# Augmented Reality: Mapping Methods and Tools for Enhancing the Human Role in Healthcare HMI
## 4 Discussion
### Visualization  
The meticulous design of realistic perceptual hints is of paramount importance for the AR visualization of medical data since a wrong perception can lead to geometric ambiguities in object scale identification, make the interaction between virtual and real information more difficult, and thus hinder the usability of AR. Artificial information and real-world images are merged through video or optical display techniques. The three main solutions to implement these displays comprise HMDs, 2D monitors (mobile phones and tablets), and projectors. The AR projector was not used in any of the reviewed studies to superimpose virtual content over physical space. Although this alternative provides the ability for the user to observe the AR content with the naked eye, without using additional instrumentation, in most of the reviewed applications, a 2D display tool was used, whether smartphone [37, 39, 40, 57, 71, 83], tablet [76], or PC [79, 45, 50, 51, 52, 53, 54, 58, 59, 60, 61, 63, 69, 70, 79, 80, 82]. This choice is justified by the fact that the virtual content was not designed for real-time interaction with the user and/or real-world objects, but mostly to increase the amount of information available at the same time. Many other studies [41, 42, 43, 44, 46, 51, 55, 56, 62, 64, 65, 66, 67, 68, 69, 72, 75, 77, 81, 84], instead, benefit from an OST HMD to show information in the user's field of view. Certainly, their use as a simple display method is favored by some of their characteristics such as weight, size, and display resolution. Although in some studies it is not well specified which HMD was used, almost the totality of the works used first- and second-generation Microsoft HoloLens to achieve their purposes. In these works, the AR content was directly superimposed on the real-world visualization, so that the user had the possibility to directly interact with the holograms to visualize anatomical models for surgical planning, could identify access points to internal anatomical structures, could replace intraoperative fluoroscopy, could guide the insertion of surgical instruments, needles, wires, and screws, or for the training of doctors and the rehabilitation of patients. However, Microsoft HoloLens, like many commercial AR devices, is originally produced as a multimedia entertaining device, hence barely suitable for clinical applications since it was not designed to provide high-precision visualization [33, 34]. Indeed, the authors reported several problems with the use of HMDs. In particular, one of the main problems encountered concerns the spatial mapping of HoloLens [75]. HoloLens' sensor cameras acquire images relying on the surrounding light reflections [64], the variability of the brightness and contrast of the environment [67], but also based on the different colors and reflectivity of the object surface [31] and the distance from the object. Thus, the performance of its sensors relies on the quality and quantity of the visual features detected, along with the environment lighting, the real-world geometrical characteristics, and the behavior of the user [88]. This means that the sensors cannot maintain 3D spatial positioning in dark rooms [64] and that the depth detection accuracy of its sensors may depend on environmental conditions, thus leading to measurement errors [31]. Furthermore, while the first generation of HoloLens lacks eye-tracking capability, making it unsuitable for surgical navigation [64], this functionality was incorporated in HoloLens 2, although its effectiveness highly depends on the user's behavior [31]. In support of these claims, some authors have noted that holograms tend to drift [24, 66, 81, 89]. Since virtual vision depends on the position of the user's eyes, a slight shift in the position of the eyes relative to the HoloLens will cause the holograms to shift, so it is critically important that HoloLens has a fixed position on the head [89].