Fabrizio Ivan Apollonio

Recent advances in digital technologies are transforming the production and visualization of 3D models for Augmented Reality (AR), Virtual Reality (VR), and cultural heritage reconstruction. Within this evolving context, ensuring scientifically grounded, transparent, and interpretable reconstruction processes remains essential, particularly for the hypothetical reconstruction of lost or unbuilt architecture. This article discusses the theoretical framework defined by the Critical Digital Model (CDM) and the Scientific Reference Model (SRM), both of which aim to define the 3D model as a scientific product generated through a transparent and falsifiable research process. The proposed approach integrates a structured reconstruction methodology based on source analysis, semantic segmentation, and iterative validation, distinguishing between Raw and Informative Models and applying these principles to selected case studies. Attention is devoted to visualization as an integral component of the reconstruction process. Rather than serving solely as a presentation tool, visualization is examined as a means of analysis, interpretation, and communication of uncertainty. Different visualization strategies—including photorealistic, non-photorealistic, uncertainty-driven, and diplomatic representations—are assessed together with XR visualization modalities, from static images and spherical panoramas to fully interactive VR environments. The results demonstrate how immersive and methodologically grounded visualization approaches can enhance both scholarly investigation and public dissemination, supporting informed choices according to specific research and communication objectives.

Over recent decades, various projects—especially at the European level—have developed platforms for storing 2D and 3D digital models of cultural heritage. These platforms aim to preserve, organise, and make heritage data accessible for research, education, and public engagement. However, they face challenges due to diverse data formats, increasing user demands, and a lack of standardisation and metadata consistency. Advancements in digital technologies have enabled more efficient systems for acquiring, processing, and preserving cultural heritage data. Three-dimensional digitisation, in particular, supports multidimensional analysis and modernises documentation practices. Despite significant experience in creating 3D data repositories, comprehensive Information Systems for managing the full lifecycle of cultural heritage—especially those that integrate existing platforms—or web-based platforms designed to support collaborative scientific research by integrating data, tools, and computational resources remain limited and are not established at national levels. This paper explores this evolving landscape, highlighting key methodological and technological foundations for future systems. It also addresses open questions, opportunities, limitations, and ongoing challenges, emphasizing the need for semantic-based approaches to integrate fragmented data and foster collaboration between public and private stakeholders.

Since 2019 the authors of this article were involved in the digital acquisition and storytelling of artifacts belonging to museum collections. This work showcases on purpose various related case studies to illustrate key issues in the field. They are the Orcinus Citoniensis paleontological finding (representative of a complex geometry to be acquired), the Annunciation by Beato Angelico (representative of materials with articulate behaviors to light such as the gold foil and tempera), and the Giovanni Battista Amici’s Microscope (representative of non-cooperative specular materials). Since they refer to such different contests, they are a valid test field to bring into the discussion methods and techniques for: — the digitization and interactive representation of multicomponent finds; — a reliable visualization of pictorial works with complex optical behaviors; — a cost-effective, rapid digitization of extensive collections with non-cooperating materials. The article outlines a pipeline from accurate digitization to 3D modeling, texturing, and optimization, culminating in the production of interactive experiences. The aim is the introduction of our experience in authoring high-resolution digital surrogates for scholars and museum visitors, to be explored in museums and cultural institutions through conventional devices or digital interactive kiosks. It emphasizes logistical challenges in photogrammetry for acquiring artifacts, including 3D or 2D objects, paleontological finds, and optically non-cooperating items, with a focus on accurate color acquisition and rendering. The developed pipeline enhances the user experience of 3D digital replicas via visualization apps designed for exhibitions and permanent collections.

Highlights: This study presents a structured methodology for the hypothetical digital 3D reconstruction of unbuilt or demolishedbuildings. The critical digital model (CDM) of the lost church of S. Margherita in Bologna designed by Agostino Barelli in 1685 was built. The reconstructive methodology is exhaustive, easily reproducible and transparent, and the 3D model is built and published in a way that is interoperableand accessible. This paper presents a structured three-dimensional (3D) reconstruction methodology of architectural heritage adopted and implemented in the context of the CoVHer (Computer-based Visualisation of architectural Heritage) Erasmus+ project. The methodology consists of a multi-step process for hypothetically reconstructing never-built or demolished architectural heritage from the past in the form of 3D digital models. This reconstruction methodology was tested over the years with professionals, scholars and laypersons, on several case studies in the context of international workshops, museum exhibitions, VR dissemination, and it was also tested with students at the architectural drawing course at the University of Bologna. This last experimentation was particularly important because fostered us to systematise its steps and make it more easily sharable and applicable while not compromising quality and robustness. The methodological steps that we are going to address and discuss in this paper are: a) data acquisition, b) critical evaluation of historical and architectural sources, c) 2D digital redrawing of graphic material, d) construction of the 3D model, e) visualisation, f) uncertainty assessment and communication, g) documentation, and h) publication with a particular focus on interoperability and accessibility. These steps are explained in detail in order to be applicable to similar case studies and foster reproducibility, comparability, accessibility, transparency, and interoperability of the digital reconstruction. These are the key principles already recommended by the FAIR principles (Findable, Accessible, Interoperable, Reusable), the Seville Principles, and the London Charter, among others. The methodology, despite being tested on various case studies and fields, has been proven to be particularly effective for never-built or demolished architectural heritage with known authors. This paper presents the case study of the reconstruction of the unbuilt Church of S. Margherita in Bologna, designed by Agostino Barelli in 1685. This exemplary case study covers all aspects of our reconstruction methodology.

The CoVHer Erasmus+ project addresses the long-standing lack of standardisation in hypothetical virtual 3D reconstruction and modelling in architectural heritage research. It aims to develop best practices for the 3D reconstruction of lost or never-built architectural heritage, enhancing research quality, transparency, and reusability. The collaborative effort includes contributions from five European universities and two private companies, focusing on architecture, archaeology, digital humanities, and art history. The presented workflow of documentation is the result of the joint effort in extensions of the approaches of the Scientific Reference Model (SRM) and Critical Digital Model (CDM), which strive for faithful reconstruction, documenting all decisions and inferences and advocate open licensing and use of non-proprietary formats to promote accessibility and reusability. The developed methodology aims to provide tools for the scientific evaluation of hypothetical reconstructions and support data exchange between researchers.

For more than thirty years, 3D digital modelling has been used more and more widely as a research tool in various disciplinary fields. Despite this, the 3D models produced by different research, investigation, and speculation activities are still only used as a basis and as sources for the production of images and scientific contributions (papers in journals, contributions in conference proceedings, etc.) in dissemination and cultural activities, but without having yet assumed full autonomy as a ‘scientific fact’, as a product of research, or as a means of scientific debate and progress. This paper outlines the context in the field of architecture and archeology in which the use of 3D models has become increasingly widespread, reaching a level of full maturity, and how the field of hypothetical reconstruction can be characterized as an autonomous/scientific discipline through the definition and adoption of a scientific, transparent, verifiable, reusable, and refutable method. In this context, the definition of the 3D model as a product of scientific speculation and research is proposed.