# AR in the Architecture Domain: State of the Art
## 2 AR in the Real-Virtual Continuum  
The augmented reality sphere relates to both the real environment and the virtual world. AR is placed within a well-known spatial interval (continuum) between real and virtual (Figure 2), named X-reality (XR), which contains real environment (RE), augmented reality (AR), augmented virtuality (AV), and virtual reality (VR) [16]. Figure 1: Diagram of the relationships between ERC sectors and the AR chain in the architecture domain. Figure 2: [PERSON] and [PERSON]’s scheme, reinterpreted and adapted for the architecture domain. Mixed reality (MR) describes the domain between AR and VR. AR's position in the continuum, close to physical reality [17], introduces virtual information into an existing reality via devices. This integration can be additive, layering information that does not exist, or subtractive, covering or deleting parts of the real world. The first case is referred to as AR, while the second as mediated reality. The latter, like AR, can be placed close to the real environment in the continuum towards AV. Within architecture, the transformation of real or virtual data towards the AR environment is supported by well-established methodologies discussed in Section 4. The connection between RE, AR, and VR (Figure 2) is made through 3D surveying and modeling methodologies that allow for the digitization of reality, such as reality-based modeling (RBM) or image-based modeling (IBM). At the same time, parametric models are artificially built through a geometry-based approach [18]. In the past decade, the definitions of AR highlighted and changed some specific elements or relations involved in the process. First, [PERSON] and [PERSON] in 1994 defined AR as: _\"any case in which an otherwise real environment is \"augmented\" by means of virtual (computer graphic) objects.\"_ This definition pointed out the secondary role of AR, as an integrative medium to better understand the real environment, or a virtual reality connected to it [17]. This meaning was modified three years later in [PERSON]'s definition: _\"A system that combines real and virtual content, provides a real-time interactive environment, and registers in 3D\"_. [19]. The new definition was focused on the real-time representation, interaction, and its location in three-dimensional space. The \"ethical\" aspect of reality integration and multisensory approaches disappeared, stating the priority of the visual system over the other senses. In the following decades, this definition and the consequent taxonomy were expanded and integrated several times. AR is a system capable of improving the understanding of the world through the integration of virtual information, enlarging immersion and environmental knowledge [9; 20; 21]. The main AR characteristics (Figure 3), according to [PERSON]'s definition and subsequent studies [22]. Outlined in the architecture domain, must present the following aspects:  
* It must merge the real architectural environment, or a portion of it, with virtual data in a common visualization platform on the device;
* It must work in real time, reaching a synchronous update between the movements in the real space and the simulated virtual one;
* It must allow a high level of engagement, not constrained to a single point of view, moving freely in the architectonic space or around it;
* It must allow a variable level of interaction between the users, the real world, and the virtual contents. Figure 3: Diagram with the main features of AR, and its relationship to the real and virtual worlds. The relationship between AR and the wider XR allows for the understanding of its interaction and immersion value [7]. Virtual reality (VR) permits a complete immersion in a digital world, presenting an environment disconnected from the real one. However, it can represent an interpretive reproduction [3], integrated with perceptual stimuli such as visual, auditory, and tactile stimuli to simulate the experience of a real environment [23]. Interaction occurs only with virtual information through different devices.