  We present RealPen, an augmented stylus for capacitive tablet screens that
recreates the physical sensation of writing on paper with a pencil, ball-point
pen or marker pen. The aim is to create a more engaging experience when writing
on touch surfaces, such as screens of tablet computers. This is achieved by
re-generating the friction-induced oscillation and sound of a real writing tool
in contact with paper. To generate realistic tactile feedback, our algorithm
analyses the frequency spectrum of the friction oscillation generated when
writing with traditional tools, extracts principal frequencies, and uses the
actuator's frequency response profile for an adjustment weighting function. We
enhance the realism by providing the sound feedback aligned with the writing
pressure and speed. Furthermore, we investigated the effects of superposition
and fluctuation of several frequencies on human tactile perception, evaluated
the performance of RealPen, and characterized users' perception and preference
of each feedback type.
