\subsection{Grasp Task}
\label{grasp}

\paragraph{Completion Time}

On the time to complete a trial,
a \LMM \ANOVA with by-participant random intercepts indicated one statistically significant effect
of \factor{Target} (\anova{7}{3385}{34.3}, \pinf{0.001})
but not of \factor{Hand} (\anova{5}{3385}{1.7}, \p{0.1}).
Targets on the back and the left (\level{B}, \level{LB}, and \level{L}) were slower than targets on the front (\level{LF}, \level{F}, and \level{RF}, \p{0.003}) {except for} \level{RB} (back-right) which was also fast.

\paragraph{Contacts}

On the number of contacts,
a \LMM \ANOVA with by-participant random intercepts indicated two statistically significant effects:
\factor{Hand} (\anova{5}{3385}{4.9}, \pinf{0.001}, see \figref{results/Grasp-ContactsCount})
and \factor{Target} (\anova{7}{3385}{20.0}, \pinf{0.001}).

Fewer contacts were made with \level{Tips} than with \level{None} (\percent{-13}, \p{0.02}) and \level{Occlusion} (\percent{-15}, \p{0.004});
and less with \level{Mesh} than with \level{None} (\percent{-15}, \p{0.006}) and \level{Occlusion} (\percent{-17}, \p{0.001}).
This result suggests that having no visible visual hand increased the number of failed grasps or cube drops.
But, surprisingly, only \level{Tips} and \level{Mesh} were statistically significantly better, not \level{Contour} nor \level{Skeleton}.

Targets on the back and left were more difficult (\level{B}, \level{LB}, and \level{L}) than targets on the front (\level{LF}, \level{F}, and \level{RF}, \pinf{0.001}).

\paragraph{Time per Contact}

On the mean time spent on each contact,
a \LMM \ANOVA with by-participant random intercepts indicated two statistically significant effects:
\factor{Hand} (\anova{5}{3385}{9.1}, \pinf{0.001}, see \figref{results/Grasp-MeanContactTime})
and \factor{Target} (\anova{7}{3385}{5.4}, \pinf{0.001}).

It was shorter with \level{None} than with \level{Tips} (\percent{-15}, \pinf{0.001}), \level{Skeleton} (\percent{-11}, \p{0.001}) and \level{Mesh} (\percent{-11}, \p{0.001});
shorter with \level{Occlusion} than with \level{Tips} (\percent{-10}, \pinf{0.001}), \level{Skeleton} (\percent{-8}, \p{0.05}), and \level{Mesh} (\percent{-8}, \p{0.04});
shorter with \level{Contour} than with \level{Tips} (\percent{-8}, \pinf{0.001}).
As for the \level{Push} task, the lack of visual hand increased the number of failed grasps or cube drops.
The \level{Tips} rendering seemed to provide one of the best feedback for the grasping, maybe thanks to the fact that it provides information about both position and rotation of the tracked fingertips.

This time was the shortest on the front \level{F} than on the other target volumes (\pinf{0.001}).

\paragraph{Grip Aperture}

On the average distance between the thumb's fingertip and the other fingertips during grasping,
a \LMM \ANOVA with by-participant random intercepts and random slopes for \factor{Hand} indicated two statistically significant effects:
\factor{Hand} (\anova{5}{19}{6.7}, \pinf{0.001}, see \figref{results/Grasp-GripAperture})
and \factor{Target} (\anova{7}{3270}{4.1}, \pinf{0.001}).

It was shorter with \level{None} than with \level{Occlusion} (\pinf{0.001}), \level{Contour} (\pinf{0.001}), \level{Skeleton} (\pinf{0.001}) and \level{Mesh} (\pinf{0.001}).
This result is an evidence of the lack of confidence of participants with no visual hand augmentation: they grasped the cube more to secure it.

The \response{Grip Aperture} was longer on the right-front (\level{RF}) target volume, indicating a higher confidence, than on back and side targets (\level{R}, \level{RB}, \level{B}, \level{L}, \p{0.03}).

\begin{subfigs}{grasp_results}{Results of the grasp task performance metrics for each visual hand augmentation.}[
    Geometric means with bootstrap \percent{95} \CI
    and Tukey's \HSD pairwise comparisons: *** is \pinf{0.001}, ** is \pinf{0.01}, and * is \pinf{0.05}.
  ][
  \item Number of contacts with the cube.
  \item Time spent on each contact.
  \item Distance between thumb and the other fingertips when grasping.
  ]
  \subfig[0.32]{results/Grasp-ContactsCount}
  \subfig[0.32]{results/Grasp-MeanContactTime}
  \subfig[0.32]{results/Grasp-GripAperture}
\end{subfigs}
