Typical interactive pens sense the x-y position as well as proximity.
Research explored additional sensing such as pressure~\cite{hinckley13}, tilt~\cite{tian08} and roll~\cite{bi08}.
Not only this extended input vocabulary can be used to map brush parameters.
-But it also enables selecting commands and offer richer interactions, whether it is with combinations of pen and touch interactions~ or by leveraging physical attributes of the pen~\cite{vogel11}.
+But it also enables selecting commands and offer richer interactions, whether it is with combinations of pen and touch interactions~\cite{hinckley10} or by leveraging physical attributes of the pen~\cite{vogel11}.
In this work we were interested in the bending of a flexible pen as additional degrees of freedom.
\subsubsection{Prototypes}
It had to be flexible enough to avoid muscle strain, but stiff enough so that users could write and draw conveniently.
Full length flexible early prototypes shown to be unconvenient for precise manipulation.
Therefore we chose to limit the flexible part to a few centimeters with a rigid part on both size to keep the benefits of both flexible and rigid pens.
+The rigid part between the tip and the flexible part is long enough to enable users gripping the stylus there.
\begin{figure}[htb]
\centering
\node[anchor=south west] (hyperbrush) at ($(hyperbrush.south west) - (0,5)$) {HyperBrush};
\end{tikzpicture}
\label{fig:penprototypes}
- \caption[Flexible pen prototypes]{Two series of flexible pen prototypes. On the top: FlexStylus with a custom bend sensor made of eroded fiber optics. On the bottom: HyperBrush with a consumer electronics bend sensors, and interchangeable flexible components.}
+ \caption[Flexible pen prototypes]{Two series of flexible pen prototypes. On the top: FlexStylus with a custom bend sensor made of eroded fiber optics. On the bottom: HyperBrush with a consumer electronics bend sensor, and interchangeable flexible components.}
\end{figure}
The second series of prototypes, called \emph{HyperBrush}~\cite{guerrero21} (\reffig{fig:penprototypes}, down), used a consumer electronics bend sensor\footnote{\href{https://www.bendlabs.com/}{https://www.bendlabs.com/}}.
Users had no cues on where to hold the stylus.
Therefore the angles inputs were relative rather than absolute.
This is the same issue Buxton reports with the iMac Round Mouse\footnote{\href{https://www.microsoft.com/buxtoncollection/detail.aspx?id=109}{https://www.microsoft.com/buxtoncollection/detail.aspx?id=109}}.
-We addressed this issue with the HyperBrush prototypes be adding a fake button that users were encouraged to keep under their index finger.
+We addressed this issue with the HyperBrush prototypes by adding a fake button that users were encouraged to keep under their index finger.
The relief of this button helped users keeping the stylus in a consistent orientation eyes-free.
%This kind of serendipitous behaviour is typically what we hope we will get with such a research project.
This is a difficulty with this type of work.
We need a design rationale to guide the implemention before we evaluate the device.
+We evaluate the interaction with the device to make sure that users are able to use it the way we wanted them to use it.
However, we want to let such serendipitous behaviour happening because it makes interaction with the device richer.
\begin{figure}[htb]
\end{figure}
-
\subsubsection{Flexural stiffness}
+We evaluated the users' ability to control the bending of FlexStylus, and compared it to pressure input~\cite{fellion17}.
+We found out that the participants of our studies were more precise with the flexible pen than with pressure input.
+Pressure pens are isometric devices, therefore it is unsurprising that they do not perform well with position control~\cite{zhai93,zhai93a}.
+Flexstylus can be considered as an elastic device.
+Therefore position control can be an issue.
+However, the classification of an flexible pen between an isotonic or elastic device essentially depends on it flexural stiffness.
+A soft flexible pen is most likely to behave like an isotonic device while a hard one should act like an elastic or even isometric device.
+Therefore with the next generation of flexible styluses, HyperBrush, we wanted to compare different flexural stiffnesses.
+
+We built several prototypes of different flexural stiffness (\reffig{fig:flexuralstiffnesses}).
+The thickness is ajusted with the thickness of the flexible parts inner walls.
+%The thicker the harder
+The measure of flexural stiffness we used is the ratio of amount of force applied per unit of deflection.
+We measured it with a hollowed cylinder cantilever beam test.
+
+We compared the performance and subjective preferences between these different flexural stiffnesses and a pressure pen for pie-menu item selection and for brush stroke precision.
+The results does no show any clear better configuration.
+This suggest that the best is to provide users several interactive pens that provide different kinds of input vocabularies and haptic feedback.
+Users will choose the one they feel appropriate for their currernt task.
+
+%evaluation, preferences, recommendations
+
\input{figures/flexuralstiffnesses.tex}
+
+
\subsection{Finger identification}
\label{sec:fingeridentification}
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