\noindent 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 \cite{ernst2002humans}.
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One of the main characteristics of a textured surface is its roughness, \ie the micro-geometry of the material \cite{klatzky2003feeling}, which is perceived equally well and similarly by both sight and touch \cite{bergmanntiest2007haptic,baumgartner2013visual,vardar2019fingertip}.
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Many haptic devices and rendering methods have been used to generate realistic virtual rough textures \cite{culbertson2018haptics}.
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One of the most common approaches is to reproduce the vibrations that occur when running across a surface, using a vibrotactile device attached to a hand-held tool \cite{culbertson2014modeling,culbertson2015should} or worn on the finger \cite{asano2015vibrotactile,friesen2024perceived}.
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By providing timely vibrations synchronized with the movement of the tool or the finger moving on a real object, the perceived roughness of the surface can be augmented \cite{culbertson2015should,asano2015vibrotactile}.
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In that sense, data-driven haptic textures have been developed as captures and models of real surfaces, resulting in the \HaTT database \cite{culbertson2014one}.
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While these virtual haptic textures are perceived as similar to real textures \cite{culbertson2015should}, they have been evaluated using hand-held tools and not yet in a direct finger contact with the surface context, in particular combined with visual textures in an immersive \VE.

Wearable haptic devices have proven to be effective in modifying the perception of a touched tangible surface, without modifying the tangible, nor covering the fingertip, forming a haptic \AE \cite{bau2012revel,detinguy2018enhancing,salazar2020altering}.

Second, many works have investigated the haptic augmentation of textures, but few have integrated them with immersive \VEs or have considered the influence of the visual rendering on their perception.
Still, it is known that the visual feedback can alter the perception of real and virtual haptic sensations \cite{schwind2018touch,choi2021augmenting} but also that the force feedback perception of grounded haptic devices is not the same in \AR and \VR \cite{diluca2011effects,gaffary2017ar}.

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

Wearable haptic devices, worn directly on the finger or hand, have been used to render a variety of tactile sensations to \VOs 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 \RE, 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 \RE, \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?

\noindentskip The contributions of this chapter are:
\begin{itemize}
  \item The rendering of virtual vibrotactile roughness textures in real time using webcam to track the finger touching.
  \item A system to provide a coherent multimodal visuo-haptic texture augmentations of the \RE in direct touch context using an immersive visual AR/VR headset and wearable haptics.
\end{itemize}

\noindentskip In the remainder of this chapter, we describe the principles of the system, how the real and virtual environments are registered, the generation of the vibrotactile textures, and measures of visual and haptic rendering latencies.

%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.
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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.
%}
