Florence Bertails-Descoubes

INRIA Researcher in BiPop team, INRIA Rhône-Alpes / LJK mail INRIA Rhône-Alpes 655 avenue de l'Europe 38 334 Saint Ismier Cedex, France office   E216 email florence (dot) descoubes (at) inria (dot) fr

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International Journals

	Super-Clothoids Florence Bertails-Descoubes. Eurographics 2012, to appear. (more to come soon) Summary: Piecewise clothoids are 2D curves with continuous, piecewise linear curvature. Due to their smoothness properties, they have been extensively used in road design and robot path planning, as well as for the compact representation of hand-drawn curves. In this paper we present the Super-Clothoid model, a new mechanical model that for the first time allows for the computing of the dynamics of an elastic, inextensible piecewise clothoid. We first show that the kinematics of this model can be computed analytically depending on the Fresnel integrals, and precisely evaluated when required. Secondly, the discrete dynamics, naturally emerging from the Lagrange equations of motion, can be robustly and efficiently computed by performing and storing formal computations as far as possible, recoursing to numerical evaluation only when assembling the linear system to be solved at each time step. As a result, simulations turn out to be both interactive and stable, even for large displacements of the rod. Finally, we demonstrate the versatility of our model by handling various boundary conditions for the rod as well as complex external constraints such as frictional contact, and show that our model is perfectly adapted to inverse statics. Compared to lower-order models, the super-clothoid appears as a more natural and aesthetic primitive for bridging the gap between 2D geometric design and physics-based deformation.
	A Hybrid Iterative Solver for Robustly Capturing Coulomb Friction in Hair Dynamics Gilles Daviet, Florence Bertails-Descoubes, Laurence Boissieux. ACM SIGGRAPH Asia 2011. [Project website] [Paper (PDF)] [Movie (MP4)] (more to come soon) Summary: Dry friction between hair fibers plays a major role in the collective hair dynamic behavior as it accounts for typical nonsmooth features such as stick-slip instabilities. However, due the challenges posed by the modeling of nonsmooth friction, previous mechanical models for hair either neglect friction or use an approximate smooth friction model, thus losing important visual features. In this paper we present a new generic robust solver for capturing Coulomb friction in large assemblies of tightly packed fibers such as hair. Our method is based on an iterative algorithm where each single contact problem is efficiently and robustly solved by introducing a hybrid strategy that combines a new zero-finding formulation of (exact) Coulomb friction together with an analytical solver as a fail-safe. Our global solver turns out to be very robust and highly scalable as it can handle up to a few thousand densely packed fibers subject to tens of thousands frictional contacts at a reasonable computational cost. It can be conveniently combined to any fiber model with various rest shapes, from smooth to curly. Our results, visually validated against real hair motions, depict typical hair collective effects and greatly enhance the realism of standard hair simulators.
	A Nonsmooth Newton Solver for Capturing Exact Coulomb Friction in Fiber Assemblies Florence Bertails-Descoubes, Florent Cadoux, Gilles Daviet, Vincent Acary. ACM Transactions on Graphics, January 2011. (Orally presented at the ACM SIGGRAPH 2011 Conference in Vancouver, in August 2011). [Project website] [Paper (PDF)] [Movie (MPG)] [Slides (PDF)][Source Code] Summary: We focus on the challenging problem of simulating thin elastic rods in contact, in the presence of friction. Most previous approaches in computer graphics rely on a linear complementarity formulation for handling contact in a stable way, and approximate Coulombs's friction law for making the problem tractable. In contrast, following the seminal work by Alart and Curnier in contact mechanics, we simultaneously model contact and exact Coulomb friction as a zero finding problem of a nonsmooth function. A semi-implicit time-stepping scheme is then employed to discretizethe dynamics of rods constrained by frictional contact: this leads to a set of linear equations subject to an equality constraint involving a non-differentiable function. To solve this one-step problem we introduce a simple and practical nonsmooth Newton algorithm, which proves to be reasonably efficient and robust for systems that are not over-constrained. We show that our method is able to finely capture the subtle effects that occur when thin elastic rods with various geometries enter into contact, such as stick-slip instabilities in free configurations, entangling curls, resting contacts in braid-like structures, or the formation of tight knots under large constraints. Our method can be viewed as a first step towards the accurate modeling of dynamic fibrous materials.
	Stable Inverse Dynamic Curves Alexandre Derouet-Jourdan, Florence Bertails-Descoubes, Joëlle Thollot. ACM SIGGRAPH Asia 2010. [Project website] [Paper (PDF)] [Movie (MP4)] (supplemental material available here) Summary: 2d animation is a traditional but fascinating domain that has recently regained popularity both in animated movies and video games. This paper introduces a method for automatically converting a smooth sketched curve into a 2d dynamic curve at stable equilibrium under gravity. The curve can then be physically animated to produce secondary motions in 2d animations or simple video games. Our approach proceeds in two steps. We first present a new technique to fit a smooth piecewise circular arcs curve to a sketched curve. Then we show how to compute the physical parameters of a dynamic rod model (super-circle) so that its stable rest shape under gravity exactly matches the fitted circular arcs curve. We demonstrate the interactivity and controllability of our approach on various examples where a user can intuitively setup efficient and precise 2d animations by specifying the input geometry.
	Linear Time Super-Helices Florence Bertails. Eurographics 2009. [Project website] [Paper (PDF)] [Movie (MPG)] Summary: Thin elastic rods such as cables, phone coils, tree branches, or hair, are common objects in the real world but computing their dynamics accurately remains challenging. The recent Super-Helix model, based on the discrete equations of Kirchhoff for a piecewise helical rod, is one of the most promising models for simulating non-stretchable rods that can bend and twist. However, this model suffers from a quadratic complexity in the number of discrete elements, which, in the context of interactive applications, makes it limited to a few number of degrees of freedom - or equivalently to a low number of variations in curvature along the mean curve. This paper proposes a new, recursive scheme for the dynamics of a Super-Helix, inspired by the popular algorithm of Featherstone for serial multibody chains. Similarly to Featherstone's algorithm, we exploit the recursive kinematics of a Super-Helix to propagate elements inertias from the free end to the clamped end of the rod, while the dynamics is solved within a second pass traversing the rod in the reverse way. Besides the gain in linear complexity, which allows us to simulate a rod of complex shape much faster than the original approach, our algorithm makes it straightforward to simulate tree-like structures of Super-Helices, which turns out to be particularly useful for animating trees and plants realistically, under large displacements.
	A Fast Variational Framework for Accurate Solid-Fluid Coupling Christopher Batty, Florence Bertails, Robert Bridson. ACM SIGGRAPH 2007. [Project website] [Paper (PDF)] [Movie (MOV)] Summary: Physical simulation has emerged as a compelling animation technique, yet current approaches to coupling simulations of fluids and solids with irregular boundary geometry are inefficient or cannot handle some relevant scenarios robustly. We propose a new variational approach which allows robust and accurate solution on relatively coarse Cartesian grids, allowing possibly orders of magnitude faster simulation. By rephrasing the classical pressure projection step as a kinetic energy minimization, broadly similar to modern approaches to rigid body contact, we permit a robust coupling between fluid and arbitrary solid simulations that always gives a well-posed symmetric positive semi-definite linear system. We provide several examples of efficient fluid-solid interaction and rigid body coupling with sub-grid cell flow. In addition, we extend the framework with a new boundary condition for free-surface flow, allowing fluid to separate naturally from solids.
	Super-Helices for Predicting the Dynamics of Natural Hair Florence Bertails, Basile Audoly, Marie-Paule Cani, Bernard Querleux, Frédéric Leroy, Jean-Luc Lévêque. ACM SIGGRAPH 2006. [Project website] [Paper (PDF)] [Movie (MPG)] Summary: Simulating human hair is recognized as one of the most difficult tasks in computer animation. In this paper, we show that the Kirchhoff equations for dynamic, inextensible elastic rods can be used for accurately predicting hair motion. These equations fully account for the nonlinear behavior of hair strands with respect to bending and twisting. We introduce a novel deformable model for solving them: each strand is represented by a Super-Helix, i.e., a piecewise helical rod which is animated using the principles of Lagrangian mechanics. This results in a realistic and stable simulation, allowing large time steps. Our second contribution is an in-depth validation of the Super-Helix model, carried out through a series of experiments based on the comparison of real and simulated hair motions. We show that our model efficiently handles a wide range of hair types with a high level of realism.

SIGGRAPH Courses/Classes

Class on Realistic Hair Simulation: Animation and Rendering, ACM SIGGRAPH 2008 Classes Organizer: Florence Bertails Lecturers: Florence Bertails, Sunil Hadap, Marie-Paule Cani, Ming Lin, Steve Marschner, Tae-Yong Kim, Zoran Kacic-Alesic, Kelly Ward. [Project website] Summary: The last five years have seen a profusion of innovative solutions to one of the most challenging tasks in character synthesis: hair simulation. This class covers both recent and novel research ideas in hair animation and rendering, and presents time tested industrial practices that resulted in spectacular imagery.

A Course on Strands and Hair, ACM SIGGRAPH 2007 Courses Organizer: Sunil Hadap Lecturers: Sunil Hadap, Marie-Paule Cani, Ming Lin, Florence Bertails, Kelly Ward, Steve Marschner, Tae-Yong Kim, Zoran Kacic-Alesic. [ Project website ] Summary: Over the past six years, there has been a Renaissance in hair modeling, rendering, and animation. This course covers the gamut of hair simulation problems and presents working solutions. Topics include recent and novel research ideas, and time-tested industrial practices that created spectacular imagery.

French Articles

Simulation Numérique des Mouvements de Chevelure Florence Bertails, Basile Audoly, Marie-Paule Cani. Interstices, October 2007. [HTML paper] (in French) Summary (French): Synthétiser le mouvement d'une chevelure suscite un intérêt croissant de la part des développeurs de jeux vidéos ou de films d'animation mais aussi des industriels en cosmétique. Des simulations très réalistes ont pu être réalisées grâce à un travail scientifique basé sur un nouveau modèle mécanique du cheveu sous forme d'hélices par morceaux. Note: A short version of this article has been published in the French journal La Recherche in December 2007. More details here .

• Romain Casati (PhD student at Grenoble University - start in 2011).
• Alexandre Derouet-Jourdan (PhD student at Grenoble University, funded by ENS Lyon - start in 2010), co-advised by Joëlle Thollot.

• Gilles Daviet (Engineer on the INRIA MECHE project - 2009/2011).
  Now at Weta Digital, NZ.
• Romain Casati (Master student at MoSIG - 2011, TER student at ENSIMAG and summer intern at INRIA - 2010 co-advised by Franck Hétroy).
  Now progressing towards PhD.
• Alexandre Derouet-Jourdan (ENS Lyon, Master student at MoSIG - 2010), co-advised by Joëlle Thollot.
  Now progressing towards PhD.
• Noura Faraj (Master student at MoSIG - 2009), co-advised by Joëlle Thollot.
  Now doing a PhD thesis at Telecom ParisTech Computer Graphics Group.
• Xavier Marchal (TER student at ENSIMAG - 2009), co-advised by Franck Hétroy. [Project website]

• I am involved in the ANR Cheveux project (2008 - 2010), which focuses on hair simulation for the feature film industry. This project gathers academic (CNRS and INRIA) as well as industrial partners. I play the role of INRIA coordinator for this project.
• I am coordinating (together with Marie-Paule Cani) the INRIA associated team SHARE (2009 - 2011), a common research group with researchers from the IMAGER Lab of the University of British Columbia.

• A few French-English translations in maths/mechanics. See also this French-English dictionary for maths.