\section{Conclusion}
\label{conclusion}

Haptic perception and manipulation of everyday objects with the hand involve exploratory movements or grasp types, respectively (\secref{hand_object_interactions}), with simultaneous sensory feedback from multiple cutaneous and kinaesthetic receptors embedded beneath the skin (\secref{haptic_sense}).
These receptors provide sensory cues about the physical properties of objects, such as roughness (texture) and hardness, which are then integrated to form a perception of the property being explored (\secref{object_properties}).
Perceptual constancy is possible in the absence of one cue by compensating with others, and enables the possibility of haptic augmentation.

\noindentskip Haptic systems aim to provide virtual interactions and sensations similar to those with real objects (\secref{wearable_haptics}).
Only a few can be considered wearable due to their compactness and portability, but they are limited to cutaneous feedback (\secref{wearable_haptic_devices}).
If their haptic rendering is timely associated with the user's touch actions on a real object, the perceived haptic properties of the object, such as its roughness and hardness, can be modified.
Wearable haptic augmentation of roughness and hardness is mostly achieved with vibrotactile feedback (\secref{tactile_rendering}).

We will use such wearable vibrotactile feedback to create texture augmentation of real objects in \partref{perception} and contact rendering of virtual objects in \partref{manipulation}.
In particular, in \chapref{vhar_system} we will propose a system that allows free exploration of texture augmentation of real surfaces with the bare hand using wearable vibrotactile.

\noindentskip \AR headsets integrate virtual content into the user's perception in an immersive way, as if it were part of the \RE, with real-time tracking of the head and hands (\secref{what_is_ar}).
Direct interaction with the hand of virtual content is often implemented using virtual hand interaction technique, which reconstructs the user's hand in the \VE and simulates its interactions with the virtual.
However, the perception and manipulation of the virtual is difficult due to the lack of haptic feedback and the mutual occlusion of the hand with the virtual content (\secref{ar_interaction}).

The lack of mutual occlusion could be improved by visual feedback of the virtual hand (\secref{ar_visual_hands}), which we will investigate in \chapref{visual_hand} by comparing the most common visual hand augmentations used in \AR in manipulation tasks with the hand of virtual objects.
Wearable haptics on the hand is another solution to improve direct hand manipulation of virtual objects, which we will explore in \chapref{visuo_haptic_hand}.

Real surrounding objects can also be used as proxies to interact with the virtual, but they may be incoherent with their visual augmentation because they are haptically passive (\secref{ar_interaction}).
Wearable haptics on the hand is again a promising solution to enable coherent visuo-haptic augmentation of real objects, as we will explore in \partref{perception}.

\noindentskip However, few wearable haptic devices have been integrated or experimentally evaluated for direct hand interaction in \AR.
Their haptic end-effector must be moved away from the inside of the hand so as not to interfere with the user's interaction with the \RE.
Many strategies for moving the actuator on the hand have been explored, but the most beneficial position for delocalized haptic feedback is still unclear (\secref{vhar_haptics}).

In \chapref{visuo_haptic_hand} we will investigate five common delocalized positions of vibrotactile feedback for rendering contact with the hand when manipulating virtual objects in \AR.
We will also investigate two contact rendering techniques and compare them with two visual hand augmentations from \chapref{visual_hand}.

\noindentskip It is also challenging to provide coherent visuo-haptic feedback when augmenting real objects and rendering virtual objects (\secref{vh_perception}).
By integrating different sensory feedback, haptic and visual, real and virtual, into a single object property, perception is somewhat robust to variations in reliability and to spatial and temporal differences.
Conversely, the same haptic rendering or augmentation can be influenced by the user's visual expectation or the visual rendering of the virtual object.

In \chapref{xr_perception} we will investigate the effect of the visual feedback of the virtual hand as well as the effect of the environment (\AR or \VR) on the perception of vibrotactile texture augmentations using the system presented in \chapref{vhar_system}.

Finally, using the same system, we will evaluate in \chapref{vhar_textures} the perceived realism, coherence and roughness of co-localized visuo-haptic texture augmentations on real surfaces seen and touched with the real hand in \AR.
