Multiple View Image Rectification

International audienceThis paper presents an extension of image rectification methods for an arbitrary number of views with aligned camera center. This technique can be used for stereoscopic rendering to enhance the perception comfort or for depth from stereo. In this paper, we first expose that epipolar geometry is not suited to solve this problem. Then we propose a non linear method that includes all the images in the rectification process. Our method only requires point correspondences between the views and can handle images with different resolutions. The tests show that the method is robust to noise and and to sparse point correspondences among the view

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

Multi-view Rendering using GPU for 3-D Displays

Creating computer graphics based content forstereoscopic and auto-stereoscopic displays require renderinga scene several times from slightly different viewpoints. In thatcase, maintaining real-time rendering can be a difficult goal if thegeometry reaches thousands of triangles. However, similaritiesexist among the vertices belonging to the different views likethe texture, some transformations or parts of the lightning. Inthis paper, we present a single pass algorithm using the GPUthat speeds-up the rendering of stereoscopic and multi-viewimages. The geometry is duplicated and transformed for thenew viewpoints using a shader program, which avoid redundantoperations on vertices

Three-dimensional quadrics in extended conformal geometric algebras of higher dimensions from control points, implicit equations and axis alignment

International audienceWe introduce the quadric conformal geometric algebra (QCGA) inside the algebra of R 9,6. In particular, this paper presents how three-dimensional quadratic surfaces can be defined by the outer product of conformal geometric algebra points in higher dimensions, or alternatively by a linear combination of basis vectors with coefficients straight from the implicit quadratic equation. These multivector expressions code all types of quadratic surfaces in arbitrary scale, location, and orientation. Furthermore, we investigate two types of definitions of axis aligned quadric surfaces, from contact points and dually from linear combinations of R 9,6 basis vectors

Distinguishing Computer Graphics from Natural Images Using Convolution Neural Networks

International audienceThis paper presents a deep-learning method for distinguishing computer generated graphics from real photographic images. The proposed method uses a Convolutional Neural Network (CNN) with a custom pooling layer to optimize current best-performing algorithms feature extraction scheme. Local estimates of class probabilities are computed and aggregated to predict the label of the whole picture. We evaluate our work on recent photo-realistic computer graphics and show that it outperforms state of the art methods for both local and full image classification

Camera array image rectification and calibration for stereoscopic and autostereoscopic displays

International audienceThis paper presents an image rectification method for an arbitrary number of views with aligned camera center. This paper also describes how to extend this method to easily perform a robust camera calibration. These two techniques can be used for stereoscopic rendering to enhance the perception comfort or for depth from stereo. In this paper, we first expose why epipolar geometry is not suited to solve this problem. Second, we propose a nonlinear method that includes all the images in the rectification process. Then, we detail how to extract the rectification parameters to provide a quasi-Euclidean camera calibration. Our method only requires point correspondences between the views and can handle images with different resolutions. The tests show that it is robust to noise and to sparse point correspondences among the views

Pyramidal Normal Map Integration for Real-time Photometric Stereo

International audienceThis paper presents a real-time photometric stereo method designed for online 3D telecommunications. This method requires only consumer grade hardware such as a webcam and a GPU. The lighting conditions are controlled from the computer screen and the photometric reconstruction is performed on the GPU. Our method reaches real-time rendering thanks to a pyramidal integration of the normal maps based on an iterative scheme. Moreover, we propose a method to relax the constraint on the synchronization between the light source controller and the camera

Least Square for Grassmann-Cayley Agelbra in Homogeneous Coordinates

International audienceThis paper presents some tools for least square computation in Grassmann-Cayley algebra, more specifically for elements expressed in homogeneous coordinates. We show that building objects with the outer product from k-vectors of same grade presents some properties that can be expressed in term of linear algebra and can be treated as a least square problem. This paper mainly focuses on line and plane fitting and intersections computation, largely used in computer vision. We show that these least square problems written in Grassmann-Cayley algebra have a direct reformulation in linear algebra, corresponding to their standard expression in projective geometry and hence can be solved using standard least square tools