\section{Introduction}
\label{intro}

Providing haptic feedback during free-hand manipulation in \AR is not a trivial issue, as wearing haptic devices on the hand might affect the tracking capabilities of the system \cite{pacchierotti2016hring}.
Moreover, it is important to leave the user capable of interacting with both virtual and real objects, avoiding the use of haptic interfaces that cover the fingertips or palm.
For this reason, it is often considered beneficial to move the point of application of the haptic feedback elsewhere on the hand (\secref[related_work]{vhar_haptics}).
However, the impact of the positioning of the haptic feedback on the hand during direct hand manipulation in \AR has not been systematically studied.

Conjointly, a few studies have explored and compared the effects of visual and haptic feedback in tasks involving the manipulation of virtual objects with the hand.
\textcite{sarac2022perceived} and \textcite{palmer2022haptic} studied the effects of providing haptic feedback about contacts at the fingertips using haptic devices worn at the wrist, testing different mappings.
Their results proved that moving the haptic feedback away from the point(s) of contact is possible and effective, and that its impact is more significant in the absence of the visual feedback of the virtual hand.
A final question is whether one or the other of these (haptic or visual) hand feedback should be preferred \cite{maisto2017evaluation,meli2018combining}, or whether a combined visuo-haptic feedback is beneficial for users.
However, these studies were conducted in non-immersive setups, with a screen displaying the \VE view.
In fact, both hand feedback can provide sufficient sensory feedback for efficient direct hand manipulation of virtual objects in \AR, or conversely, they can be shown to be complementary.

In this chapter, we aim to investigate the role of \textbf{visuo-haptic feedback of the hand when manipulating virtual object} in immersive \OST-\AR using wearable vibrotactile haptics.
We selected \textbf{four different delocalized positionings on the hand} that have been previously proposed in the literature for direct hand interaction in \AR using wearable haptic devices (\secref[related_work]{vhar_haptics}): on the nails, the proximal phalanges, the wrist, and the nails of the opposite hand.
We focused on vibrotactile feedback, as it is used in most of the wearable haptic devices and has the lowest encumbrance.
In a \textbf{user study}, using the \OST-\AR headset Microsoft HoloLens~2 and two \ERM vibrotactile motors, we evaluated the effect of the four positionings with \textbf{two contact vibration techniques} on the user performance and experience with the same two manipulation tasks as in \chapref{visual_hand}.
We additionally compared these vibrotactile renderings with the \textbf{skeleton-like visual hand augmentation} established in the \chapref{visual_hand} as a complementary visuo-haptic feedback of the hand interaction with the virtual objects.

\noindentskip The contributions of this chapter are:
\begin{itemize}
  \item The evaluation in a user study with 20 participants of the effect of providing a vibrotactile feedback of the fingertip contacts with virtual objects, during direct manipulation with bare hand in \AR, at four different delocalized positionings of the haptic feedback on the hand and with two contact vibration techniques.
  \item The comparison of these vibrotactile positionings and renderings techniques with the two most representative visual hand augmentations established in the \chapref{visual_hand}.
\end{itemize}

\noindentskip In the next sections, we first describe the four delocalized positionings and the two contact vibration techniques we considered, based on previous work. We then present the experimental setup and design of the user study. Finally, we report the results and discuss them in the context of the free hand interaction with virtual content in \AR.

\bigskip

\fig[0.6]{method/locations}{Setup of the vibrotactile positionings on the hand.}[
  To ensure minimal encumbrance, we used the same two motors throughout the experiment, moving them to the considered positioning before each new experimental block (in this case, on the co-located \level{Proximal} phalange).
  Thin self-gripping straps were placed on the five considered positionings during the entirety of the experiment.
]
