\section{Introduction}
\label{intro}

Most of the haptic augmentations of real surfaces using with wearable haptic devices, including roughness of textures (\secref[related_work]{texture_rendering}), have been studied without a visual feedback, and none have considered the influence of the visual rendering on their perception or integrated them in \AR and \VR (\secref[related_work]{texture_rendering}).
Still, it is known that the visual rendering of an object can influence the perception of its haptic properties (\secref[related_work]{visual_haptic_influence}), and that the perception of same haptic force-feedback or vibrotactile rendering can differ between \AR and \VR, probably due to difference in perceived simultaneity between visual and haptic stimuli (\secref[related_work]{ar_vr_haptic}).
Indeed, in \AR, the user can see their own hand touching, the haptic device worn and the \RE, while in \VR they are hidden by the \VE.

In this chapter, we investigate the \textbf{role of the visual virtuality} of the hand (real or virtual) and its environment (\AR or \VR) on the perception of a \textbf{real surface whose haptic roughness is augmented} with a wearable haptics. %voice-coil device worn on the finger.
To do so, we used the visuo-haptic system presented in \chapref{vhar_system} to render virtual vibrotactile patterned textures (\secref[related_work]{texture_rendering}) to augment the real surface being touched. % touched by the finger.% that can be directly touched with the bare finger.
We evaluated, in \textbf{user study with psychophysical methods and extensive questionnaire}, the perceived roughness augmentation in three visual rendering conditions: \textbf{(1) without visual augmentation}, in \textbf{(2) \OST-\AR with a realistic virtual hand} rendering, and in \textbf{(3) \VR with the same virtual hand}.
To control for the influence of the visual rendering, the real surface was not visually augmented and stayed the same in all conditions.

\noindentskip The contributions of this chapter are:
\begin{itemize}
  \item A psychophysical user study with 20 participants to evaluate the effect of visual hand rendering in \OST-\AR or \VR on the perception of haptic roughness texture augmentations, using wearable vibrotactile haptics.
  \item A discussion and recommendations on the integration of wearable haptic augmentations in direct touch context with \AR and \VR.
\end{itemize}

\noindentskip In the remainder of this chapter, we first describe the experimental design and apparatus of the user study.
We then present the results obtained, discuss them, and outline recommendations for future \AR/\VR works using wearable haptic augmentations.

%First, we present a system for rendering virtual vibrotactile textures in real time without constraints on hand movements and integrated with an immersive visual \AR/\VR headset to provide a coherent multimodal visuo-haptic augmentation of the \RE.
%An experimental setup is then presented to compare haptic roughness augmentation with an optical \AR headset (Microsoft HoloLens~2) that can be transformed into a \VR headset using a cardboard mask.
%We then conduct a psychophysical study with 20 participants, where various virtual haptic textures on a real surface directly touched with the finger are compared in a two-alternative forced choice (2AFC) task in three visual rendering conditions: (1) without visual augmentation, (2) with a realistic virtual hand rendering in \AR, and (3) with the same virtual hand in \VR.

\bigskip

\fig[0.9]{teaser/teaser2}{
  Vibrotactile textures were rendered in real time on a real surface using a wearable vibrotactile device worn on the finger.
}[%
  Participants explored this haptic roughness augmentation with (\level{Real}) their real hand alone, (\level{Mixed}) a realistic virtual hand overlay in \AR, and (\level{Virtual}) the same virtual hand in \VR.
]
