\section{Introduction}
\label{intro}

In the previous chapter, we presented a system for augmenting the visuo-haptic texture perception of real surfaces directly touched with the finger, using wearable vibrotactile haptics and an immersive \AR headset.
In this chapter and the next one, we evaluate the user's perception of such wearable haptic texture augmentation under different visual rendering conditions.

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}).
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 feedback of the virtual hand and of the environment (real or virtual) on the perception of a real surface whose haptic roughness is augmented} with wearable vibrotactile 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 next sections, 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.

\smallskip

\fig[0.55]{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.
]
