## 2 Materials and Methods
### Augmented Reality for Cultural Heritage Dissemination: A Review
* The modelling of virtual environments;
* The tracking the location of users;
* The 3D registration of real images with virtual models. As far as 3D modelling is concerned, different techniques can be used depending on the objective of the application being developed. In the simplest cases, the use of 3D design software which allows the creation of artificial virtual models, such as 3 DS Max or Blender, suffices. When the application to be performed involves the reconstruction of heritage assets, it is necessary to use other tools to generate 3D models from the real data. The most common techniques used to acquire 3D data are photogrammetry (SfM) [36] and 3D laser scanners [37]. In this case, a new problem must be faced: the adaptation of these 3D models for real-time visualisation. Some proposals regarding resolving this problem have been published recently, although it is not yet totally solved. Undoubtedly, this is an important issue whose resolution will improve the quality of AR applications in the field of cultural heritage [38]. Tracking the location of users allows one to know where they are. The 3D registration determines the users' points of view in order display on the devices used for this purpose the virtual models superimposed on the actual heritage pieces. It is also responsible for modifying the orientation and scale of the virtual models so that they are correctly seen on the real image. Both problems, tracking and registration, are related to each other and are solved by combining information from different sensors in most cases: GPS, inertial measurement units (IMU), and cameras. The information provided by the two first types of sensors is usually utilised to track the location. Meanwhile, the information obtained from the application of computer vision algorithms on the images captured by the cameras is commonly used to locate the users, although it can also be used to solve registration problems. In the case of AR applications in outdoor heritage sites, GPS or IMUs, along with cameras, are the most used sensors. For indoor heritage sites, a camera alone is usually used to locate the user and to solve the registration problem through mark recognition [39; 40]. Even so, the problem of real-time registration is still an open issue only solved for the simplest geometric shapes. Obtaining the same real-time results for free-forms and organic objects as those of cultural heritage is far from achieved. In AR applications, it is essential to avoid latency, since the delay makes the experience unsatisfactory for users, despite the perfect technical results. When designing any system or software that involves a non-expert user, it is very important to follow user-centred design (UCD) processes [41; 42]. This is particularly important in the cases discussed in this paper, as the applications designed are intended to be used by a wide range of people. In other words, in general, it must be assumed that most users have no previous technical (mobile and AR) or historical knowledge. A wide variety of tools and methodologies for research, design, rapid prototyping, and user testing of mobile AR user experiences can be found in the literature (e.g., [43]). Apart from the interface, within the design process both the amount and the flow of information the user should receive has to be decided. It is obvious that too much information could overwhelm the user. In this sense, three parameters can be considered when designing AR and VR applications: presence, affordance, and usability. Presence is usually defined as \"the degree to which a user feels like they are in reality while experiencing the extended reality\". In [PERSON]'s study [44], some parameters for measuring presence are provided. An example can be found in [45], where it is reported that some users \"lose sight of where the game ends and reality begins\". Therefore, in an application, the higher the level of presence, the better the result of the impact and assimilation of the information in the user.