An all-in-one solution to geometric and photometric calibration

We propose a fully automated approach to calibrating multiple cameras whose fields of view may not all overlap. Our technique only requires waving an arbitrary textured planar pattern in front of the cameras, which is the only manual intervention that is required. The pattern is then automatically detected in the frames where it is visible and used to simultaneously recover geometric and photometric camera calibration parameters. In other words, even a novice user can use our system to extract all the information required to add virtual 3D objects into the scene and light them convincingly. This makes it ideal for Augmented Reality applications and we distribute the code under a GPL license.

On unifying sparsity and geometry for image-based 3D scene representation

Ivana Tosic

Demand has emerged for next generation visual technologies that go beyond conventional 2D imaging. Such technologies should capture and communicate all perceptually relevant three-dimensional information about an environment to a distant observer, providing a satisfying, immersive experience. Camera networks offer a low cost solution to the acquisition of 3D visual information, by capturing multi-view images from different viewpoints. However, the camera's representation of the data is not ideal for common tasks such as data compression or 3D scene analysis, as it does not make the 3D scene geometry explicit. Image-based scene representations fundamentally require a multi-view image model that facilitates extraction of underlying geometrical relationships between the cameras and scene components. Developing new, efficient multi-view image models is thus one of the major challenges in image-based 3D scene representation methods. This dissertation focuses on defining and exploiting a new method for multi-view image representation, from which the 3D geometry information is easily extractable, and which is additionally highly compressible. The method is based on sparse image representation using an overcomplete dictionary of geometric features, where a single image is represented as a linear combination of few fundamental image structure features (edges for example). We construct the dictionary by applying a unitary operator to an analytic function, which introduces a composition of geometric transforms (translations, rotation and anisotropic scaling) to that function. The advantage of this approach is that the features across multiple views can be related with a single composition of transforms. We then establish a connection between image components and scene geometry by defining the transforms that satisfy the multi-view geometry constraint, and obtain a new geometric multi-view correlation model. We first address the construction of dictionaries for images acquired by omnidirectional cameras, which are particularly convenient for scene representation due to their wide field of view. Since most omnidirectional images can be uniquely mapped to spherical images, we form a dictionary by applying motions on the sphere, rotations, and anisotropic scaling to a function that lives on the sphere. We have used this dictionary and a sparse approximation algorithm, Matching Pursuit, for compression of omnidirectional images, and additionally for coding 3D objects represented as spherical signals. Both methods offer better rate-distortion performance than state of the art schemes at low bit rates. The novel multi-view representation method and the dictionary on the sphere are then exploited for the design of a distributed coding method for multi-view omnidirectional images. In a distributed scenario, cameras compress acquired images without communicating with each other. Using a reliable model of correlation between views, distributed coding can achieve higher compression ratios than independent compression of each image. However, the lack of a proper model has been an obstacle for distributed coding in camera networks for many years. We propose to use our geometric correlation model for distributed multi-view image coding with side information. The encoder employs a coset coding strategy, developed by dictionary partitioning based on atom shape similarity and multi-view geometry constraints. Our method results in significant rate savings compared to independent coding. An additional contribution of the proposed correlation model is that it gives information about the scene geometry, leading to a new camera pose estimation method using an extremely small amount of data from each camera. Finally, we develop a method for learning stereo visual dictionaries based on the new multi-view image model. Although dictionary learning for still images has received a lot of attention recently, dictionary learning for stereo images has been investigated only sparingly. Our method maximizes the likelihood that a set of natural stereo images is efficiently represented with selected stereo dictionaries, where the multi-view geometry constraint is included in the probabilistic modeling. Experimental results demonstrate that including the geometric constraints in learning leads to stereo dictionaries that give both better distributed stereo matching and approximation properties than randomly selected dictionaries. We show that learning dictionaries for optimal scene representation based on the novel correlation model improves the camera pose estimation and that it can be beneficial for distributed coding.

EPFL2009

Analysis of Multiview Omnidirectional Images in a Spherical Framework

Zafer Arican

With the increasing demand of information for more immersive applications such as Google Street view or 3D movies, the efficient analysis of visual data from cameras has gained more importance. This visual information permits to extract some crucial information in scenes such as similarity for recognition, 3D scene information, textures and patterns of objects. Multi-camera systems provide detailed visual information about a scene with images from different positions and viewing angles. The conventional perspective cameras that are commonly used in these systems however, have limited field of view. Therefore, they require either the deployment of many of these cameras or the capture of many images from different points to extract sufficient details about a scene. It increases the amount of data to be processed and the maintenance costs for such systems. Omnidirectional vision systems overcome this problem due to their 360-degree field of view and have found wide application areas in robotics, surveillance. These systems often use special refractive elements such as fisheye lenses or mirror-lense systems. The resulting images however inherit from the specific geometry of these units. Therefore, the analysis of these images with methods designed for perspective cameras results in degraded performance in omnivision systems. In this thesis, we focus on the analysis of multi-view omnidirectional images for efficient scene information extraction. We propose a novel spherical framework for omnidirectional image processing by exploiting the property that most omnidirectional images can be uniquely mapped on the sphere. We propose solutions for three common multiview image processing problems, namely feature detection, dense depth estimation and super-resolution for omnidirectional images. We first address the feature extraction problem in omnidirectional images. We develop a scale-invariant feature detection method which carefully handles the geometry of the images by performing the scale-space analysis directly on their native manifolds such as the parabola or the sphere. We then propose a new descriptor and a matching criteria that take into account the geometry and also eliminate the need for orientation computation. We also demonstrate that the proposed method can be used to match features in images captured by different types of sensors such as perspective, omnidirectional or spherical cameras. We then propose a dense depth estimation method to extract the 3D scene information from multiple omnidirectional images. We propose a graph-cut method adapted to the geometry of those images to minimize an energy function formulated for the dense depth estimation problem. We also propose a parallel graph-cut method that gives a significant speed improvement without a big penalty in accuracy. We show that the proposed method can be applied to multi-camera depth estimation and depth-based arbitrary view synthesis. Finally, we consider the multi-view omnidirectional images with the transformation of pure rotation. We address the view synthesis problem for these images in the framework of super-resolution. Considering also the inaccuracies in the rotation parameters, we solve an optimization problem that jointly estimates the rotation errors and reconstructs a high resolution omnidirectional image. We then extend the minimization problem with a regularization term for improved reconstruction quality with a reduced number of images. Results with both synthetic and real omnidirectional images suggest that the proposed method is a viable solution for super-resolution with omnidirectional images. Overall, this dissertation addresses three important issues of multiview omnidirectional image analysis and processing in a novel spherical framework. Our feature detection method can be used for the calibration of omnidirectional images as well as the feature matching in mobile and hybrid camera networks. Furthermore, our dense depth estimation method can impact the quality of 3D scene reconstruction and provide efficient solutions for view synthesis and multiview omnidirectional image coding. Finally, our super-resolution algorithm for omnidirectional images can promote the development of efficient acquisition systems for high resolution omnidirectional images

EPFL2010

Stream flow velocity measurement with smartphones: a technique for citizen observatories, decision-making, and water management

Tristan Jonas Brauchli, Zichong Chen, Wolf Hendrik Huwald, Steven Vincent Weijs

Stream flow velocity may be used for estimating discharge. Given the relative scarcity of direct stream flow and discharge measurements, surface velocity measurements can provide useful information for flood warning, hydropower production, hydrological science in general and water resource management. Some recent research efforts attempt involving the population in environmental sensing to complement existing static monitoring networks with alternative, spatially dense environmental information. Given the availability of sensing and the advances in image processing techniques (smartphones), there is a large potential to obtain hydrologically relevant data through motivated citizens. In this study, we investigate the feasibility of stream flow surface velocity measurements from movie clips taken by (smartphone) cameras based on specifically adapted image processing techniques. Results from movie clip derived velocity information is presented and compared to reference measurements from standard flow meters in various field experiments. The application of this technology in the context of so-called Citizen Observatories is expected to have a significant impact on hazard warning, forecasting, decision-making and water governance.