\section{Related Work}
\label{2_literature}

This Section summarizes the state of the art in visual hand rendering and (wearable) haptic rendering in AR, focusing on virtual object manipulation.

\subsection{Visual Hand Rendering in AR}
\label{2_hands}

Mutual visual occlusion between a virtual object and the real hand, \ie hiding the virtual object when the real hand is in front of it and hiding the real hand when it is behind the virtual object, is often presented as natural and realistic, enhancing the blending of real and virtual environments~\cite{piumsomboon2014graspshell, al-kalbani2016analysis}.
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In video see-through AR (VST-AR), this could be solved as a masking problem by combining the image of the real world captured by a camera and the generated virtual image~\cite{macedo2023occlusion}.
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In OST-AR, this is more difficult because the virtual environment is displayed as a transparent 2D image on top of the 3D real world, which cannot be easily masked~\cite{macedo2023occlusion}.
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Moreover, in VST-AR, the grip aperture and depth positioning of virtual objects often seem to be wrongly estimated~\cite{al-kalbani2016analysis, maisto2017evaluation}.
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However, this effect has yet to be verified in an OST-AR setup.

An alternative is to render the virtual objects and the hand semi-transparents, so that they are partially visible even when one is occluding the other, \eg in \figref{hands-none} the real hand is behind the virtual cube but still visible.
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Although perceived as less natural, this seems to be preferred to a mutual visual occlusion in VST-AR~\cite{buchmann2005interaction, ha2014wearhand, piumsomboon2014graspshell} and VR~\cite{vanveldhuizen2021effect}, but has not yet been evaluated in OST-AR.
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However, this effect still causes depth conflicts  that make it difficult to determine if one's hand is behind or in front of a virtual object, \eg in \figref{hands-none} the thumb is in front of the virtual cube, but it appears to be behind it.

In VR, as the user is fully immersed in the virtual environment and cannot see their real hands, it is necessary to represent them virtually.
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It is known that the virtual hand representation has an impact on perception, interaction performance, and preference of users~\cite{prachyabrued2014visual, argelaguet2016role, grubert2018effects, schwind2018touch}.
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In a pick-and-place task in VR, \textcite{prachyabrued2014visual} found that the virtual hand representation whose motion was constrained to the surface of the virtual objects performed the worst, while the virtual hand representation following the tracked human hand (thus penetrating the virtual objects), performed the best, even though it was rather disliked.
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The authors also observed that the best compromise was a double rendering, showing both the tracked hand and a hand rendering constrained by the virtual environment.
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It has also been shown that over a realistic avatar, a skeleton rendering (similar to \figref{hands-skeleton}) can provide a stronger sense of being in control~\cite{argelaguet2016role} and that minimalistic fingertip rendering (similar to \figref{hands-tips}) can be more effective in a typing task~\cite{grubert2018effects}.

In AR, as the real hand of a user is visible but not physically constrained by the virtual environment, adding a visual hand rendering that can physically interact with virtual objects would achieve a similar result to the promising double-hand rendering of \textcite{prachyabrued2014visual}.
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Additionally, \textcite{kahl2021investigation} showed that a virtual object overlaying a tangible object in OST-AR can vary in size without worsening the users' experience nor the performance.
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This suggests that a visual hand rendering superimposed on the real hand could be helpful, but should not impair users.

Few works have explored the effect of visual hand rendering in AR~\cite{blaga2017usability, maisto2017evaluation, krichenbauer2018augmented, yoon2020evaluating, saito2021contact}.
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For example, \textcite{blaga2017usability} evaluated a skeleton rendering in several virtual object manipulations against no visual hand overlay.
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Performance did not improve, but participants felt more confident with the virtual hand.
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However, the experiment was carried out on a screen, in a non-immersive AR scenario.
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\textcite{saito2021contact} found that masking the real hand with a textured 3D opaque virtual hand did not improve performance in a reach-to-grasp task but displaying the points of contact on the virtual object did.
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To the best of our knowledge, evaluating the role of a visual rendering of the hand displayed \enquote{and seen} directly above real tracked hands in immersive OST-AR has not been explored, particularly in the context of virtual object manipulation.


\subsection{Wearable Haptic Feedback in AR}
\label{2_haptics}

Different haptic feedback systems have been explored to improve interactions in AR, including %
grounded force feedback devices~\cite{bianchi2006high, jeon2009haptic, knorlein2009influence}, %
exoskeletons~\cite{lee2021wearable}, %
tangible objects~\cite{hettiarachchi2016annexing, detinguy2018enhancing, salazar2020altering, normand2018enlarging, xiao2018mrtouch}, and %
wearable haptic devices~\cite{pacchierotti2016hring, lopes2018adding, pezent2019tasbi, teng2021touch}.

Wearable haptics seems particularly suited for this context, as it takes into account many of the AR constraints, \eg limited impact on hand tracking performance and reduced impairment of the senses and ability of the users to interact with real content~\cite{pacchierotti2016hring, maisto2017evaluation, lopes2018adding, meli2018combining, pezent2019tasbi, teng2021touch, kourtesis2022electrotactile, marchal2022virtual}.
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For example, \textcite{pacchierotti2016hring} designed a haptic ring providing pressure and skin stretch sensations to be worn at the proximal finger phalanx, so as to improve the hand tracking during a pick-and-place task.
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\textcite{pezent2019tasbi} proposed Tasbi: a wristband haptic device capable of rendering vibrations and pressures.
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\textcite{teng2021touch} presented Touch\&Fold, a haptic device attached to the nail that provides pressure and texture sensations when interacting with virtual content, but also folds away when the user interacts with real objects, leaving the fingertip free.
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This approach was also perceived as more realistic than providing sensations directly on the nail, as in~\cite{ando2007fingernailmounted}.
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Each of these haptic devices provided haptic feedback about fingertip interactions with the virtual content on other parts of the hand.
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If it is indeed necessary to delocalize the haptic feedback, each of these positions is promising, and they have not yet been compared with each other.

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.
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\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.
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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.
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In pick-and-place tasks in AR involving both virtual and real objects, \textcite{maisto2017evaluation} and \textcite{meli2018combining} showed that having a haptic {rendering of the} fingertip interactions with the virtual objects led to better performance and perceived effectiveness than having only a visual rendering of the hand, similar to \figref{hands-tips}.
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Moreover, employing the haptic ring of~\cite{pacchierotti2016hring} on the proximal finger phalanx led to an improved performance with respect to more standard fingertip haptic devices~\cite{chinello2020modular}.
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However, the measured difference in performance could be attributed to either the device or the device position (proximal vs fingertip), or both.
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Furthermore, all of these studies were conducted in non-immersive setups, where users looked at a screen displaying the visual interactions, and only compared haptic and visual feedback, but did not examine them together.
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The improved performance and perceived effectiveness of a delocalized haptic feedback over a visual feedback alone, or their multimodal combination, remains to be verified in an immersive OST-AR setup.

\bigskip

These limitations have motivated the research presented in this paper, where we aim at filling the abovementioned gaps related to the understanding of the role of the visuo-haptic rendering of the hand during the 3D manipulation of virtual objects in AR .
