\section{Conclusion}
\label{conclusion}

Haptic perception and manipulation of everyday objects with the hand involves exploratory movements or grasp types (\secref{hand_object_interactions}), with simultaneous sensory feedback from multiple cutaneous and kinesthetic 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 sensory cue by compensating with others, and enables the possibility of haptic augmentation.

Haptic systems aim to provide virtual interactions and sensations similar to those with real objects (\secref{wearable_haptics}).
Only a few haptic systems 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}).

\AR headsets integrate virtual content into the user's perception as if it were part of the \RE, with real-time pose estimation 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}).
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 seems to be a promising solution to enable coherent and effective visuo-haptic augmentation of both virtual and real objects.

\noindentskip In this thesis, we will use wearable haptic feedback with an \AR headset to create visuo-haptic texture augmentation when touching real objects (\partref{perception}) and to improve manipulation of virtual objects (\partref{manipulation}), both directly with the bare hand.

First, it is challenging to provide coherent visuo-haptic feedback when augmenting real objects.
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 augmentation can be influenced by the user's visual expectation or the visual augmentation of the object (\secref{vh_perception}).

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.
Then, 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 above.
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.

Second, a few wearable haptic devices have been integrated or experimentally evaluated for direct hand interaction in \AR.
Their haptic end-effector can't cover the fingertips or the inside of the palm, so as not to interfere with the user's interaction with the \RE.
Many strategies have been explored to move the actuator to other parts of the hand.
However, it is still unclear which delocalized positioning is most beneficial, and how it compares or complements the visual feedback of the virtual hand (\secref{vhar_haptics}).

In \chapref{visual_hand}, we will investigate the most common visual feedback of the virtual hand as an augmentation of the real hand in \AR.
We will compare these visual hand augmentation in virtual object manipulation tasks, directly with the bare hand.
Then, in \chapref{visuo_haptic_hand}, we will evaluate five common delocalized positioning of vibrotactile feedback, combined with two contact rendering techniques and visual hand augmentations.
We will compare these visuo-haptic augmentations of the hand with the same direct hand manipulation tasks of virtual objects in \AR.