## 2 State of the Art
The result of that slicing process was then haptically presented to the user by selecting the slice associated to the user's palm position. From this work, two main limitations can be obtained. First, their system had the limitation of requiring a specific program for the generation of the 3D model, OpenSCAD. Second, their system was limited to iOS and MacOS devices, without offering possibilities for the integration of more devices. Other authors have created interaction metaphors to generate tools to manipulate digital elements using mid-air devices [44]. In [PERSON]'s work, he [45] goes one step further and removes the physical component of the interaction. Instead of having a real button, the element is presented as a haptic, augmented-reality button to the user. His work explores the idea about how to design visual and haptic interaction patterns. This work demonstrates that a physical component can be simulated through haptic interaction and that users are really able to interact with them. [PERSON] et al. [46] combined AR with mid-air haptics and conducted an evaluation with users. Their focus is on how gestures and actions are performed over the haptic device. Results show that the improved graphic components with haptic rendering happened to be more memorable. For the authors, this suggests that active exploration of haptic and AR content may be more convenient rather than in a purely receptive manner. This same train of thought has been followed in our investigation. Going forward with what will be indicated later, one of the things we seek is to know how the user explores AR models enhanced with haptic. [PERSON] et al. [47] conducted an experiment to analyze the influence of mid-air haptic feedback as supplementary feedback in Virtual Reality (VR) context. They found that visual feedback was preferred among the users as supplementary cue but that, actually, haptics did increase the accuracy when dealing with small targets. These results agree with those obtained through our validation with users. In one of the tasks (resizing an AR model with haptic clues), although the users had visual feedback, the haptic one has proved more useful in reducing errors. Mid-air haptic technologies can also be combined with gesture recognition techniques to generate attractive and powerful results. [PERSON] et al. [48] briefly explored the idea of incorporating mid-air haptic feedback to the AR experience using HoloLens and Ultrahaptics. Their work consisted of displaying a virtual sphere over an Ultrahaptics device with which the user can interact. That interaction was limited to exploring the collisions between the user's hand and the sphere. As a summary, we can draw attention over the following challenges. First, a standard way to generalize the development of applications that combine mid-air haptic interaction and AR has not been proposed. So far, each system creates an ad hoc solution. This is in part due to each AR and haptic device making use of its own development platform, which makes it difficult to establish a common framework. Second, at this moment, the generation of haptic feedback associated with AR content and its full potential has not been deeply explored. Based on these challenges, we propose an architecture that facilitates the generation and deployment of AR-haptic systems. First, it has been designed taking into account the hardware limitations of current haptic devices and that, in the future, new devices with greater potential may appear. Therefore, the architecture will be able to integrate those in a seamless way. Second, the architecture itself propitiates a workflow that encompasses the process of creating applications that combine AR and haptics. Moreover, as a result of implementations developed on the architecture, we have created a series of artifacts that can be easily integrated into future developments or adapted to new platforms. Finally, the data model that the architecture manages can be extended to more types of haptic devices, without this entailing many problems.