\section{Principles and Capabilities of AR}
\label{augmented_reality}

The first \AR headset 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{sutherland1968headmounted}).
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}.

\begin{subfigs}{sutherland1968headmounted}{Photos of the first \AR system~\cite{sutherland1968headmounted}. }[
        \item The \AR headset.
        \item Wireframe \ThreeD virtual objects were displayed registered in the real environment (as if there were part of it).
    ]
    \subfigsheight{45mm}
    \subfig{sutherland1970computer3}
    \subfig{sutherland1970computer2}
\end{subfigs}


\subsection{What is Augmented Reality?}
\label{ar_definition}

\paragraph{A Definition}

The system of \cite{sutherland1968headmounted} already fulfilled the first formal definition of \AR, proposed by \textcite{azuma1997survey} in the first survey of the domain:
\begin{enumerate}[label=(\arabic*)]
    \item combine real and virtual,
    \item be interactive in real time, and
    \item register real and virtual\footnotemark.
\end{enumerate}

%\footnotetext{There quite confusion in the literature and in (because of) the industry about the terms \AR and \MR. The term \MR is very often used as a synonym of \AR, or a version of \AR that enables an interaction with the virtual content. The title of this section refers to the title of the highly cited paper by \textcite{speicher2019what} that examines this debate.}

\footnotetext{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.

\paragraph{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 (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.

\begin{subfigs}{ar_applications}{Examples of \AR applications. }[
        \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{45mm}
    \subfig{watanabe2016transvisible}
    \subfig{bousquet2024reconfigurable}
    \subfig{hartl2013mobile}
    \subfig{roo2017inner}
\end{subfigs}


\subsection{Displays and Perception}
\label{ar_displays}

% Bimber and types of AR
% State of current HMD

\paragraph{Spatial Augmented Reality}

\paragraph{Window on World Displays}

\paragraph{Video See-Through Headsets}

\paragraph{Optical See-Through Headsets}


\subsection{Presence and Embodiment}
\label{ar_presence}

Despite the clear and acknowledged definition presented in \secref{ar_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}.

\paragraph{Presence}

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 (see \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.
\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 to what we can call \enquote{object illusion}, \ie the sense of the virtual object of \enquote{being here} in the \RE (see \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 \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.

\begin{subfigs}{presence}{The sense of immersion in virtual and augmented environments. Adapted from \textcite{stevens2002putting}. }[
        \item Place Illusion (PI) is the sense of the user of \enquote{being there} in the \VE.
        \item Objet illusion is the sense of the virtual object of \enquote{being here} in the \RE.
    ]
    \subfigsheight{35mm}
    \subfig{presence-vr}
    \subfig{presence-ar}
\end{subfigs}

\paragraph{Embodiment}

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


\subsection{Conclusion}
\label{ar_conclusion}
