Extrinsic calibration of camera networks using a sphere

In this paper, we propose a novel extrinsic calibration method for camera networks using a sphere as the calibration object. First of all, we propose an easy and accurate method to estimate the 3D positions of the sphere center w.r.t. the local camera coordinate system. Then, we propose to use orthogonal procrustes analysis to pairwise estimate the initial camera relative extrinsic parameters based on the aforementioned estimation of 3D positions. Finally, an optimization routine is applied to jointly refine the extrinsic parameters for all cameras. Compared to existing sphere-based 3D position estimators which need to trace and analyse the outline of the sphere projection in the image, the proposed method requires only very simple image processing: estimating the area and the center of mass of the sphere projection. Our results demonstrate that we can get a more accurate estimate of the extrinsic parameters compared to other sphere-based methods. While existing state-of-the-art calibration methods use point like features and epipolar geometry, the proposed method uses the sphere-based 3D position estimate. This results in simpler computations and a more flexible and accurate calibration method. Experimental results show that the proposed approach is accurate, robust, flexible and easy to use

A study on detection of risk factors of a toddler’s fall injuries using visual dynamic motion cues

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The research in this thesis is intended to aid caregivers’ supervision of toddlers to prevent accidental injuries, especially injuries due to falls in the home environment. There have been very few attempts to develop an automatic system to tackle young children’s accidents despite the fact that they are particularly vulnerable to home accidents and a caregiver cannot give continuous supervision. Vision-based analysis methods have been developed to recognise toddlers’ fall risk factors related to changes in their behaviour or environment. First of all, suggestions to prevent fall events of young children at home were collected from well-known organisations for child safety. A large number of fall records of toddlers who had sought treatment at a hospital were analysed to identify a toddler’s fall risk factors. The factors include clutter being a tripping or slipping hazard on the floor and a toddler moving around or climbing furniture or room structures. The major technical problem in detecting the risk factors is to classify foreground objects into human and non-human, and novel approaches have been proposed for the classification. Unlike most existing studies, which focus on human appearance such as skin colour for human detection, the approaches addressed in this thesis use cues related to dynamic motions. The first cue is based on the fact that there is relative motion between human body parts while typical indoor clutter does not have such parts with diverse motions. In addition, other motion cues are employed to differentiate a human from a pet since a pet also moves its parts diversely. They are angle changes of ellipse fitted to each object and history of its actual heights to capture the various posture changes and different body size of pets. The methods work well as long as foreground regions are correctly segmented

Noncentral catadioptric systems with quadric mirrors : geometry and calibration

Tese de doutoramento em Engenharia Electrotécnica (Informática) apresentada à Faculdade de Ciências e Tecnologia da Universidade de CoimbraNesta dissertação de doutoramento estudamos e analisamos a geometria dos sistema catadióptricos não-centrais compostos por uma câmara pinhole ou ortográfica e um espelho curvo, cuja forma é uma quádrica não degenerada, incluindo elipsóides, que podem ser esferas, hiperbolóides e parabolóides. A geometria destes sistemas de visão é parameterizada, analisando o fenómeno de formação da imagem, e é composta pelos parâmetros intrínsecos da câmara, os parâmetros da superfície do espelho e a posição e orientação da câmara em relação ao espelho e ao sistema de referência do mundo. A formação da imagem é estudada numa perspectiva puramente geométrica, focando principalmente o modelo de projecção e a calibração do sistema de visão. As principais contribuições deste trabalho incluem a demonstração de que num sistema catadióptrico não-central com um câmara em perspectiva e uma quádrica não degenerada, o ponto de reflexão na superfície do espelho (projectando na imagem qualquer ponto 3D do mundo) pertence a uma curva quártica que é dada pela intersecção de duas superfícies quádricas. O correspondente modelo de projecção é também desenvolvido e é expresso através de uma equação não linear implícita, dependente de um único parâmetro. Relativamente `a calibração destes sistemas de visão, foi desenvolvido um método de calibração, assumindo o conhecimento dos parâmetros intrínsecos da câmara em perspectiva e de um conjunto de pontos 3D expressos em coordenadas locais (estrutura 3D do mundo). Informação acerca do contorno aparente do espelho é também usada para melhorar a precisão da estimação. Um outro método de calibração é proposto, assumindo uma calibração prévia do sistema no sentido de um modelo geral de câmara (correspondências entre pontos na imagem e raios incidentes no espaço). Adicionalmente, a posição e orientação (pose) da câmara em relação ao espelho e ao sistema de referência do mundo são estimadas usando métricas algébricas e equações lineares (escritas para um método de calibração que também é apresentado). Considera-se a câmara como pré-calibrada. São desenvolvidas e apresentadas experiências com simulações extensivas e também com imagens reais de forma a testar a robustez e precisão dos métodos apresentados. As principais conclusões apontam para o facto de estes sistemas de visão serem altamente não lineares e a sua calibração ser possível com boa precisão, embora difícil de alcançar com precisão muito elevada, especialmente se o sistema de visão tem como objectivo aplicações direccionadas para a precisão. Apesar disso, pode observar-se que a informação da estrutura do mundo pode ser complementada com informação adicional, tal como o contorno aparente da quádrica, de forma a melhorar a qualidade dos resultados de calibração. Na verdade, o uso do contorno aparente do espelho pode, por si, melhorar drasticamente a precisão da estimação.In this PhD thesis we study and analyze the geometry of noncentral catadioptric systems composed by a pinhole or orthographic camera and a non-ruled quadric shaped mirror, that is to say an ellipsoid, which can be a sphere, a hyperboloid or a paraboloid surface. The geometry of these vision systems is parameterized by analyzing the image formation and is composed by the intrinsic parameters of the camera, the parameters of the mirror surface and the poses of the camera in relation to the mirror and to the world reference frames. Image formation is studied in a purely geometrical way, focusing mainly on the projection model and on the calibration of the vision system. The main contributions include the proof that in a noncentral catadioptric system with a perspective camera and a non degenerate quadric the reflection point on the surface (projecting any given 3D world point to the image) is on the quartic curve that is the intersection of two quadrics. The projection model related to the previous definition of the reflection point is also derived and is expressed as an implicit non linear function on a single unknown. In what concerns the calibration of these vision systems, we developed a calibration method assuming the knowledge of the intrinsic parameters of the perspective camera and of some 3D points in a local reference frame (structure) . Information about the apparent contour is also used to enhance the accuracy of the estimation. Another calibration method is proposed, assuming a previous calibration of the system in the sense of a general camera model (correspondences between image points and incident lines in space). Additionally, the camera-mirror and camera-world poses are estimated using algebraic metrics and linear equations (derived for a calibration method that is also presented). The camera is considered to be pre-calibrated. Experiments with extensive simulations and also using real images are performed to test the robustness and accuracy of the methods presented. The main conclusions are that these vision systems are highly non linear and that their calibration is possible with good accuracy but difficult to achieve with very high accuracy, specially if the vision system is aimed at being used for accuracy-driven applications. Nevertheless it is observed that structure of the world can be complemented with some additional information as the quadric apparent contour in order to improve the quality of the calibration results. Actually, the use of the apparent contour can dramatically improve the accuracy of the estimation