% Insist on the advantage of wearable : augment any surface see bau2012revel

% Even before manipulating a visual representation to induce a haptic sensation, shifts and latencies between user input and co-localised visuo-haptic feedback can be experienced differently in \AR and \VR, which we aim to investigate in this work.

%Imagine you're an archaeologist or in a museum, and you want to examine an ancient object.
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%But it is too fragile to touch directly.
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%What if you could still grasp it and manipulate it through a tangible object in your hand, whose visual appearance has been modified using Augmented Reality (AR)?
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%And what if you could also feel its shape or texture?
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%Such tactile augmentation is made possible by wearable haptic devices, which are worn directly on the finger or hand and can provide a variety of sensations on the skin, while being small, light and discreet \cite{pacchierotti2017wearable}.
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Wearable haptic devices, worn directly on the finger or hand, have been used to render a variety of tactile sensations to virtual objects seen in \VR \cite{choi2018claw,detinguy2018enhancing,pezent2019tasbi} or \AR \cite{maisto2017evaluation,meli2018combining,teng2021touch}.
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They have also been used to alter the perception of roughness, stiffness, friction, and local shape perception of real tangible objects \cite{asano2015vibrotactile,detinguy2018enhancing,salazar2020altering}.
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Such techniques place the actuator \emph{close} to the point of contact with the real environment, leaving the user free to directly touch the tangible.
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This combined use of wearable haptics with tangible objects enables a haptic \emph{augmented} reality (HAR) \cite{bhatia2024augmenting} that can provide a rich and varied haptic feedback.

The degree of reality/virtuality in both visual and haptic sensory modalities can be varied independently, but wearable haptic \AR has been little explored with \VR and (visual) \AR \cite{choi2021augmenting}.
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Although \AR and \VR are closely related, they have significant differences that can affect the user experience \cite{genay2021virtual,macedo2023occlusion}.
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%By integrating visual virtual content into the real environment, \AR keeps the hand of the user, the haptic devices worn and the tangibles touched visible, unlike \VR where they are hidden by immersing the user into a visual virtual environment.
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%Current \AR systems also suffer from display and rendering limitations not present in \VR, affecting the user experience with virtual content that may be less realistic or inconsistent with the real augmented environment \cite{kim2018revisiting,macedo2023occlusion}.
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It therefore seems necessary to investigate and understand the potential effect of these differences in visual rendering on the perception of haptically augmented tangible objects.
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Previous works have shown, for example, that the stiffness of a virtual piston rendered with a force feedback haptic system seen in \AR is perceived as less rigid than in \VR \cite{gaffary2017ar} or when the visual rendering is ahead of the haptic rendering \cite{diluca2011effects,knorlein2009influence}.
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%Taking our example from the beginning of this introduction, you now want to learn more about the context of the discovery of the ancient object or its use at the time of its creation by immersing yourself in a virtual environment in \VR.
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%But how different is the perception of the haptic augmentation in \AR compared to \VR, with a virtual hand instead of the real hand?

The goal of this paper is to study the role of the visual rendering of the hand (real or virtual) and its environment (AR or \VR) on the perception of a tangible surface whose texture is augmented with a wearable vibrotactile device worn on the finger.
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We focus on the perception of roughness, one of the main tactile sensations of materials \cite{baumgartner2013visual,hollins1993perceptual,okamoto2013psychophysical} and one of the most studied haptic augmentations \cite{asano2015vibrotactile,culbertson2014modeling,friesen2024perceived,strohmeier2017generating,ujitoko2019modulating}.
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By understanding how these visual factors influence the perception of haptically augmented tangible objects, the many wearable haptic systems that already exist but have not yet been fully explored with \AR can be better applied and new visuo-haptic renderings adapted to \AR can be designed.

Our contributions are:
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\begin{itemize}
  \item A system for rendering virtual vibrotactile roughness textures in real time on a tangible surface touched directly with the finger, integrated with an immersive visual AR/VR headset to provide a coherent multimodal visuo-haptic augmentation of the real environment.
  \item A psychophysical study with 20 participants to evaluate the perception of these virtual roughness textures in three visual rendering conditions: without visual augmentation, with a realistic virtual hand rendering in \AR, and with the same virtual hand in \VR.
\end{itemize}
%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 real environment.
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%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.
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%We then conduct a psychophysical study with 20 participants, where various virtual haptic textures on a tangible 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.

\fig[1]{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 (Real) their real hand alone, (Mixed) a realistic virtual hand overlay in \AR, and (Virtual) the same virtual hand in \VR.
]