Asymptotic models and curved surfaces : application to railway transportation

5 pagesInternational audienceSeveral wireless communication systems are developed for communication needs between train and ground or between trains in the railway or mass transit domains. In order to deploy these systems in specific environments, such as tunnels, straight or curved, rectangular or arch-shaped section, specific propagation models have to be developed. In this paper, we propose a method to model the radiowave propagation in straight arch-shaped tunnels by using asymptotic methods, such as ray tracing and ray launching, combined with the tessellation of the arched section. A method of interpolation of the facets normals is implemented in order to minimize the error made when using the tessellation. These results are compared to those found in the literature in order to validate our approach

Tessellated Voxelization for Global Illumination using Voxel Cone Tracing

Modeling believable lighting is a crucial component of computer graphics applications, including games and modeling programs. Physically accurate lighting is complex and is not currently feasible to compute in real-time situations. Therefore, much research is focused on investigating efficient ways to approximate light behavior within these real-time constraints. In this thesis, we implement a general purpose algorithm for real-time applications to approximate indirect lighting. Based on voxel cone tracing, we use a filtered representation of a scene to efficiently sample ambient light at each point in the scene. We present an approach to scene voxelization using hardware tessellation and compare it with an approach utilizing hardware rasterization. We also investigate possible methods of warped voxelization. Our contributions include a complete and open-source implementation of voxel cone tracing along with both voxelization algorithms. We find similar performance and quality with both voxelization algorithms

Towards Predictive Rendering in Virtual Reality

The strive for generating predictive images, i.e., images representing radiometrically correct renditions of reality, has been a longstanding problem in computer graphics. The exactness of such images is extremely important for Virtual Reality applications like Virtual Prototyping, where users need to make decisions impacting large investments based on the simulated images. Unfortunately, generation of predictive imagery is still an unsolved problem due to manifold reasons, especially if real-time restrictions apply. First, existing scenes used for rendering are not modeled accurately enough to create predictive images. Second, even with huge computational efforts existing rendering algorithms are not able to produce radiometrically correct images. Third, current display devices need to convert rendered images into some low-dimensional color space, which prohibits display of radiometrically correct images. Overcoming these limitations is the focus of current state-of-the-art research. This thesis also contributes to this task. First, it briefly introduces the necessary background and identifies the steps required for real-time predictive image generation. Then, existing techniques targeting these steps are presented and their limitations are pointed out. To solve some of the remaining problems, novel techniques are proposed. They cover various steps in the predictive image generation process, ranging from accurate scene modeling over efficient data representation to high-quality, real-time rendering. A special focus of this thesis lays on real-time generation of predictive images using bidirectional texture functions (BTFs), i.e., very accurate representations for spatially varying surface materials. The techniques proposed by this thesis enable efficient handling of BTFs by compressing the huge amount of data contained in this material representation, applying them to geometric surfaces using texture and BTF synthesis techniques, and rendering BTF covered objects in real-time. Further approaches proposed in this thesis target inclusion of real-time global illumination effects or more efficient rendering using novel level-of-detail representations for geometric objects. Finally, this thesis assesses the rendering quality achievable with BTF materials, indicating a significant increase in realism but also confirming the remainder of problems to be solved to achieve truly predictive image generation

Efficient multi-bounce lightmap creation using GPU forward mapping

Computer graphics can nowadays produce images in realtime that are hard to distinguish from photos of a real scene. One of the most important aspects to achieve this is the interaction of light with materials in the virtual scene. The lighting computation can be separated in two different parts. The first part is concerned with the direct illumination that is applied to all surfaces lit by a light source; algorithms related to this have been greatly improved over the last decades and together with the improvements of the graphics hardware can now produce realistic effects. The second aspect is about the indirect illumination which describes the multiple reflections of light from each surface. In reality, light that hits a surface is never fully absorbed, but instead reflected back into the scene. And even this reflected light is then reflected again and again until its energy is depleted. These multiple reflections make indirect illumination very computationally expensive. The first problem regarding indirect illumination is therefore, how it can be simplified to compute it faster. Another question concerning indirect illumination is, where to compute it. It can either be computed in the fixed image that is created when rendering the scene or it can be stored in a light map. The drawback of the first approach is, that the results need to be recomputed for every frame in which the camera changed. The second approach, on the other hand, is already used for a long time. Once a static scene has been set up, the lighting situation is computed regardless of the time it takes and the result is then stored into a light map. This is a texture atlas for the scene in which each surface point in the virtual scene has exactly one surface point in the 2D texture atlas. When displaying the scene with this approach, the indirect illumination does not need to be recomputed, but is simply sampled from the light map. The main contribution of this thesis is the development of a technique that computes the indirect illumination solution for a scene at interactive rates and stores the result into a light atlas for visualizing it. To achieve this, we overcome two main obstacles. First, we need to be able to quickly project data from any given camera configuration into the parts of the texture that are currently used for visualizing the 3D scene. Since our approach for computing and storing indirect illumination requires a huge amount of these projections, it needs to be as fast as possible. Therefore, we introduce a technique that does this projection entirely on the graphics card with a single draw call. Second, the reflections of light into the scene need to be computed quickly. Therefore, we separate the computation into two steps, one that quickly approximates the spreading of the light into the scene and a second one that computes the visually smooth final result using the aforementioned projection technique. The final technique computes the indirect illumination at interactive rates even for big scenes. It is furthermore very flexible to let the user choose between high quality results or fast computations. This allows the method to be used for quickly editing the lighting situation with high speed previews and then computing the final result in perfect quality at still interactive rates. The technique introduced for projecting data into the texture atlas is in itself highly flexible and also allows for fast painting onto objects and projecting data onto it, considering all perspective distortions and self-occlusions

View-dependent precomputed light transport using non-linear Gaussian function approximations

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2006.Includes bibliographical references (p. 43-46).We propose a real-time method for rendering rigid objects with complex view-dependent effects under distant all-frequency lighting. Existing precomputed light transport approaches can render rich global illumination effects, but high-frequency view-dependent effects such as sharp highlights remain a challenge. We introduce a new representation of the light transport operator based on sums of Gaussians. The non-linear parameters of the representation allow for 1) arbitrary bandwidth because scale is encoded as a direct parameter; and 2) high-quality interpolation across view and mesh triangles because we interpolate the average direction of the incoming light, thereby preventing linear cross-fading artifacts. However, fitting the precomputed light transport data to this new representation requires solving a non-linear regression problem that is more involved than traditional linear and non-linear (truncation) approximation techniques. We present a new data fitting method based on optimization that includes energy terms aimed at enforcing good interpolation. We demonstrate that our method achieves high visual quality for a small storage cost and fast rendering time.by Paul Elijah Green.S.M

Doctor of Philosophy

dissertationWhile boundary representations, such as nonuniform rational B-spline (NURBS) surfaces, have traditionally well served the needs of the modeling community, they have not seen widespread adoption among the wider engineering discipline. There is a common perception that NURBS are slow to evaluate and complex to implement. Whereas computer-aided design commonly deals with surfaces, the engineering community must deal with materials that have thickness. Traditional visualization techniques have avoided NURBS, and there has been little cross-talk between the rich spline approximation community and the larger engineering field. Recently there has been a strong desire to marry the modeling and analysis phases of the iterative design cycle, be it in car design, turbulent flow simulation around an airfoil, or lighting design. Research has demonstrated that employing a single representation throughout the cycle has key advantages. Furthermore, novel manufacturing techniques employing heterogeneous materials require the introduction of volumetric modeling representations. There is little question that fields such as scientific visualization and mechanical engineering could benefit from the powerful approximation properties of splines. In this dissertation, we remove several hurdles to the application of NURBS to problems in engineering and demonstrate how their unique properties can be leveraged to solve problems of interest

Realistic Visualization of Accessories within Interactive Simulation Systems for Garment Prototyping

In virtual garment prototyping, designers create a garment design by using Computer Aided Design (CAD). In difference to traditional CAD the word "aided" in this case refers to the computer replicating real world behavior of garments. This allows the designer to interact naturally with his design. The designer has a wide range of expressions within his work. This is done by defining details on a garment which are not limited to the type of cloth used. The way how cloth patterns are sewn together and the style and usage of details of the cloth's surface, like appliqués, have a strong impact on the visual appearance of a garment to a large degree. Therefore, virtual and real garments usually have a lot of such surface details. Interactive virtual garment prototyping itself is an interdisciplinary field. Several problems have to be solved to create an efficiently usable real-time virtual prototyping system for garment manufacturers. Such a system can be roughly separated into three sub-components. The first component deals with acquisition of material and other data needed to let a simulation mimic plausible real world behavior of the garment. The second component is the garment simulation process itself. Finally, the third component is centered on the visualization of the simulation results. Therefore, the overall process spans several scientific areas which have to take into account the needs of each other in order to get an overall interactive system. In my work I especially target the third section, which deals with the visualization. On the scientific side, the developments in the last years have shown great improvements on both speed and reliability of simulation and rendering approaches suitable for the virtual prototyping of garments. However, with the currently existing approaches there are still many problems to be solved, especially if interactive simulation and visualization need to work together and many object and surface details come into play. This is the case when using a virtual prototyping in a productive environment. The currently available approaches try to handle most of the surface details as part of the simulation. This generates a lot of data early in the pipeline which needs to be transferred and processed, requiring a lot of processing time and easily stalls the pipeline defined by the simulation and visualization system. Additionally, real world garment examples are already complicated in their cloth arrangement alone. This requires additional computational power. Therefore, the interactive garment simulation tends to lose its capability to allow interactive handling of the garment. In my work I present a solution, which solves this problem by moving the handling of design details from the simulation stage entirely to a completely GPU based rendering stage. This way, the behavior of the garment and its visual appearance are separated. Therefore, the simulation part can fully concentrate on simulating the fabric behavior, while the visualization handles the placing of surface details lighting, materials and self-shadowing. Thus, a much higher degree of surface complexity can be achieved within an interactive virtual prototyping system as can be done with the current existing approaches

Implementation of computer technology for more efficient industrial design processes

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Designing Gratin, A GPU-Tailored Node-Based System

International audienceNodal architectures have received an ever-increasing endorsement in computer graphics in recent years. However, creating a node-based system specifically tailored to GPU-centered applications with real-time performance is not straightforward. In this paper, we discuss the design choices we took in the making of Gratin, our open-source node-based system. This information is useful to graphics experts interested in crafting their own node-based system working on the GPU, either starting from scratch or taking inspiration from our source code. We first detail the architecture of Gratin at the graph level, with data structures permitting real- time updates even for large pipelines. We then present the design choices we made at the node level, which provide for three levels of programmability and, hence, a gentle learning curve. Finally, we show the benefits of our approach by presenting use cases in research prototyping and teaching