\section{User Study}
\label{experiment}

%The visuo-haptic rendering system, described in \secref[vhar_system]{method}, allows free exploration of virtual vibrotactile textures on real surfaces directly touched with the bare finger to simulate roughness augmentation, while the visual rendering of the hand and environment can be controlled to be in \AR or \VR.
%
%The user study aimed to investigate the effect of visual hand rendering in \AR or \VR on the perception of roughness texture augmentation of a touched real surface.
In a \TIFC task (\secref[related_work]{sensations_perception}), participants compared the roughness of different tactile texture augmentations in three visual rendering conditions: without any visual augmentation (\level{Real}, \figref{experiment/real}), in \AR with a realistic virtual hand superimposed on the real hand (\level{Mixed}, \figref{experiment/mixed}), and in \VR with the same virtual hand as an avatar (\level{Virtual}, \figref{experiment/virtual}).
In order not to influence the perception, as vision is an important source of information and influence for the perception of texture \cite{bergmanntiest2007haptic,yanagisawa2015effects,vardar2019fingertip}, the touched surface was visually a uniform white; thus only the visual aspect of the hand and the surrounding environment is changed.

\begin{subfigs}{renderings}{
    The three visual rendering conditions and the experimental procedure of the \TIFC psychophysical study.
  }[
    During a trial, two tactile textures were rendered on the augmented area of the paper sheet (black rectangle) for \qty{3}{\s} each, one after the other, then the participant chose which one was the roughest.
    The visual rendering stayed the same during the trial.
    The pictures are captured directly from the Microsoft HoloLens 2 headset.
  ][
  \item The real environment and real hand view without any visual augmentation.
  \item The real environment and hand view with the virtual hand.
  \item Virtual environment with the virtual hand.
  ]
  \subfig[0.326]{experiment/real}
  \subfig[0.326]{experiment/mixed}
  \subfig[0.326]{experiment/virtual}
\end{subfigs}

\subsection{Apparatus}
\label{apparatus}

An experimental environment was created to ensure a similar visual rendering in \AR and \VR (\figref{renderings}).
It consisted of a \qtyproduct{300 x 210 x 400}{\mm} medium-density fibreboard (MDF) box with a paper sheet glued inside and a \qtyproduct{50 x 15}{\mm} rectangle printed on the sheet to delimit the area where the tactile textures were rendered.
A single light source of \qty{800}{\lumen} placed \qty{70}{\cm} above the table fully illuminated the inside of the box.
Participants rated the roughness of the paper (without any texture augmentation) before the experiment on a 7-point Likert scale (1~=~Extremely smooth, 7~=~Extremely rough) as quite smooth (\mean{2.5}, \sd{1.3}).

The visual rendering of the virtual hand and environment was achieved using the \OST-\AR headset Microsoft HoloLens~2 (\secref[vhar_system]{virtual_real_alignment}) running at \qty{60}{FPS} a custom application made with Unity (v2021.1) and Mixed Reality Toolkit (v2.7).
An \OST-\AR headset was chosen over a \VST-\AR headset because the former only adds virtual content to the \RE, while the latter streams a real-time video capture of the \RE, and one of our objectives was to directly compare a \VE replicating a real one, not to a video feed that introduces many other visual limitations (\secref[related_work]{ar_displays}).

The \VE carefully reproduced the \RE, including the geometry of the box, textures, lighting, and shadows (\figref{renderings}, \level{Virtual}).
The virtual hand model was a gender-neutral human right hand with realistic skin texture, similar to that used by \textcite{schwind2017these}.
Its size was adjusted to match the real hand of the participants before the experiment.
The visual rendering of the virtual hand and environment is described in \secref[vhar_system]{virtual_real_alignment}.

To ensure the same \FoV in all \factor{Visual Rendering} condition, a cardboard mask was attached to the \AR headset (\figref{experiment/headset}).
In the \level{Virtual} rendering, the mask only had holes for sensors to block the view of the \RE and simulate a \VR headset.
In the \level{Mixed} and \level{Real} conditions, the mask had two additional holes for the eyes that matched the \FoV of the HoloLens~2 (\figref{experiment/headset}).
\figref{renderings} shows the resulting views in the three considered \factor{Visual Rendering} conditions.

Participants sat comfortably in front of the box at a distance of \qty{30}{\cm}, wearing the HoloLens~2 with a cardboard mask attached, so that only the inside of the box was visible, as shown in \figref{experiment/apparatus}.
The vibrotactile voice-coil actuator (HapCoil-One, Actronika) was firmly attached to the middle phalanx of the right index finger of the participants using a Velcro strap, similarly to previous studies \cite{asano2015vibrotactile,friesen2024perceived}.
The generation of the virtual texture is described in \secref[vhar_system]{texture_generation}.
They also wore headphones with a brown noise masking the sound of the voice-coil.
The user study was held in a quiet room with no windows.

\begin{subfigs}{setup}{Visuo-haptic textures rendering setup. }[][
  \item HoloLens~2 \OST-\AR headset, the two cardboard masks to switch the real or virtual environments with the same field of view, and the \ThreeD-printed piece for attaching the masks to the headset.
  \item User exploring a virtual vibrotactile texture on a real sheet of paper.
  ]
  \subfigsheight{48.5mm}
  \subfig{experiment/headset}
  \subfig{experiment/apparatus}
\end{subfigs}

\subsection{Procedure}
\label{procedure}

Participants were first given written instructions about the experimental setup and procedure, the informed consent form to sign, and a demographic questionnaire.
A calibration was then performed to adjust the HoloLens~2 to the participant's interpupillary distance (IPD), the virtual hand to the real hand size, and the fiducial marker to the finger position.
They familiarized themselves with the task by completing four training trials with the most different pair of textures.
The trials were divided into three blocks, one for each \factor{Visual Rendering} condition, with a break and questionnaire between each block.
Before each block, the experimenter ensured that the \VE and the virtual hand were correctly aligned with their real equivalents, that the haptic device was in place, and attached the cardboard mask corresponding to the next \factor{Visual Rendering} condition to the headset.

The participant started the trial by clicking the middle button of a mouse with the left hand.
The first texture was then rendered on the augmented area of the paper sheet for \qty{3}{\s} and, after a \qty{1}{\s} pause, the second texture was also rendered for \qty{3}{\s}.
The participant then had to decide which texture was the roughest by clicking the left (for the first texture) or right (for the second texture) button of the mouse and confirming their choice by clicking the middle button again.
If the participant moved their finger away from the texture area, the texture timer was paused until they returned.
Participants were asked to explore the textures as they would in real life by moving their finger back and forth over the texture area at different speeds.

One of the textures in the tested pair was always the reference texture, while the other was the comparison texture.
Participants were not told that there was a reference and a comparison texture.
The order of presentation was randomized and not revealed to the participants.
All textures were rendered as described in \secref[vhar_system]{texture_generation} with period $\lambda$ of \qty{2}{\mm}, but with different amplitudes $A$ to create different levels of roughness.
Preliminary studies allowed us to determine a range of amplitudes that could be felt by the participants and were not too uncomfortable.
The reference texture was chosen to be the one with the middle amplitude to compare it with lower and higher roughness levels and to determine key perceptual variables such as the \PSE and the \JND of each \factor{Visual Rendering} condition.
The chosen \TIFC task is a common psychophysical method used in haptics to determine \PSE and \JND by testing comparison stimuli against a fixed reference stimulus and by fitting a psychometric function to the participant's responses (\secref[related_work]{sensations_perception}).
The user study took on average one hour to complete.

\subsection{Experimental Design}
\label{experimental_design}

The user study was a within-subjects design with two factors:
\begin{itemize}
  \item \factor{Visual Rendering} consists of the augmented or virtual view of the environment, the hand and the wearable haptic device, with 3 levels: real environment and hand view without any visual augmentation (\figref{renderings}, \level{Real}), real environment and hand view with the superimposed virtual hand (\figref{renderings}, \level{Mixed}) and virtual environment with the virtual hand (\figref{renderings}, \level{Virtual}).
  \item \factor{Amplitude Difference} consists of the difference in amplitude of the comparison texture with the reference texture (which is identical for all visual renderings), with 6 levels: \qtylist{\pm 12.5; \pm 25.0; \pm 37.5}{\%}.
\end{itemize}

A trial consisted of a \TIFC task in which the participant touched two virtual vibrotactile textures one after the other and decided which one was the roughest.
To avoid any order effect, the order of \factor{Visual Rendering} conditions was counterbalanced between participants using a balanced Latin square design.
Within each condition, the presentation order of the reference and comparison textures was also counterbalanced, and all possible texture pairs were presented in random order and repeated three times.
A total of 3 visual renderings \x 6 amplitude differences \x 2 texture presentation order \x 3 repetitions = 108 trials were performed by each participant.

\subsection{Participants}
\label{participants}

Twenty participants were recruited for the study (16 males, 3 females, 1 preferred not to say), aged between 18 and 61 years (\median{26}{}, \iqr{6.8}{}).
All participants had normal or corrected-to-normal vision, and none had a known hand or finger impairment.
One was left-handed and the rest were right-handed; they all performed the task with their right index.
When rating their experience with haptics, \AR and \VR (\enquote{I use it several times a year}), 12 were experienced with haptics, 5 with \AR, and 10 with \VR.
Experience was correlated between haptics and \VR (\pearson{0.59}), and \AR and \VR (\pearson{0.67}), but not haptics and \AR (\pearson{0.20}), nor haptics, \AR, or \VR with age (\pearson{0.05} to \pearson{0.12}).
Participants were recruited at the university on a voluntary basis.
They all signed an informed consent form before the user study and were unaware of its purpose.

\subsection{Collected Data}
\label{collected_data}

For each trial, the \response{Texture Choice} by the participant as the roughest of the pair was recorded.
The \response{Response Time} between the end of the trial and the choice of the participant was also measured as an indicator of the difficulty of the task.
At each frame, the \response{Finger Position} and \response{Finger Speed} were recorded to control for possible differences in texture exploration behaviour.
After each \factor{Visual Rendering} block of trials, participants rated their experience with the vibrotactile textures (all blocks), the vibrotactile device (all blocks), the virtual hand rendering (all except \level{Mixed} block) and the \VE (\level{Virtual} block) using the questions shown in \tabref{questions1}.
They also assessed their workload with the NASA Task Load Index (\response{NASA-TLX}) questionnaire after each blocks of trials (\tabref{questions2}).
For all questions, participants were shown only labels (\eg \enquote{Not at all} or \enquote{Extremely}) and not the actual scale values (\eg 1 or 5) \cite{muller2014survey}.

The results were analyzed using R (v4.4) and the packages \textit{afex} (v1.4), \textit{ARTool} (v0.11), \textit{MixedPsy} (v1.2), \textit{lme4} (v1.1), and \textit{performance} (v0.13).

\newcommand{\scalegroup}[2]{\multirow{#1}{1\linewidth}{#2}}
\afterpage{
  \begin{tabwide}{questions1}
    {First part of the questions asked to participants after each \factor{Visual Rendering} block of trials.}
    [
      Unipolar scale questions were 5-point Likert scales (1~=~Not at all, 2~=~Slightly, 3~=~Moderately, 4~=~Very and 5~=~Extremely).
      Bipolar scale questions were 7-point Likert scales (1~=~Extremely A, 2~=~Moderately A, 3~=~Slightly A, 4~=~Neither A nor B, 5~=~Slightly B, 6~=~Moderately B, 7~=~Extremely B),
      where A and B are the two poles of the scale (indicated in brackets in the Scale column of the questions).
      Participants were shown only the labels for all questions.
    ]
    \begin{tabularx}{\linewidth}{l X p{0.2\linewidth}}
      \toprule
      \textbf{Code}        & \textbf{Question}                                                                                                  & \textbf{Scale}                       \\
      \midrule
      Texture Agency       & Did the tactile sensations of texture seem to be caused by your movements?                                         & \scalegroup{4}{Unipolar (1-5)}       \\
      Texture Realism      & How realistic were the tactile textures?                                                                           &                                      \\
      Texture Plausibility & Did you feel like you were actually touching textures?                                                             &                                      \\
      Texture Latency      & Did the sensations of texture seem to lag behind your movements?                                                   &                                      \\
      \midrule
      Vibration Location   & Did the vibrations seem to come from the surface you were touching or did you feel them on the top of your finger? & Bipolar (1=surface, 7=finger) \\
      Vibration Strength   & Overall, how weak or strong were the vibrations?                                                                   & Bipolar (1=weak, 7=strong)           \\
      Device Distraction   & To what extent did the vibrotactile device distract you from the task?                                             & \scalegroup{2}{Unipolar (1-5)}       \\
      Device Discomfort    & How uncomfortable was it to use the vibrotactile device?                                                           &                                      \\
      \midrule
      Hand Agency          & Did the movements of the virtual hand seem to be caused by your movements?                                         & \scalegroup{5}{Unipolar (1-5)}       \\
      Hand Similarity      & How similar was the virtual hand to your own hand in appearance?                                                   &                                      \\
      Hand Ownership       & Did you feel the virtual hand was your own hand?                                                                   &                                      \\
      Hand Latency         & Did the virtual hand seem to lag behind your movements?                                                            &                                      \\
      Hand Distraction     & To what extent did the virtual hand distract you from the task?                                                    &                                      \\
      Hand Reference       & Overall, did you focus on your own hand or the virtual hand to complete the task?                                  & Bipolar (1=own, 7=virtual) \\
      \midrule
      Virtual Realism      & How realistic was the virtual environment?                                                                         & \scalegroup{2}{Unipolar (1-5)}       \\
      Virtual Similarity   & How similar was the virtual environment to the real one?                                                           &                                      \\
      \bottomrule
    \end{tabularx}
  \end{tabwide}
}
