Inria / Raweb 2004
Project-Team: EVASION

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Project-Team : evasion

Section: New Results


Physically-based simulation

Participants: Florence Bertails, Marie-Paule Cani, Jean Combaz, Mathieu Coquerelle, Guillaume Dewaele, François Faure, Olivier Galizzi, Thanh Giang, Matthieu Nesme, Fabrice Neyret, Laks Raghupathi, Florence Zara.

Morphogenesis, expansion textures

Participants: Jean Combaz, Fabrice Neyret.

The purpose is to offer a modeling tool for complex surfaces whose shape results from growing phenomena (e.g. biological or geological surfaces), by allowing the user to control the growth rather than the shape itself. Control is acheived through a texture encoding of the intensity and orientation of the deformations, either explicitly (e.g. map of rifts and subductions) or generatively (e.g. reticulation, hot spots). Moreover, the control can also be interactive by `painting' the effects directly onto the surface (see figure 13). This approach allows an artist to produce folded shape using a workflow close to sculpting, while it may be difficult to obtain such shapes using a physical simulator: The initial state and the history of forces is often unknown or not easily modeled (e.g. for an unmade bed).

Jean Combaz has defended his PhD on Expansion Textures [1]. A poster [26] was presented at SCA'04.

Figure 13. Complex surfaces obtained using pre-drawn or interactively controlled expansion textures. Framed images correspond to animations. a,b,c: 0D (hot spot), 1D (hot curves) and 2D interactive expansion primitives. d,e: interactive pseudo-simulations. f: expansion controlled by a texture image. g,h: expansion controlled by an automata.
images/dilat

Virtual clay

Participants: Marie-Paule Cani, Guillaume Dewaele.

Efficiently animating virtual clay is a challenge, since neither optimisations proposed for solids (and based on a constant topology) nor for fluids (since there is a moving limit surface) are directly applicable. We, thus, proposed the first real-time model for this material [4], based on a layered approach. Three sub-models respectively handling large-scale deformations, local matter displacements, and surface tension, cooperate over time for providing the desired behaviour. Our model has been recently extended to handle an arbitrary number of tools that simultaneously interact with the clay [17]. This makes the model usable for direct hand manipulation, which is the last step in Guillaume Dewaele's thesis: the user's motion is video captured and used to control a virtual hand that serves as a multiple tool for editing the clay.

Adaptive deformation fields

Participants: Mathieu Coquerelle, François Faure, Thanh Giang.

The aim of this research is to develop novel methods for the represention and physical simulation of variations in highly deformable mesh structures for real-time animation (e.g. from the simulation of cloth to virtual surgery) through a level of detail (lod) topological approach. The research currently concentrates on the use of two primary hierarchical data structures, these being the octree and quadtree, for managing the changing multiresolution detail as well as the physical property details of mesh structures such as the likes of cloth during simulation (see Figure 14).

Figure 14. Direct manipulation of a 3D model through a deformable octree data structure
images/dino1images/dino2images/dino3

One of the primary ideas of this research is to be able to effectively use either data structure not only as a possible multiresolution paradigm for on the fly lod mesh generation during animation but also to integrate such data structures directly within the manipulation and management of the physical properties of the mesh as well. For example, collision detection could also simultaneously be handled directly through the same data structure which manages the current local mesh resolution, thus negating the neccessity of a secondary data structure for such a task.

Physically-based interactive rigid bodies

Participants: François Faure, Olivier Galizzi.

We are currently working with our partners of laboratory 3S on the software integration of several rock modeling, collision detection, and mechanical response computation methods for purposes of exhaustivity and fair comparison.

Highly colliding deformable bodies

Participants: Marie-Paule Cani, François Faure, Matthieu Nesme, Laks Raghupathi.

We address the question of simulating highly deformable objects in real-time, such as human tissues or cloth. The main problem is to detect and handle multiple (self-)collisions within the bodies. We develop a new approach for collision detection, based on a pool of "active pairs" of geometric primitives. These pairs are randomly chosen, and they iteratively converge to a local distance minimum or to a pair of colliding elements. Managing the size of the pool allows us to tune the computation time devoted to collision detection. Temporal coherence is obtained by reusing the interesting pairs from one step to another.

The application of this approach to virtual surgery has been further investigated [5][7].

We have used this method in cunjunction with hierarchical bounding volumes. This allows us to directly discard all the active pairs which do not belong to a pair of colliding bounding volumes. An application to virtual cloth simulation (see Figure 15) has been shown [19].

Beside this, we have contributed to a survey on the domain of collision detection for deformable objects [23].

Figure 15. Virtual cloth simulation with stochastic collision detection
images/dress2images/man

Robust finite elements for deformable solids

Participants: François Faure, Matthieu Nesme.

We have started a collaboration on surgical simulation with laboratory TIMC through a co-advised Ph.D. thesis. The purpose is to develop new models of finite elements for the interactive physically-based animation of human tissue. A new model of tetrahedron-based finite elements has been proposed [34]. Its main feature is to remain physically plausible even when large displacements and large deformations occur (see Figure 16), while being almost as computationally efficient as a linear finite elements method.

Figure 16. Stable large deformations produced through a robust tetrahedron-based finite elements approach
images/poutre2

Parallel simulation of cloth

Participants: François Faure, Florence Zara.

We addressed the question of simulating cloth on a PC cluster, in collaboration with laboratory ID and company Yxendis. Cloth is modeled as a physically-based deformable mesh. There are two important difficulties:

We split the cloth in compact pieces, thus reducing communication to data related to the borders of the patches. We use socket communications to transfer the data scattered in the cluster to the display machine. We have concluded this work [9] and we are currently working on a new distribution platform with our partners of laboratory ID.

Hair simulation

Participants: Florence Bertails, Marie-Paule Cani.

The adaptive model for interactive hair animation we had developed in the past few years was far from taking into account the mechanical properties of inextensible, naturally curled hair strands. We are currently investigating a way of simulating more accurate hair mechanics [30]. At the other end of the spectrum, we have developed a robust, real-time hair model for a virtual reality experiment [28].


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