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The integration of haptics within Augmented Reality may help to deliver an enriched experience, while facilitating the performance of specific actions (e.g. repositioning or resizin ) that are still dependent on the user's skills. This paper gathers the description of a flexible architecture designed to deploy haptically-enabled AR applications. The haptic feedback may be generated through a variety of devices (e.g., wearable, graspable, or mid-air ones), and the architecture facilitates handling the specificity of each. For this reason, it is discussed how to generate a haptic representation of a 3D digital object depending on the application and the target device. Additionally, it is included an analysis of practical, relevant issues that arise when setting up a system to work with specific devices like Head-Mounted Displays (e.g., HoloLens) and mid-air haptic devices (e.g., Ultrahaptics UHK), such as the alignment between the real world and the virtual one. The architecture applicability is demonstrated through the implementation of two applications: Form Inspector and Simon Game, built for HoloLens and iOS mobile phones for visualization and for UHK for mid-air haptics delivery. These applications have been used by nine users to explore the efficiency, meaningfulness, and usefulness of mid-air haptics for form perception, object resizing, and push interaction tasks. Results show that, although mobile interaction is preferred when this option is available, haptics turn out to be more meaningful in identifying shapes when compared to what users initially expect and in contributing to the execution of resizing tasks. Moreover, this preliminary user study reveals that users may be expecting a tailored interface metaphor, not necessarily inspired in natural interaction.

Augmented Reality (AR) functionalities may be effectively leveraged in collaborative service scenarios (e.g., remote maintenance, on-site building, street gaming, etc.). Standard development cycles for collaborative AR require to code for each specific visualization platform and implement the necessary control mechanisms over the shared assets. This paper describes SARA, an architecture to support cross-platform collaborative Augmented Reality applications based on microservices. The architecture is designed to work over the concept of collaboration models (turn, layer, ownership,hierarchy-based and unconstrained examples) which regulate the interaction and permissions of each user over the AR assets. Thanks to the reusability of its components, during the development of an application, SARA enables focusing on the application logic while avoiding the implementation of the communication protocol, data model handling and orchestration between the different, possibly heterogeneous,devices involved in the collaboration (i.e., mobile or wearable AR devices using different operating systems). To describe how to build an application based on SARA, a prototype for HoloLens and iOS devices has been implemented. the prototype is a collaborative voxel-based game in which several players work real time together on a piece of land, adding or eliminating cubes in a collaborative manner to create buildings and landscapes. Turn-based and unconstrained collaboration models are applied to regulate the interaction, the development workflow for this case study shows how the architecture serves as a framework to support the deployment of collaborative AR services, enabling the reuse of collaboration model components, agnostically handling client technologies.