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U. Glaeser

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and some electronic tablets. Some of these have a glass surface on the pad that is the interface to capture<br />

and display ink. Others (like the electronic tablets) have an input device that is separate from the display.<br />

Although these are usable for small quantities of input and for navigating through applications using<br />

touch-selection, they are extremely difficult from a usability standpoint for entering large quantities of<br />

digital ink. Paper-based systems provide the best user interface for inputting digital ink and hence are<br />

the best user interface for large note-taking applications. They are also the highest resolution displays<br />

for ink available today and do not suffer from issues like tolerance to glare that glass surfaces do.<br />

In systems that use paper, an additional challenge that needs to be addressed comes from the fact that<br />

pen tip contact with paper also needs to be sensed to establish when ink is deposited on paper.<br />

Pen hardware platforms available today use one of the following four kinds of technologies:<br />

Magnetic Tracking Here, sequentially energized coils embedded in the pad couple a magnetic field<br />

into a pen tank circuit (coil and capacitor). Neighboring coils pick up the magnetic field from the<br />

pen, and their relative strength determines pen location [22]. The magnetic field can also be<br />

generated in the pen, requiring a battery that increases pen weight and thickness [23].<br />

Electric Tracking Here, the conductive properties of a hand and normal pen can be used for tracking<br />

[24]. A transmitter electrode in the pad couples a small displacement current through the paper<br />

to the hand, down through the pen, and back through the paper to an array of receiver electrodes.<br />

Pen location is calculated as the “center of mass” of the received signal strengths.<br />

Ultrasonic Tracking Ultrasonic tracking is based on the relatively slow speed of sound in air (330 m/<br />

sec). A pen generates a burst of acoustic energy. Electronics in the pad measure the time of arrival<br />

to two stationary ultrasonic receivers [4,5]. The ultrasonic transmission is either synchronized to<br />

the pad, typically with an infrared signal, or a third ultrasonic receiver is used [26].<br />

Optical Tracking Technology Optical tracking systems can either provide relative tracking (like a<br />

mouse) or absolute position tracking (like a touch screen) [28,29].<br />

Bar codes printed over an entire page can provide absolute position using a tiny camera mounted in a<br />

pen [30,31]. The bar codes can also encode page number, eliminating overwrites when a person forgets to<br />

tell the digitizer they have changed pages (a challenge that pen hardware systems with paper interfaces have<br />

to address). Another approach captures a sequence of small images of handwriting and assembles them to<br />

reconstruct the entire page [32].<br />

Discussion of Input Hardware<br />

Magnetic tracking is the widest deployed system due to high spatial resolution (>1000 dpi), acceptable<br />

temporal resolution (>100 Hz), reliability, and modest cost [33].<br />

Magnetic and electric tracking require pad electronics and shielding, making them thicker and heavier<br />

than a conventional clipboard. Electric tracking uses a normal pen but has no direct way to measure pen<br />

tip contact, and must rely on less reliable pen trajectory analysis [34].<br />

Ultrasonic tracking does not require pad electronics, making it lower cost and weight. Relative tracking<br />

can reach 256 dpi, but absolute spatial resolution is limited to about 50 dpi due to air currents that cause<br />

Doppler shifts.<br />

Optical tracking offers the highest spatial (>2000 dpi) and temporal (>200 Hz) resolution, and can<br />

utilize a self-contained pen that remembers everything written. Special bar code paper provides absolute<br />

position and page tracking. Optical methods based on CMOS technology lend themselves to low-power,<br />

low-cost, and highly integrated designs. These features suggest that optical tracking will play a significant<br />

role in future pen systems.<br />

Handwriting Recognition<br />

Handwriting is a very well-developed skill that humans have used for over 5,000 years as means of<br />

communicating and recording information. With the widespread acceptance of computers, the future<br />

role of handwriting in our culture might seem questionable. However, as we discussed in introduction<br />

© 2002 by CRC Press LLC

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