Yela H., Navazo I. and Vazquez P. CEIG, 2008 PDF Volumes acquired for medical purposes are continuously increasing in size, faster than graphic cards memory capacity. Large volumetric datasets do not fit into GPU memory and therefore direct rendering is not possible. Even large volumes that still fit into GPU memory make frame rates decay. In order to reduce the size of large volumetric models, we present a new compression scheme. In this paper we present S3Dc, a lossy volume compression algorithm suitable for scalar values. It is inspired in hardware-accelerated 3Dc normal compression technique. S3Dc allows us to compress the volume in CPU up to a 4:1 or 8:1 ratio, while still yielding good quality results. We provide details on the compression scheme and show how to render directly from a S3Dc compressed texture. Furthermore, we analyze the image quality theoretical error and the average error with several images in order to assess the results. Andujar C.,Díaz J. and Brunet P. IEEE Virtual Reality Workshop on Virtual Citiscapes: Key Research Issues in Modeling Large-Scale Immersive Urban Environments, 2008 PDF Image-based rendering techniques are often the preferred choice to accelerate the exploration of massive outdoor models and complex human-made structures. In the last few years, relief mapping has been shown to be extremely useful as a compact representation of highly-detailed 3D models. In this paper we describe a rendering system for interactive, high-quality visualization of large scale urban models through a hierarchical collection of properly-oriented relief-mapped polygons. At the heart of our approach is a visibilityaware algorithm for the selection of the set of viewing planes supporting the relief maps. Our selection algorithm optimizes both the sampling density and the coverage of the relief maps and its running time is mostly independent on the underlying geometry. We show that our approach is suitable for navigating through large scale urban models at interactive rates while preserving both geometric and appearance details. Besora I., Brunet P., Callieri M., Chica A., Corsini M., Dellepiane M., Morales D., Moyés J., Ranzuglia G. and Scopigno R. 3DPVT08: Fourth International Symposium on 3D Data Processing, Visualization and Transmission, 2008 PDF The dichotomy between detail representation and data management is still a big issue in the context of the acquisition and visualization of 3D objects, especially in the field of Cultural Heritage. New technologies give the possibility to acquire very detailed geometry, but very often it’s very hard to process the amount of data produced. In this paper we present a project which aimed at virtually reconstructing the impressive (7x11 m.) portal of the Ripoll Monastery, Spain. The monument was acquired using triangulation laser scanning technology, producing a dataset of more than 2000 range maps for a total of more than 1 billion triangles. All the steps of the entire project are described, from the acquisition planning to the final setup for the dissemination to the public. In particular, we show how timeof- flight laser scanning data can be used to obtain a speed up in the alignment process, and how, after model creation and imperfections repairing, an interactive and immersive setup gives the public the possibility to navigate and visualize the high detail representation of the portal. This paper shows that, after careful planning and with the aim of new algorithms, it’s now possible to preserve and visualize the highly detailed information provided by triangulation laser scanning also for very large surfaces. Besora I.,Brunet P., Chica A., Morales D. and Moyés J. CEIG, 2008 PDF This paper presents the project of the virtual reconstruction and inspection of the "Portalada", the entrance of the Ripoll Monastery. In a first step, the monument of 7 x 11 meters was acquired using triangulation laser scanning technology, producing a dataset of more than 2000 range maps for a total of more than one billion triangles. After alignment and registration, a nearly complete digital model with 173M triangles and a sampling density of the order of one millimeter was produced and repaired. The paper describes the model acquisition and construction, the use of specific scalable algorithms for model repair and simplification, and then focuses on the design of a hierarchical data structure for data managing and view-dependent navigation of this huge dataset on a PC. Finally, the paper describes the setup for a usable, user-friendly and immersive system that induces a presence perception in the visitors. Chica A. ACM Symposium on Solid and Physical Modeling, pags. 347-352, 2008 PDF In this paper, we present a new visibility-based feature extraction algorithm from discrete models as dense point clouds resulting from laser scans. Based on the observation that one can characterize local properties of the surface by what can be seen by an imaginary creature on the surface, we propose algorithms that extract features using an intermediate representation of the model as a discrete volume for computational efficiency. We describe an efficient algorithm for computing the visibility map among voxels, based on the properties of a discrete erosion. The visibility information obtained in this first step is then used to extract the model components (faces, edges and vertices) —which may be curved— and to compute the topological connectivity graph in a very efficient and robust way. The results are discussed through several examples. Argelaguet F., Andujar C. and Trueba R. VRST: ACM, 2008 PDF Most pointing techniques for 3D selection on virtual environments rely on a ray originating at the user’s hand whose direction is controlled by the hand orientation. In this paper we study the potential mismatch between visible objects (those which appear unoccluded from the user’s eye position) and selectable objects (those which appear unoccluded from the user’s hand position). We study the impact of such eye-hand visibility mismatch on selection performance, and propose a new technique for ray control which attempts to overcome this problem. We present an experiment to compare our ray control technique with classic raycasting in selection tasks with complex 3D scenes. Our user studies show promising results of our technique in terms of speed and accuracy. Argelaguet F. and Andujar C. SmartGraphics, vol. 5166, pp. 45-47, 2008 PDF In this paper we explore the extension of 2D pointing facilitation techniques to 3D object selection. We discuss what problems must be faced when adapting such techniques to 3D interaction on VR applications, and we propose two strategies to adapt the expanding targets approach to the 3D realm, either by dynamically scaling potential targets or by using depth-sorting to guarantee that potential targets appear completely unoccluded. We also present three experiments to evaluate both strategies in 3D selection tasks with multiple targets at varying densities. Our user studies show promising results of 3D expanding targets in terms of error rates and, most importantly, user acceptance. Vazquez P., Monclus E. and Navazo I. SmartGraphics, 2008 PDF Volume data models are becoming larger and larger as the capture technology improves. Thus, their visualization requires high computational power. The automatic presentation of volume models through representative images and/or exploration paths becomes more and more useful. Representative views are also useful for document illustration, fast data quality evaluation, or model libraries documentation. Exploration paths are also useful for video demonstrations and previsualization of captured data. In this paper we present a fast, adaptive method for the selection of representative views and the automatic generation of exploration paths for volume models. Our algorithm is based on multi-scale entropy and algorithmic complexity. These views and paths reveal informative parts of a model given a certain transfer function. We show that our method is simple and easy to incorporate in medical visualization tools. Diaz J., Yela H. and Vazquez P. Computer Graphics International, 2008 PDF Volume models often show high depth com- plexity. This poses di±culties to the observer in judging the spatial relationships accurately. Illustrators usually use certain techniques such as halos or edge darkening in order to enhance depth perception of certain structures. Halos may be dark or light, and even colored. Halo con- struction on a volumetric basis impacts rendering perfor- mance due to the complexity of the construction process. In this paper we present Vicinity Occlusion Maps: a sim- ple and fast method to compute the light occlusion due to neighboring voxels. Vicinity Occlusion Maps may be used to generate flexible halos around objects or selected structures in order to enhance depth perception or ac- centuate the presence of some structures in volumetric models at a low cost. The user may freely select the struc- ture that requires the halos to be generated, its color and size, and our proposed application generates those in real time. They may also be used to perform vicinity shading in realtime, or even to combine both efects. Vazquez P. Computer Graphics Forum, 2, pag 143-156, 2007 PDF The automatic selection of good viewing parameters is a very complex problem. In most cases, the notion of good strongly depends on the concrete application. Moreover, when an intuitive definition of good view is available, it is often difficult to establish a measure that brings it to the practice. Commonly, two kinds of viewing parameters must be set: camera parameters (position and orientation) and lighting parameters (number of light sources, its position and eventually the orientation of the spot). The first ones will determine how much of the geometry can be captured and the latter will influence on how much of it is revealed (i. e. illuminated) to the user. Unfortunately, ensuring that certain parts of a scene are lit does not make sure that the details will be communicated to the user, as the amount of illumination might be too small or too high. In this paper we define a metric to calculate the amount of information relative to an object that is effectively communicated to the user given a fixed camera position. This measure is based on an information-based concept, the Shannon entropy, and will be applied to the problem of automatic selection of light positions in order to adequately illuminate an object. In order to validate the results, we have carried out an experiment on users, this experiment helped us to explore other related measures. Monclus E., Navazo I. and Vazquez P. Theory and Practice of Computer Graphics, 2007 PDF Isosurface construction and rendering based on tetrahedral grids has been adequately implemented on programmable graphics hardware. In this paper we present MTCut: a volume cutting algorithm that is able to cut isosurfaces obtained by a Marching Tetrahedra algorithm on volume data. It does not require a tetrahedal representation and runs in real time for complex meshes of up to 1.8M triangles. Our algorithm takes as input the isosurface to be cut, and produces the cut geometry in response to the user interaction with a haptic device. The result is a watertight manifold model that can be interactively recovered back to CPU upon user request. Vazquez P. and Balsa M. Theory and Practice in Computer Graphics (GRAPP), 2007 PDF There is a growing interest in simulating natural phenomena in computer graphics applications. Animating natural scenes in real time is one of the most challenging problems due to the inherent complexity of their structure, formed by millions of geometric entities, and the interactions that happen within. An example of natural scenario that is needed for games or simulation programs are forests. Forests are difficult to render because the huge amount of geometric entities and the large amount of detail to be represented. Moreover, the interactions between the objects (grass, leafs) and external forces such as wind are complex to model. In this paper we concentrate in the rendering of falling leafs at low cost. We present a technique that exploits graphics hardware in order to render thousands of leafs with different falling paths at real time and low memory requirements. Vazquez P., Götzelmann T., Hartmann K., Nürnberger A. and Strothotte1 T. 2007 PDF This paper presents the concept and an evaluation of a novel approach to support students to understand complex spatial relations and to learn unknown terms of a domain-specific terminology with coordinated textual descriptions and illustrations. Our approach transforms user interactions into queries to an information retrieval system. By selecting text segments or by adjusting the view to interesting domain objects, learners can request additional contextual information. Therefore, the system uses pre-computed multi-level representations of the content of explanatory text and of views on 3D models to suggest textual descriptions or views on 3D objects that might support the current learning task. Our experimental application is evaluated by a user study that analyzes (i) similarity measures that are used by the information retrieval system to coordinate the content of descriptive texts and computer-generated illustrations and (ii) the impact of the individual components of these measures. Our study revealed that the retrieved results match the preferences of the users. Furthermore, the statistical analysis suggests a rough value to cut-off retrieval results according to their relevancy. Vazquez P., Götzelmann T., Hartmann K.,Germer T., Nürnberger A. and Strothotte T. SCCG, 2007 PDF This paper presents a novel approach to support students to learn a comprehensive domain-specific terminology and to understand textual descriptions of complex-shaped objects. We implemented an experimental system where learners can interactively explore textual descriptions and 3D visualizations. We propose a method for hierarchical content representations of text documents and views on 3D models. Based on these data structures, user interactions on texts and interactive 3D visualizations are transformed into queries to an information retrieval system. This enables us to coordinate the content of both media, to focus the attention of the user on the most salient graphical objects, and to suggest potential relevant text segments in large text documents and appropriate views on 3D models to illustrate the spatial relations between the relevant domain objects of the query. Finally, we demonstrated this concept in an interactive tutoring environment based on standard textbooks on human anatomy. Vazquez P. and Andujar C. 3ia, 2007 PDF Automatic selection of good views of 3D models is an important problem with applications in many fields such as scene understanding, imagebased modeling, and scientific data inspection. Most of the view-quality measures proposed so far depend in some way on the geometric representation of object’s surface, as they work upon the faces of the model. Thus, most approaches are sensitive to the actual representation or level of detail of the object being analyzed. In this paper we present a new approach for tessellation-independent viewpoint selection. Our method measures the quality of a view by examining the information content of the object’s silhouette together with the information content of its depth map, resulting in a metric that overcomes the limitations of previous methods, and provides results more sensitive to human perception. Murillo S., Navazo I. and Vinacua A. Actas del XVII Congreso Español de Informatica Gráfica (CEIG'07), pg 199-208, Eurographics s.e. Zaragoza, 2007 PDF Los estudios de cardiología nuclear provenientes de captaciones cardíacas 4D gated SPECT (Single Photon Emission Computed Tomographic) aportan información sobre la perfusión del ventrículo izquierdo en diferentes fases del ciclo cardíaco. Para el pronóstico de la viabilidad del miocardio es importante comparar la perfusión del miocardio con el paciente en estado de reposo y en esfuerzo. En este artículo presentamos un método de registro para imágenes cardíacas SPECT, basado en la Información Mutua (IM) como medida de similitud, que utiliza toda la información contenida en la captación. Ésta puede representar un instante del ciclo cardíaco (registro 3D) o una serie temporal de captaciones (registro 4D). La propuesta se basa en un nuevo método de interpolación que es especialmente signicativo en el registro de imágenes de baja resolución. La utilización de toda la informaci ón disponible en la captación junto con el nuevo método de interpolación, conducen a un método de registro eficiente y de alta precisión. Andujar C., Boo J., Brunet P., Fairen M., Navazo I., Vazquez P. and Vinacua A. Computer Graphics Forum, Vol. 26 (3), Eurographics, 2007 ORIs WebPage PDF Relief impostors have been proposed as a compact and high-quality representation for high-frequency detail in 3D models. In this paper we propose an algorithm to represent a complex object through the combination of a reduced set of relief maps. These relief maps can be rendered with very few artifacts and no apparent deformation from any view direction. We present an efficient algorithm to optimize the set of viewing planes supporting the relief maps, and an image-space metric to select a sufficient subset of relief maps for each view direction. Selected maps (typically three) are rendered based on the well-known ray-height-field intersection algorithm implemented on the GPU. We discuss several strategies to merge overlapping relief maps while minimizing sampling artifacts and to reduce extra texture requirements. We show that our representation can maintain the geometry and the silhouette of a large class of complex shapes with no limit in the viewing direction. Since the rendering cost is output sensitive, our representation can be used to build a hierarchical model of a 3D scene. Andujar C., Fairen M. and Argelaguet F. In Proceedings of 3DUI 2006, The 1st IEEE Symposium on 3D User Interfaces, 2006 PDF This paper presents a new approach for fast development of application-control User Interfaces (UIs) for Virtual Environments (VEs). This approach allows developers to build sophisticated UIs containing both simple widgets (such as windows, buttons, menus and sliders) and advanced widgets (such as hierarchical views and web browsers) with minimum effort. Rather than providing a new API for defining and managing the interface components, we propose to extend current 2D toolkits such as Qt so that its full range of widgets can be displayed and manipulated either as 2D shapes on the desktop or as textured 3D objects within the virtual world. This approach allows 3D UI developers to take advantage of the increasing number of components, layout managers and graphical design tools provided by 2D UI toolkits. Resulting programs can run on platforms ranging from fully immersive systems to generic desktop workstations with little or no modification. The design of the system and the key features required on the host UI toolkit are presented and discussed. A prototype system has been implemented above Qt and evaluated on a 4-sided CAVE. The results indicate that this approach provides an efficient and cost-effective way for porting and developing application-control GUIs on VEs and thus it can greatly enhance the possibilities of many VE applications. Andujar C., Brunet P., Chica A., Rossignac J., Navazo I. and Vinacua A. Computer-Aided Design, 37 (8), pp.847-857, 2005 PDF Since the publication of the original Marching Cubes algorithm, numerous variations have been proposed for guaranteeing water-tight constructions of triangulated approximations of isosurfaces. Most approaches divide the 3D space into cubes that each occupy the space between eight neighboring samples of a regular lattice. The portion of the isosurface inside a cube may be computed independently of what happens in the other cubes, provided that the constructions for each pair of neighboring cubes agree along their common face. The portion of the isosurface associated with a cube may consist of one or more connected components, which we call sheets. The topology and combinatorial complexity of the isosurface is influenced by three types of decisions made during its construction: (1) how to connect the four intersection points on each ambiguous face, (2) how to form interpolating sheets for cubes with more than one loop, and (3) how to triangulate each sheet. To determine topological properties, it is only relevant whether the samples are inside or outside the object, and not their precise value, if there is one. Previously reported techniques make these decisions based on local —per cube— criteria, often using precomputed look-up tables or simple construction rules. Instead, we propose global strategies for optimizing several topological and combinatorial measures of the isosurfaces: triangle count, genus, and number of shells. We describe efficient implementations of these optimizations and the auxiliary data structures developed to support them. Vazquez P. and Sbert M. Future Generation Computer Systems International Journal. 20 (8), 1251-1262, 2004 PDF This paper explores a set of techniques to reduce the bandwidth required by remote navigation systems. These systems, such as exploration of virtual 3D worlds or remote surgery, usually require higher bandwidth than the Internet connection commonly available at home. Our system consists of a client PC equipped with a graphics card, and a remote high-end server. The server hosts the remote environment and does the actual rendering of the scenes for several clients, and the new image is passed to them. This scheme is suitable when the data has a copyright or when its size may exceed the rendering capabilities of the client. The general scheme is the following: each time the position changes, the new view is predicted by both the client and the server and the difference information between the predicted view and the correct one is sent to the client. To reduce bandwidth two kind of improvements can be made: improve the prediction method, and improve the transmission system. We present here two groups of techniques: First, a set of lossless methods which achieve reductions of up to a 9:1 ratio. These are a combination of a two level forward warping, that takes advantage of spatial coherence, and a masking method, which allows transmitting only the information that really needs to be updated. Second, a set of lossy methods suitable for very low bandwidth environments which involve both progressive transmission and image reuse. They consider relevant parameters such as the number of pixels, the amount of information they provide, and their colour deviation in order to create a strategy for prioritizing the information transmission. This system allows improving up to an additional 4:1 ratio. The quality of the generated images is very high, and often indistinguishable from the correct ones. These techniques can be applied to head-mounted displays or any remote navigation software. Patow G., Pueyo X. and Vinacua A. Journal of Shape Modelling, 10 (2), pp. 211-235, 2004 PDF This paper proposes a technique for the design of reflector shapes from prescribed optical properties (far field radiance distribution) and geometrical constraints, which is of high importance in the field of Lighting Engineering, more specifically for Luminaire Design. The reflector shape to be found is just a part of a set of pieces of what is known in lighting engineering as an optical set, and is composed of a lamp (light source), a reflector, a holding case and a glass that protects the system from dust and other environmental phenomena. Thus, we aim at the design and development of a system capable of generating a reflector shape in a way such that the optical set emits a given, user defined, far field radiance distribution. This problem can be put in the mathematical context of inverse problems, which refer to all the problems where, contrary to what happens with traditional direct problems, several aspects of the scene are unknown. Then, the algorithm is allowed to work backwards to establish the missing parameters. In order to do so, light propagation inside and outside the optical set must be computed and the resulting radiance distribution compared to the desired one. Finally, constraints on the shape imposed by industry needs must be taken into account, bounding the set of possible shape definitions. The general approach taken is based on a minimization procedure on the space of possible reflector shapes. The algorithm moves towards minimizing the distance, in the l^2 metric, between the resulting illumination far from the reflector and a prescribed, ideal optical radiance distribution specified at the far field by the user. Andujar C., Vazquez P. and Fairen M. Computer Graphics Forum, 23 (3), pp. 499-508, 2004 PDF The exploration of complex walkthrough models is often a difficult task due to the presence of densely occluded regions which pose a serious challenge to online navigation. In this paper we address the problem of algorithmic generation of exploration paths for complex walkthrough models.We present a characterization of suitable properties for camera paths and we discuss an efficient algorithm for computing them with little or no user intervention. Our approach is based on identifying the free-space structure of the scene (represented by a cell and portal graph) and an entropy-based measure of the relevance of a view-point. This metric is key for deciding which cells have to be visited and for computing critical way-points inside each cell. Several results on different model categories are presented and discussed. Sainz M., Pajarola R., Susin A. and Mercade A. IEEE Journal of Computer Graphics and Applications, 24 (4), pp. 24-33, 2004 PDF Point-based object representations are a powerful alternative to traditional polygonal object representations. Points are, in fact, the natural raw output data from the capturing stage in most 3D geometry acquisition systems. Capturing 3D geometry is a mission- critical content acquisition technique in application domains such as virtual reality, CAD/CAM, and physical asset management. The advantage in reducing the geometry to points is that it permits various processing and display simpliÞcations. Furthermore, a closed and continuous surface may not always be necessary for the visualization tasks involved in a given application. However, generating usable point-based models isn't easy, particularly with a system built around simple, low-cost components. In this paper, we describe SPOC, a simple, point-based object capturing system we've geometry for point-based image rendering. We also identify the problems involved and describe the technical solutions we've implemented. Finally, we address several algorithmic issues in capturing point models using a simple digital camera and turntable setup, and in processing and rendering point clouds. Fairen M., Brunet P. and Techmann T. Computers and Graphics, 28 (2), pp. 289-296, 2004 PDF This is a new virtual reality system designed to be small enough to be totally portable. It is a semiimmersive interaction system based on a movable stereoscopic projection screen with a tracking system to capture the movement of the screen with respect to the virtual model. The system can be used for cooperative inspection of very complex computer-aided design environments, allowing very simple interaction among users and designers at distant locations. Andujar C., Brunet P., Chica A., Navazo I., Rossignac J. and Vinacua A. Computer Graphics Forum, 23 (3), pp. 401-410, 2004 PDF The computation of the largest planar region approximating a 3D object is an important problem with wide applications in modeling and rendering. Given a voxelization of the 3D object, we propose an efficient algorithm to solve a discrete version of this problem. The input of the algorithm is the set of grid edges connecting the interior and the exterior of the object (called sticks). Using a voting-based approach, we compute the plane that slices the largest number of sticks and is orientation-compatible with these sticks. The robustness and efficiency of our approach rests on the use of two different parameterizations of the planes with suitable properties. The first of these is exact and is used to retrieve precomputed local solutions of the problem. The second one is discrete and is used in a hierarchical voting scheme to compute the global maximum. This problem has diverse applications that range from finding object signatures to generating simplified models. Here we demonstrate the merits of the algorithm for efficiently computing an optimized set of textured impostors for a given polygonal model. Boada I. and Navazo I. Computers and Graphics, 27 (1), pp. 41-49, 2003 PDF In this paper, the visualization of hybrid scenes that contain volume data and a fitted extracted surface is addressed. The proposed algorithm is based on a integrated octree-based representation: the "hybrid octree". The hybrid octree allows to obtain multiresolution representation of the volume data and it also maintains a decimated surface codification. The proposed visualization approach uses three-dimensional-textures for the visualization of the volume data and integrates the surface polygons using the information represented in the octree structure. The main characteristics of the method are: its capabilities to perform multiresolution hybrid visualizations and its efficient use of texture memory space. Navazo I., Rossignac J., Jou J. and Shariff R. H. Computer Graphics Forum, 22 (3), pp. 291-302, 2003 PDF We present an algorithm to generate a cell-and-portal decomposition of general indoor scenes. The method is an adaptation of the 3D watershed transform, computed on a distance-to-geometry sampled field. The watershed is processed using a flooding analogy in the distance field space. Flooding originates from local minima, each minimum producing a region. Portals are built as needed to avoid the merging of regions during their growth. As a result, the cell-and-portal decomposition is closely linked to the structure of the models. In a building, the algorithm finds all the rooms, doors and windows. To restrict the memory load, a hierarchical implementation of the algorithm is presented. We also explain how to handle possible model degeneracies -such as cracks, holes and interpenetrating geometries- using a pre-voxelisation step. The hierarchical algorithm, preceded when necessary by the pre-voxelisation, was tested on a large range of models. We show that it is able to deal with classical architectural models, as well as cave-like environments and large mixed indoor/outdoor scenes. Thanks to the intermediate distance field representation, the algorithm can be used regardless of the way the model is represented: it deals with parametric curves, implicit surfaces, volumetric data and polygon soups in a unified way. Vazquez P., Feixas M., Sbert M. and Heidrich W. Computer Graphics Forum, 22(4), pp. 689-700, 2003 PDF In the last decade a new family of methods, namely Image-Based Rendering, has appeared. These techniques rely on the use of precomputed images to totally or partially substitute the geometric representation of the scene. This allows to obtain realistic renderings even with modest resources. The main problem is the amount of data needed, mainly due to the high redundancy and the high computational cost of capture. In this paper we present a new method to automatically determine the correct camera placement positions in order to obtain a minimal set of views for Image-Based Rendering. The input is a 3D polyhedral model including textures and the output is a set of views that sample all visible polygons at an appropriate rate. The viewpoints should cover all visible polygons with an adequate quality, so that we sample the polygons at sufficient rate. This permits to avoid the excessive redundancy of the data existing in several other approaches. We also reduce the cost of the capturing process, as the number of actually computed reference views decreases. The localization of interesting viewpoints is performed with the aid of an information theory-based measure, dubbed viewpoint entropy. This measure is used to determine the amount of information seen from a viewpoint. Next we develop a greedy algorithm to minimize the number of images needed to represent a scene. In contrast to other approaches, our system uses a special preprocess for textures to avoid artifacts appearing in partially occluded textured polygons. Therefore no visible detail of these images is lost. Franquesa M. and Brunet P. Winter School on Computer Graphics. WSCG'04, 2004 PDF In this paper, the collision prediction between polyhedra under screw motions and a static scene using a new K dimensional tree data structure (Multiresolution Kdtree, MKtree) is introduced. In a complex scene containing a high number of individual objects, the MKtree represents a hierarchical subdivision of the scene objects that guarantees a small space overlap between node regions. The proposed MKtree data structure succeeds in performing simultaneously space and scene subdivision. MKtrees are useful for broad phase collision and proximity detection tests and for time-critical rendering in large environments requiring external memory storage. The paper proposes an efficient broad phase collision prediction algorithm. Examples in ship design applications are presented and discussed. Theoktisto V. and Fairen M. Computers and Graphics, vol: 5, num: 29, 2004 PDF We derive a complete component framework for transforming standalone VR applications into full-fledged multithreaded Collaborative Virtual Reality Environments (CVREs), after characterizing existing implementations into a feature-rich superset. Our main contribution is placing over the existing VR tool a very concise and extensible class framework as an add-on component that provides emerging collaboration features. The enhancements include: a scalable arbitrated peer-to-peer topology for scene sharing; multi-threaded components for graphics rendering, user interaction and network communications; a streaming message protocol for client communications; a collaborative user interface model for session handling; and interchangeable user roles with multi-camera perspectives, avatar awareness and shared 3D annotations. We validate the framework by converting the existing ALICE VR Navigator into complete CVRE, with experimental results showing good performance in the collaborative inspection and manipulation of complex models. Sainz M., Susin A. and Bagherzadeh N. International Conference in Image Processing, 2003 PDF Camera calibration is a critical problem in application such as augmented reality and image based model reconstruction. When constructing a 3D model of an object from an uncalibrated video sequence, large amounts of frames and self occlusions of parts of the object are common and difficult problems. In this paper we present a fast and robust algorithm that uses a divide and conquer strategy to split the video sequence into sub-sequences containing only the most relevant frames. Then a robust stratified linear based algorithm is able to calibrate each of the sub-sequences to a metric structure and finally the sub-sequences are merged together and a final non-linear optimization refines the solution. Examples of the real data reconstructions are presented. Vazquez .P and Sbert M. Conference on Computational Science and its Applications, ICCSA'2003. (LNCS 2669), 2003 PDF Automatic computation of best views of objects is very userful. For exemple, they can be used as the starting point of a scene exploration, or to enrich galleries of objects available through the Internet; adding a good image to a model may give a good idea of it before deciding to download. To select the most interesting viewpoint of an object, we use the so-called viewpoint entropy. We assume the best view is the one which gives the most information of the object being inspected. In this paper we present an adaptive method to compute best views. Our adaptive scheme allows to improve over other approaches with ratios of up to 360:1 on the selection of best views, and therefore achieve a nearly interactive rate. Esteve J., Brunet P. and Vinacua A. Computer Graphics Forum, 20 (1), 47-58, 2001 PDF This paper describes a multiresolution method for implicit curves and surfaces. The method is based on wavelets, and is able to simplify the topology. The implicit curves and surfaces are defined as the zero-valued piece-wise algebraic isosurface of a tensor-product uniform cubic B-spline. A wavelet multiresolution method that deals with uniform cubic B-splines on bounded domains is proposed. In order to handle arbitrary domains the proposed algorithm dynamically adds appropriate control points and deletes them in the synthesis phase Brunet P., Navazo I., Rossignac J. and Saona C. Computer Graphics Forum, 20 (1), 47-58, 2001 PDF Most visibility culling algorithms require convexity of occluders. Occluder synthesis algorithms attempt to construct large convex occluders inside bulky non-convex sets. Occluder fusion algorithms generate convex occluders that are contained in the umbra cast by a group of objects given an area light. In this paper we prove that convexity requirements can be shifted from the occluders to their umbra with no loss of efficiency, and use this property to show how some special non-planar, non-convex closed polylines that we call "hoops" can be used to compute occlusion efficiently for objects that have no large interior convex sets and were thus rejected by previous approaches. Boada I., Navazo I. and Scopigno R. Visual Computer, 17 (3), pp. 185-197, 2001 PDF Although 3D texture-based volume rendering guarantees image quality almost interactively, it is difficult to maintain an interactive rate when the technique has to be exploited on large datasets. In this paper, we propose a new texture memory representation and a management policy that substitute the classical one-texel per voxel approach for a hierarchical approach. The hierarchical approach benefits nearly homogeneous regions and regions of lower interest. The proposed algorithm is based on a simple traversal of the octree representation of the volume data. Driven by a user-defined image quality, defined as a combination of data homogeneity and importance, a set of octree nodes (the cut) is selected to be rendered. The degree of accuracy applied for the representation of each one of the nodes of the cut in the texture memory is set independently according to the user-defined parameters. The variable resolution texture model obtained reduces the texture memory size and thus texture swapping, improving rendering speed. Andujar C., Ayala D. and Brunet P. ACM Transactions on Graphics, 20 (6), pp. 88-105, 2002 PDF This paper presents a new approach for generating coarse-level approximations of topologically complex models. Dramatic topology reduction is achieved by converting a 3D model to and from a volumetric representation. Our approach produces valid, error-bounded models and supports the creation of approximations that do not interpenetrate the original model, either being completely contained in the input solid or bounding it. Several simple to implement versions of our approach are presented and discussed. We show that these methods perform significantly better than other surface-based approaches when simplifying topologically-rich models such as scene parts and complex mechanical assemblies. Andujar C., Fairen M. and Brunet P. 2001 PDF This paper describes an affordable Virtual Reality system designed and developed by a group of researchers at the Polytechnic University of Catalunya (UPC). The system allows direct selection and manipulation of virtual 3D objects. The interaction is based on stereoscopic images projected over the user's working space and on devices tracking the user's natural movements. The system includes a screen being adjustable both in orientation and height, sensors tracking the head and hand movements, and a tactile device for the forefinger providing touch sense. A prototype of the system is currently exhibited at the Virtual Reality Center of Barcelona and it is being used in different application fields like architecture, medicine and industrial design. Rodriguez J.A., Brunet P., Ezquerra N. and Palomar J. E. CEIG'2005, 2005 PDF Progressive lenses are lenses that allow focusing objects at any distance. The main problem of these lenses is the arise of marginal zones that present aberrations in which vision is defective. The interest of our research is to study the perceptual effect of these distortions using Virtual Reality techniques. To achieve this goal we are developing a lens simulator that permits having a correct perception of the lens aberrations. We will also perform an usability test that will let us find out if users have the same perception with real lenses and with the simulation. Rodriguez J., Sainz M. and Susin A. Short presentations EG'05, pp 85-88, 2005 PDF In this paper we present a very fast method for body-cloth animation. The usual bottle-neck in cloth simulation performance is collision detection, which becomes more difficult to solve when a complex geometry, like a human body, is involved. Recent image based methods, that use depth images to detect collisions, usually relays on CPU for collision correction. In our work we implement a GPU based simulation that takes care both of cloth simulation and body-cloth collisions when the humanoid is moving. Our solution is based on a hierarchic depth map structure. A high frame rate is obtained with both structured and unstructured cloth meshes with thousands of particles. Garcia O. and Susin A. IEEE Computers in Cardiology 2002. vol. 29, pp 621/624, 2002 PDF Our approach describes the three-dimensional reconstruction of the internal and external surfaces of the human's left ventricle from actual SPECT data. The reconstruction is a first process fitting in a complete VR application that will serve as an important diagnosis tool for hospitals. Beginning with the surfaces reconstruction, the application will provide volume and interactive real-time manipulation with the model. We focus on speed, precision and smoothness for the final surfaces. We present several recoveries with non-missing data and partial missing data, performed using different fillings for a PHANTOM test volume. We also present a reconstructed cardiac cycle from an actual patient data, all along with its associated ejection fraction estimation. Mero M.G. and Susin A. IEEE Computers in Cardiology 2002. vol. 29,pp 617/620, 2002 PDF In this paper , we enhance existing techniques for simulating flexible volumetric objects. The idea is to use a mixed model of Finite Element and Mesh Free Methods. From this approach we will be able to build a 3D deformable model which can be included in a general application of virtual reality for a medical surgery simulator. The final model will be the heart of a patient builded from their actual SPECT data. The mixed approach allow us to construct a multiresolution model that can be used to obtain real time response when an external user interacts with the model. Chica A., Williams J., Andujar C., Brunet P., Navazo I., Rossignac J. and Vinacua A. LSI Research Report (LSI-06-9-R), 2006 PDF We explore the automatic recovery of solids from their volumetric discretizations. In particular, we propose an approach, called Pressing, for smoothing isosurfaces extracted from binary volumes while recovering their large planar regions (flats). Pressing yields a surface that is guaranteed to contain the samples of the volume classified as interior and exclude those classified as exterior. It uses global optimization to identify flats and constrained bilaplacian smoothing to eliminate sharp features and high-frequencies from the rest of the isosurface. It recovers sharp edges between flat regions and between flat and smooth regions. Hence, the resulting isosurface is usually a much more accurate approximation of the original solid than isosurfaces produced by previously proposed approaches. Furthermore, the segmentation of the isosurface into flat and curved faces and the sharp/smooth labelling of their edges may be valuable for shape recognition, simplification, compression, and various reverse engineering and manufacturing applications. Esteve J., Brunet P. and Vinacua A. Computer Graphics Forum, Volume 24 , Number 4 ,pp 791-808, 2005 PDF This paper describes a method to obtain a closed surface that approximates a general 3D data point set with non-uniform density. Aside from the positions of the initial data points, no other information is used. Particularly, neither the topological relations between the points nor the normal to the surface at the data points are needed. The reconstructed surface does not exactly interpolate the initial data points, but approximates them with a bounded maximum distance. The method allows to reconstruct closed surfaces with arbitrary genus and closed surfaces with disconnected shells. Theoktisto V., Fairen M., Navazo I. and Monclus E. Workshop on Virtual Reality Interaction and Physical Simulation, 2005 PDF Rendering haptic textures seamlessly out of triangle meshes just by using geometry requires heavy work and does not allow high sampling rates for detailed, rugged models. Better approaches simulate surface texture without increasing much the complexity or processing cost, at the expense of fidelity of perception. We propose a method for rendering local height field maps out of an underlying triangle mesh, which relies in a space subdivision representation based on octrees for collision detection, and rendering individual surface detail by modulating force response using local height fields. We compare our method against a force mapping implementation for rendering/perceiving the same models with textures using normal maps. The proposed technique allows for real time perception of 3D surface detail, allowing the user to perceive the best haptic rendering alternative for a given model. Some experimental results are presented to show the goodness of the approach. Vazquez P., Feixas M., Sbert M. and Llobet A. Computers and Graphics,Volume 30, Issue 1, Pages 98-110, 2006 PDF The investigation of molecular structures often requires the use of graphics software to display different representations of the molecule of interest. Unfortunately, the commonly available visualization software is generally quite complex and requires a high degree of expertise for the user to obtain the desired images. Often, the selection of interesting views implies a considerable time and effort for non experienced users. Characterizing the desired properties the users may need is often impossible. In this paper we present a method to automatically determine certain views of molecules that can be used to study their chemical or physical properties. We have used Information Theory's Shannon entropy in order to characterize two kinds of views: views which show most of the structure of a molecule and views which show a low amount of information of an arrangement of molecules. The first ones can be used to study the composition of the molecule, that is to study certain chemical properties. The latter easily show how molecules are ordered in space and therefore are suitable to infer physical properties of compounds, such as resistance. Finally, we also present an adaptive, hardware accelerated algorithm that makes use of the features of graphics cards to make this calculation in realtime. Our method has proven to give good results as in most cases the views generated by our application can completely replace human involvement. For highly complex compounds, they can be either enough, or a good starting point. Often, our application also provides several views that could be missed by the users. Andujar C. and Argelaguet F. 12th Eurographics Symposium on Virtual Environments, 2006 PDF The accommodation of conventional 2D GUIs with Virtual Environments (VEs) can greatly enhance the possibilities of many VE applications. In this paper we present a variation of the well-known ray-casting technique for fast and accurate selection of 2D widgets over a virtual window immersed into a 3D world. The main idea is to provide a new interaction mode where hand rotations are scaled down so that the ray is constrained to intersect the active virtual window. This is accomplished by changing the control-display ratio between the orientation of the user’s hand and the ray used for selection. Our technique uses a curved representation of the ray providing visual feedback of the orientation of both the input device and the selection ray. The users’ feeling is that they control a flexible ray that gets curved as it moves over a virtual friction surface defined by the 2D window. We have implemented this technique and evaluated its effectiveness in terms of accuracy and performance. Our experiments on a four-sided CAVE indicate that the proposed technique can increase the speed and accuracy of component selection in 2D GUIs immersed into 3D worlds. Murillo S., Navazo I. and Vinacua A. International Conference on Medical Information Visualisation--BioMedical Visualisation (MedVis), 2006 PDF The registration of cardiac images is relevant for the diagnosis of cardiac pathologies. In the case of SPECT (Single Photon Emission Computer Tomography) datasets, this is difficult because of the low resolution of the images. In this paper we present a new registration method for 3D images with poor resolution. Our method is based on the maximization of the mutual information (MI) using a new interpolation method which significantly improves the result of an MI-based strategy on low resolution images. We also use a different optimization strategy than other MI-based algorithms, typically achieving errors of the order of one pixel. We also discuss experimental results of our method. Rodríguez L., Navazo I. and Vinacua A. Proceedings of Ibero-American Symposium on Computer Graphics, 2006 PDF Given a tetrahedral mesh immersed in a voxel model, we present a method to refine the mesh to reduce the discrepancy between interpolated values based on either scheme at arbitrary locations. An advantage of the method presented is that it requires few subdivisions and all decisions are made locally at each tetrahedron. We discuss the algorithm’s performance and applications. Esteve J., Vinacua A. and Brunet P. Report LSI-05-44-R, 2005 PDF This paper describes a constrained fairing method for implicit surfaces de¯ned on a voxelization. This method is suitable for computing a closed smooth surface that approximates an initial set of face connected voxels. The implicit surface is de¯ned as the zero-set of a tensor-product uniform cubic Bspline. The fairing process is based on increasing the Bspline continuity from C2 to C3 on the boundary faces of the voxels. The ¯nal surface is guaranteed to stab a prede¯ned subset of voxels. Andujar C. and Argelaguet F. IEEE Symposium on 3D User Interfaces, 2007 PDF The ability to access external 2D applications from within 3D worlds can greatly enhance the possibilities of many VE applications. In this paper we present a new interaction metaphor for fast, accurate and comfortable manipulation of external GUIs displayed as texture-mapped rectangles. The main idea is to decouple the motor space from the visual space so that the external application can be manipulated within a user-defined working volume whose location and size is completely independent from the application’s visual representation. This decoupling is accomplished through a virtual pad which receives user actions and maps them into cursor movements. The main advantage of our approach is that both the working space and the visual space can be adjusted independently to suit user preferences. This allows users to seamlessly balance speed and accuracy without affecting the visual representation of the application’s GUI. We have implemented an interaction technique adopting our metaphor in combination with a pointing technique and we have evaluated its effectiveness in terms of task performance and user preference. Our experiments indicate that the proposed technique increases user’s comfort while providing dynamic management of speed/accuracy tradeoff. Andujar C. and Argelaguet F. Computers and Graphics, Volume 31, Issue 1, 2007 PDF The accommodation of conventional 2D GUIs with Virtual Environments (VEs) can greatly enhance the possibilities of many VE applications. In this paper we present a variation of the well-known raycasting technique for fast and accurate selection of 2D widgets over a virtual window immersed into a 3D world. The main idea is to provide a new interaction mode where hand rotations are scaled down so that the ray is constrained to intersect the active virtual window. This is accomplished by changing the control-display ratio between the orientation of the user’s hand and the ray used for selection. Our technique uses a curved representation of the ray providing visual feedback of the orientation of both the input device and the selection ray. We have implemented this technique and evaluated its effectiveness in terms of performance and user preference. Our experiments on a four-sided CAVE indicate that the proposed technique can increase the speed and accuracy of component selection in 2D GUIs immersed into 3D worlds. Fairen M. and Trueba R. Winter School in Computer Graphics, 2007 PDF In this paper we present a new technique for view-dependent LOD rendering, where the scene is represented through an octree model from which we can obtain a triangle mesh corresponding to a view-dependent LOD. We present the construction of this octree model and the visualization algorithm that generates on-the-fly a closed and valid triangle mesh for each frame of the visualization. This visualization algorithm is a depth-first traversal algorithm which also allows to re-use triangles from one frame to another.