Volume 13, Number 1-3 (2005)
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Item Denoising 2-D vector fields by vector wavelet thresholding(Václav Skala - UNION Agency, 2005) Westenberg, Michael A.; Ertl, Thomas; Skala, VáclavNoise reduction is an important preprocessing step for many visualization techniques that make use of feature extraction. We propose a method for denoising 2-D vector fields that are corrupted by additive noise. The method is based on the vector wavelet transform, which transforms a vector input signal to wavelet coefficients that are also vectors. We introduce modifications to scalar wavelet coefficient thresholding for dealing with vector-valued coefficients. We compare our wavelet-based denoising method with Gaussian filtering, and test the effect of these methods on the signal-to-noise ratio (SNR) of the vector fields before and after denoising. We also compare our method with component-wise scalar wavelet thresholding. Furthermore, we use a vortex measure to study the performances of the methods for retaining relevant details for visualization. The results show that for very low SNR, Gaussian filtering with large kernels has a slightly better performance than the wavelet-based method in terms of SNR. For larger SNR, the wavelet-based method outperforms Gaussian filtering, because Gaussian filtering removes small details that are preserved by the wavelet-based method. Component-wise denoising has a lower performance than our method.
Item VSA-based fractal image compression(Václav Skala - UNION Agency, 2005) Wang, Huaqing; Wang, Meiqing; Hintz, Tom; Wu, Qiang; He, Xiangjian; Skala, VáclavSpiral Architecture (SA) is a novel image structure which has hexagons but not squares as the basic elements. Apart from many other advantages in image processing, SA has shown two unbeatable characters that have potential to improve image compression performance, namely, Locality of Pixel Density and Uniform Image Partitioning. Fractal image compression is a relatively recent image compression method which exploits similarities in different parts of the image. The basic idea is to represent an image as fixed points of Iterated Function Systems (IFS). Therefore, an input image can be represented by a series of IFS codes rather than pixels. In this way, an amazing compression ratio 10000:1 can be achieved. The application of fractal image compression presented in this paper is based on Spiral Architecture. Since there is no mature capture and display device for hexagon-based images, the experiments are implemented on a newly proposed mimic scheme, called Virtual Spiral Architecture (VSA). The experimental results in the paper have shown that introducing Spiral Architecture into fractal image compression will improve the compression performance in image quality with little trade-off in compression ratio. A lot of research work exists in this area to further improve the results.Item Go with the winners strategy in path tracing(Václav Skala - UNION Agency, 2005) Szirmay-Kalos, László; György, Antal; Sbert, Mateu; Skala, VáclavThis paper proposes a new random walk strategy that minimizes the variance of the estimate using statistical estimations of local and global features of the scene. Based on the local and global properties, the algorithm decides at each point whether a Russian-roulette like random termination is worth performing, or on the contrary, we should split the path into several child paths. In this sense the algorithm is similar to the go-with-the-winners strategy invented in general Monte Carlo context. However, instead of establishing thresholds to make decisions, we compute the number of child paths on a continuous level and show that Russian roulette can be interpreted as a kind of splitting using fractional number of children. The new method is built into a path tracing algorithm, and a minimum cost heuristic is proposed for choosing the number of rejected rays. Comparing it with the classical path tracing approach we concluded that the new method reduced the variance significantly.Item Shape from silhouette: image pixels for marching cubes(Václav Skala - UNION Agency, 2005) Mercier, Bruno; Meneveaux, Daniel; Skala, VáclavIn this paper, we propose to use image pixels for geometry reconstruction with a shape from silhouette approach. We aim at estimating shape and normal for the surface of a single object seen through calibrated images. From the voxelbased shape obtained with the algorithm proposed by R. Szeliski in [18], our main contribution concerns the use of image pixels together with marching cubes for constructing a triangular mesh. We also provide a mean for estimating a normal inside each voxel with two different methods: (i) using marching cubes triangles and (ii) using only voxels. As seen in the results, our method proves accurate even for real objects acquired with a usual camera and an inexpensive acquisition system.Item Silhouette enhanced point-based rendering(Václav Skala - UNION Agency, 2005) Luz, José Luiz; Velho, Luiz; Carvalho, Paulo Cezar P.; Skala, VáclavWith the recent advances in the 3D scanning field, the size of datasets to be displayed has increased up to billions of points. Typically, we have a dense, unstructured set of points without connectivity information. Most researchers have proposed the point-surfel association to represent the surface’s geometry and to render it using a planar approximation for each point. This paper proposes an alternative approximation, where curved surface elements (c-surfels) are employed, in order to get better adaptation to the surface to be rendered. We also use texture mapping and blending, to produce a perceptually better visualization. Improvements caused by using curved surfels instead of planar ones are especially noticeable at the object’s silhouette.Item Parameterization of tubular surfaces on the cylinder(Václav Skala - UNION Agency, 2005) Huysmans, Toon; Sijbers, Jan; Verdonk, Brigitte; Skala, VáclavIn this paper we develop a method to parameterize tubular surfaces onto the cylinder. The cylinder can be seen as the natural parameterization domain for tubular surfaces since they share the same topology. Most present algorithms are designed to parameterize disc-like surfaces onto the plane. Surfaces with a different topology are cut into disc-like patches and the patches are parameterized separately. This introduces discontinuities and constrains the parameterization. Also the semantics of the surface are lost. We avoid this by parameterizing tubular surfaces on, their natural domain, the cylinder. Since the cylinder is locally isometric to the plane we can do calculations on the cylinder without loosing efficiency. For speeding up the calculation we use a progressive parameterization technique, as suggested in recent literature. Together, this results in a robust, efficient, continuous, and semantics preserving parameterization method for arbitrary tubular surfaces.Item Tree growth visualization(Václav Skala - UNION Agency, 2005) Linsen, Lars; Karis, Brian J.; McPherson, E. Gregory; Hamann, Bernd; Skala, VáclavIn computer graphics, models describing the fractal branching structure of trees typically exploit the modularity of tree structures. The models are based on local production rules, which are applied iteratively and simultaneously to create a complex branching system. The objective is to generate three-dimensional scenes of often many realisticlooking and non-identical trees. Our goal, instead, is to visualize the growth of a prototypical tree of certain species. It is supposed to look realistic but, more importantly, has to conform with real, measured data. We construct a tree model being similar to existing ones and extend it by coupling the branching production rules with dynamic tree-growth rules. The latter are based on equations derived from measured street tree data for London Plane tree (Platanus acerifolia) such as tree height, diameter-at-breast-height, crown height, crown diameter, and leaf area. We map the global, measured parameters to the local parameters used in the tree model. The mapping couples knowledge from plant biology and arboriculture, as we deal with trees that are trained and manipulated to achieve desired forms and functions within highly urbanized environments.Item Texture painting from video(Václav Skala - UNION Agency, 2005) Hu, Jinhui; You, Suya; Neumann, Ulrich; Skala, VáclavTexture mapping is an important research topic in computer graphics. Traditional static texture-maps are limiting for capturing a dynamic and up-to-date picture of the environment. This paper presents a new technique called texture painting from video. By employing live video as the texture resource, we are not only able to create an accurate and photo-realistic rendering of the scene, but also can support dynamic spatio-temporal update in the structure of texture model, database, and rendering system. We present our approaches towards the system requirements and experimental results for both simulation and real datasets.Item Efficiently keeping an optimal stripification over a CLOD mesh(Václav Skala - UNION Agency, 2005) Porcu, Massimiliano B.; Sanna, Nicola; Scateni, Ricardo; Skala, VáclavIn this paper we present an algorithm of simple implementation but very effective that guarantees to keep an optimal stripification (in term of frames per seconds) over a progressive mesh. The algorithm builds on-the-fly the stripification on a mesh at a selected level-of-details (LOD) using the stripifications built, during a pre-processing stage, at the lowest and highest LODs. To reach this goal the algorithm uses two different operations on the dual graph of the mesh: when the user changes the mesh resolution the mesh+strips local configuration is looked up in a table and, after a vertex split operation, the strips are rearranged accordingly, immediately after a sequence of special topological operation called “tunneling” with short tunnel length are started till the number of isolated triangles in the mesh get under 10% of the total number of strips. Moreover, when the user select a relevant LOD it can trigger a tunnelling with higher tunnel length to optimize the stripification. Using these operations we are able to keep the progressive mesh stripified in a time of the same order of magnitude of the time needed to change the resolution and, only if required, to perform a time-demanding optimization. Only the stripifications generated by explicit user requests are stored to serve as optimal starting points for further inspection. In this way we can always feed the graphics board with a triangle strip representation of the mesh at any LOD. The results we present demonstrate that we can tightly couple each sequence of vertex splits used to increase the resolution of the progressive mesh with: a simple rearrangement of the strips followed by a very cheap stripification search with a predetermined strategy. A strong feature of the method is that the local rearrangement leads to an implementation that keeps almost constant the execution time. The results of the visualization benchmarks are very good: comparing the rendering of the stripified (using this strategy) and the non stripified meshes we can, on average, double the frames per seconds rate.Item Broadcast GL: an alternative method for distributing OpenGL API calls to multiple rendering slaves(Václav Skala - UNION Agency, 2005) Ilmonen, Tommi; Reunanen, Markku; Kontio, Petteri; Skala, VáclavThis paper describes the use of UDP/IP broadcast for distributing OpenGL API calls. We present an overview of the system and benchmark its performance against other common distribution methods. The use of network broadcasts makes this approach highly scalable. The method was found effective for applications that need to transmit changing vertex arrays or textures frequently.Item Fast and accurate Hausdorff distance calculation between meshes(Václav Skala - UNION Agency, 2005) Guthe, Michael; Borodin, Pavel; Klein, Reinhard; Skala, VáclavComplex models generated e.g. with a laser range scanner often consist of several thousand or million triangles. For efficient rendering this high number of primitives has to be reduced. An important property of mesh reduction – or simplification – algorithms used for rendering is the control over the introduced geometric error. In general, the better this control is, the slower the simplification algorithm becomes. This is especially a problem for out-of-core simplification, since the processing time quickly reaches several hours for high-quality simplification. In this paper we present a new efficient algorithm to measure the Hausdorff distance between two meshes by sampling the meshes only in regions of high distance. In addition to comparing two arbitrary meshes, this algorithm can also be applied to check the Hausdorff error between the simplified and original meshes during simplification. By using this information to accept or reject a simplification operation, this method allows fast simplification while guaranteeing a user-specified geometric error.Item The adaptive thin shell tetrahedral mesh(Václav Skala - UNION Agency, 2005) Erleben, Kenny; Dohlmann, Henrik; Sporring, Jon; Skala, VáclavTetrahedral meshes are often used for simulating deformable objects. Unlike engineering disciplines that often focuses on accuracy, computer graphics is biased towards stable, robust, and fast methods. In that spirit we present an approach for building an adaptive inward shell of the surface of an object. The goal is to device a simple and fast algorithm capable of building a topologically consistent tetrahedral mesh. The tetrahedral mesh can be used with several different simulation method, such as the finite element method (FEM), and the main contribution of this paper is a novel tetrahedral mesh generation method based on adaptive surface extrusion.Item 3D free-form modeling with variational surfaces(Václav Skala - UNION Agency, 2005) Cuno, Alvaro; Esperança, Claudio; Cavalcanti, Paulo Roma; Farias, Ricardo; Skala, VáclavWe describe a free-form stroke-based modeling system where objects are primarily represented by means of variational surfaces. Although similar systems have been described in recent years, our approach achieves both a good performance and reduced surface leak problems by employing a coarse mesh as support for constraint points. The prototype implements an adequate set of modeling operations, “undo” and “redo” facilities and a clean interface capable of resolving ambiguities by means of suggestion thumbnails.Item Direct volume rendering of unstructured grids in a PC based VR environment(Václav Skala - UNION Agency, 2005) Benölken, Paul; Graf, Holger; Skala, VáclavIn this paper we present our solution for fast, direct volume rendering of unstructured grids on standard PC workstations. We describe our modification of the incremental slicing approach for achieving high performance as well as the application of geometry-compression methods for minimizing the memory requirements. Furthermore, we show our implementation for the interactive modification of transfer functions (classification) in a virtual reality environment by using 3D interaction widgets. Finally we present and discuss the results, we achieved with our application in a VR environment.Item Statistical shape analysis for computer aided spine deformity detection(Václav Skala - UNION Agency, 2005) Bendels, Gerhard H.; Klein, Reinhard; Samimi, Mandana; Schmitz, Alfred; Skala, VáclavIn this paper we describe a medical application where we exploit surface properties (measured in form of 3D-Range scans of the human back) to derive a-priori unknown additional properties of the proband, that otherwise can only be acquired using multiple x-ray recordings or volumetric scans as CT or MRI. On the basis of 274 data sets, we perform classification using statistical shape analysis methods. Consistent parameterization and alignment is achieved on the basis of only few anatomic landmarks. As our choice of landmarks is easy to detect on the human body, our approach is feasible for screening applications that can be expected to have much impact on the early detection and later treatment of spine deformities, in particular scoliosis.Item Label layout for interactive 3D illustrations(Václav Skala - UNION Agency, 2005) Ali, Kamran; Hartmann, Knut; Strothotte, Thomas; Skala, VáclavHand-made illustrations in scientific and technical textbooks commonly use internal and external labels or legends to establish co-referential relation between pictorial elements and textual expressions. By analyzing the most complex examples, we extracted several label layout styles and classified them. We propose a variety of real-time label layout algorithms that aim to produce nice and clean layouts. In order to achieve a frame-coherent label layout during user interactions, the algorithms consider layout decisions from previous frame. Moreover, several evaluation criteria to measure the quality of static as well as dynamic label layouts are presented.
