## 1 Introduction
### State of the Art on XR and Accessibility for Cultural Heritage
The project was based on an open platform developed to share content and create apps for guided tours of cultural sites. Generally, the most complex and delicate part of multimedia and technological enhancement is the experience design linked to the digital content since there are countless factors to consider [8]. It is necessary to make choices and be aware that quality design cannot please all users and adapt to all contexts. The integration of real and virtual content in museums and archaeological sites can be achieved through multiple technological solutions adapted to various needs. The eXtended Reality (XR) includes all combinations of real and virtual environments and the various human-machine interactions generated by computer technology [9; 10; 11]. Following the reality/virtuality continuum [12], between the real environment and virtual reality (VR), we find mixed reality (MR), which aims at blending real and virtual environments, and includes various intermediate levels: augmented reality (AR), more in dialogue with the real world, and augmented virtuality (AV), closer to VR. AR improves the perception and understanding of the natural world by superimposing virtual information through an interposed device (a smartphone, tablet or glasses) or a projected pattern. It is possible to apply it both at the architectural scale to see reconstructive hypothesis in situ [13] or contextualise diagnostic analyses and layers on 3D models [14] and at the scale of detail to interact with virtual artefacts [15] or visualise additional information about them through the revealing flashlight technology [16]. In AV, the blending of real and virtual worlds is maintained, but the aim is to augment the virtual one with elements from the real one. This application is not particularly common in archaeology and cultural heritage, whereas it is more widely used in 3D videoconferencing, telepresence, infotainment, gaming and education. Recent open standards such as WebXR allow immersive VR and AR presentations of 3D content through a simple web page without any installation. However, this standard is still in the process of being integrated into browsers in commonly used devices. MR or AR experiences can be developed in various ways and at various levels of technological complexity [17]. AR in the proper sense is based on the principle of tracking. The software automatically recognises some significant and well-defined points (features) of the real space (geometry of a building or space) and 'hooks' the corresponding virtual contents onto them by means of geolocalisation, feature recognition, tracking technology, optical sensors, accelerometers, gyroscopes, etc. The correct overlap between real and virtual content can be calculated in real time, based on specific software algorithms, or offline. In the truest concept of MR/AR, feature tracking is calculated in real time, and the experience must necessarily be lived on-site, with all the environmental and logistical variables that this entails. The calculation requires the use of dedicated and generally proprietary devices and software platforms. There are some device and platform solutions implemented by large industries (Google, Apple, etc. ), but they are not simply usable by all people, requiring non-trivial spatial calibration and setup processes before being used. Despite these sophisticated systems being much more projected into the future, they are currently still imperfect in their aesthetic rendering and usability, with the consequence of a possible penalisation of the cultural experience. The focus in these cases shifts more to technological innovation rather than to content. Offline calculation refers instead to pre-processed visualisations in which the camera tracking between real and virtual content has been previously calculated and the output of the scene is a movie in which the two layers, real and virtual, are already merged and rendered together.