Romain Raffin

Geometric modeling of pelvic organs

The pelvic floor can be subjected to different disorders, coming from a physiological change in the spatial configuration of the organs of interest: the bladder, the rectum, the uterus and the vagina. However, resort to surgery to replace them is complicated to achieve. In order to support the decision of the surgeon as to the invasive method to use for the patient, the MoDyPe (Pelvis Dynamics Modeling) project was launched, aiming at building a patient specific pelvic organ behavior. Our approach consists in creating thick surfaces of hollow organs, using periodic B-splines and offsets, then in controlling their discretization and in exporting a hexahedral model to provide input data for the study on the dynamics of the soft bodies of interest. From a segmentation step providing a dataset of 3D points, a function is built to measure the bidirectional distance between the surface and the data. It is minimized with an alternate iterative Hoschek-like method, by updating the parametric map and moving the control points. Several offsets of the base surface are then created to build up the thickness of the organ.

Variational geometric modeling with black box constraints and DAGs

CAD modelers enable designers to construct complex 3D shapes with high-level B-Rep operators. This avoids the burden of low level geometric manipulations. However a gap still exists between the shape that the designers have in mind and the way they have to decompose it into a sequence of modeling steps. To bridge this gap, Variational Modeling enables designers to specify constraints the shape must respect. The constraints are converted into an explicit system of mathematical equations (potentially with some inequalities) which the modeler numerically solves. However, most of available programs are 2D sketchers, basically because in higher dimension some constraints may have complex mathematical expressions. This paper introduces a new approach to sketch constrained 3D shapes. The main idea is to replace explicit systems of mathematical equations with (mainly) Computer Graphics routines considered as Black Box Constraints. The obvious difficulty is that the arguments of all routines must have known numerical values. The paper shows how to solve this issue, i.e., ? how to solve and optimize without equations. The feasibility and promises of this approach are illustrated with the developed DECO (Deformation by Constraints) prototype. Extending geometric constraints with black box constraints which have no equation.Solving and optimizing without equation.This approach applies to all history-based or parametric modelers.

Free Form Deformations or Deformations Non-Constrained by Geometries or Topologies

Free-form deformations are widely used to model 3D objects. In these methods “free-form” designates: “whatever the object is, whatever its description and topology, we are able to deform it”. They limit the user interaction to pull some points of an embedding rough mesh. From the user point-of-view, it does not matter if the object manipulated is 3-dimensional, of 0-genus or a parametric surface, he or she always uses the same process to model a complex object: load an initial object from a library and deform it via Ffd methods to follow his (her) needs. A large number of deformation methods have been published, allowing new deformations, new kinds of controls or enhancing the description of resulting objects. In fact advantage of deformation non-constrained by geometries is also a drawback: as it only manipulates points it could only result in points, so its necessary to use and maintain the neighborhood (the topology) or the surface expression on a second hand, as these methods do not care of object description.

Geometric modeling of pelvic organs with thickness

Physiological changes in the spatial configuration of the internal organs in the abdomen can induce different disorders that need surgery. Following the complexity of the surgical procedure, mechanical simulations are necessary but the in vivo factor makes complicate the study of pelvic organs. In order to determine a realistic behavior of these organs, an accurate geometric model associated with a physical modeling is therefore required. Our approach is integrated in the partnership between a geometric and physical module. The Geometric Modeling seeks to build a continuous geometric model: from a dataset of 3D points provided by a Segmentation step, surfaces are created through a B-spline fitting process. An energy function is built to measure the bidirectional distance between surface and data. This energy is minimized with an alternate iterative Hoschek-like method. A thickness is added with an offset formulation, and the geometric model is finally exported in a hexahedral mesh. Afterward, the Physical Modeling tries to calculate the properties of the soft tissues to simulate the organs displacements. The physical parameters attached to the data are determined with a feedback loop between finite-elements deformations and ground-truth acquisition (dynamic MRI).

B-spline Surface Reconstruction by Inverse Subdivisions

This paper presents a method to reconstruct a B-spline surface from a quadrangular mesh, using an inverse Catmull-Clark subdivision. We want to minimize the surface contraction due to the approximating subdivision scheme. We introduce geometrical operations which minimize the impact of the subdivision approximation and can be used in the parametric surface reconstruction. The quality of the method is evaluated by criteria of distances, curvatures or computing time on experimental results.

Interactive Hyper Spectral Image Rendering on GPU.

In this paper, we describe a framework focused on spectral images rendering. The rendering of a such image leads us to three major issues: the computation time, the footprint of the spectral image, and the memory consumption of the algorithm. The computation time can be drastically reduced by the use of GPUs, however, their memory capacity and bandwidth (compared to their compute power) are limited. When the spectral dimension of the image will raise, the straightforward approach of the Path Tracing will lead us to high memory consumption and latency problems. To overcome these problems, we propose the DPEPT (Deferred Path Evaluation Path Tracing) which consists in decoupling the path evaluation from the path generation. This technique reduces the memory latency and consumption of the Path Tracing. It allows us to use an efficient wavelength samples batches parallelization pattern to optimize the path evaluation step and outperforms the straightforward approach when the spectral resolution of the simulated image increases.

A Review of Two Approaches for Joining 3D Meshes

The construction of smooth surfaces of 3D complex objects is an im- portant problem in many graphical applications. Unfortunately, cracks or holes may appear on their surfaces caused by the limitation of scanners or the differ- ence in resolution levels and subdivision schemes between adjacent faces. In this paper, we introduce two approaches for joining 3D meshes of different resolu- tions to remove the cracks or holes. These approaches use a wavelet transform and a RBF local interpolation or a tangent plane local approximation. They guar- antee that the discrete continuity between meshes is preserved and the connecting mesh can change gradually in resolution between coarse and fine mesh areas.

Indexation and visualization of documents in a 3D model of the St Trophime cloister over the Internet

Historical buildings are more and more digitized, photographed or measured with precision to facilitate their conservation. A second widely addressed theme is the conservation of documents relative to an archeological site, together with the growth of functions in databases that can also cover the 3D space. We propose to join these two topics using 3D geometric objects to index a spatial database. St Trophime Cloister of Arles (France) has been a scientific playground for geometry processing and 3D visualization over the Internet. In order to propose to archaeologist and cultural heritage managers an interface that will offer access to all documents related to the Cloister we developed a 3D database indexation and its selection system. Gracefully to the standards evolution of the Internet, this is made possible in a simple web browser. The documents linked to a part of the building are then selected by a visual choice in 3D. According to the user position the geometry is dynamically loaded/unloaded, the details in the visualization engine are adapted to keep a high-performance system and a sufficient quality. The support of a well known communication protocol (HTTP) also enhances the access to the application and makes the platform accessible from the public or the scientists but introduces drawbacks in the capabilities of the application (geometry loadings, memory management, network latency).

Discrete geometric modeling of thick pelvic organs with a medial axis

Modeling of soft pelvic organs and their thicknesses is a difficult task, especially when inputs are noisy and scattered. In order to define the geometric step for a global pelvic surgery simulator, we define a new method based only on geometry while considering the problem of error transfer between outer and inner organ surfaces. We compare this approach with a parametric formulation and a mass-spring system.