## 9 Conclusions and Future Work  
In this paper, we have presented our HARP system to enhance augmented reality perception by means of _harptic_ elements, i.e., by adding haptic representation to AR elements. As a result of the whole process, we have generated a set of libraries and artifacts ready to be incorporated into future developments, which will speed up the process. At the moment, these artifacts support systems that handle Unity platform and iOS devices. However, by following the established guidelines, it is easy to adapt the HARP architecture to other platforms. Besides that, the possibility of integrating new metaphors of interaction based on haptic feedback is also a promising field. In this paper, we have explored and made use of a literal definition of _haptic representation_, focusing the process on the mid-air context. However, it is possible to integrate another type of haptic feedback such as discrete vibration. One possible application of this would be to provide haptic feedback through devices such as smartphones or smartwatches, which are increasingly more accessible. To do this, it would be enough to update the logic of the _haptic connector_ component on the clients, and we are already taking steps in this direction.  
Moreover, both prototypes presented in this paper are limited to only one haptic device (Ultrahaptics UHDK5). Nevertheless, the HARP architecture allows adding more instances of the _Haptic client_, each one of them associated with its own haptic device and with its own implementation. This opens the door to future scenarios in which various haptic devices can cooperate. An example of this may be, for example, that while a user is receiving feedback from Ultrahaptics, more external information is provided through haptic clues by a smartwatch.  
The main objective of the user tests is to understand how haptic works to complement interactive systems based on AR. In this paper, we have focused on the users' performance when interacting with a haptic, mid-air device such as the Ultrahaptics. This implies a bias, since both prototypes used for the tests have the limitations associated with this type of system.  
Facing the future, there are two pillars we are working on. First, we are currently working on integrating this architecture into an bigger, external one, which is focused on collaboration between users in AR environments. Thanks to this integration, it will be possible to haptically augment arbitrary AR content to applications and systems that were not developed for that purpose. Second, we are considering the integration of more haptic devices, in this case, wristbands and smartphones. In addition to the use cases highlighted in this paper, we believe they can be used for more advanced interaction metaphors. A good example of this would be, for example, to use them to transmit texture information to the user. If the model to be inspected has a smooth texture, the haptic feedback could be imperceptible. However, if roughness is present, its degree could be transmitted by the vibration of the device.  
Both authors designed the prototypes and experiments in the paper, analysed the data and contributed to the writing. [PERSON] implemented the technical layer and conducted the user tests, while [PERSON] defined the initial research idea.  
This work was supported by UPM Project RP150955017 and by the Spanish Ministry of Economy and Competitiveness under grant TEC2017-88048-C2-1-R. Additionally, the authors want to thank the trial's participants and [PERSON] for her technical help.  
The authors declare no conflict of interest.