\section{Introduction}
\label{intro}

In the previous chapter, we investigated the role of the visual feedback of the virtual hand and the environment (\AR \vs \VR) on the perception of wearable haptic texture augmentation.
In this chapter, we explore the perception of wearable visuo-haptic texture augmentation of real surfaces touched directly with the finger.

When we look at the surface of an everyday object, we then touch it to confirm or contrast our initial visual impression and to estimate the properties of the object, particularly its texture (\secref[related_work]{visual_haptic_influence}).
Among the various haptic texture augmentations, data-driven methods allow to capture, model and reproduce the roughness perception of real surfaces when touched by a hand-held stylus (\secref[related_work]{texture_rendering}).
Databases of visuo-haptic textures have been developed in this way \cite{culbertson2014one,balasubramanian2024sens3}, but they have not yet been explored in a direct touch context with \AR and wearable haptics.

In this chapter, we consider simultaneous and \textbf{co-localized visual and wearable haptic texture augmentation of real surfaces} with an \OST-\AR headset and wearable vibrotactile feedback.
We investigate how these textures can be perceived in a coherent and realistic manner, and to what extent each sensory modality would contribute to the overall perception of the augmented texture.
We used nine pairs of \textbf{data-driven visuo-haptic textures} from the \HaTT database \cite{culbertson2014one}, which we rendered using the wearable visuo-haptic augmentation system presented in \chapref{vhar_system}. %, an \OST-\AR headset, and a wearable voice-coil device worn on the finger.
In a \textbf{user study}, 20 participants freely explored in direct touch the combination of the visuo-haptic texture pairs to rate their coherence, realism and perceived roughness.
We aimed to assess \textbf{which haptic textures were matched with which visual textures}, how the roughness of the visual and haptic textures was perceived, and whether \textbf{the perceived roughness} could explain the matches made between them.

\noindentskip The contributions of this chapter are:
\begin{itemize}
  \item Transposition of data-driven visuo-haptic textures to augment real objects in a direct touch context in \AR.
  \item A user study evaluating with 20 participants the coherence, realism and perceived roughness of nine pairs of these visuo-haptic texture augmentations.
\end{itemize}

\smallskip

\fig[0.55]{experiment/view}{First person view of the user study.}[
  The visual texture overlays were statically displayed on the surfaces, allowing the user to move around to view them from different angles.
  The haptic texture augmentations were generated based on \HaTT data-driven texture models and finger speed, and were rendered on the middle index phalanx.
]

\noindentskip In the next sections, we first describe the apparatus of the user study experimental design, including the two tasks performed. We then present the results obtained and discuss them before concluding.
