International audience

Abstract We present a novel approach for the generation of hexahedral meshes in a volume domain given its meso‐skeleton. This compact representation of the topology and geometry, composed of both curve and surface parts, is used to produce a raw decomposition of the domain into hexahedral blocks. Analysis of the different local configurations of the skeleton leads to the construction of a set of connection surfaces that are used as a scaffold onto which the hexahedral blocks are assembled. These local configurations of the skeleton completely determine the singularities of the final mesh, and by following the skeleton, the geometry of the produced mesh naturally follows the geometry of the domain. Depending on the end user needs, the obtained mesh can be further adapted, refined or optimized, for example to better fit the boundary of the domain. Our algorithm does not involve the resolution of any global problem, most decisions are taken locally and it is thus highly suitable for parallel processing. This efficiency allows the user to stay in the loop for the correction or edition of the meso‐skeleton for which a first sketch can be given by an existing automatic extraction algorithm.

In this paper we present an interaction analytics system we are working on. With this system we intend to simplify the evaluation and classification of eXtented Reality devices and interaction techniques. Our final objective is to release it as an open Cloud platform that will allow researchers to compare their respective results in the field of Human-Computer Interaction with ease. To achieve this, we use the UMI3D exchange protocol to design device-independent 3D environments, and a Cloud analytics platform which stores experimental raw data from these environments as well as from the different devices. Finally, we present our first results using the platform to compute statistical analyzes and create dashboards comparing a set of devices and interaction techniques.

UMI3D is a recent model which allows the generic description of 3D environments through a network. These generic environments can run outside of any device, on a server or on a separate computer. A unique dedicated application that we have named UMI3D client allows a VR, AR or MR device to interact with all the existing UMI3D environments. Moreover, the client-server architecture allows these environments to be collaborative. In this paper, we introduce a toolbox which allows the creation and running of UMI3D collaborative environments using Unity 3D. It contains tools to address the main concerns of computer-supported cooperative work (CSCW) systems in UMI3D environments. After introducing the contribution of this toolbox to the design of 3D CSCW systems, we present a user evaluation of an environment designed with this toolbox. Finally, we discuss the limits of the UMI3D model and the improvements that could be made to this model.

Stereo and head tracking are considered as distance perception cues in virtual environments. Several studies have investigated their influence on several tasks. Results were different among studies. In this paper, we conducted a complete experiment investigating the influence of the stereo and the head tracking in the specific context of virtual visits of houses during architectural project review with clients. We manipulated the stereo and the head tracking in four conditions and we examined the effects of the two factors on distance estimations (walls size, habitability, etc.), task difficulty, presence and simulator sickness. Results revealed a significant effect of the stereo on the judgement of the habitability, the dimensions of the rooms and task difficulty. However, for both features, no significant effects were found on the presence and simulator sickness.

The Virtual (VR), Augmented (AR) and Mixed Reality (MR) devices are currently evolving at a very fast pace. This rapid evolution affects significantly the maintainability and portability of the applications. In this paper, we present a model for designing VR, AR and MR applications independently of any device. To do this, we use degrees of freedom to define an abstraction layer between the tasks to be performed and the interaction device.

In architectural project review, the perception of the near surrounding ground is important for the evaluation of the virtual environment (VE). This near surrounding ground is missing when using wall screens. To address this problem we suggest increasing the Vertical Field of View (VFoV). One solution is the use of the rendering approach such as non-planar projection. However, simply increasing the FoV of the rendering leads to much distortion in the VE which is not suitable for architectural project review. We propose a new hybrid projection combining a perspective projection in the center of the screen and a cylindrical-like projection on the top and bottom boarders. By this, we increase VFoV without incurring large deformations to preserve the perception of distances and to allow seeing the surrounding ground. We also report the results of an experiment we conducted to evaluate distance perception with our projection.

Abstract Morphing is an important technique for the generation of special effects in computer animation. However, an analogous technique has not yet been applied to the increasingly prevalent animation representation, i.e. 3D mesh sequences. In this paper, a technique for morphing between two mesh sequences is proposed to simultaneously blend motions and interpolate shapes. Based on all possible combinations of the motions and geometries, a universal framework is proposed to recreate various plausible mesh sequences. To enable a universal framework, we design a skeleton‐driven cage‐based deformation transfer scheme which can account for motion blending and geometry interpolation. To establish one‐to‐one correspondence for interpolating between two mesh sequences, a hybrid cross‐parameterization scheme that fully utilizes the skeleton‐driven cage control structure and adapts user‐specified joint‐like markers, is introduced. The experimental results demonstrate that the framework, not only accomplishes mesh sequence morphing, but also is suitable for a wide range of applications such as deformation transfer, motion blending or transition and dynamic shape interpolation.

In this paper, we present an efficient method for detecting collisions and self-collisions on articulated models deformed by Position Based Skinning. Position Based Skinning is a real-time skinning method, which produces believable skin deformations, and avoids artifacts such as the well-known " candy-wrapper " effect and joint-bulging. The proposed method employs spatial hashing with a uniform grid to detect collisions and self collisions. All the mesh primitives are mapped to a hash table, where only primitives mapped to the same hash index indicate a possible collision and need to be tested for intersections. Being based on spatial hashing, our method requires neither expensive set-up nor complex data structures and is hence suitable for articulated characters with deformable soft tissues. We exploit the skeletal nature of the deformation to only update the hash table when required. The resulting algorithm is simple to implement and fast enough for real-time applications. We demonstrate the efficiency of our method on various animation examples. A quantitative experiment is also presented to evaluate our method.

Distances are perceived as being more compressed in immersive virtual environments (IVEs) than in real environments. The goal of this study is to identify the most important factors that influence decision making and accuracy of distance perception in the context of architectural project reviews. Technical factors such as field of view, display devices and motion parallax were widely studied. In this paper, we have investigated other individual and contextual factors. We conducted a between-subject experiment using an immersive large screen display to examine the influence of the three factors: 1) the cognitive profile of the user (visual, auditory and kinesthetic - VAK), 2) the furnishing of the house, and 3) the locomotion speed, on distance perception. Results reveal that participants with visual profile were more accurate in distance estimation. Further, furnished houses were more suitable for virtual visits. The locomotion speed also seems to influence virtual visits which were better with a slow locomotion speed. Based on the results of our study, we finally present guidelines for setting up architectural project review tools which employ similar setup.

Abstract We present CPH (Compact Primal Hierarchy): a compact representation of the hierarchical connectivity of surface and volume manifold meshes generated through primal subdivision refinements. CPH is consistently defined in several dimensions and supports multiple kinds of tessellations and refinements, whether regular or adaptive. The basic idea is to store only the finest mesh, encoded in a classical monoresolution structure that is enriched with a minimal set of labels. These labels allow traversal of any intermediate level of the mesh concurrently without having to extract it in an additional structure. Our structure allows attributes to be stored on the cells not only on the finest level, but also on any intermediate level. We study the trade‐off between the memory cost of this compact representation and the time complexity of mesh traversals at any resolution level.

We present Bubble Bee - an extension for the 3D bubble cursor in Virtual Environments (VEs). This technique provides an alternative to arrows for pointing out a direction in a 3D scene.

We present a new Virtual Reality (VR) interaction technique, called Cursor On Surface (CrOS). It is an interaction technique for cursor manipulation on 2-manifolds using touch screen input. The objective is to provide a technique to easily perform complex modelling operations in VR. CrOS is based on an algorithm which maps 2-D inputs into cursor displacements on 3-D surfaces. The technique relies on two principles. Firstly, it restricts the manipulation space to a 2-D space. Secondly, it reduces the complexity of the task through an automatic orientation algorithm that prevent the user from switching between edition task and object repositioning task.

We present Starfish - a new target selection technique for virtual reality (VR) environments. This technique provides a solution to accurately select targets in high-density 3D scenes. The user controls a 3D pointer surrounded by a starfish-shaped closed surface. The extremity of each branch ends exactly on preselected near targets. The shape is an implicit surface built on the segments going from the pointer to each of these targets. As the pointer moves across the scene, the starfish shape is dynamically rebuilt. When it is locked the pointer is allowed to move inside the volume, slide down the desired branch, reach and select the corresponding target. Since the pointer stays within the shape, targets are easy to reach and select.

The use of Touch Screen Interaction (TSI) for 3-D interaction entails both the addition of new haptic cues and the separation of the manipulation of the Degrees of Freedom (DoF) of the task: a 3 DoF task must be transformed into a 2-D+1-D task to be completed using a touch screen. In this paper, we investigate the impact of these two factors in the context of a 3-D positioning task. Our goal is to identify their respective influence on subjective preferences and performance measurements. To that purpose, we conducted an experimental comparison of five positioning techniques, isolating the influence of each of these two factors. The results we obtained suggest that the addition of haptic cues does not influence the user precision. However, the decomposition of the task has a strong influence on accuracy. More precisely, separating the manipulation of the depth dimension leads to an increased precision while isolating other dimensions does not influence the results. To explain this result, we realised a behavioural analysis of the data. This study suggests that the differences in the performance may be linked to the perceptual structure of the techniques. A technique isolating the manipulation of the depth seems to have a more adapted perceptual structure than a technique separating the height, even if those two dimensions are equally involved in the realisation of the task.

Reconstructing tubular structures with high-order branching is a difficult task to perform automatically. Medical applications in particular demand accurate models of such objects that fulfill specific topological and geometric criteria. Indeed, the reconstructed object should be a 2-manifold surface with compact, adaptive geometry. We present a generic algorithm for automatically reconstructing n-furcated tubular surfaces. Our approach relies on a strong underlying topological structure and a novel n-furcation reconstruction algorithm using convex entities.

In this paper we present a new interaction technique based on degrees of freedom (DoF) decomposition for accurate positioning in virtual reality environments. This technique (called DIOD for Decomposition and Integration Of Degrees of freedom) is based on an adaptation of the Two-Component Model. It provides two different control levels regarding DoF coordination, one integrating and one separating the manipulation of the DoF. Our hypothesis is that each control level is appropriated to a different phase of the positioning task. During the ballistic phase, users manipulate all the dimensions of the task at the same time. However, during the control phase, users try to manipulate specific dimensions individually. The results of a preliminary study we conducted seem to indicate that the DIOD technique is more efficient than existing techniques.

In this paper we investigate the influence of the integration and separation of the Degrees Of Freedom (DOF) on the users' performances during 3-D orientation tasks. For this purpose, we compare the performances and the level of DOF's coordination users reached using two interaction techniques, one integrating and the other separating the task's DOF. To evaluate the degree of coordination, we propose a new behavioural measurement (called Magnitude of Degrees of Freedom's Separation), which provides the number of DOF simultaneously manipulated during an orientation task. The results of our study suggest that users are not able to integrate the manipulation of all the DOF during the whole task even using a direct manipulation technique. Moreover, if the interaction eases the task's decomposition, its use can lead to significant improvements regarding the achievement times. This result suggests that the simultaneous manipulation of all the DOF does not necessary lead to the best performances.

The spatial deformation methods are a family of modeling and animation techniques for indirectly reshaping an object by warping the surrounding space, with results that are similar to molding a highly malleable substance. They have the virtue of being computationally efficient (and hence interactive) and applicable to a variety of object representations. In this article we survey the state of the art in spatial deformation. Since manipulating ambient space directly is infeasible, deformations are controlled by tools of varying dimension—points, curves, surfaces and volumes—and it is on this basis that we classify them. Unlike previous surveys that concentrate on providing a single underlying mathematical formalism, we use the user-centered criteria of versatility, ease of use, efficiency and correctness to compare techniques.

In this paper we propose a new volumetric implicit surface data structure as an alternative representation to perform mesh processing algorithms. We introduce a new triangulation algorithm suitable for our voxel-based volumetric structure which is a vector field distance transform from an explicit mesh. This new representation is an extension of the implicit scalar field distance function of a mesh. We show our new vector field function is more accurate than the classic scalar field function by comparing both representations with an error metric evaluation. We adapt to this new vector representation mesh integration operation in the reconstruction process of 3D objects from scanned data which was previously performed on the implicit scalar field function. Results show that using this new vector field representation, our mesh integration and mesh triangulation algorithm designs outperform the previous processes based on the implicit scalar field function representation.

In this paper we propose an extension of the well-known circular menu paradigm, in virtual environments. We propose a menu where items are arranged on a portion of a circle and controlled by rotating the wrist in the horizontal plane. Experience shows that this manipulation is easy to carry out even for novice users, and precision is high thanks to an innovative filtering technique we called the reduced hotspot. The limitation of the number of items in the menu is solved using hierarchy and an adequate layout is proposed, the stacked rings. All hypothesis are validated by user tests.

In this paper, we present a modelling operation dedicated to animation. This operation, called thickening, is a generalization of the extrusion operation. It allows to build from a graph 3-D objects with circular section and 4-D objects with spherical or toric section. Moreover, the date and the number of mergings and splittings in the associated animation are perfectly controlled, and the trajectory of each of the animated objects is completely established. Several types of thickening are presented in this paper, to build a surface or a volume, starting from a graph drawn in a surface or in a volume. Copyright © 2000 John Wiley & Sons, Ltd.

This article presents the properties of animation with space‐time objects. A space‐time object means here a geometrical object embedded in R4 with a volumic topology. Resulting animations are obtained by deforming space‐time objects with a free‐form deformation model. In this way topological modifications, such as disconnecting or hole making, as well as classical geometrical modifications, can be created in an animated object.

We present an application to facial animation of the DOGMA (Deformation Of Geometrical Model Animated) model. We describe an order-controlled animation, where a first set of orders simulate the contraction of different facial muscles, a second some facial expressions. The main interest of this method is the manipulation of faces with various shape. Since the deformations given by the animation model DOGMA are space deformations, the only necessity is a hierarchical structuring of the different face components. With this method the same facial animation order can easily be used for various objects with different shapes, if they present the same hierarchical structuring.

Abstract This paper introduces an animation system directed by order. High‐level operations such as moving, turning, rolling and bouncing on a flat surface provide an easy‐to‐use interface to build animations. The underlying animation system relies on a constraint‐based deformation model. Previously, to build an animation the user had to break up the desired animation into a list of deformations composed of a set of constraints. In addition to each constraint, he had to control the size and the shape of the deformed area as well as the shape of the deformation. The goal of the order‐controlled animation is to encapsulate all the parameters of the deformations. Indeed, using these high‐level operations the underlying deformations are completely transparent to the user. Before introducing these operations, we will present some extensions of the deformation model such as the generalized shape of the deformed area and a rotating movement combined with the deformation. We also explain how to control velocity and acceleration.

Abstract To animate deformable objects through time and space, a new technique that will be referred to as DOGMA is proposed here. It is well suited to animation for both interactive design and real‐time visualization. The concept of animation primitives based on an extension to animation of the space deformation model is introduced. Animation primitives can be added to any geometrical modeller because they are independent of the underlying geometrical model of the animated objects. It encapsulates animation through space and time under a unique formalism. A local control of deformations both through space and time is easily specified. A direct manipulation of objects is provided. At any time, animated objects are continuously computed from their corresponding animation primitives. This dispenses with the necessity for major storage of the interpolation process. A fully interactive system offers an interactive environment which enables the user to animate deformable objects in real time.