## 5 DT Improvements Brought about by eXtended Reality Technologies
### Interaction with the Model  
A system as complex as a DT needs intuitive and user-friendly interactions for the user [25, 26, 77, 84]. The most common interaction methods are through the use of standard inputs (e.g., touch screen and virtual keyboard for HHD, virtual buttons for HMD and smart-glasses) [25, 70, 76, 77, 78]. However, through the use of an embedded camera and specific sensors, the HMD can offer natural interaction methods to the user. Using image recognition algorithms and an embedded microphone, gesture and voice recognition can be performed by the user to provide natural interaction with the virtual interface [25, 26, 53, 80]. However, in an industrial environment, external factors can disrupt these interactions (e.g., dust that disturbs image recognition and noise that complicates speech recognition). Other interaction methods can be used to overcome this problem, such as using the location of the user to display specific augmentations [25, 47, 75, 76].  
To locate the user in the real environment in comparison with the targeted equipment, some authors have decided to use the embedded GPS (Global Positioning System) of the devices (either HMD or HHD) [76]. The location it provides can then be used to filter the information to display, depending on the distance between him and the equipment or on the room he or she is in [25, 47, 75, 76]. [PERSON] et al. proposed a solution using a device's built-in GPS to display a simulated system configuration when the user is standing at a specific location [76]. However, the accuracy of GPS is too wide for effective use in indoor environments (precision of 5 to 10 m) [85]. [PERSON] et al. presented another example of the use of this interaction method by allowing the creation of specific zones in which the user must stand in order to view the information displayed to him [25] (see Figure 7).  
Some authors have proposed using another interaction method to also estimate the orientation of the user, namely gaze tracking, which can be coupled with head tracking [25, 66, 71, 72]. One of the main uses that can be made of gaze/head tracking is to select the augmentations that will be displayed, such as the time-series specific to a piece of equipment and which is displayed only when the user aims at it, as shown by [PERSON] et al. [25]. This method can also be used to interact directly with the displayed augmentations through the creation of a virtual point where the user's gaze impacts with the  
Figure 6: (**a**) Handheld device [47]; (**b**) Occluded HMD example: HTC VIVE pro [83]; (**c**) See-through HMD example: Hololens 2 [82].  
augmentation (e.g., selecting the documentation to display with the Hololens by looking at a specific button [72]). The main advantage of this method is that it allows the user to interact with the digital environment, even if his or her hands are busy, or if the environment is too noisy for voice recognition. Some devices are emerging that directly integrate these features, such as Hololens 2 [82] and Vive Pro Eye [83].  
Even if these interactions are mainly used to select the augmentations to be displayed (e.g., augmented instruction and real-time data of a specific equipment), they can also be used to take control of the physical equipment more precisely (e.g., controlling the trajectory of an industrial crane by looking at a digital representation of its end point [26].