Mise en correspondance active et passive pour la vision par ordinateur multivue

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal

Development of a SGM-based multi-view reconstruction framework for aerial imagery

Advances in the technology of digital airborne camera systems allow for the observation of surfaces with sampling rates in the range of a few centimeters. In combination with novel matching approaches, which estimate depth information for virtually every pixel, surface reconstructions of impressive density and precision can be generated. Therefore, image based surface generation meanwhile is a serious alternative to LiDAR based data collection for many applications. Surface models serve as primary base for geographic products as for example map creation, production of true-ortho photos or visualization purposes within the framework of virtual globes. The goal of the presented theses is the development of a framework for the fully automatic generation of 3D surface models based on aerial images - both standard nadir as well as oblique views. This comprises several challenges. On the one hand dimensions of aerial imagery is considerable and the extend of the areas to be reconstructed can encompass whole countries. Beside scalability of methods this also requires decent processing times and efficient handling of the given hardware resources. Moreover, beside high precision requirements, a high degree of automation has to be guaranteed to limit manual interaction as much as possible. Due to the advantages of scalability, a stereo method is utilized in the presented thesis. The approach for dense stereo is based on an adapted version of the semi global matching (SGM) algorithm. Following a hierarchical approach corresponding image regions and meaningful disparity search ranges are identified. It will be verified that, dependent on undulations of the scene, time and memory demands can be reduced significantly, by up to 90% within some of the conducted tests. This enables the processing of aerial datasets on standard desktop machines in reasonable times even for large fields of depth. Stereo approaches generate disparity or depth maps, in which redundant depth information is available. To exploit this redundancy, a method for the refinement of stereo correspondences is proposed. Thereby redundant observations across stereo models are identified, checked for geometric consistency and their reprojection error is minimized. This way outliers are removed and precision of depth estimates is improved. In order to generate consistent surfaces, two algorithms for depth map fusion were developed. The first fusion strategy aims for the generation of 2.5D height models, also known as digital surface models (DSM). The proposed method improves existing methods regarding quality in areas of depth discontinuities, for example at roof edges. Utilizing benchmarks designed for the evaluation of image based DSM generation we show that the developed approaches favorably compare to state-of-the-art algorithms and that height precisions of few GSDs can be achieved. Furthermore, methods for the derivation of meshes based on DSM data are discussed. The fusion of depth maps for 3D scenes, as e.g. frequently required during evaluation of high resolution oblique aerial images in complex urban environments, demands for a different approach since scenes can in general not be represented as height fields. Moreover, depths across depth maps possess varying precision and sampling rates due to variances in image scale, errors in orientation and other effects. Within this thesis a median-based fusion methodology is proposed. By using geometry-adaptive triangulation of depth maps depth-wise normals are extracted and, along the point coordinates are filtered and fused using tree structures. The output of this method are oriented points which then can be used to generate meshes. Precision and density of the method will be evaluated using established multi-view benchmarks. Beside the capability to process close range datasets, results for large oblique airborne data sets will be presented. The report closes with a summary, discussion of limitations and perspectives regarding improvements and enhancements. The implemented algorithms are core elements of the commercial software package SURE, which is freely available for scientific purposes

Méthodes de rendu à base de vidéos et applications à la réalité Virtuelle

Given a set images of the same scene, the goal of video-based rendering methods is to compute new views of this scene from new viewpoints. The user of this system controls the virtual camera's movement through the scene. Nevertheless, the virtual images are computed from static cameras. A first approach is based on a reconstruction of the scene and can provide accurate models but often requires lengthy computation before visualization. Other methods try to achieve real-time rendering. Our main contribution to video-base rendering concerns the plane sweep method which belongs to the latter family. The plane sweep method divides space in parallel planes. Each point of each plane is processed independently in order to know if it lies on the surface of an object of the scene. These informations are used to compute a new view of the scene from a new viewpoint. This method is well suited to an implementation using graphic hardware and thus to reach realtime rendering. Our main contribution to this method concerns the way to consider whether a point of a plane lies on the surface of an object of the scene. We first propose a new scoring method increasing the visual quality of the new images. Compared with previous approaches, this method implies fewer constraints on the position of the virtaul camera, i.e. this camera does not need to lie between the input camera's area. We also present an adaptation of the plane sweep algorithm that handles partial occlusions. According to video-based rendering practical applications in virtual reality, we propose an improvement of the plane sweep method dealing with stereoscopic images computation that provides visualization of the virtual scene in relief. Our enhancement provides the second view with only low additional computation time whereas most of the others techniques require to render the scene twice. This improvement is based on a sharing of the informations common to the two stereoscopic views. Finally, we propose a method that removes pseudoscopic movements in a virtual reality application. These pseudoscopic movements appear when the observer moves in front of the stereoscopic screen. Then the scene roportions seem to be distorted and the observer sees the objects of the scene moving in an anormal way. The method we propose is available either on a classical stereoscopic rendering method or on the Plane Seep algorithm. Every method we propose widely uses graphic harware through to shader programs and provides real-time rendering. These methods only require a standard computer, a video acquisition device and a powerful enough graphic card. There exists a lot of practicalapplications of the plane sweep method, especially in fields like virtual reality, video games, 3d television or security.Etant donné un ensemble de caméras filmant une même scène, le rendu à base de vidéos consiste à générer de nouvelles images de cette scène à partir de nouveaux points de vue. L'utilisateur a ainsi l'impression de pouvoir déplacer une caméra virtuelle dans la scène alors qu'en réalité, toutes les caméras sont fixes. Certaines méthodes de rendu à base de vidéos coûteuses en temps de calcul se basent sur une reconstruction 3d de la scène et produisent des images de très bonne qualité. D'autres méthodes s'orientent plutôt vers le rendu temps réel. C'est dans cette dernière catégorie que s'inscrit la méthode de Plane Sweep sur laquelle porte la majeure partie de nos travaux. Le principe de la méthode des Plane Sweep consiste à discrétiser la scène en plans parallèles et à traiter séparément chaque point de ces plans afin de déterminer s'ils se trouvent ou non sur la surface d'un objet de la scène. Les résultats obtenus permettent de générer une nouvelle image de la scène à partir d'un nouveau point de vue. Cette méthode est particulièrement bien adaptée à une utilisation optimale des ressources de la carte graphique ce qui explique qu'elle permette d'effectuer du rendu en temps réel. Notre principale contribution à cette méthode concerne la façon d'estimer si un point d'un plan représente la surface d'un objet. Nous proposons d'une part un nouveau mode de calcul permettant d'améliorer le résultat visuel tout en rendant la navigation de la caméra virtuelle plus souple. D'autre part, nous présentons une adaptation de la méthode des Plane Sweep permettant de gérer les occlusions partielles. Compte tenu des applications du rendu à base de vidéos en réalité virtuelle, nous proposons une amélioration des Plane Sweep appliquée à la réalité virtuelle avec notamment la création de paires d'images stéréoscopiques permettant de visualiser en relief la scène reconstruite. Notre amélioration consiste à calculer la seconde vue à moindre coût alors qu'une majorité des méthodes concurrentes sont contraintes d'effectuer deux rendus indépendants. Cette amélioration est basée sur un partage des données communes aux deux vues stéréoscopiques. Enfin, dans le cadre de l'utilisation des Plane Sweep en réalité virtuelle, nous présentons une méthode permettant de supprimer les mouvements pseudoscopiques. Ces mouvements pseudoscopiques apparaissent lorsque l'observateur se déplace devant une image stéréoscopique, il ressent alors une distorsion des proportions de la scène virtuelle et voit les objets se déplacer de façon anormale. La méthode de correction que nous proposons est applicable d'une part à des méthodes classiques de rendu d'images de synthèse et d'autre part à la méthode des Plane Sweep. Toutes les méthodes que nous présentons utilisent largement les possibilités du processeur de la carte graphique à l'aide des shader programs et génèrent toutes des images en temps réel. Seuls un ordinateur grand public, un dispositif d'acquisition vidéo et une bonne carte graphique sont suffisants pour les faire fonctionner. Les applications des Plane Sweep sont nombreuses, en particulier dans les domaines de la réalité virtuelle, du jeu vidéo, de la télévision 3d ou de la sécurité