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1-background/related-work/3-augmented-reality.tex
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\section{Manipulating Objects with the Hands in AR}
\label{augmented_reality}
As with haptic systems (\secref{wearable_haptics}), visual \AR devices generate and integrate virtual content into the user's perception of the \RE, creating the illusion of the presence of the virtual.
Immersive systems such as headsets leave the hands free to interact with \VOs, promising natural and intuitive interactions similar to those with everyday real objects.
%As with haptic systems (\secref{wearable_haptics}), visual
\AR devices generate and integrate virtual content into the user's perception of their real environment (\RE), creating the illusion of the \emph{presence} of the virtual \cite{azuma1997survey,skarbez2021revisiting}.
Immersive systems such as headsets leave the hands free to interact with virtual objects (\VOs), promising natural and intuitive interactions similar to those with everyday real objects \cite{billinghurst2021grand,hertel2021taxonomy}.
%\begin{subfigs}{sutherland1968headmounted}{Photos of the first \AR system \cite{sutherland1968headmounted}. }[
% \item The \AR headset.
@@ -17,7 +18,7 @@ Immersive systems such as headsets leave the hands free to interact with \VOs, p
\label{what_is_ar}
The first \AR headset was invented by \textcite{sutherland1968headmounted}: With the technology available at the time, it was already capable of displaying \VOs at a fixed point in space in real time, giving the user the illusion that the content was present in the room.
Fixed to the ceiling, the headset displayed a stereoscopic (one image per eye) perspective projection of the virtual content on a transparent screen, taking into account the user's position, and thus already following the interaction loop presented in \figref[introduction]{interaction-loop}.
Fixed to the ceiling, the headset displayed a stereoscopic (one image per eye) perspective projection of the virtual content on a transparent screen, taking into account the user's position, and thus already following our interaction loop presented in \figref[introduction]{interaction-loop}.
\subsubsection{A Definition of AR}
\label{ar_definition}
@@ -26,8 +27,8 @@ Fixed to the ceiling, the headset displayed a stereoscopic (one image per eye) p
The first formal definition of \AR was proposed by \textcite{azuma1997survey}: (1) combine real and virtual, (2) be interactive in real time, and (3) register real and virtual\footnotemark.
Each of these characteristics is essential: the real-virtual combination distinguishes \AR from \VR, a movie with integrated digital content is not interactive and a \TwoD overlay like an image filter is not registered.
There are also two key aspects to this definition: it does not focus on technology or method, but on the user's perspective of the system experience, and it does not specify a particular human sense, \ie it can be auditory \cite{yang2022audio}, haptic \cite{bhatia2024augmenting}, or even olfactory \cite{brooks2021stereosmell} or gustatory \cite{brooks2023taste}.
Yet, most of the research have focused on visual augmentations, and the term \AR (without a prefix) is almost always understood as visual \AR.
There are also two key aspects of this definition: it does not focus on technology or method, but on the user's perspective of the system experience, and it does not specify a particular human sense, \ie it can be auditory \cite{yang2022audio}, haptic \cite{bhatia2024augmenting}, or even olfactory \cite{brooks2021stereosmell} or gustatory \cite{brooks2023taste}.
Yet, most research has focused on visual augmentation, and the term \AR (without a prefix) is almost always understood as visual \AR.
\footnotetext{This third characteristic has been slightly adapted to use the version of \textcite{marchand2016pose}, the original definition was: \enquote{registered in \ThreeD}.}
%For example, \textcite{milgram1994taxonomy} proposed a taxonomy of \MR experiences based on the degree of mixing real and virtual environments, and \textcite{skarbez2021revisiting} revisited this taxonomy to include the user's perception of the experience.
@@ -35,11 +36,11 @@ Yet, most of the research have focused on visual augmentations, and the term \AR
\subsubsection{Applications of AR}
\label{ar_applications}
Advances in technology, research and development have enabled many usages of \AR, including medicine, education, industrial, navigation, collaboration and entertainment applications \cite{dey2018systematic}.
For example, \AR can provide surgery training simulations in safe conditions \cite{harders2009calibration} (\figref{harders2009calibration}), or improve the learning of students with complex concepts and phenomena such as optics or chemistry \cite{bousquet2024reconfigurable}.
Advances in technology, research, and development have enabled many uses of \AR, including medical, educational, industrial, navigation, collaboration, and entertainment applications \cite{dey2018systematic}.
For example, \AR can provide surgical training simulations in safe conditions \cite{harders2009calibration} (\figref{harders2009calibration}), or improve student learning of complex concepts and phenomena such as optics or chemistry \cite{bousquet2024reconfigurable}.
It can also guide workers in complex tasks, such as assembly, maintenance or verification \cite{hartl2013mobile} (\figref{hartl2013mobile}), reinvent the way we interact with desktop computers \cite{lee2013spacetop} (\figref{lee2013spacetop}), or can create complete new forms of gaming or tourism experiences \cite{roo2017inner} (\figref{roo2017inner}).
Most of (visual) \AR/\VR experience can now be implemented with commercially available hardware and software solutions, in particular for tracking, rendering and display.
Yet, the user experience in \AR is still highly dependent on the display used.
Most of (visual) \AR/\VR experiences can now be implemented with commercially available hardware and software solutions, especially for tracking, rendering and display.
However, the user experience in \AR is still highly dependent on the display used.
\begin{subfigs}{ar_applications}{Examples of \AR applications. }[][
\item Visuo-haptic surgery training with cutting into virtual soft tisues \cite{harders2009calibration}.
@@ -57,7 +58,8 @@ Yet, the user experience in \AR is still highly dependent on the display used.
\subsubsection{AR Displays}
\label{ar_displays}
To experience a virtual content combined and registered with the \RE, an output \UI that display the \VE to the user is necessary.
To experience a virtual content combined and registered with the \RE, an output device that display the \VE to the user is necessary.
%An output device is more formally defined as an output \emph{\UI}
There is a large variety of \AR displays with different methods of combining the real and virtual content, and different locations on the \RE or the user \cite[p.126]{billinghurst2015survey}.
In \emph{\VST-\AR}, the virtual images are superimposed to images of the \RE captured by a camera \cite{marchand2016pose}, and the combined real-virtual image is displayed on a screen to the user, as illustrated in \figref{itoh2022indistinguishable_vst}, \eg \figref{hartl2013mobile}.
@@ -96,22 +98,24 @@ Finally, \AR displays can be head-worn like \VR \emph{headsets} or glasses, prov
Presence and embodiment are two key concepts that characterize the user experience in \AR and \VR.
While there is a large literature on these topics in \VR, they are less defined and studied for \AR \cite{genay2022being,tran2024survey}.
Still, these concepts are useful to design, evaluate and discuss our contributions in the next chapters.
These concepts will be useful for the design, evaluation, and discussion of our contributions:
In particular, we will investigate the effect of the visual feedback of the virtual hand when touching haptic texture augmentation (\chapref{xr_perception}) and manipulating \VOs (\chapref{visual_hand}), and explore the plausibility of visuo-haptic textures (\chapref{visuo_haptic}).
\paragraph{Presence}
\label{ar_presence}
\AR and \VR are both essentially illusions as the virtual content does not physically exist but is just digitally simulated and rendered to the user's senses through display \UIs.
Such experience of disbelief suspension in \VR is what is called \emph{presence}, and it can be decomposed into two dimensions: place illusion and plausibility \cite{slater2009place}.
Place illusion is the sense of the user of \enquote{being there} in the \VE (\figref{presence-vr}).
\AR and \VR are both essentially illusions as the virtual content does not physically exist but is just digitally simulated and rendered to the user's senses through display devices.
Such experience of disbelief suspension in \VR is what is called \emph{presence}, and it can be decomposed into two dimensions: place illusion and plausibility \cite{slater2009place,slater2022separate}.
\emph{Place illusion} is the sense of the user of \enquote{being there} in the \VE (\figref{presence-vr}).
It emerges from the real time rendering of the \VE from the user's perspective: to be able to move around inside the \VE and look from different point of views.
Plausibility is the illusion that the virtual events are really happening, even if the user knows that they are not real.
It doesn't mean that the virtual events are realistic, but that they are plausible and coherent with the user's expectations.
\emph{Plausibility} is the illusion that the virtual events are really happening, even if the user knows that they are not real.
It doesn't mean that the virtual events are realistic, \ie that reproduce the real world with high fidelity \cite{skarbez2017survey}, but that they are believable and coherent with the user's expectations.
In the same way, a film can be plausible even if it is not realistic, such as a cartoon or a science-fiction movie.
%The \AR presence is far less defined and studied than for \VR \cite{tran2024survey}
For \AR, \textcite{slater2022separate} proposed to invert place illusion to what we can call \enquote{object illusion}, \ie the sense of the \VO to \enquote{feels here} in the \RE (\figref{presence-ar}).
As with \VR, \VOs must be able to be seen from different angles by moving the head but also, this is more difficult, be consistent with the \RE, \eg occlude or be occluded by real objects \cite{macedo2023occlusion}, cast shadows or reflect lights.
The plausibility can be applied to \AR as is, but the \VOs must additionally have knowledge of the \RE and react accordingly to it.
As with \VR, \VOs must be able to be seen from different angles by moving the head, but also, this is more difficult, appear to be coherent enough with the \RE \cite{skarbez2021revisiting}, \eg occlude or be occluded by real objects \cite{macedo2023occlusion}, cast shadows or reflect lights.
The plausibility can be applied to \AR as is, but the \VOs must additionally have knowledge of the \RE and react accordingly to it to be, again, perceived as coherently behaving with the real world \cite{skarbez2021revisiting}.
%\textcite{skarbez2021revisiting} also named place illusion for \AR as \enquote{immersion} and plausibility as \enquote{coherence}, and these terms will be used in the remainder of this thesis.
%One main issue with presence is how to measure it both in \VR \cite{slater2022separate} and \AR \cite{tran2024survey}.
@@ -129,7 +133,7 @@ The plausibility can be applied to \AR as is, but the \VOs must additionally hav
\paragraph{Embodiment}
\label{ar_embodiment}
The sense of embodiment is the \enquote{subjective experience of using and having a body} \cite{blanke2009fullbody}, \ie the feeling that a body is our own.
The \emph{sense of embodiment} is the \enquote{subjective experience of using and having a body} \cite{blanke2009fullbody}, \ie the feeling that a body is our own.
In everyday life, we are used to being, seeing and controlling our own body, but it is possible to embody a virtual body as an avatar while in \AR \cite{genay2022being} or \VR \cite{guy2023sense}.
This illusion arises when the visual, proprioceptive and (if any) haptic sensations of the virtual body are coherent \cite{kilteni2012sense}.
It can be decomposed into three subcomponents: \emph{Agency}, which is the feeling of controlling the body; \emph{Ownership}, which is the feeling that \enquote{the body is the source of the experienced sensations}; and \emph{Self-Location}, which is the feeling \enquote{spatial experience of being inside [the] body} \cite{kilteni2012sense}.
@@ -138,19 +142,22 @@ In \AR, it could take the form of body accessorization, \eg wearing virtual clot
\subsection{Direct Hand Manipulation in AR}
\label{ar_interaction}
A user in \AR must be able to interact with the virtual content to fulfil the second point of \textcite{azuma1997survey}'s definition (\secref{ar_definition}) and complete the interaction loop (\figref[introduction]{interaction-loop}). %, \eg through a hand-held controller, a real object, or even directly with the hands.
A user in \AR must be able to interact with the virtual content to fulfil the second point of \textcite{azuma1997survey}'s definition (\secref{ar_definition}) and complete our proposed visuo-haptic interaction loop (\figref[introduction]{interaction-loop}). %, \eg through a hand-held controller, a real object, or even directly with the hands.
In all examples of \AR applications shown in \secref{ar_applications}, the user interacts with the \VE using their hands, either directly or through a physical interface.
\subsubsection{User Interfaces and Interaction Techniques}
\subsubsection{User Inputs and Interaction Techniques}
\label{interaction_techniques}
For a user to interact with a computer system (desktop, mobile, \AR, etc.), they first perceive the state of the system and then acts upon it through an input \UI.
For a user to interact with a computer system (desktop, mobile, \AR, etc.), they first perceive the state of the system and then acts upon it through an input device \cite[p.145]{laviolajr20173d}.
Such input devices form an input \emph{\UI} that captures and translates user's actions to the computer.
Similarly, an output \UI render and display the state of the system to the user (such as a \AR/\VR display, \secref{ar_display}, or an haptic actuator, \secref{wearable_haptic_devices}).
Inputs \UI can be either an \emph{active sensing}, a held or worn device, such as a mouse, a touch screen, or a hand-held controller, or a \emph{passive sensing}, that does not require a contact, such as eye trackers, voice recognition, or hand tracking \cite[p.294]{laviolajr20173d}.
The information gathered from the sensors by the \UI is then translated into actions within the computer system by an \emph{interaction technique} (\figref{interaction-technique}).
The captured information from the sensors is then translated into actions within the computer system by an \emph{interaction technique}. %(\figref{interaction-technique}).
For example, a cursor on a screen can be moved using either with a mouse or with the arrow keys on a keyboard, or a two-finger swipe on a touchscreen can be used to scroll or zoom an image.
Choosing useful and efficient \UIs and interaction techniques is crucial for the user experience and the tasks that can be performed within the system.
\fig[0.5]{interaction-technique}{An interaction technique map user inputs to actions within a computer system. Adapted from \textcite{billinghurst2005designing}.}
%\fig[0.5]{interaction-technique}{An interaction technique map user inputs to actions within a computer system. Adapted from \textcite{billinghurst2005designing}.}
\subsubsection{Tasks with Virtual Environments}
\label{ve_tasks}
@@ -170,28 +177,30 @@ Wayfinding is the cognitive planning of the movement, such as path finding or ro
The \emph{system control tasks} are changes to the system state through commands or menus such as creating, deleting, or modifying \VOs, \eg as in \figref{roo2017onea}. It is also the input of text, numbers, or symbols.
In this thesis we focus on manipulation tasks of virtual content directly with the hands, more specifically on touching visuo-haptic textures with a finger (\partref{perception}) and positioning and rotating \VOs pushed and grasp by the hand.
\begin{subfigs}{interaction-techniques}{Interaction techniques in \AR. }[][
\item Spatial selection of virtual item of an extended display using a hand-held smartphone \cite{grubert2015multifi}.
\item Displaying as an overlay registered on the \RE the route to follow \cite{grubert2017pervasive}.
\item Virtual drawing on a real object with a hand-held pen \cite{roo2017onea}.
\item Simultaneous Localization and Mapping (SLAM) algorithms such as KinectFusion \cite{newcombe2011kinectfusion} reconstruct the \RE in real time and enables to register the \VE in it.
]
\subfigsheight{36mm}
\subfig{grubert2015multifi}
\subfig{grubert2017pervasive}
\subfig{roo2017onea}
\subfig{newcombe2011kinectfusion}
\subfigsheight{35.5mm}
\subfigbox{grubert2015multifi}
\subfigbox{grubert2017pervasive}
\subfigbox{roo2017onea}
\subfigbox{newcombe2011kinectfusion}
\end{subfigs}
\subsubsection{Reducing the Real-Virtual Gap}
\subsubsection{The Gap between Real and Virtual}
\label{real_virtual_gap}
In \AR and \VR, the state of the system is displayed to the user as a \ThreeD spatial \VE.
In an immersive and portable \AR system, this \VE is experienced at a 1:1 scale and as an integral part of the \RE.
The rendering gap between the real and virtual elements, as described on the interaction loop in \figref[introduction]{interaction-loop}, is thus experienced as narrow or even not consciously perceived by the user.
The rendering gap between the real and virtual elements, as described on our interaction loop in \figref[introduction]{interaction-loop}, is thus experienced as narrow or even not consciously perceived by the user.
This manifests as a sense of presence of the virtual, as described in \secref{ar_presence}.
As the gap between real and virtual rendering is reduced, one could expects a similar and seamless interaction with the \VE as with a \RE, which \textcite{jacob2008realitybased} called \emph{reality based interactions}.
As the gap between real and virtual rendering is reduced, one could expect a similar and seamless interaction with the \VE as with a \RE, which \textcite{jacob2008realitybased} called \emph{reality based interactions}.
As of today, an immersive \AR system tracks itself with the user in \ThreeD, using tracking sensors and pose estimation algorithms \cite{marchand2016pose}, \eg as in \figref{newcombe2011kinectfusion}.
It enables the \VE to be registered with the \RE and the user simply moves to navigate within the virtual content.
%This tracking and mapping of the user and \RE into the \VE is named the \enquote{extent of world knowledge} by \textcite{skarbez2021revisiting}, \ie to what extent the \AR system knows about the \RE and is able to respond to changes in it.
@@ -212,13 +221,13 @@ The issue with these \emph{space-multiplexed} interfaces is the large number and
An alternative is to use a single \emph{universal} tangible object like a hand-held controller, such as a cube \cite{issartel2016tangible} or a sphere \cite{englmeier2020tangible}.
These \emph{time-multiplexed} interfaces require interaction techniques that allow the user to pair the tangible with any \VO, \eg by placing the tangible into the \VO and pressing the fingers \cite{issartel2016tangible} (\figref{issartel2016tangible}), similar to a real grasp (\secref{grasp_types}).
Still, the virtual visual rendering and the real haptic sensations can be inconsistent.
Still, the virtual visual rendering and the real haptic sensations can be incoherent.
Especially in \OST-\AR, since the \VOs are inherently slightly transparent allowing the paired real objects to be seen through them.
In a pick-and-place task with real objects, a difference in size \cite{kahl2021investigation} (\figref{kahl2021investigation}) and shape \cite{kahl2023using} (\figref{kahl2023using_1}) of the \VOs does not affect user performance or presence, and that small variations (\percent{\sim 10} for size) were not even noticed by the users.
This suggests the feasibility of using simplified real obejcts in \AR whose spatial properties (\secref{object_properties}) abstract those of the \VOs.
Similarly, in \secref{tactile_rendering} we described how a material property (\secref{object_properties}) of a touched real object can be modified using wearable haptic devices \cite{detinguy2018enhancing,salazar2020altering}: It could be used to render coherent visuo-haptic material perceptions directly touched with the hand in \AR.
Similarly, in \secref{tactile_rendering} we described how a material property (\secref{object_properties}) of a touched real object can be modified using wearable haptic devices \cite{detinguy2018enhancing,salazar2020altering}: it could be used to render coherent visuo-haptic material perceptions directly touched with the hand in \AR.
\begin{subfigs}{ar_tangibles}{Manipulating \VOs with tangibles. }[][
\begin{subfigs}{ar_tangibles}{Manipulating \VOs through real objects. }[][
\item Ubi-Touch paired the movements and screw interaction of a virtual drill with a real vaporizer held by the user \cite{jain2023ubitouch}.
\item A real cube that can be moved into the \VE and used to grasp and manipulate \VOs \cite{issartel2016tangible}.
\item Size and
@@ -234,13 +243,12 @@ Similarly, in \secref{tactile_rendering} we described how a material property (\
\subsubsection{Manipulating with Virtual Hands}
\label{ar_virtual_hands}
Natural \UIs allow the user to use their body movements directly as inputs to the \VE, as defined by \textcite[p.172]{billinghurst2015survey}.
In daily life, our hands allow us to manipulate real objects with both strength and precision (\secref{grasp_types}), so virtual hand interaction techniques seem to be the most natural way to manipulate virtual objects \cite[p.400]{laviolajr20173d}.
It is also called mid-air interaction.
Initially tracked by active sensing devices such as gloves or controllers, it is now possible to track hands in real time using cameras and computer vision algorithms natively integrated into \AR/\VR headsets \cite{tong2023survey}.
%can track the user's movements and use them as inputs to the \VE \textcite[p.172]{billinghurst2015survey}.
Initially tracked by active sensing devices such as gloves or controllers, it is now possible to track hands in real time using passive sensing (\secref{interaction_techniques}) and computer vision algorithms natively integrated into \AR/\VR headsets \cite{tong2023survey}.
Our hands allow us to manipulate real everyday objects (\secref{grasp_types}), so virtual hand interaction techniques seem to be the most natural way to manipulate virtual objects \cite[p.400]{laviolajr20173d}.
The user's hand is therefore tracked and reconstructed as a \emph{virtual hand} model in the \VE \cite[p.405]{laviolajr20173d}.
The simplest models represent the hand as a rigid \ThreeD object that follows the movements of the real hand with \qty{6}{\DoF} (position and orientation in space) \cite{talvas2012novel}.
The user's hand being tracked is reconstructed as a \emph{virtual hand} model in the \VE \cite[p.405]{laviolajr20173d}.
The simplest models represent the hand as a rigid \ThreeD object that follows the movements of the real hand with \qty{6}{DoF} (position and orientation in space) \cite{talvas2012novel}.
An alternative is to model only the fingertips (\figref{lee2007handy}) or the whole hand (\figref{hilliges2012holodesk_1}) as points.
The most common technique is to reconstruct all the phalanges of the hand in an articulated kinematic model (\secref{hand_anatomy}) \cite{borst2006spring}.
@@ -248,7 +256,7 @@ The contacts between the virtual hand model and the \VOs are then simulated usin
Heuristic techniques use rules to determine the selection, manipulation and release of a \VO (\figref{piumsomboon2013userdefined_1}).
However, they produce unrealistic behaviour and are limited to the cases predicted by the rules.
Physics-based techniques simulate forces at the points of contact between the virtual hand and the \VO.
In particular, \textcite{borst2006spring} have proposed an articulated kinematic model in which each phalanx is a rigid body simulated with the god-object method \cite{zilles1995constraintbased}:
In particular, \textcite{borst2006spring} proposed an articulated kinematic model in which each phalanx is a rigid body simulated with the god-object method \cite{zilles1995constraintbased}:
The virtual phalanx follows the movements of the real phalanx, but remains constrained to the surface of the \VOs during contact.
The forces acting on the object are calculated as a function of the distance between the real and virtual hands (\figref{borst2006spring}).
More advanced techniques simulate the friction phenomena \cite{talvas2013godfinger} and finger deformations \cite{talvas2015aggregate}, allowing highly accurate and realistic interactions, but which can be difficult to compute in real time.
@@ -260,28 +268,28 @@ More advanced techniques simulate the friction phenomena \cite{talvas2013godfing
\item A kinematic hand model with rigid-body phalanges (in beige) that follows the real tracked hand (in green) but kept physically constrained to the \VO. Applied forces are shown as red arrows \cite{borst2006spring}.
]
\subfigsheight{37mm}
\subfig{lee2007handy}
\subfig{hilliges2012holodesk_1}
\subfig{piumsomboon2013userdefined_1}
\subfig{borst2006spring}
\subfigbox{lee2007handy}
\subfigbox{hilliges2012holodesk_1}
\subfigbox{piumsomboon2013userdefined_1}
\subfigbox{borst2006spring}
\end{subfigs}
However, the lack of physical constraints on the user's hand movements makes manipulation actions tiring \cite{hincapie-ramos2014consumed}.
While the user's fingers traverse the \VO, a physics-based virtual hand remains in contact with the object, a discrepancy that may degrade the user's performance in \VR \cite{prachyabrued2012virtual}.
Finally, in the absence of haptic feedback on each finger, it is difficult to estimate the contact and forces exerted by the fingers on the object during grasping and manipulation \cite{maisto2017evaluation,meli2018combining}.
While a visual rendering of the virtual hand in \VR can compensate for these issues \cite{prachyabrued2014visual}, the visual and haptic rendering of the virtual hand, or their combination, in \AR is under-researched.
While a visual feedback of the virtual hand in \VR can compensate for these issues \cite{prachyabrued2014visual}, the visual and haptic feedback of the virtual hand, or their combination, in \AR needs to be investigated as well.
\subsection{Visual Rendering of Hands in AR}
\subsection{Visual Feedback of Virtual Hands in AR}
\label{ar_visual_hands}
In \VR, since the user is fully immersed in the \VE and cannot see their real hands, it is necessary to represent them virtually (\secref{ar_embodiment}).
When interacting with a physics-based virtual hand method (\secref{ar_virtual_hands}), the visual rendering of the virtual hand has an influence on perception, interaction performance, and preference of users \cite{prachyabrued2014visual,argelaguet2016role,grubert2018effects,schwind2018touch}.
In a pick-and-place manipulation task in \VR, \textcite{prachyabrued2014visual} and \textcite{canales2019virtual} found that the visual hand rendering whose motion was constrained to the surface of the \VOs similar as to \textcite{borst2006spring} (\enquote{Outer Hand} in \figref{prachyabrued2014visual}) performed the worst, while the visual hand rendering following the tracked human hand (thus penetrating the \VOs, \enquote{Inner Hand} in \figref{prachyabrued2014visual}) performed the best, though it was rather disliked.
\textcite{prachyabrued2014visual} also found that the best compromise was a double rendering, showing both the virtual hand and the tracked hand (\enquote{2-Hand} in \figref{prachyabrued2014visual}).
%In \VR, since the user is fully immersed in the \VE and cannot see their real hands, it is necessary to represent them virtually (\secref{ar_embodiment}).
When interacting with a physics-based virtual hand method (\secref{ar_virtual_hands}) in \VR, the visual feedback of the virtual hand has an influence on perception, interaction performance, and preference of users \cite{prachyabrued2014visual,argelaguet2016role,grubert2018effects,schwind2018touch}.
In a pick-and-place manipulation task in \VR, \textcite{prachyabrued2014visual} and \textcite{canales2019virtual} found that the visual hand feedback whose motion was constrained to the surface of the \VOs similar as to \textcite{borst2006spring} (\enquote{Outer Hand} in \figref{prachyabrued2014visual}) performed the worst, while the visual hand feedback following the tracked human hand (thus penetrating the \VOs, \enquote{Inner Hand} in \figref{prachyabrued2014visual}) performed the best, though it was rather disliked.
\textcite{prachyabrued2014visual} also found that the best compromise was a double feedback, showing both the virtual hand and the tracked hand (\enquote{2-Hand} in \figref{prachyabrued2014visual}).
While a realistic rendering of the human hand increased the sense of ownership \cite{lin2016need}, a skeleton-like rendering provided a stronger sense of agency \cite{argelaguet2016role} (\secref{ar_embodiment}), and a minimalist fingertip rendering reduced typing errors \cite{grubert2018effects}.
A visual hand rendering while in \VE also seems to affect how one grasps an object \cite{blaga2020too}, or how real bumps and holes are perceived \cite{schwind2018touch}.
A visual hand feedback while in \VE also seems to affect how one grasps an object \cite{blaga2020too}, or how real bumps and holes are perceived \cite{schwind2018touch}.
\fig{prachyabrued2014visual}{Visual hand renderings affect user experience in \VR \cite{prachyabrued2014visual}.}
\fig{prachyabrued2014visual}{Visual hand feedback affect user experience in \VR \cite{prachyabrued2014visual}.}
Conversely, a user sees their own hands in \AR, and the mutual occlusion between the hands and the \VOs is a common issue (\secref{ar_displays}), \ie hiding the \VO when the real hand is in front of it, and hiding the real hand when it is behind the \VO (\figref{hilliges2012holodesk_2}).
%For example, in \figref{hilliges2012holodesk_2}, the user is pinching a virtual cube in \OST-\AR with their thumb and index fingers, but while the index is behind the cube, it is seen as in front of it.
@@ -292,25 +300,25 @@ While in \VST-\AR, this could be solved as a masking problem by combining the re
%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.
%However, this effect still causes depth conflicts that make it difficult to determine if one's hand is behind or in front of a \VO, \eg the thumb is in front of the virtual cube, but could be perceived to be behind it.
Since the \VE is intangible, adding a visual rendering of the virtual hand in \AR that is physically constrained to the \VOs would achieve a similar result to the promising double-hand rendering of \textcite{prachyabrued2014visual}.
Since the \VE is intangible, adding a visual feedback of the virtual hand in \AR that is physically constrained to the \VOs would achieve a similar result to the double-hand feedback of \textcite{prachyabrued2014visual}.
A \VO overlaying a real object object in \OST-\AR can vary in size and shape without degrading user experience or manipulation performance \cite{kahl2021investigation,kahl2023using}.
This suggests that a visual hand rendering superimposed on the real hand as a partial avatarization (\secref{ar_embodiment}) might be helpful without impairing the user.
This suggests that a visual hand feedback superimposed on the real hand as a partial avatarization (\secref{ar_embodiment}) might be helpful without impairing the user.
Few works have compared different visual hand rendering in \AR or with wearable haptic feedback.
Few works have compared different visual feedback of the virtual hand in \AR or with wearable haptic feedback.
Rendering the real hand as a semi-transparent hand in \VST-\AR is perceived as less natural but seems to be preferred to a mutual visual occlusion for interaction with real and virtual objects \cite{buchmann2005interaction,piumsomboon2014graspshell}.
%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.
Similarly, \textcite{blaga2017usability} evaluated direct hand manipulation in non-immersive \VST-\AR with a skeleton-like rendering \vs no visual hand rendering: while user performance did not improve, participants felt more confident with the virtual hand (\figref{blaga2017usability}).
Similarly, \textcite{blaga2017usability} evaluated direct hand manipulation in non-immersive \VST-\AR with a skeleton-like rendering \vs no visual hand feedback: while user performance did not improve, participants felt more confident with the virtual hand (\figref{blaga2017usability}).
%\textcite{krichenbauer2018augmented} found that participants were \percent{22} faster in immersive \VST-\AR than in \VR in the same pick-and-place manipulation task, but no visual hand rendering was used in \VR while the real hand was visible in \AR.
In a collaborative task in immersive \OST-\AR \vs \VR, \textcite{yoon2020evaluating} showed that a realistic human hand rendering was the most preferred over a low-polygon hand and a skeleton-like hand for the remote partner.
\textcite{genay2021virtual} found that the sense of embodiment with robotic hands overlay in \OST-\AR was stronger when the environment contained both real and virtual objects (\figref{genay2021virtual}).
Finally, \textcite{maisto2017evaluation} and \textcite{meli2018combining} compared the visual and haptic rendering of the hand in \VST-\AR, as detailed in the next section (\secref{vhar_rings}).
Taken together, these results suggest that a visual rendering of the hand in \AR could improve usability and performance in direct hand manipulation tasks, but the best rendering has yet to be determined.
Finally, \textcite{maisto2017evaluation} and \textcite{meli2018combining} compared the visual and haptic feedback of the hand in \VST-\AR, as detailed in the next section (\secref{vhar_rings}).
Taken together, these results suggest that a visual augmentation of the hand in \AR could improve usability and performance in direct hand manipulation tasks, but the best rendering has yet to be determined.
%\cite{chan2010touching} : cues for touching (selection) \VOs.
%\textcite{saito2021contact} found that masking the real hand with a textured \ThreeD opaque virtual hand did not improve performance in a reach-to-grasp task but displaying the points of contact on the \VO did.
%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 \VO manipulation.
\begin{subfigs}{visual-hands}{Visual hand renderings in \AR. }[][
\item Grasping a \VO in \OST-\AR with no visual hand rendering \cite{hilliges2012holodesk}.
\begin{subfigs}{visual-hands}{Visual feedback of the virtual hand in \AR. }[][
\item Grasping a \VO in \OST-\AR with no visual hand feedback \cite{hilliges2012holodesk}.
\item Simulated mutual-occlusion between the hand grasping and the \VO in \VST-\AR \cite{suzuki2014grasping}.
\item Grasping a real object with a semi-transparent hand in \VST-\AR \cite{buchmann2005interaction}.
\item Skeleton rendering overlaying the real hand in \VST-\AR \cite{blaga2017usability}.
@@ -331,7 +339,7 @@ Taken together, these results suggest that a visual rendering of the hand in \AR
\AR systems integrate virtual content into the user's perception as if it were part of the \RE.
\AR headsets now enable real-time tracking of the head and hands, and high-quality display of virtual content, while being portable and mobile.
They enable highly immersive \AEs that users can explore with a strong sense of the presence of the virtual content.
However, without a direct and seamless interaction with the \VOs using the hands, the coherence of the \AE experience is compromised.
In particular, there is a lack of mutual occlusion and interaction cues between the hands and virtual content when manipulating \VOs in \OST-\AR that could be mitigated by a visual rendering of the hand.
A common alternative approach is to use real objects as a proxy for interaction with \VOs, but this raises concerns about their consistency with the visual rendering.
However, without direct and seamless interaction with the \VOs using the hands, the coherence of the \AE experience is compromised.
In particular, when manipulating \VOs in \OST-\AR, there is a lack of mutual occlusion and interaction cues between the hands and the virtual content, which could be mitigated by a visual augmentation of the hand.
A common alternative approach is to use real objects as proxies for interaction with \VOs, but this raises concerns about their coherence with visual augmentations.
In this context, the use of wearable haptic systems worn on the hand seems to be a promising solution both for improving direct hand manipulation of \VOs and for coherent visuo-haptic augmentation of touched real objects.