Section: New Results
Participants : Marie-Paule Cani, Philippe Decaudin, Christine Depraz, Julien Diener, François Faure, Midori Hyndman, Paul Kry, Xing Mei, Matthieu Nesme, Fabrice Neyret, Lionel Reveret, Maxime Tournier, Xiaomao Wu, Qizhi Yu.
Highly colliding deformable bodies
Participant : François Faure.
We address the question of simulating highly deformable objects. This year, we have focused on the collision detection and response between arbitrary deformable or rigid bodies, using density fields, as illustrated in figure 6 . A new method [Oops!] was implemented in SOFA and will be submitted to a high level international conference.
Robust finite elements for deformable solids
We continue a collaboration on surgical simulation with laboratory TIMC through a co-advised Ph.D. thesis. Its purpose is to develop new models of finite elements for the interactive physically-based animation of human tissue.
Based on the hexahedral finite elements we developed last year, we are creating a new multigrid method to speed up the computations. An example of multigrid model is shown in figure 7 .
In the next future, we plan to apply this new approach to the simulation of patient-specific organ models.
Simulation of 1D models and application to hair
Realistically predicting the shape and motion of human hair requires an accurate mechanical model for strands, a convincing extension to wisp dynamics and a good model for their interactions within hair. Following our collaboration with L'Oréal research labs and with Basile Audoly from CNRS, Paris, which lead to the Super-Helices model for hair, we presented a synthesis of the different techniques we developed for hair modeling, animation and rendering within a course presented at SIGGRAPH 2007 [Oops!] . We also developed, in collaboration with Florence Bertails (postdoc at UBC, Canada) an interface for creating a physically-based hair-style from a single sketch: we used a few strokes to define some sample hair strands and to tune the hair volume, and used this information to infer the parameters of the static version of super-helices (Figure 8 ). This work, which also enables to create hair-styles by annotating a real photograph, was presented at the IEEE Shape Modeling and Applications conference [Oops!] .
Real-time animation of liquids and river surfaces
Qizhi Yu is working on this topic as a Marie Curie PhD student (Visitor program), supervised by Fabrice Neyret. The purpose is to obtain a realistical detailed appareance of landscape-long animated rivers in real-time, with user-editable features. The idea is to separate the river simulation into 3 scales, corresponding to different specification and simulation tools: macroscale for the topographic shape and global flow characteristics (relying on simple CFD at coarse resolution), mesoscopic scale for the local wave patterns (relying on dedicated phenomenological models), microscopic scale for the details (relying on textural procedural schemes). Note that this topic is included in the scope of the NatSim collaboration (see Section 8.2.2 ).
The PhD of Mathieu Coquerelle, co-advised by Marie-Paule Cani and Georges-Henri Cottet, explores the use of vortex particules for animating liquids and gases and to simulate their interactions with rigid solids.
Hydraulic erosion simulation and visualization
Natural mountains and valleys are gradually eroded by rainfall and river flows. Physically-based modeling of this complex phenomenon is a major concern in producing realistic synthesized terrains. The objective of this project is to develop proper models for simulating and visualizing these specific natural phenomena. We take advantage of the high parallel computing ability and new features of the latest graphics hardware to accelerate the simulation and visualization process. We proposed a new erosion simulation method based on an efficient shallow water model carefully designed to run entirely on GPU [Oops!] .
Motion capture of animal motion
The motion of animals is still a challenging problem in 3D animation, both for articulated motion and deformation of the skin and fur.
In several domains of character animation, footsteps are one of the most important constraints. It guarantees one of the main aspects of a realistic animation of locomotion. This task, when done manually, is even more complex for quadrupeds. Being able to automatically predict the footstep information from a video footage is thus an important contribution. The method developed is based on the design of a dedicated image filter to detect the pattern of animal legs. Along the time range of the video, the positive filter responses are clustered so that a single trajectory point is given per leg. As 2D images are considered (profile view), there exist ambiguities in the prediction of each individual foot position when side views of legs are crossing each other (typically left and right side of the animal, and front and back legs for higher velocities). A motion model has been developed to take into account this problem. This work has been done in collaboration with the University of Washington, in Seatle, USA.
As for now, these works on footsteps are still in progress and are not published yet. A collaboration with the National Museum of Natural History has been started to develop these video techniques in the framework of a new theoritical perspective investigated recently by the Museum about a parametric description of animal gaits. Works are done on a massive database of video of military dogs provided by the Museum.
In addition to footstep analysis, most of the activity on motion from video for this year has been dedicated to the ANR project Kameleon (Fig. 10 ). The experimental set-up has been finalized for this project and it is fully operationnal since june 2007. This set-up uses 4 high-speed cameras synchronized with x-ray video. These five views are now correctly calibrated. Several projects can now be started, especially the learning of motion patterns for head and limbs in rat locomotion. In parallel, a new post-doc has been appointed and one of the first achievement has been to develop a real-time solution for 3D reconstruction of rat skeleton from static x-ray images using latest GPU technique. This 3D skeleton has been used to develop a physical model of rat legs that can be animated from x-ray videos. Results can be seen on the project website. http://www-evasion.inrialpes.fr/people/Lionel.Reveret/kameleon
Motion capture of tree motion
This work investigates how the complex motion of plants and trees under wind effect can be analyzed from video and retarget to complex 3D model to create realistic animation. In 2006, a first method has been proposed and published at the SCA conference in 2006 in collaboration with the U. of Toronto. This work is now continued in cooperation with the laboraties from INRA dedicated to the physiology of trees and the fluid mechanics laboratory of Ecole polytechnique, within the ANR project Chêne et Roseaux started in 2007. One of the achievement of this collaboration has been to develop a visualisation software of the mechanical modes computed for tree structures.
In addition, Julien Diener is currently spending a 6 month-stay at the DGP laboratory of U. of Toronto to perform a user study on a motion sketching tool adapted to the measurement of tree motion from video.
Probabilistic editing of human motion from motion capture database
This is a new project motivated by previous works of Maxime Tournier in the context of a collaboration between EVASION and Ubisoft. This collaboration has not been continued in 2007 but a new project has started with other video game companies (see the GENAC project Section 7.5 ). The goal of this work is to develop new mathematical foundation of mutlivariate analysis on unit quaternions. Unit quaternions are the canonical representation of 3D rotations involved for articulated body description and multilinear analysis on such parameters is challenging due to the intrinsic non-linearity of unit quaternion interpolation. Currently, a formulation of PCA as Principal Geodesic Analysis applied on the hypersphere of unit quaternions is investigated. A collaboration on this subject has also been started withtin the ARC Fantastik (see Section 8.2.4 ).