## 2 State of the Art
The vibration of the device is perceived by the user through cutaneous perception that the user is accustomed to, such as when they receive an alert on the smartphone. However, this kind of device presents some limitations; for example, users may find it difficult to understand complicated vibration patterns. Some examples of this kind of device are vibration wristbands [19] (discontinued from 2018) [27]. Also, there are possibilities like fingertip devices [28] or smartphone vibration itself. * _Active surfaces_. This devices uses physical components to represent the feedback, for example, by using vertical bars. This kind of device is good for rendering surfaces, having nice resolution and accuracy. However, they have limited functionality due to the actuators required to perform the feedback. The biggest limitation of this kind of device is the lack of portability, since they are usually required to be installed in a certain manner. Some examples of this kind of device are Relief project [29] and Materiable [30]. * _Mid-air_ devices provide haptic feedback without requiring the user to wear any device (in contrast with devices such as gloves, bracelets, or rings). The main difference with the previous group is that, in the previous one, the user came into physical contact with the haptic device, while in this group, that interaction occurs in the air (hence, the name). There are different kinds of mid-air haptic devices, but they can be divided into three groups according to the technology they use: air vortices [31], air-jets [32], and ultrasounds [33; 34]. Ultrasound-based devices use ultrasonic speakers to focus sound waves at a point in the space above them. At the point where the waves converge, a pressure point is generated. That pressure point is strong enough to be noticed in the user's hand and, thus, to induce a tactile sensation. By changing the frequency of the ultrasounds, they are also able to control the strength of the sensation. A representative of this type of device is Ultrahaplics [33; 35]. Some works have already explored the idea of using those haptic points to render 3D objects [36]. In this case, to render means to dispose of the haptics points and to let the user perceive them with her hands. Some authors justify the use of technology based on air-vortex-ring generators. [PERSON] and [PERSON] [37] presented an approach to augment 2D and 3D displays with haptic capabilities. Their reasons for the justification were the limitations that ultrasound presented for their system: weak perceived sensation, limited range, and volume of interaction limited by the area of the array. The use of haptics to interact with digital assets has been in literature for HMD [38], physical traditional screens [39], and custom hardware solutions [40; 41]. In comparison, the idea of combining AR and MR with mid-air haptic feedback has not been so thoroughly explored. Moreover, literature related with this topic tends to be focused on the application stage. Some works are only focused on the haptic side of the equation, such as the work of [PERSON] [42]. In his work, he replaced visual aids with haptic feedback using Ultrahaplics UHDK5 device. These sensations that surround the hand like a haptic aura guided the user towards a physical control element with the objective to provide information about its functionality and type of control. Briefly, they made use of haptic feedback to provide information about a physical real-world element. In the same line of not incorporating AR information, [PERSON] and [PERSON] [43] presented the GraVVITAS system, a multi-modal system for presenting 2D graphics. The system they exposed had the limitation of not being able to display 3D graphics, and to solve this problem, they extended the system to integrate the Ultrahaplics UHEV1 device. In that system, a 3D model was generated using an external tool. In the next step, that 3D model was divided into slices according to its volume.