\section{Discussion}
\label{sec:discussion}

We evaluated sixteen visuo-haptic renderings of the hand, in the same two virtual object manipulation tasks in AR as in the first experiment, as the combination of two vibrotactile contact techniques provided at four delocalized positions on the hand with the two most representative visual hand renderings established in the first experiment.

In the Push task, vibrotactile haptic hand rendering has been proven beneficial with the Proximal positioning, which registered a low completion time, but detrimental with the Fingertips positioning, which performed worse (see \figref{results/Push-CompletionTime-Location-Overall-Means}) than the Proximal and Opposite (on the contralateral hand) positionings.
%
The cause might be the intensity of vibrations, which many participants found rather strong and possibly distracting when provided at the fingertips.
%
This result was also observed by \textcite{bermejo2021exploring}, who provided vibrotactile cues when pressing a virtual keypad.
%
Another reason could be the visual impairment caused by the vibrotactile motors when worn on the fingertips, which could have disturbed the visualization of the virtual cube.

We observed different strategies than in the first experiment for the two tasks.
%
During the Push task, participants made more and shorter contacts to adjust the cube inside the target volume (see \figref{results/Push-Contacts-Location-Overall-Means} and \figref{results/Push-TimePerContact-Location-Overall-Means}).
%
During the Grasp task, participants pressed the cube 25~\% harder on average (see \figref{results/Grasp-GripAperture-Location-Overall-Means}).
%
The Fingertips and Proximal positionings led to a slightly larger grip aperture than the others.
%
We think that the proximity of the vibrotactile rendering to the point of contact made users to take more time to adjust their grip in a more realistic manner, \ie closer to the surface of the cube.
%
This could also be the cause of the higher number of failed grasps or cube drops: indeed, we observed that the larger the grip aperture, the higher the number of contacts.
%
Consequently, the Fingertips positioning was slower (see \figref{results/Grasp-CompletionTime-Location-Overall-Means}) and more prone to error (see \figref{results/Grasp-Contacts-Location-Overall-Means}) than the Opposite and Nowhere positionings.

In both tasks, the Opposite positioning also seemed to be faster (see \figref{results/Push-CompletionTime-Location-Overall-Means}) than having no vibrotactile hand rendering (Nowhere positioning).
%
However, participants also felt more workload (see \figref{questions}) with this positioning opposite to the site of the interaction.
%
This result might mean that participants focused more on learning to interpret these sensations, which led to better performance in the long run.

Overall, many participants appreciated the vibrotactile hand renderings, commenting that they made the tasks more realistic and easier.
%
However, the closer to the contact point, the better the vibrotactile rendering was perceived (see \figref{questions}).
%
This seemed inversely correlated with the performance, except for the Nowhere positioning, \eg both the Fingertips and Proximal positionings were perceived as more effective, useful, and realistic than the other positionings despite lower performance.

Considering the two tasks, no clear difference in performance or appreciation was found between the two contact vibration techniques.
%
While the majority of participants discriminated the two different techniques, only a minority identified them correctly (see \secref{technique_results}).
%
It seemed that the Impact technique was sufficient to provide contact information compared to the Distance technique, which provided additional feedback on interpenetration, as reported by participants.

No difference in performance was found between the two visual hand renderings, except for the Push task, where the Skeleton hand rendering resulted again in longer contacts.
%
Additionally, the Skeleton rendering was appreciated and perceived as more effective than having no visual hand rendering, confirming the results of our first experiment.
%
Participants reported that this visual hand rendering provided good feedback on the status of the hand tracking while being constrained to the cube, and helped with rotation adjustment in both tasks.
%
However, many also felt that it was a bit redundant with the vibrotactile hand rendering.
%
Indeed, receiving a vibrotactile hand rendering was found by participants as a more accurate and reliable information regarding the contact with the cube than simply seeing the cube and the visual hand reacting to the manipulation.
%
This result suggests that providing a visual hand rendering may not be useful during the grasping phase, but may be beneficial prior to contact with the virtual object and during position and rotation adjustment, providing valuable information about the hand pose.
%
It is also worth noting that the improved hand tracking and grasp helper improved the manipulation of the cube with respect to the first experiment, as shown by the shorter completion time during the Grasp task.
%
This improvement could also be the reason for the smaller differences between the Skeleton and the None visual hand renderings in this second experiment.

In summary, the positioning of the vibrotactile haptic rendering of the hand affected on the performance and experience of users manipulating virtual objects with their bare hands in AR.
%
The closer the vibrotactile hand rendering was to the point of contact, the better it was perceived in terms of effectiveness, usefulness, and realism.
%
These subjective appreciations of wearable haptic hand rendering for manipulating virtual objects in AR were also observed by \textcite{maisto2017evaluation} and \textcite{meli2018combining}.
%
However, the best performance was obtained with the farthest positioning on the contralateral hand, which is somewhat surprising.
%
This apparent paradox could be explained in two ways.
%
On the one hand, participants behave differently when the haptic rendering was given on the fingers, close to the contact point, with shorter pushes and larger grip apertures.
%
This behavior has likely given them a better experience of the tasks and more confidence in their actions, as well as leading to a lower interpenetration/force applied to the cube~\cite{pacchierotti2015cutaneous}.
%
On the other hand, the unfamiliarity of the contralateral hand positioning caused participants to spend more time understanding the haptic stimuli, which might have made them more focused on performing the task.
%
In terms of the contact vibration technique, the continuous vibration technique on the finger interpenetration (Distance technique) did not make a difference to performance, although it provided more information.
%
Participants felt that vibration bursts were sufficient (Impact technique) to confirm contact with the virtual object.
%
Finally, it was interesting to note that the visual hand renderings was appreciated but felt less necessary when provided together with vibrotactile hand rendering, as the latter was deemed sufficient for acknowledging the contact.

As we already said in \secref[visual_hand]{discussion}, these results have some limitations as they address limited types of visuo-haptic renderings and manipulations were restricted to the thumb and index fingertips.
%
While the simpler vibration technique (Impact technique) was sufficient to confirm contacts with the cube, richer vibrotactile renderings may be required for more complex interactions, such as collision or friction renderings between objects~\cite{kuchenbecker2006improving, pacchierotti2015cutaneous} or texture rendering~\cite{culbertson2014one, asano2015vibrotactile}.
%
More generally, a broader range of haptic sensations should be considered, such as pressure or stretching of the skin~\cite{maisto2017evaluation, teng2021touch}.
%
However, moving the point of application of the sensation away may be challenging for some types of haptic rendering.
%
Similarly, as the interactions were limited to the thumb and index fingertips, positioning a delocalized haptic rendering over a larger area of the hand could be challenging and remains to be explored.
%
Also, given that some users found the vibration rendering too strong, adapting/personalizing the haptic feedback to one's preference (and body positioning) might also be a promising approach.
%
Indeed, personalized haptics is recently gaining interest in the community~\cite{malvezzi2021design, umair2021exploring}.
