ControlMat: A Controlled Generative Approach to Material Capture

Material reconstruction from a photograph is a key component of 3D content creation democratization. We propose to formulate this ill-posed problem as a controlled synthesis one, leveraging the recent progress in generative deep networks. We present ControlMat, a method which, given a single photograph with uncontrolled illumination as input, conditions a diffusion model to generate plausible, tileable, high-resolution physically-based digital materials. We carefully analyze the behavior of diffusion models for multi-channel outputs, adapt the sampling process to fuse multi-scale information and introduce rolled diffusion to enable both tileability and patched diffusion for high-resolution outputs. Our generative approach further permits exploration of a variety of materials which could correspond to the input image, mitigating the unknown lighting conditions. We show that our approach outperforms recent inference and latent-space-optimization methods, and carefully validate our diffusion process design choices. Supplemental materials and additional details are available at: https://gvecchio.com/controlmat/

The Computation of Surface Lightness in Simple and Complex Scenes

The present thesis examined how reflectance properties and the complexity of surface mesostructure (small-scale surface relief) influence perceived lightness in centresurround displays. Chapters 2 and 3 evaluated the role of surface relief, gloss, and interreflections on lightness constancy, which was examined across changes in background albedo and illumination level. For surfaces with visible mesostructure (“rocky” surfaces), lightness constancy across changes in background albedo was better for targets embedded in glossy versus matte surfaces. However, this improved lightness constancy for gloss was not observed when illumination varied. Control experiments compared the matte and glossy rocky surrounds to two control displays, which matched either pixel histograms or a phase-scrambled power spectrum. Lightness constancy was improved for rocky glossy displays over the histogram-matched displays, but not compared to phase-scrambled variants of these images with equated power spectrums. The results were similar for surfaces rendered with 1, 2, 3 and 4 interreflections. These results suggest that lightness perception in complex centre-surround displays can be explained by the distribution of contrast across space and scale, independently of explicit information about surface shading or specularity. The results for surfaces without surface relief (“homogeneous” surfaces) differed qualitatively to rocky surfaces, exhibiting abrupt steps in perceived lightness at points at which the targets transitioned from being increments to decrements. Chapter 4 examined whether homogeneous displays evoke more complex mid-level representations similar to conditions of transparency. Matching target lightness in a homogeneous display to that in a textured or rocky display required varying both lightness and transmittance of the test patch on the textured display to obtain the most satisfactory matches. However, transmittance was only varied to match the contrast of targets against homogeneous surrounds, and not to explicitly match the amount of transparency perceived in the displays. The results suggest perceived target-surround edge contrast differs between homogeneous and textured displays. Varying the mid-level property of transparency in textured displays provides a natural means for equating both target lightness and the unique appearance of the edge contrast in homogeneous displays

The Computation of Surface Lightness in Simple and Complex Scenes

The present thesis examined how reflectance properties and the complexity of surface mesostructure (small-scale surface relief) influence perceived lightness in centresurround displays. Chapters 2 and 3 evaluated the role of surface relief, gloss, and interreflections on lightness constancy, which was examined across changes in background albedo and illumination level. For surfaces with visible mesostructure (“rocky” surfaces), lightness constancy across changes in background albedo was better for targets embedded in glossy versus matte surfaces. However, this improved lightness constancy for gloss was not observed when illumination varied. Control experiments compared the matte and glossy rocky surrounds to two control displays, which matched either pixel histograms or a phase-scrambled power spectrum. Lightness constancy was improved for rocky glossy displays over the histogram-matched displays, but not compared to phase-scrambled variants of these images with equated power spectrums. The results were similar for surfaces rendered with 1, 2, 3 and 4 interreflections. These results suggest that lightness perception in complex centre-surround displays can be explained by the distribution of contrast across space and scale, independently of explicit information about surface shading or specularity. The results for surfaces without surface relief (“homogeneous” surfaces) differed qualitatively to rocky surfaces, exhibiting abrupt steps in perceived lightness at points at which the targets transitioned from being increments to decrements. Chapter 4 examined whether homogeneous displays evoke more complex mid-level representations similar to conditions of transparency. Matching target lightness in a homogeneous display to that in a textured or rocky display required varying both lightness and transmittance of the test patch on the textured display to obtain the most satisfactory matches. However, transmittance was only varied to match the contrast of targets against homogeneous surrounds, and not to explicitly match the amount of transparency perceived in the displays. The results suggest perceived target-surround edge contrast differs between homogeneous and textured displays. Varying the mid-level property of transparency in textured displays provides a natural means for equating both target lightness and the unique appearance of the edge contrast in homogeneous displays

Voice and poetry as inspiration and material in acousmatic composition

This thesis combines both practice-based research, in the form of acousmatic composition, and theoretical research, addressing voice and poetry both as inspiration and material. It includes a portfolio of original compositions and a written text with aesthetic ideas that informed the compositional process. The aim of the research was to propose a particular creative strategy, based on Chilean poet Vicente Huidobro’s aesthetic theory; a system which aims to create artistic works independent of real world by taking materials from reality and combining them in unexpected ways through an equilibrium between rationality and intuition. This theory, alongside various other theoretical and artistic sources informing the creative process, is explained in a section entitled Compositional Rationale. The broader thesis is divided into two parts, each starting with a methodology relating to the compositions described within: Part 1: Octophonic cycle La lumière artificielle and Part 2: Three acousmatic tributes. In order to examine how the Compositional Rationale operates within the portfolio’s pieces, an analytical methodology has been proposed. This is described in an Analytical methodology section and considers the use of two parts of the tripartite model proposed by Nattiez (1990) and developed for electroacoustic music by Roy (2004). The two parts of the tripartite model are poietic analysis and neutral analysis. The first describes the creative process and compositional considerations of the author, and the second details the constitutive elements of each piece within five areas; Pierre Schaeffer’s notion of sound objects (1966), Denis Smalley’s notion of spectromorphological functions (1997), levels of spatial function by Annette Vande Gorne (2010), and finally two more types of analyses developed by the author: voice type and speech-sound type. Taken as a whole, the analysis demonstrates the structural constitution of each piece, and thus shows how Huidobro’s creative system, called creacionismo, has been applied successfully to acousmatic composition, generating the notion of acousmatic-creationist as nomenclature for the process. This is the main outcome of this thesis, a new artistic strategy which balances rationality and intuition within acousmatic composition and places poetry as a driven force in the use of voice, merging artistic practice and theory in a recursive action

Flexural fatigue of high-strength plain and fiber-reinforced concrete: influence of mesostructure and study of size effect

Equal fatigue tests in concrete, meaning those performed on exactly equal or homothetic specimens subjected to the same stress levels, present two problems yet to be solved. On the one hand, the high dispersion of the results, which causes the fatigue life in two apparently identical elements to vary by up to two or more orders of magnitude. On the other hand, the size effect, whereby fatigue strength decreases with increasing element size. The aim of this research work is to study these two issues in depth. Regarding the first one, the influence of the dispersion of concrete mesostructure on the dispersion of fatigue results will be studied; in particular, it will be evaluated whether the stochastic arrangement of fibers (in fiber concrete) or pores (in plain concrete) explains the variability of fatigue life. As for the size effect, fatigue tests will be performed on geometrically similar specimens subjected to the same equivalent stress levels. Macroscopic damage indicators, such as crack opening, will be used to determine the magnitude of the size effect and its variation according to the type of concrete (plain concrete and fiber-reinforced with different fiber contents). The results reveal that the arrangement of fibers and pores varies significantly in apparently identical specimens, and that this partly explains the dispersion of fatigue results. However, the estimation of the fatigue response of concrete from its mesostructure requires complex models, comprising several geometrical parameters of various components. In this work, a methodological approach to the problem is made, proposing mesostructure parameters that are reasonably predictive. With respect to the size effect, it is observed that the presence of fibers reduces the size effect on fatigue life, almost nullifying it. Furthermore, it is concluded that the secondary crack opening rate (dCMOD/dn) is an adequate parameter to explain the fatigue life in general, and the size effect in particular.Los ensayos de fatiga en hormigón iguales, entendiéndose como tales, los realizados en probetas exactamente iguales u homotéticas sometidas a los mismos niveles de tensiones, presentan dos problemáticas aún por resolver. Por un lado, la elevada dispersión de los resultados, que provoca que la vida a fatiga en dos elementos aparentemente idénticos pueda variar hasta en dos o más órdenes de magnitud. Por otro lado, el efecto tamaño, por el cual la resistencia a fatiga disminuye con el aumento del tamaño del elemento. En este trabajo de investigación se pretende profundizar en el estudio de estas dos cuestiones. Con respecto a la primera, se va a estudiar la influencia de la dispersión de la mesoestructura del hormigón en la dispersión de los resultados de fatiga; en particular, se evaluará si la disposición estocástica de las fibras (en hormigón con fibras) o de los poros (en hormigón en masa) explica la variabilidad de la vida a fatiga. En cuanto al efecto tamaño, se realizarán ensayos de fatiga en probetas geométricamente similares sujetas a los mismos niveles de tensión equivalente. A través de indicadores macroscópicos de daño, como la apertura de fisura, se determinará la magnitud del efecto tamaño y su variación en función del tipo de hormigón (en masa y con diferentes contenidos de fibras). Los resultados revelan que la disposición de las fibras y los poros varía significativamente en probetas aparentemente idénticas, y que esto explica en parte la dispersión de los resultados de fatiga. No obstante, la estimación de la respuesta en fatiga del hormigón a partir de su mesoestructura requiere de modelos complejos, que comprendan varios parámetros geométricos de diversos componentes. En este trabajo se realiza una aproximación metodológica al problema, proponiendo parámetros de la mesoestructura que son razonablemente predictivos. Con respecto al efecto tamaño, se observa que la presencia de fibras reduce el efecto tamaño en la vida fatiga, hasta prácticamente anularlo. Además, se concluye que la tasa de apertura de fisura secundaria (dCMOD/dn) es un parámetro adecuado para explicar la vida a fatiga en general, y el efecto tamaño en particular

Realistic Visualization of Animated Virtual Cloth

Photo-realistic rendering of real-world objects is a broad research area with applications in various different areas, such as computer generated films, entertainment, e-commerce and so on. Within photo-realistic rendering, the rendering of cloth is a subarea which involves many important aspects, ranging from material surface reflection properties and macroscopic self-shadowing to animation sequence generation and compression. In this thesis, besides an introduction to the topic plus a broad overview of related work, different methods to handle major aspects of cloth rendering are described. Material surface reflection properties play an important part to reproduce the look & feel of materials, that is, to identify a material only by looking at it. The BTF (bidirectional texture function), as a function of viewing and illumination direction, is an appropriate representation of reflection properties. It captures effects caused by the mesostructure of a surface, like roughness, self-shadowing, occlusion, inter-reflections, subsurface scattering and color bleeding. Unfortunately a BTF data set of a material consists of hundreds to thousands of images, which exceeds current memory size of personal computers by far. This work describes the first usable method to efficiently compress and decompress a BTF data for rendering at interactive to real-time frame rates. It is based on PCA (principal component analysis) of the BTF data set. While preserving the important visual aspects of the BTF, the achieved compression rates allow the storage of several different data sets in main memory of consumer hardware, while maintaining a high rendering quality. Correct handling of complex illumination conditions plays another key role for the realistic appearance of cloth. Therefore, an upgrade of the BTF compression and rendering algorithm is described, which allows the support of distant direct HDR (high-dynamic-range) illumination stored in environment maps. To further enhance the appearance, macroscopic self-shadowing has to be taken into account. For the visualization of folds and the life-like 3D impression, these kind of shadows are absolutely necessary. This work describes two methods to compute these shadows. The first is seamlessly integrated into the illumination part of the rendering algorithm and optimized for static meshes. Furthermore, another method is proposed, which allows the handling of dynamic objects. It uses hardware-accelerated occlusion queries for the visibility determination. In contrast to other algorithms, the presented algorithm, despite its simplicity, is fast and produces less artifacts than other methods. As a plus, it incorporates changeable distant direct high-dynamic-range illumination. The human perception system is the main target of any computer graphics application and can also be treated as part of the rendering pipeline. Therefore, optimization of the rendering itself can be achieved by analyzing human perception of certain visual aspects in the image. As a part of this thesis, an experiment is introduced that evaluates human shadow perception to speedup shadow rendering and provides optimization approaches. Another subarea of cloth visualization in computer graphics is the animation of the cloth and avatars for presentations. This work also describes two new methods for automatic generation and compression of animation sequences. The first method to generate completely new, customizable animation sequences, is based on the concept of finding similarities in animation frames of a given basis sequence. Identifying these similarities allows jumps within the basis sequence to generate endless new sequences. Transmission of any animated 3D data over bandwidth-limited channels, like extended networks or to less powerful clients requires efficient compression schemes. The second method included in this thesis in the animation field is a geometry data compression scheme. Similar to the BTF compression, it uses PCA in combination with clustering algorithms to segment similar moving parts of the animated objects to achieve high compression rates in combination with a very exact reconstruction quality.Realistische Visualisierung von animierter virtueller Kleidung Das photorealistisches Rendering realer Gegenstände ist ein weites Forschungsfeld und hat Anwendungen in vielen Bereichen. Dazu zählen Computer generierte Filme (CGI), die Unterhaltungsindustrie und E-Commerce. Innerhalb dieses Forschungsbereiches ist das Rendern von photorealistischer Kleidung ein wichtiger Bestandteil. Hier reichen die wichtigen Aspekte, die es zu berücksichtigen gilt, von optischen Materialeigenschaften über makroskopische Selbstabschattung bis zur Animationsgenerierung und -kompression. In dieser Arbeit wird, neben der Einführung in das Thema, ein weiter Überblick über ähnlich gelagerte Arbeiten gegeben. Der Schwerpunkt der Arbeit liegt auf den wichtigen Aspekten der virtuellen Kleidungsvisualisierung, die oben beschrieben wurden. Die optischen Reflektionseigenschaften von Materialoberflächen spielen eine wichtige Rolle, um das so genannte look & feel von Materialien zu charakterisieren. Hierbei kann ein Material vom Nutzer identifiziert werden, ohne dass er es direkt anfassen muss. Die BTF (bidirektionale Texturfunktion)ist eine Funktion die abhängig von der Blick- und Beleuchtungsrichtung ist. Daher ist sie eine angemessene Repräsentation von Reflektionseigenschaften. Sie enthält Effekte wie Rauheit, Selbstabschattungen, Verdeckungen, Interreflektionen, Streuung und Farbbluten, die durch die Mesostruktur der Oberfläche hervorgerufen werden. Leider besteht ein BTF Datensatz eines Materials aus hunderten oder tausenden von Bildern und sprengt damit herkömmliche Hauptspeicher in Computern bei weitem. Diese Arbeit beschreibt die erste praktikable Methode, um BTF Daten effizient zu komprimieren, zu speichern und für Echtzeitanwendungen zum Visualisieren wieder zu dekomprimieren. Die Methode basiert auf der Principal Component Analysis (PCA), die Daten nach Signifikanz ordnet. Während die PCA die entscheidenen visuellen Aspekte der BTF erhält, können mit ihrer Hilfe Kompressionsraten erzielt werden, die es erlauben mehrere BTF Materialien im Hauptspeicher eines Consumer PC zu verwalten. Dies erlaubt ein High-Quality Rendering. Korrektes Verwenden von komplexen Beleuchtungssituationen spielt eine weitere, wichtige Rolle, um Kleidung realistisch erscheinen zu lassen. Daher wird zudem eine Erweiterung des BTF Kompressions- und Renderingalgorithmuses erläutert, die den Einsatz von High-Dynamic Range (HDR) Beleuchtung erlaubt, die in environment maps gespeichert wird. Um die realistische Erscheinung der Kleidung weiter zu unterstützen, muss die makroskopische Selbstabschattung integriert werden. Für die Visualisierung von Falten und den lebensechten 3D Eindruck ist diese Art von Schatten absolut notwendig. Diese Arbeit beschreibt daher auch zwei Methoden, diese Schatten schnell und effizient zu berechnen. Die erste ist nahtlos in den Beleuchtungspart des obigen BTF Renderingalgorithmuses integriert und für statische Geometrien optimiert. Die zweite Methode behandelt dynamische Objekte. Dazu werden hardwarebeschleunigte Occlusion Queries verwendet, um die Sichtbarkeitsberechnung durchzuführen. Diese Methode ist einerseits simpel und leicht zu implementieren, anderseits ist sie schnell und produziert weniger Artefakte, als vergleichbare Methoden. Zusätzlich ist die Verwendung von veränderbarer, entfernter HDR Beleuchtung integriert. Das menschliche Wahrnehmungssystem ist das eigentliche Ziel jeglicher Anwendung in der Computergrafik und kann daher selbst als Teil einer erweiterten Rendering Pipeline gesehen werden. Daher kann das Rendering selbst optimiert werden, wenn man die menschliche Wahrnehmung verschiedener visueller Aspekte der berechneten Bilder analysiert. Teil der vorliegenden Arbeit ist die Beschreibung eines Experimentes, das menschliche Schattenwahrnehmung untersucht, um das Rendern der Schatten zu beschleunigen. Ein weiteres Teilgebiet der Kleidungsvisualisierung in der Computergrafik ist die Animation der Kleidung und von Avataren für Präsentationen. Diese Arbeit beschreibt zwei neue Methoden auf diesem Teilgebiet. Einmal ein Algorithmus, der für die automatische Generierung neuer Animationssequenzen verwendet werden kann und zum anderen einen Kompressionsalgorithmus für eben diese Sequenzen. Die automatische Generierung von völlig neuen, anpassbaren Animationen basiert auf dem Konzept der Ähnlichkeitssuche. Hierbei werden die einzelnen Schritte von gegebenen Basisanimationen auf Ähnlichkeiten hin untersucht, die zum Beispiel die Geschwindigkeiten einzelner Objektteile sein können. Die Identifizierung dieser Ähnlichkeiten erlaubt dann Sprünge innerhalb der Basissequenz, die dazu benutzt werden können, endlose, neue Sequenzen zu erzeugen. Die Übertragung von animierten 3D Daten über bandbreitenlimitierte Kanäle wie ausgedehnte Netzwerke, Mobilfunk oder zu sogenannten thin clients erfordert eine effiziente Komprimierung. Die zweite, in dieser Arbeit vorgestellte Methode, ist ein Kompressionsschema für Geometriedaten. Ähnlich wie bei der Kompression von BTF Daten wird die PCA in Verbindung mit Clustering benutzt, um die animierte Geometrie zu analysieren und in sich ähnlich bewegende Teile zu segmentieren. Diese erkannten Segmente lassen sich dann hoch komprimieren. Der Algorithmus arbeitet automatisch und erlaubt zudem eine sehr exakte Rekonstruktionsqualität nach der Dekomprimierung

Material Recognition Meets 3D Reconstruction : Novel Tools for Efficient, Automatic Acquisition Systems

For decades, the accurate acquisition of geometry and reflectance properties has represented one of the major objectives in computer vision and computer graphics with many applications in industry, entertainment and cultural heritage. Reproducing even the finest details of surface geometry and surface reflectance has become a ubiquitous prerequisite in visual prototyping, advertisement or digital preservation of objects. However, today's acquisition methods are typically designed for only a rather small range of material types. Furthermore, there is still a lack of accurate reconstruction methods for objects with a more complex surface reflectance behavior beyond diffuse reflectance. In addition to accurate acquisition techniques, the demand for creating large quantities of digital contents also pushes the focus towards fully automatic and highly efficient solutions that allow for masses of objects to be acquired as fast as possible. This thesis is dedicated to the investigation of basic components that allow an efficient, automatic acquisition process. We argue that such an efficient, automatic acquisition can be realized when material recognition "meets" 3D reconstruction and we will demonstrate that reliably recognizing the materials of the considered object allows a more efficient geometry acquisition. Therefore, the main objectives of this thesis are given by the development of novel, robust geometry acquisition techniques for surface materials beyond diffuse surface reflectance, and the development of novel, robust techniques for material recognition. In the context of 3D geometry acquisition, we introduce an improvement of structured light systems, which are capable of robustly acquiring objects ranging from diffuse surface reflectance to even specular surface reflectance with a sufficient diffuse component. We demonstrate that the resolution of the reconstruction can be increased significantly for multi-camera, multi-projector structured light systems by using overlappings of patterns that have been projected under different projector poses. As the reconstructions obtained by applying such triangulation-based techniques still contain high-frequency noise due to inaccurately localized correspondences established for images acquired under different viewpoints, we furthermore introduce a novel geometry acquisition technique that complements the structured light system with additional photometric normals and results in significantly more accurate reconstructions. In addition, we also present a novel method to acquire the 3D shape of mirroring objects with complex surface geometry. The aforementioned investigations on 3D reconstruction are accompanied by the development of novel tools for reliable material recognition which can be used in an initial step to recognize the present surface materials and, hence, to efficiently select the subsequently applied appropriate acquisition techniques based on these classified materials. In the scope of this thesis, we therefore focus on material recognition for scenarios with controlled illumination as given in lab environments as well as scenarios with natural illumination that are given in photographs of typical daily life scenes. Finally, based on the techniques developed in this thesis, we provide novel concepts towards efficient, automatic acquisition systems

Image based surface reflectance remapping for consistent and tool independent material appearence

Physically-based rendering in Computer Graphics requires the knowledge of material properties other than 3D shapes, textures and colors, in order to solve the rendering equation. A number of material models have been developed, since no model is currently able to reproduce the full range of available materials. Although only few material models have been widely adopted in current rendering systems, the lack of standardisation causes several issues in the 3D modelling workflow, leading to a heavy tool dependency of material appearance. In industry, final decisions about products are often based on a virtual prototype, a crucial step for the production pipeline, usually developed by a collaborations among several departments, which exchange data. Unfortunately, exchanged data often tends to differ from the original, when imported into a different application. As a result, delivering consistent visual results requires time, labour and computational cost. This thesis begins with an examination of the current state of the art in material appearance representation and capture, in order to identify a suitable strategy to tackle material appearance consistency. Automatic solutions to this problem are suggested in this work, accounting for the constraints of real-world scenarios, where the only available information is a reference rendering and the renderer used to obtain it, with no access to the implementation of the shaders. In particular, two image-based frameworks are proposed, working under these constraints. The first one, validated by means of perceptual studies, is aimed to the remapping of BRDF parameters and useful when the parameters used for the reference rendering are available. The second one provides consistent material appearance across different renderers, even when the parameters used for the reference are unknown. It allows the selection of an arbitrary reference rendering tool, and manipulates the output of other renderers in order to be consistent with the reference

Differentiable Display Photometric Stereo

Photometric stereo leverages variations in illumination conditions to reconstruct per-pixel surface normals. The concept of display photometric stereo, which employs a conventional monitor as an illumination source, has the potential to overcome limitations often encountered in bulky and difficult-to-use conventional setups. In this paper, we introduce Differentiable Display Photometric Stereo (DDPS), a method designed to achieve high-fidelity normal reconstruction using an off-the-shelf monitor and camera. DDPS addresses a critical yet often neglected challenge in photometric stereo: the optimization of display patterns for enhanced normal reconstruction. We present a differentiable framework that couples basis-illumination image formation with a photometric-stereo reconstruction method. This facilitates the learning of display patterns that leads to high-quality normal reconstruction through automatic differentiation. Addressing the synthetic-real domain gap inherent in end-to-end optimization, we propose the use of a real-world photometric-stereo training dataset composed of 3D-printed objects. Moreover, to reduce the ill-posed nature of photometric stereo, we exploit the linearly polarized light emitted from the monitor to optically separate diffuse and specular reflections in the captured images. We demonstrate that DDPS allows for learning display patterns optimized for a target configuration and is robust to initialization. We assess DDPS on 3D-printed objects with ground-truth normals and diverse real-world objects, validating that DDPS enables effective photometric-stereo reconstruction

Enhancing detailed haptic relief for real-time interaction

The present document exposes a different approach for haptic rendering, defined as the simulation of force interactions to reproduce the sensation of surface relief in dense models. Current research shows open issues in timely haptic interaction involving large meshes, with several problems affecting performance and fidelity, and without a dominant technique to treat these issues properly. Relying in pure geometric collisions when rendering highly dense mesh models (hundreds of thousands of triangles) sensibly degrades haptic rates due to the sheer number of collisions that must be tracked between the mesh's faces and a haptic probe. Several bottlenecks were identified in order to enhance haptic performance: software architecture and data structures, collision detection, and accurate rendering of surface relief. To account for overall software architecture and data structures, it was derived a complete component framework for transforming standalone VR applications into full-fledged multi-threaded Collaborative Virtual Reality Environments (CVREs), after characterizing existing implementations into a feature-rich superset. Enhancements include: a scalable arbitrated peer-to-peer topology for scene sharing; multi-threaded components for graphics rendering, user interaction and network communications; a collaborative user interface model for session handling; and interchangeable user roles with multi-camera perspectives, avatar awareness and shared annotations. We validate the framework by converting the existing ALICE VR Navigator into a complete CVRE, showing good performance in collaborative manipulation of complex models. To specifically address collision detection computation, we derive a conformal algebra treatment for collisions among points, segments, areas, and volumes, based on collision detection in conformal R{4,1} (5D) space, and implemented in GPU for faster parallel queries. Results show orders of magnitude time reductions in collisions computations, allowing interactive rates. Finally, the main core of the research is the haptic rendering of surface mesostructure in large meshes. Initially, a method for surface haptic rendering was proposed, using image-based Hybrid Rugosity Mesostructures (HRMs) of per-face heightfield displacements and normalmaps layered on top of a simpler mesh, adding greater surface detail than actually present. Haptic perception is achieved modulating the haptic probe's force response using the HRM coat. A usability testbed framework was built to measure experimental performance with a common set tests, meshes and HRMs. Trial results show the goodness of the proposed technique, rendering accurate 3D surface detail at high sampling rates. This local per-face method is extended into a fast global approach for haptic rendering, building a mesostructure-based atlas of depth/normal textures (HyRMA), computed out of surface differences of the same mesh object at two different resolutions: original and simplified. For each triangle in the simplified mesh, an irregular prism is considered defined by the triangle's vertices and their normals. This prism completely covers the original mesh relief over the triangle. Depth distances and surfaces normals within each prism are warped from object volume space to orthogonal tangent space, by means of a novel and fast method for computing barycentric coordinates at the prism, and storing normals and relief in a sorted atlas. Haptic rendering is effected by colliding the probe against the atlas, and effecting a modulated force response at the haptic probe. The method is validated numerically, statistically and perceptually in user testing controlled trials, achieving accurate haptic sensation of large meshes' fine features at interactive rendering rates, with some minute loss of mesostructure detail.En aquesta tesi es presenta un novedós enfocament per a la percepció hàptica del relleu de models virtuals complexes mitjançant la simulació de les forces d'interacció entre la superfície i un element de contacte. La proposta contribueix a l'estat de l'art de la recerca en aquesta àrea incrementant l'eficiència i la fidelitat de la interacció hàptica amb grans malles de triangles. La detecció de col·lisions amb malles denses (centenars de milers de triangles) limita la velocitat de resposta hàptica degut al gran nombre d'avaluacions d'intersecció cara-dispositiu hàptic que s'han de realitzar. Es van identificar diferents alternatives per a incrementar el rendiment hàptic: arquitectures de software i estructures de dades específiques, algorismes de detecció de col·lisions i reproducció hàptica de relleu superficial. En aquesta tesi es presenten contribucions en alguns d'aquests aspectes. S'ha proposat una estructura completa de components per a transformar aplicacions de Realitat Virtual en Ambients Col·laboratius de Realitat Virtual (CRVEs) multithread en xarxa. L'arquitectura proposada inclou: una topologia escalable punt a punt per a compartir escenes; components multithread per a visualització gràfica, interacció amb usuaris i comunicació en xarxa; un model d'interfície d'usuari col·laboratiu per a la gestió de sessions; i rols intercanviables de l'usuari amb perspectives de múltiples càmeres, presència d'avatars i anotacions compartides. L'estructura s'ha validat convertint el navegador ALICE en un CVRE completament funcional, mostrant un bon rendiment en la manipulació col·laborativa de models complexes. Per a incrementar l'eficiència del càlcul de col·lisions, s'ha proposat un algorisme que treballa en un espai conforme R{4,1} (5D) que permet detectar col·lisions entre punts, segments, triangles i volums. Aquest algorisme s'ha implementat en GPU per obtenir una execució paral·lela més ràpida. Els resultats mostren reduccions en el temps de càlcul de col·lisions permetent interactivitat. Per a la percepció hàptica de malles complexes que modelen objectes rugosos, s'han proposat diferents algorismes i estructures de dades. Les denominades Mesoestructures Híbrides de Rugositat (HRM) permeten substituir els detalls geomètrics d'una cara (rugositats) per dues textures: de normals i d'alçades. La percepció hàptica s'aconsegueix modulant la força de resposta entre el dispositiu hàptic i la HRM. Els tests per avaluar experimentalment l'eficiència del càlcul de col·lisions i la percepció hàptica utilitzant HRM respecte a modelar les rugositats amb geometria, van mostrar que la tècnica proposada va ser encertada, permetent percebre detalls 3D correctes a altes tases de mostreig. El mètode es va estendre per a representar rugositats d'objectes. Es proposa substituir l'objecte per un model simplificat i un atles de mesoestructures en el que s'usen textures de normals i de relleus (HyRMA). Aquest atles s'obté a partir de la diferència en el detall de la superfície entre dos malles del mateix objecte: l'original i la simplificada. A partir d'un triangle de la malla simplificada es construeix un prisma, definit pels vèrtexs del triangle i les seves normals, que engloba el relleu de la malla original sobre el triangle. Les alçades i normals dins del prisma es transformen des de l'espai de volum a l'espai ortogonal tangent, amb mètode novedós i eficient que calcula les coordenades baricèntriques relatives al prisma, per a guardar el mapa de textures transformat en un atles ordenat. La percepció hàptica s'assoleix detectant les col·lisions entre el dispositiu hàptic i l'atles, i modulant la força de resposta d'acord al resultat de la col·lisió. El mètode s'ha validat numèricament, estadística i perceptual en tests amb usuaris, aconseguint una correcta i interactiva sensació tàctil dels objectes simulats mitjançant la mesoestructura de les mallesEn esta tesis se presenta un enfoque novedoso para la percepción háptica del relieve de modelos virtuales complejos mediante la simulación de las fuerzas de interacción entre la superficie y un elemento de contacto. La propuesta contribuye al estado del arte de investigación en este área incrementando la eficiencia y fidelidad de interacción háptica con grandes mallas de triángulos. La detección de colisiones con mallas geométricas densas (cientos de miles de triángulos) limita la velocidad de respuesta háptica debido al elevado número de evaluaciones de intersección cara-dispositivo háptico que deben realizarse. Se identificaron diferentes alternativas para incrementar el rendimiento háptico: arquitecturas de software y estructuras de datos específicas, algoritmos de detección de colisiones y reproducción háptica de relieve superficial. En esta tesis se presentan contribuciones en algunos de estos aspectos. Se ha propuesto una estructura completa de componentes para transformar aplicaciones aisladas de Realidad Virtual en Ambientes Colaborativos de Realidad Virtual (CRVEs) multithread en red. La arquitectura propuesta incluye: una topología escalable punto a punto para compartir escenas; componentes multithread para visualización gráfica, interacción con usuarios y comunicación en red; un modelo de interfaz de usuario colaborativo para la gestión de sesiones; y roles intercambiables del usuario con perspectivas de múltiples cámaras, presencia de avatares y anotaciones compartidas. La estructura se ha validado convirtiendo el navegador ALICE en un CVRE completamente funcional, mostrando un buen rendimiento en la manipulación colaborativa de modelos complejos. Para incrementar la eficiencia del cálculo de colisiones, se ha propuesto un algoritmo que trabaja en un espacio conforme R4,1 (5D) que permite detectar colisiones entre puntos, segmentos, triángulos y volúmenes. Este algoritmo se ha implementado en GPU a efectos de obtener una ejecución paralelamás rápida. Los resultadosmuestran reducciones en el tiempo de cálculo de colisiones permitiendo respuesta interactiva. Para la percepción háptica de mallas complejas que modelan objetos rugosos, se han propuesto diferentes algoritmos y estructuras de datos. Las denominadasMesoestructuras Híbridas de Rugosidad (HRM) permiten substituir los detalles geométricos de una cara (rugosidades) por una textura de normales y otra de alturas. La percepción háptica se consigue modulando la fuerza de respuesta entre el dispositivo háptico y la HRM. Los tests realizados para evaluar experimentalmente la eficiencia del cálculo de colisiones y la percepción háptica utilizando HRM respecto a modelar las rugosidades con geometría, mostraron que la técnica propuesta fue acertada, permitiendo percibir detalles 3D correctos a altas tasas de muestreo. Este método anterior es extendido a un procedimiento global para representar rugosidades de objetos. Para hacerlo se propone sustituir el objeto por un modelo simplificado y un atlas de mesostructuras usando texturas de normales y relieves (HyRMA). Este atlas se obtiene de la diferencia en detalle de superficie entre dos mallas del mismo objeto: la original y la simplificada. A partir de un triángulo de la malla simplificada se construye un prisma definido por los vértices del triángulo a lo largo de sus normales, que engloba completamente el relieve de la malla original sobre este triángulo. Las alturas y normales dentro de cada prisma se transforman del espacio de volumen al espacio ortoganal tangente, usando un método novedoso y eficiente que calcula las coordenadas baricéntricas relativas a cada prisma para guardar el mapa de texturas transformado en un atlas ordenado. La percepción háptica se consigue detectando directamente las colisiones entre el dispositivo háptico y el atlas, y modulando la fuerza de respuesta de acuerdo al resultado de la colisión. El procedmiento se ha validado numérica, estadística y perceptualmente en ensayos con usuarios, consiguiendo a tasas interactivas la correcta sensación táctil de los objetos simulados mediante la mesoestructura de las mallas, con alguna pérdida muy puntual de detall