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Item Surfel Octrees: A New Scheme for Interactive Inspection of Anatomy Atlases in Client-Server Applications(The Eurographics Association, 2015) Surinyac, Jordi; Brunet, Pere; Mateu Sbert and Jorge Lopez-MorenoNowadays, an increasing interest on tele-medicine and tele-diagnostic solutions can be observed, with client/server architectures for remote inspection of volume image-based medical data which are becoming more and more popular. The use of portable devices is gradually spreading due to their portability and easy maintenance. In this paper, we present an efficient data model for segmented volume models based on a hierarchical data structure of surfels per anatomical structure. Surfel Octrees are compact enough for transmission through networks with limited bandwidth, and provide good visual quality in the client devices at a limited footprint. Anatomy atlases are represented as octree forests, supporting local interaction in the client device and selection of groups of medical organs. After presenting the octree generation and interaction algorithms, we present several examples and discuss the interest of the proposed approach in low-end devices such as mobiles and tablets.Item Downsampling and Storage of Pre-Computed Gradients for Volume Rendering(The Eurographics Association, 2017) DÃaz-GarcÃa, Jesús; Brunet, Pere; Navazo, Isabel; Vázquez, Pere-Pau; Fco. Javier Melero and Nuria PelechanoThe way in which gradients are computed in volume datasets influences both the quality of the shading and the performance obtained in rendering algorithms. In particular, the visualization of coarse datasets in multi-resolution representations is affected when gradients are evaluated on-the-fly in the shader code by accessing neighbouring positions. This is not only a costly computation that compromises the performance of the visualization process, but also one that provides gradients of low quality that do not resemble the originals as much as desired because of the new topology of downsampled datasets. An obvious solution is to pre-compute the gradients and store them. Unfortunately, this originates two problems: First, the downsampling process, that is also prone to generate artifacts. Second, the limited bit size of storage itself causes the gradients to loss precision. In order to solve these issues, we propose a downsampling filter for pre-computed gradients that provides improved gradients that better match the originals such that the aforementioned artifacts disappear. Secondly, to address the storage problem, we present a method for the efficient storage of gradient directions that is able to minimize the minimum angle achieved among all representable vectors in a space of 3 bytes. We also provide several examples that show the advantages of the proposed approaches.Item Integrating Occlusion Culling and Levels of Detail through Hardly-Visible Sets(Blackwell Publishers Ltd and the Eurographics Association, 2000) Andujar, Carlos; Saona-Vazquez, Carlos; Navazo, Isabel; Brunet, PereOcclusion culling and level-of-detail rendering have become two powerful tools for accelerating the handling of very large models in real-time visualization applications. We present a framework that combines both techniques to improve rendering times. Classical occlusion culling algorithms compute potentially visible sets (PVS), which are supersets of the sets of visible polygons. The novelty of our approach is to estimate the degree of visibility of each object of the PVS using synthesized coarse occluders. This allows to arrange the objects of each PVS into several Hardly-Visible Sets (HVS) with similar occlusion degree. According to image accuracy and frame rate requirements, HVS provide a way to avoid sending to the graphics pipeline those objects whose pixel contribution is low due to partial occlusion. The image error can be bounded by the user at navigation time. On the other hand, as HVS offer a tighter estimation of the pixel contribution for each scene object, it can be used for a more convenient selection of the level-of-detail at which objects are rendered. In this paper, we describe the new framework technique, provide details of its implementation using a visibility octree as the chosen occlusion culling data structure and show some experimental results on the image quality.Item GEOMETRIC MODELLING USING EXACT OCTREE REPRESENTATION OF POLYHEDRAL OBJECTS(The Eurographics Association, 1985) Brunet, Pere; Navazo, Isabel; C.E. VandoniIn the last few years, octrees have been proposed for the representation of solids in Geometric Modelling systems. Using them, boolean operations are simple; however, they use a great amount of memory and it is very difficult to obtain smooth graphical outputs from them or to recompute the boundary model. In the present paper, a new class of Octtrees is introduced, which allows face, edge and vertex nodes as well as the classical white, black and grey nodes. The required amount of storage is less than in the case of classical Octtrees, and boolean operations are also simple. Futhermore, there exists an algorithm for the exact recomputation of the boundary model.Item Table of Contents CGF 22-3(Blackwell Publishers, Inc and the Eurographics Association, 2003) Fellner, Dieter W.; Brunet, Pere-Item Multiresolution for Algebraic Curves and Surfaces using Wavelets(Blackwell Publishers Ltd and the Eurographics Association., 2001) Esteve, Jordi; Brunet, Pere; Vinacua, AlvarThis 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.Item VR-assisted Architectural Design in a Heritage Site: the Sagrada FamÃlia Case Study(The Eurographics Association, 2018) Andujar, Carlos; Brunet, Pere; Buxareu, Jerónimo; Fons, Joan; Laguarda, NarcÃs; Pascual, Jordi; Pelechano, Nuria; Sablatnig, Robert and Wimmer, MichaelVirtual Reality (VR) simulations have long been proposed to allow users to explore both yet-to-built buildings in architectural design, and ancient, remote or disappeared buildings in cultural heritage. In this paper we describe an on-going VR project on an UNESCO World Heritage Site that simultaneously addresses both scenarios: supporting architects in the task of designing the remaining parts of a large unfinished building, and simulating existing parts that define the environment that new designs must conform to. The main challenge for the team of architects is to advance towards the project completion being faithful to the original GaudÃ's project, since many plans, drawings and plaster models were lost. We analyze the main requirements for collaborative architectural design in such a unique scenario, describe the main technical challenges, and discuss the lessons learned after one year of use of the system.Item Real-Time Exploration of the Virtual Reconstruction of the Entrance of the Ripoll Monastery(The Eurographics Association, 2008) Besora, Isaac; Brunet, Pere; Chica, Antoni; Morales, Daniel; Moyes, Jordi; Luis Matey and Juan Carlos TorresThis 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 visitorsItem EG Editorial(The Eurographics Association and Blackwell Publishing Ltd., 2005) Brunet, Pere; Willis, Phil; Seidel, Hans-PeterItem Extended Octtrees, between CSG Trees and Boundary Representations(Eurographics Association, 1987) Navazo, Isabel; Fontdecaba, Josep; Brunet, PereBesides the most widely used models in the Geometric Modeling Systems, Constructive Solid Geometry (CSG) and Boundary Representations (BR), Octtrees have appeared as an alternative representation scheme which is particularly well suited for the solid boolean operation algorithms. Extended Octtrees, which incorporates three additional node types containing part of the surface of the object, are much more compact and allow the exact representation of plane faced objects, while supporting also low complexity algorithms for boolean operations. Due to the limitations of the two main models, CSG and Boundary Representation , a number of Hybrid Systems have appeared, which support both schemes and perform every operation in the most suitable model. However, algorithms for the boundary evaluation of CSG trees are complex, and at the moment little is known on algorithms for the inverse conversion, from BR to CSG. In the present paper, the use of the Extended Octtree model as an intermediate tool in the conversions between. CSG trees and BR is studied. In the case of CSG trees build from primitives with plane faces, an algorithm for the conversion from the CSG model to the Extended Octtree representation is presented. Its complexity is linear with respect to the numbe of nodes in the Octtree. The use of this algorithm in model-to-model conversions is discussed, together with the BR to Extended Octtree and Extended Octtree to BR conversion algorithms, that present also linear complexity with respect to the total number of nodes.