamin; Agarwala, Aseem; Agrawala, Maneesh; Ramamoorthi, Ravi; Nicolas Holzschuch and Szymon Rusinkiewicz
Cinemagraphs are a popular new type of visual media that lie in-between photos and video; some parts of the frame are animated and loop seamlessly, while other parts of the frame remain completely still. Cinemagraphs are especially effective for portraits because they capture the nuances of our dynamic facial expressions. We present a completely automatic algorithm for generating portrait cinemagraphs from a short video captured with a hand-held camera. Our algorithm uses a combination of face tracking and point tracking to segment face motions into two classes: gross, large-scale motions that should be removed from the video, and dynamic facial expressions that should be preserved. This segmentation informs a spatially-varying warp that removes the large-scale motion, and a graph-cut segmentation of the frame into dynamic and still regions that preserves the finer-scale facial expression motions. We demonstrate the success of our method with a variety of results and a comparison to previous work.
Computational Simulation of Alternative Photographic Processes
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Echevarria, Jose I.; Wilensky, Gregg; Krishnaswamy, Aravind; Kim, Byungmoon; Gutierrez, Diego; Nicolas Holzschuch and Szymon Rusinkiewicz
We present a novel computational framework for physically and chemically-based simulations of analog alternative photographic processes. In the real world, these processes allow the creation of very personal and unique depictions due to the combination of the chemicals used, the physical interaction with liquid solutions, and the individual craftsmanship of the artist. Our work focuses not only on achieving similar compelling results, but on the manual process as well, introducing a novel exploratory approach for interactive digital image creation and manipulation. With such an emphasis on the user interaction, our simulations are devised to run on tablet devices; thus we propose the combination of a lightweight data-driven model to simulate the chemical reactions involved, with efficient fluids simulations that modulate them. This combination allows realistic gestures-based user interaction with constant visual feedback in real-time. Using the proposed framework, we have built two prototypes with different tradeoffs between realism and flexibility, showing its potential to build novel image editing tools.
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.
Factorized Point Based Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wang, Beibei; Huang, Jing; Buchholz, Bert; Meng, Xiangxu; Boubekeur, Tamy; Nicolas Holzschuch and Szymon Rusinkiewicz
The Point-Based Global Illumination (PBGI) algorithm is composed of two major steps: a caching step and a multiview rasterization step. At caching time, a dense point-sampling of the scene is shaded and organized in a spatial hierarchy, with internal nodes approximating the radiance of their subtrees using spherical harmonics. At rasterization time, a microbuffer is instantiated at the unprojected position of each image pixel (receiver). Then, a view-adaptive level-of-detail of the scene is extracted in the form of a tree cut and rasterized in the receiver's microbuffer, solving for visibility using a local variant of the z-buffer. Finally, the pixel color is computed by convolving its filled microbuffer with the surface BRDF. This noise-free indirect lighting method is widely used in the industry and captures several critical lighting effects, including ambient occlusion, color bleeding, (indirect) soft-shadows and environment lighting. However, we observe a large redundancy in this algorithm, both in cuts and receivers'microbuffers, which stems from their relatively low resolution. In this paper, we propose an evolution of PBGI which exploits spatial coherence to reduce these redundant computations. Starting from a similarity-based variational clustering of the receivers, we compute a single tree cut and rasterize a single microbuffer for each cluster. This per-cluster microbuffer provides a faithful approximation of the incident radiance for distant nodes and is composited over a receiver-specific microbuffer rasterizing only the closest nodes of the cluster's cut. This factorized approach is easy to integrate in any existing PBGI implementation and offers a significant rendering speed-up for a negligible and controllable approximation error.
Generating Pointillism Paintings Based on Seurat's Color Composition
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wu, Yi-Chian; Tsai, Yu-Ting; Lin, Wen-Chieh; Li, Wen-Hsin; Nicolas Holzschuch and Szymon Rusinkiewicz
This paper presents a novel example-based stippling technique that employs a simple and intuitive concept to convert a color image into a pointillism painting. Our method relies on analyzing and imitating the color distributions of Seurat's paintings to obtain a statistical color model. Then, this model can be easily combined with the modified multi-class blue noise sampling to stylize an input image with characteristics of color composition in Seurat's paintings. The blue noise property of the output image also ensures that the color points are randomly located but remain spatially uniform. In our experiments, the multivariate goodness-of-fit tests were adopted to quantitatively analyze the results of the proposed and previous methods, further confirming that the color composition of our results are more similar to Seurat's painting style than that of previous approaches. Additionally, we also conducted a user study participated by artists to qualitatively evaluate the synthesized images of the proposed method.
Interactive Lighting Design with Hierarchical Light Representation
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lin, Wen-Chieh; Huang, Tsung-Shian; Ho, Tan-Chi; Chen, Yueh-Tse; Chuang, Jung-Hong; Nicolas Holzschuch and Szymon Rusinkiewicz
Lighting design plays a crucial role in indoor lighting design, computer cinematograph and many other applications. Computer-assisted lighting design aims to find a lighting configuration that best approximates the illumination effect specified by designers. In this paper, we present an automatic approach for lighting design, in which discrete and continuous optimization of the lighting configuration, including the number, intensity, and position of lights, are achieved. Our lighting design algorithm consists of two major steps. The first step estimates an initial lighting configuration by light sampling and clustering. The initial light clusters are then recursively merged to form a light hierarchy. The second step optimizes the lighting configuration by alternatively selecting a light cut on the light hierarchy to determine the number of representative lights and optimizing the lighting parameters using the simplex method. To speed up the optimization computation, only illumination at scene vertices that are important to rendering are sampled and taken into account in the optimization. Using the proposed approach, we develop a lighting design system that can compute appropriate lighting configurations to generate the illumination effects iteratively painted and modified by a designer interactively.
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.
Optimizing Disparity for Motion in Depth
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Kellnhofer, Petr; Ritschel, Tobias; Myszkowski, Karol; Seidel, Hans-Peter; Nicolas Holzschuch and Szymon Rusinkiewicz
Beyond the careful design of stereo acquisition equipment and rendering algorithms, disparity post-processing has recently received much attention, where one of the key tasks is to compress the originally large disparity range to avoid viewing discomfort. The perception of dynamic stereo content however, relies on reproducing the full disparity-time volume that a scene point undergoes in motion. This volume can be strongly distorted in manipulation, which is only concerned with changing disparity at one instant in time, even if the temporal coherence of that change is maintained. We propose an optimization to preserve stereo motion of content that was subject to an arbitrary disparity manipulation, based on a perceptual model of temporal disparity changes. Furthermore, we introduce a novel 3D warping technique to create stereo image pairs that conform to this optimized disparity map. The paper concludes with perceptual studies of motion reproduction quality and task performance in a simple game, showing how our optimization can achieve both viewing comfort and faithful stereo motion.
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.
Practical Real-Time Lens-Flare Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lee, Sungkil; Eisemann, Elmar; Nicolas Holzschuch and Szymon Rusinkiewicz
We present a practical real-time approach for rendering lens-flare effects. While previous work employed costly ray tracing or complex polynomial expressions, we present a coarser, but also significantly faster solution. Our method is based on a first-order approximation of the ray transfer in an optical system, which allows us to derive a matrix that maps lens flare-producing light rays directly to the sensor. The resulting approach is easy to implement and produces physically-plausible images at high framerates on standard off-the-shelf graphics hardware.
Preface and Table of Contents
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Nicolas Holzschuch and Szymon Rusinkiewicz
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.
A Shape-Aware Model for Discrete Texture Synthesis
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Landes, Pierre-Edouard; Galerne, Bruno; Hurtut, Thomas; Nicolas Holzschuch and Szymon Rusinkiewicz
We present a novel shape-aware method for synthesizing 2D and 3D discrete element textures consisting of collections of distinct vector graphics objects. Extending the long-proven point process framework, we propose a shape process, a novel stochastic model based on spatial measurements that fully take into account the geometry of the elements. We demonstrate that our approach is well-suited for discrete texture synthesis by example. Our model enables for both robust statistical parameter estimation and reliable output generation by Monte Carlo sampling. Our numerous experiments show that contrary to current state-of-the-art techniques, our algorithm manages to capture anisotropic element distributions and systematically prevents undesirable collisions between objects.
Sorted Deferred Shading for Production Path Tracing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Eisenacher, Christian; Nichols, Gregory; Selle, Andrew; Burley, Brent; Nicolas Holzschuch and Szymon Rusinkiewicz
Ray-traced global illumination (GI) is becoming widespread in production rendering but incoherent secondary ray traversal limits practical rendering to scenes that fit in memory. Incoherent shading also leads to intractable performance with production-scale textures forcing renderers to resort to caching of irradiance, radiosity, and other values to amortize expensive shading. Unfortunately, such caching strategies complicate artist workflow, are difficult to parallelize effectively, and contend for precious memory. Worse, these caches involve approximations that compromise quality. In this paper, we introduce a novel path-tracing framework that avoids these tradeoffs. We sort large, potentially out-of-core ray batches to ensure coherence of ray traversal. We then defer shading of ray hits until we have sorted them, achieving perfectly coherent shading and avoiding the need for shading caches.
Spherical Visibility Sampling
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Eikel, Benjamin; Jähn, Claudius; Fischer, Matthias; Heide, Friedhelm Meyer auf der; Nicolas Holzschuch and Szymon Rusinkiewicz
Many 3D scenes (e.g. generated from CAD data) are composed of a multitude of objects that are nested in each other. A showroom, for instance, may contain multiple cars and every car has a gearbox with many gearwheels located inside. Because the objects occlude each other, only few are visible from outside. We present a new technique, Spherical Visibility Sampling (SVS), for real-time 3D rendering of such - possibly highly complex - scenes. SVS exploits the occlusion and annotates hierarchically structured objects with directional visibility information in a preprocessing step. For different directions, the directional visibility encodes which objects of a scene's region are visible from the outside of the regions' enclosing bounding sphere. Since there is no need to store a separate view space subdivision as in most techniques based on preprocessed visibility, a small memory footprint is achieved. Using the directional visibility information for an interactive walkthrough, the potentially visible objects can be retrieved very efficiently without the need for further visibility tests. Our evaluation shows that using SVS allows to preprocess complex 3D scenes fast and to visualize them in real time (e.g. a Power Plant model and five animated Boeing 777 models with billions of triangles). Because SVS does not require hardware support for occlusion culling during rendering, it is even applicable for rendering large scenes on mobile devices.
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.
A Topological Approach to Voxelization
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Laine, Samuli; Nicolas Holzschuch and Szymon Rusinkiewicz
We present a novel approach to voxelization, based on intersecting the input primitives against intersection targets in the voxel grid. Instead of relying on geometric proximity measures, our approach is topological in nature, i.e., it builds on the connectivity and separability properties of the input and the intersection targets. We discuss voxelization of curves and surfaces in both 2D and 3D, and derive intersection targets that produce voxelizations with various connectivity, separability and thinness properties. The simplicity of our method allows for easy proofs of these properties. Our approach is directly applicable to curved primitives, and it is independent of input tessellation.

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