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The already arduous task of collecting, processing and managing heterogeneous cultural heritage data is getting more intense in terms of indexing, interaction and dissemination. This paper presents the creation of a 4D web-based platform as a centralized data hub, moving beyond advanced photogrammetric techniques for 3D capture and multi-dimensional documentation. Precise metric data, generated by a combination of image-based, range and surveying techniques, are spatially, logically and temporally correlated with cultural and historical resources, in order to form a critical knowledge base for multiple purposes and user types. Unlike conventional information systems, the presented platform, which adopts a relational database model, has the following front-end functionalities: (i) visualization of high-resolution 3D models based on distance dependent Level of Detail (LoD) techniques; (ii) web Augmented Reality; and (iii) interactive access and retrieval services. Information deduced from the developed services is tailored to different target audiences: scientific community, private sector, public sector and general public. The case study site is the UNESCO world heritage site of Meteora, Greece, and particularly, two inaccessible huge rocks, the rock of St. Modestos, known as Modi, and the Alyssos rock.

Cultural Heritage (CH) management software represents virtual information in various ways aiming either at usability and long-term preservation or interactivity and immersiveness. A single web-based framework that couples the organization of geospatial, multimedia and relational data with 4D visualization, Virtual Reality (VR) and Augmented Reality (AR) is presented in this paper (https://meteora.topo.auth.gr/5dmeteora.php). It comprises the 5dMeteora platform and the Content Management System (CMS) for uploading, processing, publishing and updating its content. The 5dMeteora platform integrates a responsive 3D viewer of high-resolution models in the basis of 3DHOP (3D Heritage Online Platform) and Nexus.js multi-resolution library. It offers data retrieval and interpretation mechanisms through navigation tools, clickable geometries in the 3D scene, named hotspots, and semantic organization of metadata. Its content and interactive services are differentiated, based on the scientific specialty or the field of interest of the users. To achieve the sense of spatial presence, VR and AR viewports are designed to give a clearer understanding of spatial bounds and context of 3D CH assets. The proposed CMS allows dynamic content management, automation of 3DHOP’s operations regarding 3D data uploading and hotspots defining, real-time preview of the 3D scene as well as extensibility at all levels (e.g., new data types). It is built upon a MySQL Database Management System and developed with PHP scripting, backend JavaScript and Ajax controllers as well as front-end web languages. The database maintains and manages the entities of every type of data supported by the platform, while encryption methods guarantee data confidentiality and integrity. The presented work is the first valid attempt of open-source software that automates the dissemination of 3D and 2D content for customized eXtented Reality (XR) experiences and reaches multiple levels of interactivity for different users (experts, non-experts). It can meet the needs of domain experts that own or manage multimodal heritage data.

The purpose of this paper is the presentation of a novel algorithm for automatic estimation of the exterior orientation parameters of image datasets, which can be applied in the case that the scene depicted in the images has a planar surface (e.g., roof of a building). The algorithm requires the measurement of four coplanar ground control points (GCPs) in only one image. It uses a template matching method combined with a homography-based technique for transfer of the GCPs in another image, along with an incremental photogrammetry-based Structure from Motion (SfM) workflow, coupled with robust iterative bundle adjustment methods that reject any remaining outliers, which have passed through the checks and geometric constraints imposed during the image matching procedure. Its main steps consist of (i) determination of overlapping images without the need for GPS/INS data; (ii) image matching and feature tracking; (iii) estimation of the exterior orientation parameters of a starting image pair; and (iv) photogrammetry-based SfM combined with iterative bundle adjustment methods. A developed software solution implementing the proposed algorithm was tested using a set of UAV oblique images. Several tests were performed for the assessment of the errors and comparisons with well-established commercial software were made, in terms of automation and correctness of the computed exterior orientation parameters. The results show that the estimated orientation parameters via the proposed solution have comparable accuracy with those ones computed through the commercial software using the highest possible accuracy settings; in addition, double manual work was required by the commercial software compared to the proposed solution.

Augmented Reality (AR) is more than an added value for Cultural Heritage (CH); it is vital for its sustainability, promotion and dissemination, increasing accessibility in CH even during difficult periods of time, like the Covid-19 pandemic. In order to be meaningful and engaging, an AR application should have the following characteristics: easiness of use, high-quality representations and compatibility. This paper presents a marker-less mobile AR application for the display and inspection of high-resolution 3D cultural assets, overlayed on a particular location in the real-world scene. Instead of predefined markers, an image captured by the user is exploited as a pattern for real-time feature matching, pose estimation and scene augmentation. Our approach is based on pure computer vision and photogrammetric techniques, implemented using native C++ and Java code for Android mobile platforms. It is built with the use of the OpenCV library and the OpenGL ES graphics API without any dependencies of AR Software Development Kits (SDKs). Therefore, it supports cross-vendor portability regarding mobile model devices and hardware specifications. The evaluation of the developed application examines the performance of various matching techniques and the overall responsiveness of processing and 3D rendering on mid-range and low-end smartphones. The results showcase the reliability and responsiveness of the pattern recognition as well as the potential of the 3D graphics engine to render and overlay complex 3D models balancing between visual quality and time. The proposed methodology is applied to the Ciborium of the church of St. Charalabos, located at St. Stephen’s Monastery in Meteora, Greece.

The mission of digital Cultural Heritage (CH) repositories goes beyond engagement and dissemination; their research data should be approached and correlated spatially and semantically in the service of conservation and accessibility. Therefore, a flexible system is needed to expand, reuse and repurpose their content, addressing online collaboration, real time progress and ease of use. Towards this direction, the “METEORA” web-based platform is a novel and holistic approach to the organization, management and visualization of the 3D and 2D documentation products of CH sites. It integrates a multi-scale 3D viewer based on the 3DHOP framework, personalized information access and interactive tools for data retrieval and presentation. Emphasis is given in the development of a mid-level interface for creating, updating and maintaining 3DHOP’s functionalities and customizing information based on the user’s scientific specialty and field of interest. The proposed admin control panel updates the state of the current visualization, allows the integration of 3D models and multimedia and automates the creation of clickable points of interest on top of the 3D surfaces. It is based on a MySQL Relational Database Management System (RDBMS) that supports the back-end User Interface (UI) with create, read, update and delete (CRUD) management functions of a typical RESTful service. Moreover, all the control mechanisms as well as the procedures of authentication, authorization and encryption of the credentials of the certified users are configured through the PHP scripting language and JSON files. The resulting platform achieves maximum adaptability in digital archiving of heterogeneous datasets and has increasing efficiency in representing all types of data supported by 3DHOP.

In this paper, a methodology for the calculation of rough exterior orientation (EO) parameters of multiple large-scale overlapping oblique aerial images, in the case that GPS/INS information is not available (e.g., for old datasets), is presented. It consists of five main steps; (a) the determination of the overlapping image pairs and the single image in which four ground control points have to be measured; (b) the computation of the transformation parameters from every image to the coordinate reference system; (c) the rough estimation of the camera interior orientation parameters; (d) the estimation of the true horizon line and the nadir point of each image; (e) the calculation of the rough EO parameters of each image. A developed software suite implementing the proposed methodology is tested using a set of UAV multi-perspective oblique aerial images. Several tests are performed for the assessment of the errors and show that the estimated EO parameters can be used either as initial approximations for a bundle adjustment procedure or as rough georeferencing information for several applications, like 3D modelling, even by non-photogrammetrists, because of the minimal user intervention needed. Finally, comparisons with a commercial software are made, in terms of automation and correctness of the computed EO parameters.

The complexity of modern urban environments and civil demands for fast, reliable and affordable decision-making requires not only a 3D Land Information System, which tends to replace traditional 2D LIS architectures, but also the need to address the time and scale parameters, that is, the 3D geometry of buildings in various time instances (4th dimension) at various levels of detail (LoDs - 5th dimension). This paper describes and proposes solutions for technical aspects that need to be addressed for the 5D modelling pipeline. Such solutions include the creation of a 3D model, the application of a selective modelling procedure between various time instances and at various LoDs, enriched with cadastral and other spatial data, and a procedural modelling approach for the representation of the inner parts of the buildings. The methodology is based on automatic change detection algorithms for spatial-temporal analysis of the changes that took place in subsequent time periods, using dense image matching and structure from motion algorithms. The selective modelling approach allows a detailed modelling only for the areas where spatial changes are detected. The procedural modelling techniques use programming languages for the textual semantic description of a building; they require the modeller to describe its part-to-whole relationships. Finally, a 5D viewer is developed, in order to tackle existing limitations that accompany the use of global systems, such as the Google Earth or the Google Maps, as visualization software. An application based on the proposed methodology in an urban area is presented and it provides satisfactory results.

Advances in the scientific fields of photogrammetry and computer vision have led to the development of automated multi-image methods that solve the problem of 3D reconstruction. Simultaneously, 3D scanners have become a common source of data acquisition for 3D modeling of real objects/scenes/human bodies. This article presents a comprehensive overview of different 3D modeling technologies that may be used to generate 3D reconstructions of outer or inner surfaces of different kinds of targets. In this context, it covers the topics of 3D modeling using images via different methods, it provides a detailed classification of 3D scanners by additionally presenting the basic operating principles of each type of scanner, and it discusses the problem of generating 3D models from scans. Finally, it outlines some applications of 3D modeling, beyond well-established topographic ones.

This entry provides a comprehensive overview of methods used in image matching. It starts by introducing area-based matching, outlining well-established techniques for determining correspondences. Then, it presents the concept of feature-based image matching, covering feature point detection and description issues, including both handcrafted and learning-based operators. Brief presentations of frequently used detectors and descriptors are included, followed by a presentation of descriptor matching and outlier rejection techniques. Finally, the entry provides a brief overview of relational matching.

In the context of web augmented reality (AR), 3D rendering that maintains visual quality and frame rate requirements remains a challenge. The lack of a dedicated and efficient 3D format often results in the degraded visual quality of the original data and compromises the user experience. This paper examines the integration of web-streamable view-dependent representations of large-sized and high-resolution 3D models in web AR applications. The developed cross-platform prototype exploits the batched multi-resolution structures of the Nexus.js library as a dedicated lightweight web AR format and tests it against common formats and compression techniques. Built with AR.js and Three.js open-source libraries, it allows the overlay of the multi-resolution models by interactively adjusting the position, rotation and scale parameters. The proposed method includes real-time view-dependent rendering, geometric instancing and 3D pose regression for two types of AR: natural feature tracking (NFT) and location-based positioning for large and textured 3D overlays. The prototype achieves up to a 46% speedup in rendering time compared to optimized glTF models, while a 34 M vertices 3D model is visible in less than 4 s without degraded visual quality in slow 3D networks. The evaluation under various scenes and devices offers insights into how a multi-resolution scheme can be adopted in web AR for high-quality visualization and real-time performance.