%\noindent La principale augmentation haptique de texture consiste à simuler la rugosité d'une surface périodique de grille

%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}.
%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.
%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.
%Such techniques place the actuator \emph{close} to the point of contact with the \RE, leaving the user free to directly touch the tangible.

In this chapter, we propose a system for rendering coherent visual and haptic virtual textures that augment tangible surfaces.
It is implemented with an immersive \OST-\AR headset Microsoft HoloLens~2 and a wearable vibrotactile (voice-coil) device worn on the outside of finger (not covering the fingertip, \secref[related_work]{vhar_haptics}).
The visuo-haptic augmentations can be viewed freely from any angle and touched directly with the bare finger, as if they were real textures.
To ensure real-time and realible renderings, the hand and the tangibles are tracked using a webcam and marker-based tracking.
The haptic textures are rendered as a real-time vibrotactile signal representing a patterned grating texture that is synchronized with the finger movement on the augmented tangible surface.

\noindentskip The contributions of this chapter are:
\begin{itemize}
  \item The rendering of virtual vibrotactile roughness textures representing a patterned grating texture in real time from free finger movements.
  \item A system to provide a coherent visuo-haptic texture augmentations of the \RE in a direct touch context using an immersive \AR 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.