Information Technology Reference
In-Depth Information
2.1.4 Capturing Pen Input
Pen-input is an important style of interaction with paper documents. Users take
notes, make annotations and sketches, or interact with printed user interfaces, e.g.
by tapping with a pen on printed buttons. There is a wide range of technologies
for capturing pen input. Technology for capturing pen input on real paper should
offer high tracking performance while restricting the natural interaction as little as
possible. A first class of approaches tracks the relative position of a pen with respect
to a separate tracking device, e.g. a camera. A second class of capturing approaches
is able to directly capture the absolute position on a page.
All approaches have in common to generate not only a two-dimensional repre-
sentation of the pen traces (as does a photo or a scan of the document). In addition,
they record temporal information of how the traces are made over time. Some tech-
nologies also track the force with which the pen tip is pressed onto the paper sheet.
At regular intervals, e.g. 50 times per second, a so-called sample is captured. This
sample contains the current position of the pen, the current time and optionally the
pen tip force. A set of samples is called digital ink data. By interpolating curves
through the sample coordinates, the pen traces can be visualized.
Camera-based Capturing
Above we have discussed how camera-based capturing can be used for detecting
touch input. Similar approaches have been presented for tracking pen input. The
DigitalDesk [167] analyzes images of the desk that an overhead camera is continu-
ously capturing. It detects not the pen itself, but the traces that are made with a pen.
While this allows the system to record traces, it is not possible to detect interactions
that leave no visible ink traces, such as tapping with the pen.
Other approaches aim at tracking the pen itself, not only the visible traces the
user has made with it. Typically one or several visual markers are attached to the
pen. This allows the system to robustly identify the pen in the camera image. These
markers can be active markers that emit light signals [70] or passive markers that
reflect light of a given wavelength [40].
Ultrasonic Pens
A second technique relies on ultrasonic triangulation. The digital pen continuously
emits an ultrasonic signal which is not audible by humans. An additional separate
tracking device is attached to the paper sheet(s). It has two or more reference points
that capture the ultrasonic signal. By calculating the time difference with which the
signal reaches the reference points, the position of the pen with respect to these
reference points can be calculated.
The temporal and spatial resolution of this technology is high enough for captur-
ing handwriting. The tracking does not depend on the material the pen is used upon
