%Providing haptic feedback during free-hand manipulation in \AR is not trivial, as wearing haptic devices on the hand might affect the tracking capabilities of the system.
%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 rendering elsewhere on the hand.% (\secref{haptics}).

%  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.
% 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 when the visual feedback is limited.
%A final question is whether one or the other of these (haptic or visual) hand renderings should be preferred \cite{maisto2017evaluation, meli2018combining}, or whether a combined visuo-haptic rendering is beneficial for users.
%In fact, both hand renderings can provide sufficient sensory cues for efficient manipulation of virtual objects in \AR, or conversely, they can be shown to be complementary.

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 \VOs, during direct manipulation with bare hand in \AR, at four different delocalized positionings of the haptic rendering 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 renderings of the hand established in the \chapref{visual_hand}.
\end{itemize}

\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 proximal phalanges, \emph{Prox}).
  Thin self-gripping straps were placed on the five considered positionings during the entirety of the experiment.
}
