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Visual object recognition and tracking require a good resolution of the object to accurately model its appearance. In addition, tracking systems must be able to robustly recover moving target trajectory, and possibly cope with fast motion and large displacements. Wide angle static cameras capture a global view of the scene but they suffer from a lack of resolution in the case of a large distance between the objects and the sensor. On the contrary, dynamic sensors such as Pan-Tilt-Zoom (PTZ) cameras are controlled to focus on a 3D point in the scene and give access to high resolution images by adapting their zoom level. However, when a PTZ camera focuses on a target, its very limited field of view makes the tracking difficult. To overcome these limitations, hybrid sensor systems composed of a wide angle static camera and a dynamic camera can be used. Coupling these two types of sensors enables the exploitation of their complementary desired properties while limiting their respective drawbacks. Calibration is required to enable information exchange between the two sensors to produce collaborative algorithms. The calibration of our system is difficult because of changes of both intrinsic (focal length, central point, distortion) and extrinsic (position, orientation) parameters of the dynamic sensor during system exploitation. Two approaches for dynamic stereo sensor calibration are possible: • Strong calibration involves a complete modeling of the system. Intrinsic parameters of each camera and extrinsic parameters are estimated. This approach enables the projection of 3D points, expressed in the world frame, in 2D points expressed in each image frame. • Weak calibration does not target to estimate intrinsic or extrinsic parameters. The objective is only to estimate the direct relation between pixels of the different sensors. From a pixel in the first camera, which is the projection of a given 3D point, the calibration gives the projection of the same 3D point to the second camera. In this approach, the recovery of 3D point coordinates is not more difficult.

The dynamic solicitations imposed on modern production machines make the rigid body hypothesis obsolete in mechanical design. To increase productivity while ensuring effector trajectory quality, numerical control of the machine has to integrate the elastic deformations of machine parts. To reach this objective, the elastic behaviour of the machine has to be accurately predicted by a simple but efficient elastic model. This model should involve low-cost computation to be implemented in a real-time control loop, and also necessitates an accurate identification process to attain the required precision. The paper proposes to model the elastic behaviour of the machine using an efficient mass–spring–damper model. The main contribution lies in a new approach to accurately identifying the elastic model parameters. Instead of using modal analysis, the new approach is based on time domain identification using a specific metrological device. The measurement system developed and described in this paper is based on combined computer vision and inertial sensor data. First, the displacements of points of interest on the machine part are tracked and measured by a standard digital camera (0.8 Mega pixels and 30 frames/s). A subpixelar detector allows us to reach a level of accuracy equal to 30 μm (maximum error) for a displacement up to 300 mm away from the sensor, but 30 Hz image sampling does not provide enough information to get all details of the overall movement. So secondly, accelerometers are added to get the required information to over-sample the vision data without additional error; by doing so the effective sampling frequency increases from 30 Hz to 15 kHz. The proposed identification method is applied to a real industrial system; the results obtained in elastic deformation prediction are quite satisfactory and validate the new approach proposed in this paper.

The problem of the self-calibration of Scheimpflug cameras is addressed. The approach aims to calibrate by handpositioning a roughly-known calibration pattern, thanks to a bundle adjustment technique. The purpose of this paper is to design an easy protocol accordingly. A simple motion of the calibration target is introduced and a minimal number of recordings are obtained from the simulated data. The precision required for the calibration pattern is also discussed. Finally, the suggested protocol is tested in real conditions, especially in the stereo particle image velocimetry configuration.

International audience; Basé sur la littérature, cet article décrit trois environnements d'apprentissage conçus dans le but d'aider les étudiants novices à acquérir les compétences de la programmation informatique: Alice, Greenfoot et Prog&Play, en vue de présenter leurs mécanismes de représentation des concepts fondamentaux de la Programmation Orienté-Objet, et de vérifier leur adéquation aux profils d'étudiants débutants et leur impact sur l'apprentissage de ces étudiants. Abstract. Based on the literature, this article describes three learning environments that have been designed to help novice students to develop computer programming skills: Alice, Greenfoot and Prog&Play, in a view to present their principals of how they represent abstract fundamentals of Object-Oriented Programming, and to verify their adequacy for novice students and their impact on the students learning.

Volcanoes subjected to hydrothermal activity develop weak cores as a result of alteration and due to elevated pore pressures. Edifices constructed at the angle of repose of volcanoclastics, or at even more gentle slopes, respond to internal weakening by initially deforming slowly, but may then collapse catastrophically. Such a process has so far been described for only a few volcanoes, such as Casita, Nicaragua; however, the conditions for flank spreading are widespread and many, if not most volcanoes should suffer some alteration-related flank spreading. We provide analogue models that characterise the structure — surface deformation fields and internal structures — of a spreading flank. Deformation creates a characteristic concave-convex-concave flank profile producing structures such as basal thrusts, summit normal faults, grabens and strike-slip relay faults. Three deformation regimes are found: a ‘pit collapse’ regime is associated with very small volumes of ductile material located far from the edifice surface. This would not appear in nature, as time for deformation is greater than the lifetime of a volcano, unless very low rock viscosities are present. The other two regimes are flank spreading regimes, one symmetric and one asymmetric. The latter is the most common, as most volcanic structures are asymmetrical in form and in distribution of physical properties. The deformation is controlled by altered region dimensions, volume and position relative to the edifice, and to a lesser extent by its shape. As the flanks spread, landslides are created, initially on the steepened portion, but also from fault scarps. Major flank collapse may occur leading to explosive hydrothermal decompression and to a debris avalanche rich in hydrothermally altered material. We provide several new examples of volcanoes that have structures and morphologies compatible with flank spreading. We suggest that it is a common feature, important in the tectonics and hazards of many volcanoes.

We present a method of reconstructing volcanic morphology using multiple digital views ( N -view), captured at different angles around an object. This approach uses recent advances in the field of Computer Vision to provide accurate 3-D measurements of volcanic surfaces. Videogrammetry (digital image reconstruction) is used, as it is best adapted to numerical processing. The method is tested and now used in the laboratory on analogue volcanic cones. The method begins with calibrating the camera and finding image positions, using an accurate N -view calibration method. This is done by estimating internal and external parameters of the camera using several views of a specially constructed calibration target. The N -view reconstruction of the real object is then done by iteratively deforming an initial theoretical model of the surface. Laboratory tests show that reconstruction accuracy is about 10 −4 m for a distance between the object and the camera of 0.5 m, and is potentially several orders of magnitude higher for surfaces of finer texture and using higher precision sensors. This is easily high enough to be useful for the accuracy required for morphological studies. It is also sufficient for monitoring most types of volcano deformation. The technique has the potential to detect morphology changes of the order of mm. Use of the method in the field requires a slightly different approach from that in the laboratory: textures and lighting are more variable, and the sensor and ground control point location and model calibration must be done differently. We provide case studies from laboratory tests and qualitative image analysis for two field cases: Piton de la Fournaise (Indian Ocean) and Santaguito (Guatemala). These illustrate the technique’s potential and explore problems of field application. Using current sensors, the method has the potential to provide sufficient precision for fine (mm–cm) scale reconstruction, and will represent a valuable, simple and flexible tool when compared with classical stereophotogrammetry techniques.

The introduction of touch-sensitive artifacts in education and their use by teachers bring to light new issues about Technology Enhanced Learning (TEL). Indeed learning activities based on these innovative artifacts create new needs during the design and orchestration of learning scenario. Thus this article is for us a means to find an answer which is adequate both for teachers' new needs and new interactions generated by touch-sensitive artifacts. We suggest a comprehensive approach in the instrumentation of learning scenario design based on four concepts: resources, real and/or virtual objects, technologies and learning activities., L'introduction d'artéfacts tactiles dans l'éducation, et leurs utilisations par les enseignants, met en lumière de nouvelles problématiques dans les plateformes de formation. En effet, les activités pédagogiques s'appuyant sur ces artéfacts innovants déterminent de nouveaux besoins dans la phase d'instrumentation de scénarios pédagogiques et d'orchestration instrumentale. Ainsi, nous essayons d'apporter à travers cet article une réponse en adéquation avec les nouveaux besoins de l'enseignant et les nouvelles interactions induites par les artéfacts tactiles. Nous proposons une approche générique ouverte pour instrumenter la scénarisation d'activités pédagogiques qui s'appuie sur quatre concepts : des ressources, des objets réels et/ou virtuels, des technologies et des activités pédagogiques