# AR in the Architecture Domain: State of the Art
## 5 AR System
### Tracking and Registration
#### 5.2.1 Marker Tracking  
Image recognition algorithms automatically recognize AR markers in the scene [119]. Some of these fiducial markers (Figure 12), such as QR codes (quick response codes) and Maxi codes since the late 90s, are commonly considered to be two-dimensional cryptograms or barcodes that redirect to other databases and programs, typically online, activatingmultimedia information or applications. In the last decade, they have also been used in AR [120]. An AR code is usually a QR code with a marker that redirects the user to the AR web app or mobile app. The markers can be passive--printed on any surface--or active, defined by infrared emission. In both cases, the calibration and the tracking activities occur with cameras that work on the visible or infrared spectra. Most AR applications use the first type, while the second is helpful in AR indoors, where the non-ideal lighting conditions can cause greater complexity in recognizing passive markers. The use of AR markers in architecture can depend on several factors, first of all:  
* The relationship established between the marker and the real scene;
* The possibility of preserving its location;
* The lighting conditions. Regarding the first point, it is not always possible to directly place the marker on the artifact to preserve the surface without altering the original aspect. In this sense, using toms with markers next to the architecture can solve the problem, planning their spatial position concerning the building and urban design simulation carefully [121]. Moreover, the second aspect concerns the conservation of the marker in terms of preservation of position and support. This limitation is particularly evident outdoors, where atmospheric agents and less scene control can make its application very difficult. The factor presents a minor impact in indoor environments, mainly if there is an active surveillance system; this is the reason why markers are often used in museum contexts [122]. Finally, lighting conditions strongly influence the target recognition activity. Although this problem is more limited in outdoor conditions, it can become significant in indoor ones. Active or passive IR markers can be used in such situations [123]. In the case of active IR markers, diodes that periodically emit IR light are used. Moreover, passive markers consist of retroreflective materials that reflect incoming IR light to the source. Multiple cameras frame a scene illuminated with IR light, paying attention to preserving the line of sight between the IR source and the reflector, in the meantime avoiding ambient IR radiation that may disturb this reception. In general, there is no relation between marker recognition and network data access, since the process depends only on the device's camera. Therefore, lack of network access is not binding, as AR content can be uploaded either in an external online server or directly on the offline device's local application. Another tracking possibility is based on sensors outside the camera. These can be electromagnetic, inertial, acoustic, or hybrid. The former bases its measurements on the intensity of the magnetic field existing between a base station and a measurement point, according to different directions and orientations; it has low latency and high responsiveness, but the measurement is subject to interference from other magnetic fields near the tracking space, so it requires a controlled environment. The inertial tracking system uses gyroscopes and accelerometers to measure the rotation and motion of a given target, allowing the calculation of position and speed. This method allows for a high  
Figure 12: Some examples of markers recognizable through image recognition. update rate with low latency and low cost but suffers from the accumulation of minor positioning errors made by the gyroscope and accelerometer, so it is often integrated with other tracking systems.