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Item Linear-Time Dynamics of Characters with Stiff Joints(The Eurographics Association, 2021) Hernández, Fernando; Garre, Carlos; Casillas, Rubén; Otaduy, Miguel A.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.Characters, like other articulated objects and structures, are typically simulated using articulated dynamics algorithms. There are efficient linear-time algorithms for the simulation of open-chain articulated bodies, but complexity grows notably under additional constraints such as joint limits, loops or contact, or if the bodies undergo stiff joint forces. This paper presents a linear-time algorithm for the simulation of open-chain articulated bodies with joint limits and stiff joint forces. This novel algorithm uses implicit integration to simulate stiff forces in a stable manner, and avoids drift by formulating joint constraints implicitly. One additional interesting feature of the algorithm is that its practical implementation entails only small modifications to a popular algorithm.Item A Simple Surface Tracking Method for Physically-Based 3D Water Simulations(The Eurographics Association, 2021) Amador, G.; Gomes, A.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.Water simulation, and more generically fluid simulation, is an important research topic in computer graphics. In 3D Eulerian Navier-Stokes-based water simulations, surface tracking and rendering are two delicate problems. The existing solutions to these problems (i.e., implicit surfaces-based approaches, height-fields, ray-tracing), are either to computationally intensive for real-time scenarios, or present bulge water surfaces (i.e., blobby water surfaces). In this paper, we propose a novel tracking algorithm for rendering water surfaces. Instead of tracking the flow of water using either level sets or height-fields, each cell of an 3D grid density value is directly measured in order to determine if it is either water, air, or water-air contact surface. The information in each cell is later used for the water surface splat rendering, using OpenGL vertex buffer objects.Item Procedural Modeling of Suspension Bridges(The Eurographics Association, 2021) Patow, Gustavo; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.In this paper we introduce a method for designing a class of engineering structures, namely suspension bridges. These bridges are ubiquitous in the industrialized countries, often appearing in known city landscapes, yet they are complex enough that hand-based modeling is tedious and time consuming. We present a method that finds the right proportions for such a structure through an optimization method that tries to distribute the tower positions while maintaining cable width to be a finite number. By simultaneously optimizing the span and sag of the cables of a bridge, we optimize the geometry and soundness of the structure. We present the details of our technique together with examples illustrating its use, including comparisons with real structures.Item Real-Time Modelling and Rendering of Sprayed Concrete(The Eurographics Association, 2021) Vélez, G.; Matey, L.; Amundarain, A.; Ordás, F.; Marín, J.A.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.This paper presents a new real-time method to model and render how sprayed concrete is spread on a surface. The method not only models and renders deposits sprayed from any angle, any distance and with any concrete flow, but it is also able to compute the amount of deposited volume taking into account the percentage of material that rebounds. The proposed method has been developed for a real-time training simulator for concrete spraying machinery, where most of the algorithm is parallelised and computed in the GPU, leaving the CPU free for other computations. In this research, the method has been validated for its use on plain surfaces and tunnel walls, but it can be extended to other types of surfaces.Item Efficient Propagation of Light Field Edits(The Eurographics Association, 2021) Jarabo, Adrian; Masia, Belen; Gutierrez, Diego; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.Light field editing is a complex task, due to the large amount of data and the need to keep consistency between views. This has hampered the creation of efficient edit propagation methods, similar to those existing for single images. We propose a framework to edit light fields at interactive rates, by propagating some sparse user edits in the full light field. This propagation is guided by a novel affinity function, which forces similar pixels (defined by our affinity space) to receive similar edits, thus ensuring consistency. To manage the light field's large amount of data, we propose a novel multi-dimensional downsampling technique: we first cluster pixels with high affinity, and then perform edit propagation over the downsampled data. We finally upsample back to the original full resolution, maintaining visual fidelity and view consistency between views.Item Augmented Reality for Web - A New Interaction Method Without Markers(The Eurographics Association, 2021) Silva, Luis F. A.; Silva, Frutuoso G. M.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.In the new age of omnipresent internet, browsers are accessible on every device in everyday's life. Thus it is also needed new interaction methods to facilitate the interaction through browser. Augmented reality is one of the techniques that is available now via browser. In this paper we present a new method of interaction without physical markers for augmented reality via browser. The first part of the paper describes the use of augmented reality via browser using physical markers and its limitations. In the second part, we propose a solution to solve the limitations of augmented reality using physical markers, i.e. we propose a new interaction method to manipulate virtual objects without using physical markers. Finally, we show a simple augmented web game based on the new interaction method proposed.Item Volume-Surface Collision Detection(The Eurographics Association, 2021) Ortegano, J.; Navarro, H.; Carmona, R.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.The presence or not of collisions between objects is usually required to study the interaction between them, increasing the realism in virtual environments. Collision detection between polygonal objects has been widely studied, and more recently some studies have been made concerning collisions between volume objects. Collision detection between volume datasets and polygonal objects is introduced in this work. This kind of mixed scenes appears naturally in many applications such as surgery simulation and volume edition. To detect the collision, first the volume dataset is represented by a single 3D texture. Then, a mapping from eye space to volume space is established, such as each mesh fragment has a 3D texture coordinate. The collision is verified by fragment during the rasterization stage. We use OpenGL R occlusion query extension to count the number of mesh fragments colliding with the volume. Our tests show that up to 3800 pairs of volume-mesh may be evaluated in one second.Item Inexpensive 3D Stereo for Two Users Using a Single Display(The Eurographics Association, 2021) Re, Armando de la; Martorell, Eduardo; Camahort, Emilio; Abad, Francisco; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.Multiuser computer systems allow several users to collaborate on the same task. Medium sized displays, like workbenches or tabletops provide a shared view of the task and a set of input devices to interface with the system. Some of the existing installations also provide 3D stereoscopic viewing, often also shared by all the participants and therefore incorrect for all of them but one. In these cases a different stereoscopic view for each user is desirable. We implemented two hardware configurations that support independent stereo viewing for two users sharing a common display using a 120Hz monitor and a 120Hz projector. Our prototypes use off-the-shelf parts to create a functional, responsive and inexpensive two-user 3D stereo display system. We also apply our systems to a competitive game, where each user has a private stereoscopic view of the game table, and a two-user multimedia player, where each user can watch and hear a different video on the same display.Item 3DWikiU - 3D Wiki For Urban Environments(The Eurographics Association, 2021) Melo, M.; Silva, P.; Magalhães, L.; Bessa, M.; Coelho, A.; Moura, J.; Rocha, A.; Ferreira, F.; Cruz, J.; Sousa, A.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.Virtual Reality (VR) systems may be used with great effectiveness in applications such as urban planning, virtual tourism, cultural heritage and education. In fact, the VR systems allow the visualization and interaction with 3D models, which can work as a natural metaphor due to their similarities with the objects that they intend to recreate. This is the perfect scenario for this project which intends to recreate virtual urban environments and complement them with digital services useful for people's everyday tasks. On the other hand, the implementation and management of a virtual environment is complex and requires a numerous development team with different skills to build it properly. The purpose of this project is to build a collaborative environment where the users have the power to contribute and edit objects in the referred environment, exploring the wiki concept. This paper presents a collaborative VR system that aims to provide a more natural and intuitive interface to access diversified services on 3D urban environments - 3D Wiki for urban environments. These services shall contemplate citizenship activities, entrepreneurship activities, promotion of the natural and cultural heritage and support of the cooperation and data sharing among citizens.Item High-quality Compression of Mipmapped Textures(The Eurographics Association, 2021) Andujar, C.; Silva, F. and Gutierrez, D. and Rodríguez, J. and Figueiredo, M.High-quality texture minification techniques, including trilinear and anisotropic filtering, require texture data to be arranged into a collection of prefiltered texture maps called mipmaps. In this paper we present a compression scheme for mipmapped textures which achieves much higher quality than current native schemes by exploiting image coherence across mipmap levels. The basic idea is to use a high-quality native compressed format for the upper levels of the mipmap pyramid (to retain efficient minification filtering) together with a novel compact representation of the detail provided by the highest-resolution mipmap. Key elements of our approach include delta-encoding of the luminance signal, efficient encoding of coherent regions through texel runs following a Hilbert scan, a scheme for run encoding supporting fast random-access, and a predictive approach for encoding indices of variable-length blocks. We show that our scheme clearly outperforms native 6:1 compressed texture formats in terms of image quality while still providing real-time rendering of trilinearly-filtered textures.