# 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 E111 email florence (dot) descoubes (at) inria (dot) fr

## Short Bio

I am working as a junior researcher ('Chargée de Recherche') at INRIA in Grenoble, France, in the BiPop research group which is specialized in the modeling and simulation of nonsmooth dynamic phenomena. In 2006 - 2007, I did a post-doc at the IMAGER Lab of the University of British Columbia, in beautiful Vancouver, where I have been working with Robert Bridson and Christopher Batty on physically-based models for coupling fluid and solid structures. During my Ph.D. thesis completed at Grenoble INP (awarded the 2006 SPECIF Prize), I have been working on hair simulation under the supervision of Marie-Paule Cani (Grenoble INP) and Basile Audoly (CNRS).

## What's new

• I will serve as a Program Co-Chair for the Symposium on Computer Animation in 2015 (SCA'15).
• The source code accompanying some of our recent papers on the modeling and simulation of fibers and frictional contact has been recently released, check out the full list of available ressources here.

## Research Interests

My research interests deal with the mathematical modeling and the numerical simulation of complex mechanical objects featuring a rich shape and/or motion. Typical applications include computer graphics and virtual prototyping for industry. More specifically, I am interested in the following topics:
• Discrete modeling of dynamic slender structures (such as rods and plates)
• Geometric representation and design of space curves
• Simulation of contact and friction, within the nonsmooth dynamic settings
• Mathematical modeling of heterogeneous materials (such as granulars and fibrous materials)
• Parameters identification (inverse modeling problems), and control of physics-based simulators.
The main challenge consists in finding appropriate and compact discrete models for capturing nonlinear (and sometimes nonsmooth) phenomena, in a both robust and efficient way.

## Selected Publications

The full list of my publications is given here. Below is a list of selected publications.

### International Journals

 Inverse Dynamic Hair Modeling with Frictional Contact Alexandre Derouet-Jourdan, Florence Bertails-Descoubes, Gilles Daviet, Joëlle Thollot. ACM SIGGRAPH Asia 2013, to appear. [Project website] [Paper (PDF)] [Movie (MP4)][Other ressources] Summary: In the latest years, considerable progress has been achieved for accurately acquiring the geometry of human hair, thus largely improving the realism of virtual characters. In parallel, rich and robust physics-based simulators have been successfully designed to capture the intricate dynamics of hair due to contact and friction. However, at the moment there exists no consistent pipeline for converting a given hair geometry into a realistic physics-based hair model. Current approaches simply initialize the hair simulator with the input geometry in the absence of external forces. This results in an undesired sagging effect when the dynamic simulation is started, which basically ruins all the efforts put into the accurate design and/or capture of the input hairstyle. In this paper we propose the first method which consistently and robustly accounts for surrounding forces $-$ gravity and frictional contacts, including hair self-contacts $-$ when converting a geometric hairstyle into a physics-based hair model. Taking an arbitrary hair geometry as input together with a corresponding body mesh, we interpret the hair shape as a static equilibrium configuration of a hair simulator, in the presence of gravity as well as hair-body and hair-hair frictional contacts. Assuming hair parameters are homogeneous and lie in a plausible range of physical values, we show that this large, underdetermined inverse problem can be formulated as a well-posed constrained optimization problem, which can be robustly and efficiently solved by leveraging the frictional contact solver of the direct hair simulator. Our method was successfully applied to the animation of various hair geometries, ranging from synthetic hairstyles manually designed by an artist to the most recent human hair data reconstructed from capture. Super Space Clothoids Romain Casati, Florence Bertails-Descoubes. ACM SIGGRAPH 2013. [Project website] [Paper (PDF)] [Movie (MP4)] [Supplemental material (PDF)][Inria highlight ][Source code] Summary: Thin elastic filaments in real world such as vine tendrils, hair ringlets or curled ribbons often depict a very smooth, curved shape that low-order rod models $-$ e.g., segment-based rods $-$ fail to reproduce accurately and compactly. In this paper, we push forward the investigation of high-order models for thin, inextensible elastic rods by building the dynamics of a $G^2$- continuous piecewise 3D clothoid: a smooth space curve with piecewise affine curvature. With the aim of precisely integrating the rod kinematic problem, for which no closed-form solution exists, we introduce a dedicated integration scheme based on power series expansions. It turns out that our algorithm reaches machine precision orders of magnitude faster compared to classical numerical integrators. This property, nicely preserved under simple algebraic and differential operations, allows us to compute all spatial terms of the rod kinematics and dynamics in both an efficient and accurate way. Combined with a semi-implicit time-stepping scheme, our method leads to the efficient and robust simulation of arbitrary curly filaments that exhibit rich, visually pleasing configurations and motion. Our approach was successfully applied to generate various scenarios such as the unwinding of a curled ribbon as well as the aesthetic animation of spiral-like hair or the fascinating growth of twining plants. Floating Tangents for Approximating Spatial Curves with $G^1$ Piecewise Helices Alexandre Derouet-Jourdan, Florence Bertails-Descoubes, Joëlle Thollot. Computer-Aided Geometric Design, June 2013. [Editor link][Preprint (PDF)][Movie (MP4)](more to come soon) Summary: Curves are widely used in computer science to describe real-life objects such as slender deformable structures. Using only 3 parameters per element, piecewise helices offer an interesting and compact way of representing digital curves. In this paper, we present a robust and fast algorithm to approximate Bézier curves with $G^1$ piecewise helices. Our approximation algorithm takes a Bézier spline as input along with an integer N and returns a piecewise helix with N elements that closely approximates the input curve. The key idea of our method is to take N+1 evenly distributed points along the curve, together with their tangents, and interpolate these tangents with helices by slightly relaxing the points. Building on previous work, we generalize the proof for Ghosh's co-helicity condition, which serves us to guarantee the correctness of our algorithm in the general case. Finally, we demonstrate both the efficiency and robustness of our method by successfully applying it on various datasets of increasing complexity, ranging from synthetic curves created by an artist to automatic image-based reconstructions of real data such as hair, heart muscular fibers or magnetic field lines of a star. Super-Clothoids Florence Bertails-Descoubes. Eurographics 2012. [Project website] [Paper (PDF)] [Movie (MP4)] (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 Coulomb'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.

### Scientific Mediation (in French)

 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 .

## Software

We freely distribute a number of source codes accompanying some of our recent papers on the modeling and simulation of fibers and frictional contact. All source codes are distributed under the GNU GPLv.3 licence. Proprietary licenses are also available upon request.

## Teaching

Since 2009 I have been involved in the teaching of Numerical Optimization at ENSIMAG.
I have been participating to the Spring School on Nonsmooth Mechanics 2010 and Summer School on Nonsmooth Mechanics 2012 organized by BiPop.
In 2009 I have participated to the teaching of the Mobinet classes.
During my Ph.D., I have been working at Département Télécom and ENSIMAG as a teaching assistant (CIES). I have participated to the teaching of the following topics and projects: Probability and Statistics, Theory of Codes and Algorithmic, Applied Analysis, and Assembler Project in C.

## Students and Engineers

Current
Past
Feel free to contact me if you are interested in any project related to physics-based simulation.

## Collaborations

Current
• I am regularly involved in direct parternships with industries, mostly on the topic of digital hair.
Past
• I was 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 played the role of INRIA coordinator for this project.
• I was 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.

## Professional activities

• SIGGRAPH Program Committee 2014, 2013
• Eurographics Program Committee 2015, 2014
• SCA Program Committee 2014, 2012, 2011, 2010, 2009