Benoît Crespin

Implicit Sweep Objects

In the field of geometric modeling, the implicit representation is a convenient model to define complex objects composed of several blended primitives. One particular subset of implicit surfaces is especially concerned by this feature. This subset includes many models proposed in the literature, namely blobs [3], metaballs [12], soft objects [13], distance surfaces [5] and convolution surfaces [6]. In this paper, all these models will be grouped under the generic name: iso-surfaces. In the iso-surface model, an object is defined by a set of primitives that can be considered as potential fields. The boundary surface of the object is then represented by the set of points for which the sum of the potential values equates a given isovalue. Two competitive approaches can be taken to create complex shapes with the iso-surface model. Either the object may be described by a very large number of simple primitives (e.g. spheres or ellipsoids) or by a small-number of complex ones (e.g. distance or convolution surfaces). The first approach is well adapted to the automatic matching of 3D experimental data [11] while the second one is merely used in a design process which allows the user to define interactively each primitive.

A Survey of Ocean Simulation and Rendering Techniques in Computer Graphics

This paper presents a survey of ocean simulation and rendering methods in computer graphics. To model and animate the ocean’s surface, these methods mainly rely on two main approaches: on the one hand, those which approximate ocean dynamics with parametric, spectral or hybrid models and use empirical laws from oceanographic research. We will see that this type of methods essentially allows the simulation of ocean scenes in the deep water domain, without breaking waves. On the other hand, physically‐based methods use Navier–Stokes equations to represent breaking waves and more generally ocean surface near the shore. We also describe ocean rendering methods in computer graphics, with a special interest in the simulation of phenomena such as foam and spray, and light’s interaction with the ocean surface.

High-Speed Private Information Retrieval Computation on GPU

A Private Information Retrieval (PIR) scheme is a protocol in which a user retrieves a record out of n from a replicated database, while hiding from the database which record has been retrieved, as long as the different replicas do not collude. A specially interesting sub-field of research, called single-database PIR, deals with the schemes that allow a user to retrieve privately an element of a non-replicated database. In these schemes, user privacy is related to the intractability of a mathematical problem, instead of being based on the assumption that different replicas exist and do not collude against their users. Single-database PIR schemes have generated an enormous amount of research in the privacy protection field during the last two decades. However, many scientists believe that these are theoretical tools unusable in almost any situation. It is true that these schemes usually require the database to use a lot of computational power, but considering the large number of applications these protocols have, it is important to develop practical approaches that provide acceptable performances for as many applications as possible. We present in this article a proof-of-concept implementation of a single-database PIR scheme proposed by Aguilar and Gaborit [2, 3]. This implementation can run in a CPU or in a GPU using CUDA, nVidias library for General Purpose computing on Graphics Processing Units (GPGPU). The performance results highlight that linear algebra PIR schemes allow to process database contents several orders of magnitude faster than previous protocols.

Dynamic triangulation of variational implicit surfaces using incremental Delaunay tetrahedralization

In this paper, we present a novel method to triangulate variational implicit surfaces. The core of the algorithm is an incremental Delaunay tetrahedralization of the constraint points defining the surface; it can be refined over time by adding new points around the surface as needed. Each tetrahedron that crosses the surface can then be triangulated to locally approximate the surface. This method allows getting several meshes of the same shape at different resolutions, which can be updated dynamically when adding new constraint points. This level-of-detail property makes variational surfaces more appealing for applications such as interactive modeling.

Generalized maps for erosion and sedimentation simulation

Abstract We propose a new approach, based on dynamic animation, to simulate geomorphological events such as erosion, sediment transportation and deposition. It relies on a generalized map representation of different geological layers. The topological evolution of these layers is driven by a set of displacements applied onto the vertices. The topological consistency of the model is guaranteed by a collision detection system that handles vertices, edges, or faces through generic operations. Experimental results show the ability of this approach to simulate various evolution scenarios studied in geology. Fluid simulation is added to implement fluid–solid interactions based on a hydrology model. These interactions generate animations of hydraulic erosion and sedimentation phenomena.

Hybrid Physical -- Topological Modeling of Physical Shapes Transformations

This work is grounded on the idea that physical and topological properties are complementary features in visual objects and their transformations. Indeed, physical properties delimit the quality of the dynamics whereas topological properties focus on the spatial organization. In this paper, we propose a new cooperation between mass-interaction models for modeling motion and topological maps for modeling topology. We experiment it through two basic examples: the tearing of a fabric and the breaking-sticking of a cohesive material.

A Hierarchical Hashing Scheme for Nearest Neighbor Search and Broad-Phase Collision Detection

Increasing computational power allows computer graphics researchers to model spectacular phenomena such as fluids and their interactions with deformable objects and structures. Particle-based (or Lagrangian) fluid and solid simulations are commonly managed separately and mixed together for the collision detection phase. We present a unified dynamic acceleration model to be used for particle neighborhood queries and broad-phase collision detection, based on a hierarchical hash table data structure. Our method is able to significantly reduce computations in large, empty areas, and thus gives better results than existing acceleration techniques, such as multilevel hashing schemes or KD-trees, in most situations.

A New Force Model for Controllable Breaking Waves

(a) Multiple waves displayed from the side (b) from above (c) Waves interacting with static blocks Figure 1: Breaking waves obtained with our model. An ocean scene with multiple waves is displayed (a) from the side and (b) from above; (c) and breaking waves and backflow waves colliding with blocks. The waves combine naturally, while each of them is modeled with its specific parameters (height, width, speed, orientation, crest slope, breaking time). Abstract This paper presents a new method for controlling swells and breaking waves using fluid solvers. With conventional approaches that generate waves by pushing particles with oscillating planes, the resulting waves cannot be controlled easily, and breaking waves are even more difficult to obtain in practice. Instead, we propose to use a new wave model that physically describes the behavior of wave forces. We show that mapping those forces to particles produces various types of waves that can be controlled by the user with only a few parameters. Our method is based on a 2D representation that describes wave speed, width, and height. It handles many swell and wave configurations, with various breaking situations. Crespin / A New Force Model for Controllable Breaking Waves

Topological Operations for Geomorphological Evolution

Geomorphological processes sculpt the shape of our everyday landscapes and must therefore be simulated to generate plausible digital landscapes. In particular, topological changes must be taken into account during the formation of complex geometries such as natural arches, bridges or tunnels. We present a novel approach to simulate the geomorphological evolution of a 3D terrain represented as a set of volumes stored in a topological model, and describe a set of atomic operations to handle topological events in a robust way. These operations form the basis to successfully implement more complex evolution scenarios in a modelling software based on generalized maps, which could be used to reduce the storage needed by other methods relying on voxel grids or layered data structures.

Colloidal suspension by SRD–MD simulation on GPU

Abstract In this paper, we focus on a coarse-grained model for fluid simulation named Stochastic Rotation Dynamics (SRD) combined with colloids simulated with Molecular Dynamics (MD). In this method, the fluid is represented by point particles with continuous velocities distributed in small cells. The coupling between the dynamics of the fluid particles and the dynamics of colloids is performed by applying repulsive forces between these two kinds of particles at each MD step. As this method is computationally expensive for large systems, we propose an approach adapted to graphical processing units (GPU) that outperforms existing methods by associating each colloid to a block of parallel threads. This strategy offers a better workload balance on the GPU even for systems with a large number of neighbors.