Dipartimento di Ingegneria Meccanica, Energetica e Gestionale

This thesis is concerned with optical surveying techniques for 3D reconstruction in underwater environment. In particular, it focuses on 3D modelling of archaeological areas, lying both in shallow and deep water. Virtual representations of archaeological sites are required by researchers in order to enhance study and preservation methodologies and also to ease promotion and dissemination. The presented work mainly follows two research lines. First, it tackles the main problems arisen for standard 3D reconstruction algorithms and procedures when applied on submarine datasets. Multi-View mapping algorithm has been investigated and enhanced in order to achieve high accuracy reconstruction of two submerged villae, despite the challenging context of Baia Archaeological Park. Then, the textured and high detailed models can be employed for different purposes; in particular, some examples of virtual exploration, simulation for device design and for intervention planning and monitoring are shown. The second research line deals with the implementation of a Simultaneous Localization and Mapping (SLAM) algorithm for Unmanned Underwater Vehicles (UUVs). Some datasets acquired by an UUV equipped with typical navigation sensors and with a stereo-camera have been processed through the implemented Extended Kalman Filter (EKF), in order to perform sensor fusion and enhance both vehicle tracking and 3D scene reconstruction. Additionally, Augmented State Kalman Filter (ASKF) has been implemented in order to exploit optical data for loop closure detection in order to reduce drift error, typical of dead-reckoning navigation.

This dissertation focuses on the use of multi-view 3D reconstruction techniques in the field of cultural heritage. To name just a few applications, a digital 3D acquisition can be used for documentation purposes in the event of destruction or damage of an artefact, or for the creation of museums and virtual tourism, education, structural studies, restoration, etc... All these applications require high precision and accuracy to reproduce the details, but there are other important characteristics such as low cost, ease of use, the level of knowledge needed to operate the systems, which have also to be taken into account. At the present time, the interest is growing around the use of images for the digital documentation of cultural heritage, because it is possible to obtain a 3D model by the means of common photographic equipment. In this work, we have investigated multi-view 3D reconstruction techniques in two specific fields that have not been treated in literature: the 3D reconstruction of small objects (from few mm to few cm) and the survey of submerged archaeological finds. As for the 3D reconstruction of small objects, a new methodology based on multi-view and image fusion techniques has been developed. The used approach solves the problems related to the use of macro lenses in photogrammetry, such as the very small depth of field and the loss of quality due to diffraction. Since image matching algorithms cannot work on blurred areas, each image of the sequence is obtained by merging pictures acquired at different focus planes. The methodology has been applied on different case studies, and the results have shown that it is possible to reconstruct small complex objects with a resolution of 20 microns and an accuracy of 10 microns. For which concerns the underwater imaging, a preliminary comparative study between active and passive techniques in turbid water has been conducted. The experimental setup consists in a 3D scanner designed for underwater survey, composed by two cameras and a projector. An analysis on the influence of the colour channel has been conducted, showing how it is possible to obtain a cleaner reconstruction by using the green channel only. The results have shown a denser point cloud when using the passive technique, characterized by missing areas since the technique is more sensible to turbidity. By contrast, the reconstruction conducted with the active technique have shown more stable results as the turbidity increases, but a greater noise. A multi-view passive technique has been experimented for the survey of a submerged structure located at a depth of 5 meters, on a seabed characterized by poor visibility conditions and the presence of marine flora and fauna. We performed an analysis of the performances of a multi-view technique commonly used in air in the first instance, highlighting the limits of the current techniques in underwater environment. In such conditions, in fact, it has not been possible to obtain a complete reconstruction of the scene. The second stage of the process was the testing of image enhancement algorithms in order to improve matching performances in poor visibility conditions. In particular, a variational analysis of the factors that influence the quality of the 3D reconstruction, such as the image resolution and the colour channel, has been performed. For this purpose, the data related to the parameters of interest, such as the number of features extracted or the number of oriented cameras, have been evaluated. The statistical analysis has allowed to find the best combination of factors for a complete and accurate 3D reconstruction of the submerged scenario.

Dipartimento di Ingegneria Meccanica, Energetica e Gestionale - Tesi di Dottorato

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