Section: New Results
Modeling, editing and processing geometry
Participants : Grégoire Aujay, Georges-Pierre Bonneau, Marie-Paule Cani, Christine Depraz, François Faure, Laurent Favreau, Franck Hétroy, Paul Kry, Olivier Palombi, Adeline Pihuit, Lionel Reveret, Jamie Wither.
Multiresolution geometric modeling with constraints
Participant : Georges-Pierre Bonneau.
This work is done in collaboration with Stefanie Hahmann from LJK. A collaboration is also taking place on this topic with Prof. Gershon Elber from Technion, in the framework of the Aim@Shape Network of Excellence (see Section 8.1.1 ). The purpose of this research is to allow complex nonlinear geometric constraints in a multiresolution geometric modeling environment. This year, constraints of constant volume for the multiresolution deformation of BSpline tensor-product surfaces as well as subdivision surfaces have been investigated. Fig. 4 illustrates the deformation of a subdivision surface with constant volume. The work on multiresolution subdivision surfaces has been published at Eurographics in [Oops!] . The work on BSpline tensor-product surfaces has been accepted for publication in the journal CAGD, and will appear next year. A survey on integrating constraints into multiresolution models has been written in collaboration with Prof. Elber, and published in [Oops!] . The work on multiresolution curves has also been applied to the problem of morphing - or deforming - one curve into another. It has been published in [Oops!] .
This year, our work on interactive sculpting techniques focused on developing ways to interact with virtual clay.
The layered volumetric model we previously developed for virtual clay achieves the desired plausibility in real-time, but opens the problem of providing intuitive interaction tools. The master thesis of Adeline Pihuit, co-advised by Paul Kry and Marie-Paule Cani in 2007, addressed this problem. She combined the virtual clay model with a compliant virtual hand, used as a deformable tool for sculpting it. She experimented with several devices for controlling the motion and deformation of the virtual hand. In particular, she developed a prototype where a soft ball serving as an avatar for the virtual clay is attached to a force feedback device (phantom). The ball is augmented with force sensors, to ease the control of the deformable virtual hand that sculpts the clay.
The first results were presented at the French conference on virtual reality [Oops!] . User studies will be held soon to validate our contributions, thanks to funds from the PPF "Multimodal interaction" (see Section 8.3.3 ).
Modeling by sketching
Sketch-based techniques are currently attracting more and more attention, being recognized as a fast and intuitive way of creating digital content. We are exploring these techniques from two different view-points:
A first class of sketching techniques directly infer free-form shapes in 3D from arbitrary progressive sketches, without any a priori knowledge on the objects being represented. In collaboration with Loic Barthes from the IRIT lab in Toulouse, we are studying the use of convolution surfaces for achieving this goal: the user paints a 2D projection of the shape. A skeleton (or medial axis), taking the form of a set of branching curves, is reconstructed from this 2D region. It is converted into a close form convolution surface whose radius varies along the skeleton. The resulting 3D shape can be extended by sketching over it from a different viewpoint, while the blending operator used adapts its action so that no detail is blurred during the process. This work was supported by a direct industrial contract with the firm Axiatec (see Section 7.2 ), and lead to the development to a first prototype of the MaTISSe software. This project will go on through the PhD of Adeline Pihuit, started in October 2007, under the supervision of Olivier Palombi and Marie-Paule Cani: the aim will be to explore the use of sketch-based modeling in the context of the teaching of anatomy, where a professor progressively sketches a simplified view of one or several organs and explains their action, which could lead to some animation.
Other sketching techniques are able to create a complex shape from a single sketch, using some a priori knowledge on the object being drawn for inferring the missing 3D information. This is the topic of Jamie Wither's PhD, advised by Marie-Paule Cani. In collaboration with Alla Sheffer from UBC, Canada, we studied the reconstruction of developable surfaces from sketches and applied it to the sketching of objects made of metal, leather or cloth [Oops!] . We are currently studying the sketching of fractal-like objects, such as trees (in collaboration with Frédéric Boudon from CIRAD, Montpellier) or clouds, whose shape can be sketched and refined at different scales. This last part of the work is one of our contributions to the ANR project NatSim (see Section 8.2.2 ).
Automatic computation of animation skeletons
Animation of a 3D model is usually made using a hierarchical representation of its articulations called the animation skeleton. Creation of this animation skeleton is a laborious task since it is made by hand. During his Master thesis, Grégoire Aujay has developed an algorithm which uses geometrical information on the model to automatically compute a hierarchical geometric skeleton, which can be converted into an animation skeleton. This method has then been improved to create skeletons which match animation skeletons in both the biped and quadruped cases [Oops!] . User intervention is restricted to the selection of one or a few points at the very beginning of the process, but the algorithm is customizable and users can adjust the skeleton to their needs. This work has been done in collaboration with Francis Lazarus, from the GIPSA-Lab in Grenoble.
Geometrical methods for skinning character animations
Skinning, which consists in computing how vertices of a character mesh (representing its skin) are moved during a deformation w.r.t. the skeleton bones, is currently the most tedious part in the skeleton-based character animation process. We propose new geometrical tools to enhance current methods. First, we develop a new skinning framework inspired from the mathematical concept of atlas of charts: we segment a 3D model of a character into overlapping parts, each of them being anatomically meaningful (e.g., a region for each arm, leg, etc., with overlaps around joints), then during deformation the position of each vertex in an overlapping area is updated thanks to the movement of neighboring bones. This work has been done in collaboration with Boris Thibert from the MGMI team of the LJK, Cédric Gérot and Annick Montanvert from the GIPSA-Lab in Grenoble, and Lin Lu from the University of Hong Kong.
Detection and quantification of brain aneurysms
Aneurysms are excrescences on blood vessels. They can break, letting the blood propagate outside the vessel, which often leads to death. In some cases, the blood clots sufficiently fast so that people survive. However, a neurosurgeon or a neuroradiologist should intervene very quickly in order to repare the vessel before the aneurysm breaks once more.
The purpose of this research is to help neurosurgeons and neuroradiologists to plan surgery, by giving them quantitative information about the size, shape and geometry position of aneurysms. The first part of this work has been done during the internship of Sahar Hassan: we have developed a simple algorithm for the automatic detection of aneurysms on CTA images.
This work continued in 2007 with the Master thesis of Sahar Hassan, detection and masure of aneurysms is implemented in an application which will be evaluated by a radiologist at the Grenoble Universitary Hospital. We plan to publish the method in the medical literature.