Alexis Angelidis

Hierarchical pattern mapping

We present a multi-scale algorithm for mapping a texture defined by an input image onto an arbitrary surface. It avoids the generation and storage of a new, specific texture. The idea is to progressively cover the surface by texture patches of various sizes and shapes, selected from a single input image. The process starts with large patches. A mapping that minimizes the texture fitting error with already textured neighbouring patches is selected. When this error is above a threshold, the patch is split into smaller ones, and the algorithm recursively looks for good fits at a smaller scale. The process ends when the surface is entirely covered. Our results show that the method correctly handles a wide set of texture patterns, which can be used at different mapping scales. Hierarchical texture mapping only outputs texture coordinates in the original texture for each triangle of the initial mesh. Rendering is therefore easy and memory cost minimal. Moreover the initial geometry is preserved.

A controllable, fast and stable basis for vortex based smoke simulation

We introduce a novel method for describing and controlling a 3D smoke simulation. Using harmonic analysis and principal component analysis, we define an underlying description of the fluid flow that is compact and meaningful to non-expert users. The motion of the smoke can be modified with high level tools, such as animated current curves, attractors and tornadoes. Our simulation is controllable, interactive and stable for arbitrarily long periods of time. The simulations computational cost increases linearly in the number of motion samples and smoke particles. Our adaptive smoke particle representation conveniently incorporates the surface-like characteristics of real smoke.

Swirling-sweepers: constant volume modeling

Swirling-sweepers is a new method for modeling shapes while preserving volume. The artist describes a deformation by dragging a point along a path. The method is independent of the geometric representation of the shape. It preserves volume and avoids self-intersections, both local and global. It is capable of unlimited stretching and the deformation can be constrained to affect only apart of the model. We argue that all of these properties are necessary for interactive modeling if the user is to have the impression that he or she is shaping a real material. Our method is the first to implement all five.

Simulation of smoke based on vortex filament primitives

We describe a method that permits the high performance simulation of fluid phenomena such as smoke, with high-level control for the artist. Our key primitives are vortex filament and vortex ring: vorticity defines a flow as well as velocity does, and for numerous interesting flows such as smoke or explosions this information is very compact and tightly linked to the visual features of the fluid. We treat these vortices as ID Lagrangian primitives (i.e. connected particles), which permit unbounded fluids and very accurate positioning of the features. The simulation passive density particles for rendering is totally independent of the fluid animation itself. Thus, the animation can be efficiently simulated, edited and even stored, while the fluid resolution used for rendering can be arbitrarily high. We aim at plausible fluids rather than physical accuracy. For efficiency and stability, we introduce a new formalization of the Biot-Savart law and a modified Biot-Savart Kernel. Our model also introduces a hierarchical filament structure for animation LOD, turbulent noise, and an original scheme for density particles.

Kinodynamic skinning using volume-preserving deformations

We present a new approach to character skinning where divergence-free vector fields induced by skeletal motion, describe the velocity of skin deformation. The joint transformations for a pose relative to a rest pose create a bend deformation field, resulting in pose-dependent or kinematic skin deformations, varying smoothly across joints. The bend deformation parameters are interactively controlled to capture the varying deformability of bone and other anatomic tissue within an overall fold-over free and volume-preserving skin deformation. Subsequently, we represent the dynamics of skeletal motion, tissue elasticity, muscular tension and the environment as forces that are mapped to vortices at tissue interfaces. A simplified Biot-Savart law in the context of elastic deformation recovers a divergence-free velocity field from the vorticity. Finally, we apply a new stable technique to efficiently integrate points along their deformation trajectories. Adding these dynamic forces over a window of time prior to a given pose provides a continuum of user controllable kinodynamic skinning. A comprehensive implementation using a typical animator workflow in Maya shows our approach to be effective for complex character skinning.

A layered model of a virtual human intestine for surgery simulation.

In this paper, we propose a new approach to simulate the small intestine in a context of laparoscopic surgery. The ultimate aim of this work is to simulate the training of a basic surgical gesture in real-time: moving aside the intestine to reach hidden areas of the abdomen. The main problem posed by this kind of simulation is animating the intestine. The problem comes from the nature of the intestine: a very long tube which is not isotropically elastic, and is contained in a volume that is small when compared to the intestines length. It coils extensively and collides with itself in many places. To do this, we use a layered model to animate the intestine. The intestines axis is animated as a linear mechanical component. A specific sphere-based model handles contacts and self-collisions. A skinning model is used to create the intestines volume around the axis. This paper discusses and compares three different representations for skinning the intestine: a parametric surface model and two implicit surface models. The first implicit surface model uses point skeletons while the second uses local convolution surfaces. Using these models, we obtained good-looking results in real-time. Some videos of this work can be found in the online version at doi: 10.1016/j.media.2004.11.006 and at www-imagis.imag.fr/Publications/2004/FLAMCFC04.

Adaptive implicit modeling using subdivision curves and surfaces as skeletons

Recent work has shown that implicit modeling using levels of details (LODs) is possible thanks to the use of subdivision-curves as skeletons. However, the geometric skeleton of a 3D shape is, in the general case, a graph of interconnected curve segments and surface patches, the exclusive use of curve skeletons is therefore not sufficient. We present a model that uses a graph of interconnected subdivision curves and surfaces as a skeleton, on which a varying radius can be specified in order to control surface thickness. The subdivision levels of the skeleton provide levels of detail for the field function that defines the implicit surface. Its visualization is achieved by generating a coarse mesh that surrounds the skeleton. At high valence skeleton vertices, triangulation topology issues are managed by locally overlapping the iso-surface triangulations. The mesh is then adaptively refined in order to sample the current LOD of the implicit surface within an error tolerance. The last contribution is a new solution to the unwanted blending problem. It avoids blending between parts of the surface that do not correspond to neighboring skeletal elements, and ensures continuity everywhere. All these methods are integrated into an interactive modeling system, where the user can create, view and edit complex shapes at different levels of detail.

Space deformations and their application to shape modeling

We present an overview of a set of techniques called space deformations, also known as free-form deformations, warps, skinning or deformers. This family of techniques has various applications in modeling, animation, rendering or simulation, and we focus especially on their application to modeling. Space deformation techniques are mappings of space onto another space, and can therefore be applied conveniently to any embedded geometry. This independence from the underlying geometric representation of deformed shape makes even the simplest and earliest deformation techniques still applicable and popular in current industrial practice.

Controlled blending in contact situations

Interactive implicit modelling with complex skeletons was improved with the introduction of primitives defined at different LODs. These implicit primitives use a subdivision curve as a skeleton. In this paper an extension to this representation is presented, in order to make it suitable for the interactive modelling and animation of soft objects in contact situations. The first contribution improves the display method for subdivision-based implicit primitives; this uses an adaptive polygonisation which locally refines, where necessary, according to the currently selected LOD. The second contribution consists of a new method for preventing unwanted blending when a skeleton-curve folds back onto itself. The third introduces local deformations where surfaces that should not blend come into contact. We illustrate the benefits of this methodology by describing two applications: the interactive modelling of complex organic shapes in contact situations and the physically-based animation of such organic shapes.

Fur simulation with spring continuum

We propose a practical method for generating and animating fur on parametrized surfaces using a spring continuum. Springs are physically simulated at the vertices of the polygon mesh, and spring behavior is interpolated across the mesh to provide realistic dense fur animation. Our method handles collisions between furry surfaces using a procedural model and self-collisions with a statistical model. The goal of this method is to make it easy for an animator to generate realistic dynamic fur which is efficient to simulate. The technique is most applicable to short fur, rather than long hair.