Multi-Modal Signal Processing

Chapter | 6 Processing of Handwriting and Sketching Dynamics

123

nition, sketching, gesturing all the way to biometric user recognition

by signature analysis. This chapter provides an overview to technical

concepts of the most common sensors today, reviews principles for the

analysis of signals derived from writing and drawing dynamics and

discusses different recognition objectives. Finally, we briefly review

algorithms for two exemplary applications: pen-based sketches for

interactive interface design [5] and similarity search in databases of

handwriting digital documents [6].

6.3 BASICS IN ACQUISITION, EXAMPLES FOR

SENSORS

The variety of sensors for sampling pen-based dynamics, also referred

to as digitiser tablets, which is available today is based on different

physical phenomena. This section provides a short review of acquisi-

tion technologies and sensors, based on optical, electromagnetic and

resistive/capacitive film methods.

Besides the aforementioned light pen technology, there exists at

least one more concept for optically tracing the pen movement, which

is based on a defined pattern structure, which encode the position by

almost invisible patterns on pre-printed paper. In this technology,

pens are equipped with tiny digital cameras, integrated in the pen

tip, which continuously perform video recordings of the area below

the pen tip. A pattern recognition algorithm, executed by a computer

device embedded in the pen, determines the pen position based on

co-ordinates encoded by the patterns printed on the pen surface. Sen-

sors of this type have been developed, commercialised and licensed

to other organisations for example by a company named Anoto AB

(www.anoto.com).

Besides more exotic methods based on ultrasonic triangulation,

acceleration and force sensors attached to the writing surface, today

the majority of digitiser tablets are either electromagnetically oper-

ated or are based on resistive/capacitive film technology. The first

category is typically based on inductive coupling between an induc-

tor in the pen tip and a matrix of circuit tracks embedded in the writing

surface (e.g., the tablet PC display), powered by AC voltage at differ-

ent frequencies. By frequency analysis of the voltage induced at the

pen tip, the pen position can be determined. Tablets based on resistive