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Normal vectors are essential for many point cloud operations, including segmentation, reconstruction and rendering. The robust estimation of normal vectors from 3D range scans is a challenging task due to undersampling and noise, specially when combining points sampled from multiple sensor locations. Our error model assumes a Gaussian distribution of the range error with spatially-varying variances that depend on sensor distance and reflected intensity, mimicking the features of Lidar equipment. In this paper we study the impact of measurement errors on the covariance matrices of point neighborhoods. We show that covariance matrices of the true surface points can be estimated from those of the acquired points plus sensor-dependent directional terms. We derive a lower bound on the neighbourhood size to guarantee that estimated matrix coefficients will be within a predefined error with a prescribed probability. This bound is key for achieving an optimal trade-off between smoothness and fine detail preservation. We also propose and compare different strategies for handling neighborhoods with samples coming from multiple materials and sensors. We show analytically that our method provides better normal estimates than competing approaches in noise conditions similar to those found in Lidar equipment.

The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depending on the clinical protocols, the volume analysis has to be performed on MRI of the unprepared colon without contrast administration. In such circumstances, existing measurement procedures are cumbersome and time-consuming for the specialists. The algorithm presented in this paper permits a quasi-automatic segmentation of the unprepared colon on T2-weighted MRI scans. The segmentation algorithm is organized as a three-stage pipeline. In the first stage, a custom tubularity filter is run to detect colon candidate areas. The specialists provide a list of points along the colon trajectory, which are combined with tubularity information to calculate an estimation of the colon medial path. In the second stage, we delimit the region of interest by applying custom segmentation algorithms to detect colon neighboring regions and the fat capsule containing abdominal organs. Finally, within the reduced search space, segmentation is performed via 3D graph-cuts in a three-stage multigrid approach. Our algorithm was tested on MRI abdominal scans, including different acquisition resolutions, and its results were compared to the colon ground truth segmentations provided by the specialists. The experiments proved the accuracy, efficiency, and usability of the algorithm, while the variability of the scan resolutions contributed to demonstrate the computational scalability of the multigrid architecture. The system is fully applicable to the colon measurement clinical routine, being a substantial step towards a fully automated segmentation.

The use of virtual prototypes and digital models containing thousands of individual objects is commonplace in complex industrial applications like the cooperative design of huge ships. Designers are interested in selecting and editing specific sets of objects during the interactive inspection sessions. This is however not supported by standard visualization systems for huge models. In this paper we discuss in detail the concept of rendering front in multiresolution trees, their properties and the algorithms that construct the hierarchy and efficiently render it, applied to very complex CAD models, so that the model structure and the identities of objects are preserved. We also propose an algorithm for the interactive inspection of huge models which uses a rendering budget and supports selection of individual objects and sets of objects, displacement of the selected objects and real-time collision detection during these displacements. Our solution-based on the analysis of several existing view-dependent visualization schemes-uses a Hybrid Multiresolution Tree that mixes layers of exact geometry, simplified models and impostors, together with a time-critical, view-dependent algorithm and a Constrained Front. The algorithm has been successfully tested in real industrial environments; the models involved are presented and discussed in the paper. Widely used rendering fronts are formalized and characterized for real-time renders.A user study of perception of quality for different hierarchical representations.New object-aware multiresolution scheme with monotonic costs and benefits per node.Time-critical- view-dependent rendering algorithm for commodity hardware.Navigation of huge models with simple selection and manipulation of separate objects.

The final publication is available at Springer via http://dx.doi.org/10.1007/s00371-016-1253-9 Medical datasets are continuously increasing in size. Although larger models may be available for certain research purposes, in the common clinical practice the models are usually of up to 512x512x2000 voxels. These resolutions exceed the capabilities of conventional GPUs, the ones usually found in the medical doctors’ desktop PCs. Commercial solutions typically reduce the data by downsampling the dataset iteratively until it fits the available target specifications. The data loss reduces the visualization quality and this is not commonly compensated with other actions that might alleviate its effects. In this paper, we propose adaptive transfer functions, an algorithm that improves the transfer function in downsampled multiresolution models so that the quality of renderings is highly improved. The technique is simple and lightweight, and it is suitable, not only to visualize huge models that would not fit in a GPU, but also to render not-so-large models in mobile GPUs, which are less capable than their desktop counterparts. Moreover, it can also be used to accelerate rendering frame rates using lower levels of the multiresolution hierarchy while still maintaining high-quality results in a focus and context approach. We also show an evaluation of these results based on perceptual metrics.

Mobile devices have experienced an incredible market penetration in the last decade. Currently, medium to premium smartphones are relatively affordable devices. With the increase in screen size and resolution, together with the improvements in performance of mobile CPUs and GPUs, more tasks have become possible. In this paper we explore the rendering of medium to large volumetric models on mobile and low performance devices in general. To do so, we present a progressive ray casting method that is able to obtain interactive frame rates and high quality results for models that not long ago were only supported by desktop computers.

About 50% of the patients consulting a gastroenterology clinic report symptoms without detectable cause. Clinical researchers are interested in analyzing the volumetric evolution of colon segments under the effect of different diets and diseases. These studies require non-invasive abdominal MRI scans without using any contrast agent. In this work, we propose a colon segmentation framework designed to support T2-weighted abdominal MRI scans obtained from an unprepared colon. The segmentation process is based on an efficient and accurate quasi-automatic approach that drastically reduces the specialist interaction and effort with respect other state-of-the-art solutions, while decreasing the overall segmentation cost. The algorithm relies on a novel probabilistic tubularity filter, the detection of the colon medial line, probabilistic information extracted from a training set and a final unsupervised clustering. Experimental results presented show the benefits of our approach for clinical use.

In this paper, we present an inexpensive approach to create highly detailed reconstructions of the landscape surrounding a road. Our method is based on a space-efficient semi-procedural representation of the terrain and vegetation supporting high-quality real-time rendering not only for aerial views but also at road level. We can integrate photographs along selected road stretches. We merge the point clouds extracted from these photographs with a low-resolution digital terrain model through a novel algorithm which is robust against noise and missing data. We pre-compute plausible locations for trees through an algorithm which takes into account perceptual cues. At runtime we render the reconstructed terrain along with plants generated procedurally according to pre-computed parameters. Our rendering algorithm ensures visual consistency with aerial imagery and thus it can be integrated seamlessly with current virtual globes.

An error-aware robust estimation of the normal vectors for noisy point clouds.A panorama-based 3D representation for gigantic point clouds.An efficient algorithm for interactive navigation of gigantic point clouds. Obtaining 3D realistic models of urban scenes from accurate range data is nowadays an important research topic, with applications in a variety of fields ranging from Cultural Heritage and digital 3D archiving to monitoring of public works. Processing massive point clouds acquired from laser scanners involves a number of challenges, from data management to noise removal, model compression and interactive visualization and inspection. In this paper, we present a new methodology for the reconstruction of 3D scenes from massive point clouds coming from range lidar sensors. Our proposal includes a panorama-based compact reconstruction where colors and normals are estimated robustly through an error-aware algorithm that takes into account the variance of expected errors in depth measurements. Our representation supports efficient, GPU-based visualization with advanced lighting effects. We discuss the proposed algorithms in a practical application on urban and historical preservation, described by a massive point cloud of 3.5 billion points. We show that we can achieve compression rates higher than 97% with good visual quality during interactive inspections.

Graphical abstractDisplay Omitted Highlights? An energy minimization formulation that preserves the initial example mesh. ? An adaptive optimization algorithm based on iterating local operators. ? It uses volume information in the areas that can be unambiguously segmented. ? It imports the example shape in the areas with poor volume information. ? Segmentation of problematic cases with very limited user intervention. In this paper, we propose a novel strategy to automatically segment volume data using a high-quality mesh segmentation of an "example" model as a guiding example. The example mesh is deformed until it matches the relevant volume features. The algorithm starts from a medical volume model (scalar field of densities) to be segmented, together with an already existing segmentation (polygonal mesh) of the same organ, usually from a different person. The pre-process step computes a suitable attracting scalar field in the volume model. After an approximate 3D registration between the example mesh and the volume (this is the only step requiring user intervention), the algorithm works by minimizing an energy and adapts the shape of the polygonal mesh to the volume features in order to segment the target organ. The resulting mesh adapts to the volume features in the areas which can be unambiguously segmented, while taking the shape of the example mesh in regions which lack relevant volume information. The paper discusses several examples involving human foot bones, with results that clearly outperform present segmentation schemes.

Computer graphics and virtual reality technologies provide powerful tools for visualizing, documenting and disseminating cultural heritage. Virtual inspection tools have been used proficiently to show cultural artifacts either through the web or in museum exhibits. The usability of the user interface has been recognized to play a crucial role in overcoming the typical fearful attitude of the cultural heritage community towards 3D graphics. In this paper we discuss the design of the user interface for the virtual inspection of the impressive entrance of the Ripoll Monastery in Spain. The system was exhibited in the National Art Museum of Catalonia (MNAC) and it is now part of the Romanesque exhibition at the MAPFRE foundation. The MNAC is the third most visited art museum in Spain, and features the world's largest collection on Romanesque Art. We analyze the requirements from museum curators and discuss the main interface design decisions. The user interface combines (a) focus-plus-context visualization, with focus (detail view) and context (overview) being shown at separate displays, (b) touch-based camera control techniques, and (c) continuous feedback about the exact location of the detail area within the entrance. The interface allows users to aim the camera at any point of the entrance with centimeter accuracy using a single tap. We provide the results of a user study comparing our user interface with alternative approaches. We also discuss the benefits the exhibition had to the cultural heritage community.

In this paper, we present an efficient approach for the interactive rendering of large-scale urban models, which can be integrated seamlessly with virtual globe applications. Our scheme fills the gap between standard approaches for distant views of digital terrains and the polygonal models required for close-up views. Our work is oriented towards city models with real photographic textures of the building facades. At the heart of our approach is a multi-resolution tree of the scene defining multi-level relief impostors. Key ingredients of our approach include the pre-computation of a small set of zenithal and oblique relief maps that capture the geometry and appearance of the buildings inside each node, a rendering algorithm combining relief mapping with projective texture mapping which uses only a small subset of the pre-computed relief maps, and the use of wavelet compression to simulate two additional levels of the tree. Our scheme runs considerably faster than polygonal-based approaches while producing images with higher quality than competing relief-mapping techniques. We show both analytically and empirically that multi-level relief impostors are suitable for interactive navigation through large urban models.

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.

Recent advances in 3D scanning technologies have opened new possibilities in a broad range of applications including cultural heritage, medicine, civil engineering, and urban planning. Virtual Reality systems can provide new tools to professionals that want to understand acquired 3D models. In this review paper, we analyze the concept of data comprehension with an emphasis on visualization and inspection tools on immersive setups. We claim that in most application fields, data comprehension requires model measurements, which in turn should be based on the explicit visualization of uncertainty. As 3D digital representations are not faithful, information on their fidelity at local level should be included in the model itself as uncertainty bounds. We propose the concept of Measurable 3D Models as digital models that explicitly encode such local uncertainty bounds. We claim that professionals and experts can strongly benefit from immersive interaction through new specific, fidelity-aware measurement tools, which can facilitate 3D data comprehension. Since noise and processing errors are ubiquitous in acquired datasets, we discuss the estimation, representation, and visualization of data uncertainty. We show that, based on typical user requirements in Cultural Heritage and other domains, application-oriented measuring tools in 3D models must consider uncertainty and local error bounds. We also discuss the requirements of immersive interaction tools for the comprehension of huge 3D and nD datasets acquired from real objects.

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. Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Computational Geometry and Object Modeling

This paper is about the major peculiarities and difficulties we encounter when trying to validate research results in fields such as virtual reality (VR) and 3D user interfaces (3DUI). We review the steps in the empirical method and discuss a number of challenges when conducting human-subject experiments. These challenges include the number of independent variables to control to get useful findings, the within-subjects or between-subjects dilemma, hard-to-collect data, experimenter effects, ethical issues, and the lack of background in the community for proper statistical analysis and interpretation of the results. We show that experiments involving human-subjects hinder the adoption of traditional experimental principles (comparison, repeatability, reproducibility, justification and explanation) and propose some ideas to improve the reliability of findings in VR and 3DUI disciplines.

This paper describes the visibility octree, a data structure to accelerate 3D navigation through very complex scenes. A conservative visibility algorithm that computes and hierarchically stores the structure at a preprocessing stage is presented. The Visibility Octree is used during navigation and its main contribution is its ability to provide an effective control over the coarseness of the visibility approximation. Tests with indoor ship scenes show that the visibility octree performs well on densely occluded environments.

Remote exploration of medical volume models is nowadays a challenging problem. Interactive visualization algorithms must be able to send and render in real-time the volume model at the maximum possible visual quality, while adapting to network bandwidth limitations and to hardware constraints in the client device. In this paper we present a transfer function-aware scheme for the remote interactive inspection of volume models in client-server architectures with the objectives of supporting multi-resolution, avoiding gradient computations in the client device and sending a very limited amount of information through the network. Gradient Octrees can be progressively transmitted to the clients in a strongly compact way while achieving a minimum loss of visual quality as compared to state of the art ray-casting renderings. Visual volume understanding can be complemented by showing 2D sections of the original volume data on demand.

The paper discusses proposed solutions for constraint-based modelling, with special emphasis on constructive approaches. A new constructive scheme is proposed that is based on a nonevaluated, constructive solid model. The model definition language is presented and discussed, with the general architecture of the system and the structure of the internal model representation. The proposed approach supports the instantiation of predefined models, parametric geometric operations 1D, 2D and 3D, variable topologies, and operations with structural constraints.

The dichotomy between full detail representation and the efficient management of data digitization is still a big issue in the context of the acquisition and visualization of 3D objects, especially in the field of the cultural heritage. Modern scanning devices enable very detailed geometry to be acquired, but it is usually quite hard to apply these technologies to large artifacts. In this article we present a project aimed at virtually reconstructing the impressive (7×11 m.) portal of the Ripoll Monastery, Spain. The monument was acquired using triangulation laser scanning technology, producing a dataset of 2212 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 dissemination to the public. We show how time-of-flight laser scanning data can be used to speed-up the alignment process. In addition we show how, after creating a model and repairing imperfections, an interactive and immersive setup enables the public to navigate and display a fully detailed representation of the portal. This article shows that, after careful planning and with the aid of state-of-the-art algorithms, it is now possible to preserve and visualize highly detailed information, even for very large surfaces.

Real-time inspection and visualization of huge terrain models is a promising topic with many potential applications. Interactive rendering of planet-size, photo-textured digital elevation models hosted on remote servers is a challenging problem, requiring a careful management of the data. Typical user requirements include the access to as many accessible datasets as possible, scalable algorithms and a client—server architecture. Besides efficient handling of out-of-core gigantic models, our work has been specially focused to urban rendering and to video mapping. Our technique for the visualization of large scale urban models is based on the image-based rendering of a hierarchical collection of relief-mapped polygons. Moreover, real-time video mapping of captured video from flying cameras onto the digital terrain model can be a powerful tool for environmental security and protection.

Solid modelers must be based on reliable and fast algorithms for Boolean operations. The octree model, as well as several generalizations (polytrees, integrated polytrees, extended octrees), is specially well suited for these algorithms and can be used either as a primary or as a secondary model in solid modeling systems. This paper is concerned with a precise definition of the extended octree model that allows the representation of nonmanifold objects with planar faces and, consequently, is closed under Boolean operations on polyhedrons. Boolean nodes and nearly vertex nodes are introduced, and the model is discussed in comparison with related representations. A fast algorithm for the direct generation of the extended octree from the geometry of the base polygon in extrusion solids is presented, and its complexity is studied. Boolean operation algorithms are introduced.

In this paper we present a new simplification approach intended for scenes containing a huge number of simple objects forming a topologically complex assembly. Our method combines appearance preservation and topology reduction by converting a 3D model to and from an intermediate octree representation. During the conversion of the input mesh into an octree, appearance attributes such as colour are stored in the octree nodes. Unlike related approaches, the inside/outside values at octree vertices are computed according to neighbourhood configuration rather than by direct sampling. This allows the reconstructed surface to span only a reduced subset of the terminal nodes of the octree (those which are classified as border nodes), thus avoiding small cracks and removing internal structures not visible from the outside. The reconstruction step of our method succeeds in preserving the appearance of most of the scene objects while drastically simplifying the geometry and topology.

This paper describes 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.

Virtual Reality (VR) Systems allow immersive inspection of very complex environments, with direct interaction with the objects in the virtual scene. Low cost and affordable VR systems are becoming essential in application areas like Industrial Design, Medical Applications and Cultural Heritage. The specific requirements in these areas will be discussed, together with the advantages of using VR devices and immersive inspection techniques.

Medical imaging is constantly advancing due to technological improvements in both data gathering devices and data processing of their output. In this progress the heart presents special demands because of its incessant and brisk movement, and intrinsic characteristics. In this paper we briefly survey techniques that can be used to construct a three-dimensional model of a heart, and discuss our approach within an ongoing effort to produce accurate models of a patient's beating heart for both diagnosis and medical training.

This lecture presents and discusses the use of discrete models for modeling geometric objects. Discrete models (voxel representations, octrees, KD-trees, interval solids or even point representations) are emerging as a flexible tool for geometric modeling. These algorithms exploit the trade-off between robustness and memory. The availability at low cost of large amounts of memory affords thus completely robust models. Discrete bands can be used for instance for reconstructing valid closed models from point clouds, and to obtain different kinds of smooth Boundary Representations. On the other hand, discrete representations are specially well suited for error-bounded geometry and topology simplification, being also useful for occlusion culling in the inspection and navigation of very large virtual environments. Basic tools for modeling, surface extraction and visualization, simplification and relaxation will be described, and a number of potential applications will be presented.

This work is focused on the interactive navigation through highly polygon-populated ship environments. A conservative visibility algorithm is presented that hierarchically computes visibility for different observer positions and stores it in a new octree based structure, the Visibility Octree (VO) The VO is used during the navigation. Its main contribution is the ability to provide an effective control over the coarseness of the visibility approximation. Some results are presented for indoor ship scenes that show that the VO is efficient on densely occluded scenes.

We present an automatic simplification algorithm that produces multiresolution models with piecewise algebraic implicit surfaces. The method is based on a spatial decomposition —initial voxelization of the model— and wavelet simplification. Like other spatial decomposition simplification schemes (MDCO, BSpline filtering) it simplifies geometry and topology It can be used for navigation with LOD models in virtual environments or for approximate collision detection. The implicit simplified surface is defined as the zero-valued algebraic isosurface of a 4D functional tensor-product unform cubic BSpline. A wavelet multiresolution scheme that deals with uniform cubic BSplines on bounded domains has been constructed. One dimensional wavelet analysis and synthesis are defined on intervals with an even number (2n) of coefficient data, instead of 2 n . Storage requirements for coefficients are of the order of number of initial voxels (surface area of the object). A suitable data structure and a way to estimate / reject data coefficients at each multiresolution step is also proposed.

A new geometric relaxation algorithm for solving geometric constraint-based 2D models is proposed. Different geometric relaxation strategies and a new algorithm based on an iterative global deformation of the system is presented and discussed, and it is shown that it avoids local minima. The performance of hybrid algorithms involving global deformation and individual constraint relaxation is also discussed on several practical cases. The presented solver can easily be extended for 3D point location as is also show on practical cases.

In this paper we introduce the Discretized Polyhedra Simplification (DPS), a framework for polyhedra simplification using space decomposition models. The DPS is based on a new error measurement and provides a sound scheme for error-bounded, geometry and topology simplification while preserving the validity of the model. A method following this framework, Direct DPS, is presented and discussed. Direct DPS uses an octree for topology simplification and error control, and generates valid solid representations. Our method is also able to generate approximations which do not interpenetrate the original model, either being completely contained in the input solid or bounding it. Unlike most of the current methods, restricted to triangle meshes, our algorithm can deal and also produces faces with arbitrary complexity.

In this chapter, iterative algorithms for solving geometric constraint-based models are discussed, and a new relaxation algorithm is proposed. The algorithm starts from a constructive symbolic representation of objects (Constructive Parametric Solid Model, CPSM) and it proceeds by iterative relaxation of the geometric constraints. Systems that can be reduced to distance and angle constraints can be handled. The convergence of the proposed algorithm is discussed and an example is presented and discussed.

Multiresolution models with different levels of detail are a basic tool for handling very large and complex systems in computer aided design applications such as ship design. In this work, we discuss several automatic simplification algorithms that generate a multiresolution family of solid models that approximate with different levels of detail the boundary representation of a given polyhedral solid. Methods based on the simplification of triangular meshes are discussed together with octree based algorithms. Comparison criteria include the possibility of geometry and topology simplification, the availability of approximation bounds and feature preservation.

Computer graphics education is becoming more and more relevant in technical studies in Spain. This positive evolution is partly due to the changes in the curricula structure in the undergraduate studies in Spain in recent years. New and updated curricula are being set up in most universities with a semester structure for their courses. As a consequence, a good number of computer graphics courses have been created, and the number of universities offering graphics courses has clearly increased.Due to the converging interests in computer graphics education, a Computer Graphics Education Group has existed within the Spanish chapter of the Eurographics Association since 1992. The main activities of the group include annual meetings to discuss curricula, covered topics, methodology and resources. Relevant information and support materials are accessible and shared through the Internet ( http://smagris.eleinf.uv.es/DOCEN-CIA_IG/docencia.html ).Undergraduate university studies in Spain are either technical (three years) or high (five years). Most of the computer graphics courses are offered in both curricula. Graduate courses are organized in Ph.D. programs. Ph.D. programs in computer graphics are emerging as the new universities settle their studies. In the next two sections we briefly summarize the organization and contents of the computer graphics courses in Spain.

This paper describes the authors experience in the organization and teaching of courses of Computer Graphics for both undergraduate and graduate students. Curriculum contents and teaching methodologies are discussed, taking into account practical constraints related to the facilities available at laboratories and curriculum structures.

The objective of geometric modeling is to provide efficient, flexible and powerful tools to represent three-dimensional objects and to manipulate such representations. Such manipulations consist in creating new objects, applying geometric transformations, editing the shape or performing other operations to generate new shapes, interrogating the model and rendering it.

Occlusion 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 to combine both techniques that improves rendering times. Classical occlusion culling algorithms compute potentially visible sets (PVS), overestimations 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 different level-of-detail for the occluders. This allows to arrange the objects of each PVS into several Hardly-Visible Sets (HVS) by similar occlusion percentage. According to image accuracy and frame ratio 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 loss can be bounded by the user at navigation time. On the other hand, as HVS offers 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.

The paper focuses on automatic simplification algorithms for the generation of a multiresolution family of solid models from an initial boundary representation of a polyhedral solid. An algorithm for general polyhedra based on an intermediate octree representation is proposed. Simplified elements of the multiresolution family approximate the initial solid within increasing tolerances. A discussion among different octree-based simplification methods and the standard marching cubes algorithm is presented.

Interactive visualization of volume models in standard mobile devices is a challenging present problem with increasing interest from new application fields like telemedicine. The complexity of present volume models in medical applications is continuously increasing, therefore increasing the gap between the available models and the rendering capabilities in low-end mobile clients. New and efficient rendering algorithms and interaction paradigms are required for these small platforms. In this paper, we propose a transfer function-aware compression and interaction scheme, for client-server architectures with visualization on standard mobile devices. The scheme is block-based, supporting adaptive ray-casting in the client. Our two-level ray-casting allows focusing on small details on targeted regions while keeping bounded memory requirements in the GPU of the client. Our approach includes a transfer function-aware compression scheme based on a local wavelet transformation, together with a bricking scheme that supports interactive inspection and levels of detail in the mobile device client. We also use a quantization technique that takes into account a perceptive metrics of the visual error. Our results show that we can have full interaction with high compression rates and with transmitted model sizes that can be of the order of a single photographic image.