\section{Conclusion}
\label{conclusion}

In this chapter, we addressed the challenge of touching, grasping and manipulating virtual objects directly with the hand in immersive \OST-\AR.
To do so, we proposed to evaluate visual renderings of the virtual hand as augmentation of the real hand.
Superimposed on the user's hand, these visual renderings provide feedback from the virtual hand, which tracks the real hand, and simulates the interaction with virtual objects as a proxy.
We first selected and compared the six most popular visual hand augmentations used to interact with virtual objects in \AR.
Then, in a user study with 24 participants and an immersive \OST-\AR headset, we evaluated the effect of these six visual hand augmentations on the user performance and experience in two representative manipulation tasks.

Our results showed that a visual hand augmentation improved the performance, perceived effectiveness and confidence of participants compared to no augmentation.
A skeleton rendering, which provided a detailed view of the tracked joints and phalanges while not hiding the real hand, was the most performant and effective.
The contour and mesh renderings were found to mask the real hand, while the tips rendering was controversial.
The occlusion rendering had too much tracking latency to be effective.
This is consistent with similar manipulation studies in \VR and in non-immersive \VST-\AR setups.

This study suggests that a \ThreeD visual hand augmentation is important in \AR when interacting with a virtual hand technique, particularly when it involves precise finger movements in relation to virtual content, \eg \ThreeD windows, buttons and sliders, or more complex tasks, such as stacking or assembly.
A minimal but detailed rendering of the virtual hand that does not hide the real hand, such as the skeleton rendering we evaluated, seems to be the best compromise between the richness and effectiveness of the feedback.
%Still, users should be able to choose and adapt the visual hand augmentation to their preferences and needs.

In addition to visual augmentation of the hand, direct manipulation of virtual objects with the hand can also benefit from wearable haptic feedback.
In the next chapter, we explore two wearable vibrotactile contact feedback devices in a user study, located at four positionings on the hand so as to not cover the fingertips.
We evaluate their effect on user performance and experience in the same manipulation tasks as in this chapter, with the best visual hand augmentation found in this study.

%\noindentskip This work was published in Transactions on Haptics:

%Erwan Normand, Claudio Pacchierotti, Eric Marchand, and Maud Marchal.
%\enquote{Visuo-Haptic Rendering of the Hand during 3D Manipulation in Augmented Reality}.
%In: \textit{IEEE Transactions on Haptics}. 27.4 (2024), pp. 2481--2487.
