\section{Hand Interaction in Augmented Reality}
\label{augmented_reality}

%Based on the interaction loop presented in \figref[introduction]{interaction-loop}, we briefly detail the fundamental main components that compose any AR application: tracking, rendering and display.

\subsection{Principles and Capabilities of AR}
\label{ar_intro}

\subsubsection{What is Augmented Reality?}

The first \AR \HMD was invented by \textcite{sutherland1968headmounted}: With the technology available at the time, it was already capable of displaying virtual objects at a fixed point in space in real time, giving the user the illusion that the content was present in the room (see \figref{sutherland1970computer3}).
%
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}.

This system also already fulfilled the first formal definition of \AR, proposed by \textcite{azuma1997survey} in the first survey of the domain: (1) \emph{combine real and virtual}, (2) \emph{be interactive in real time} and (3) \emph{register real and virtual}\footnote{This third characteristic has been slightly adapted to use the version of \textcite{marchand2016pose}, the original definition was: \enquote{registered in \ThreeD}.}.
%
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 \v-\AR.

%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.

% debate on the definition of AR
% taxonomy of Milgram/Skarbez

\subsubsection{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 help surgeons to visualize \ThreeD images of the brain overlaid on the patient's head prior or during surgery (see \figref{watanabe2016transvisible}) or improve the learning of students with complex concepts and phenomena such as optics or chemistry (see \figref{bousquet2024reconfigurable}).
%
It can also guide workers in complex tasks, such as assembly, maintenance or verification (see \figref{hartl2013mobile}), or can create complete new forms of gaming or tourism experiences (see \figref{roo2017inner}).
%
Most of \AR/\VR experience can now be implemented with commercially available hardware and software solutions, in particular for tracking, rendering and display.

\begin{subfigs}{augmented-reality}{Examples of \AR applications. }[
        \item The first \AR \HMD displaying wireframe \ThreeD virtual objects registered in the real environment~\cite{sutherland1968headmounted}.
        \item Neurosurgery visualization of the brain on a patient's head~\cite{watanabe2016transvisible}.
        \item HOBIT is a spatial, tangible \AR table simulating an optical bench for educational experimentations~\cite{bousquet2024reconfigurable}.
        \item \AR can interactively guide in document verification tasks by recognizing and comparing with virtual references
        ~\cite{hartl2013mobile}.
        \item Inner Garden is a visually augmented zen garden for relaxation and meditation~\cite{roo2017inner}.
    ]
    \subfigsheight{36mm}
    \subfig{sutherland1970computer3}
    \subfig{watanabe2016transvisible}
    \subfig{bousquet2024reconfigurable}
    \subfig{hartl2013mobile}
    \subfig{roo2017inner}
\end{subfigs}


\subsubsection{Displays}

% Bimber and types of AR
% State of current HMD


\subsubsection{On Presence and Embodiment}

Despite this clear and acknowledged definition and the viewpoint of this thesis that \AR and \VR are two type of \MR experience with different levels of mixing real and virtual environments, as presented in \secref[introduction]{visuo_haptic_augmentations}, there is still a debate on defining \AR and \MR as well as how to characterize and categorized such experiences~\cite{speicher2019what,skarbez2021revisiting}.

\emph{Presence} is one of the key concept to characterize a \VR experience.
%
\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 perception through a user interface and the user's senses.
%
Such experience of disbelief suspension in \VR is what is called presence, and it can be decomposed into two dimensions: \PI and \PSI~\cite{slater2009place}.
%
\PI is the sense of the user of \enquote{being there} in the \VE, and it emerges from the real time rendering of the \VE from the user's perspective, the displayed content conforming and being consistent with the proprioception and actions of the user.
%
\PSI 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.
%
A third strong illusion in \VR is the \SoE, which is the illusion that the virtual body is one's own~\cite{slater2022separate,guy2023sense}.

The \AR presence is far less defined and studied than for \VR~\cite{tran2024survey}, but it will be useful to design, evaluate and discuss our contributions in the next chapters.
%
Thereby, \textcite{slater2022separate} proposed to invert \PI as bring the virtual into the physical world, \ie \enquote{place it here}.
%
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 \PSI can be applied to \AR as is, but the \VOs must additionally have knowledge of the \RE and react accordingly to it.
%
\textcite{skarbez2021revisiting} also named \PI for \AR as \enquote{immersion} and \PSI as \enquote{coherence}, and these terms will be used in the remainder of this thesis.

As presence, \SoE in \AR is a recent topic and little is known about its perception on the user experience~\cite{genay2021virtual}.


\subsection{How Virtual is Perceived in AR}
\label{ar_perception}


\subsection{Interacting with Virtual and Augmented Content}
\label{ar_interaction}

\subsubsection{Virtual Hands in AR}
\label{ar_interaction_hands}


\subsection{Conclusion}
\label{ar_conclusion}