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16 1 Basic Notions

Example of a neural net application: Foetal weight estimation is important for
assessing antepartum delivery risk. For that purpose a set of echographic
measurements of the foetus are obtained and a neural net is trained in order to
provide a useful estimate of the foetal weight.

1.4.4 Structural PR

Structural pattern recognition is the approach followed whenever one needs to take
into consideration the set of relations applying to the parts of the object to be
recognized. Sometimes the recognition assumes the form of structural matching,
when one needs to assess how well an unknown object or part of it relates to some
prototype. A nmtching score is then computed for this purpose, which does not
necessarily have the usual properties of a distance measure.
A particular type of structural PR, known as syntactic PR, may be followed
when one succeeds in formalizing rules for describing the relations among the
object's parts. The goal of the recognizing machine is then to verify whether a
sequence of pattern primitives obeys a certain set of rules, known as syntactic rules
or grrrmrnur. For that purpose a syntactic analyser or purser is built and the
sequence of primitives inputted to it.
Structural analysis is quite distinctive from the other approaches. It operates
with symbolic information, often in the form of strings, therefore using appropriate
non-numeric operators. It is sometimes used at a higher level than the other
methods, for instance in image interpretation, after segmenting an image into
primitives using a statistical or a neural net approach, the structure or relation
linking these primitives can be elucidated using a structural approach.
Some structural approaches can be implemented using neural nets. We will see
an example of this in Chapter 6.
Example of structural analysis: Given the foetal heart rate tracings mentioned in
section 1.2.3 design a parser that will correctly describe these tracings as sequences
of wave events such as spikes, accelerations and decelerations.

1.5 PR Project

1.5.1 Project Tasks
PR systems, independent of the approach followed to design them, have specific
functional units as shown in Figure 1.12. Some systems do not have pre-processing
andlor post-processing units.
The PR system units and corresponding project tasks are:

1. Puttern acqui.sition, which can take severd forms: signal or image acquisition,
data collection.
2. Feature extraction, in the form of measurements, extraction of primitives, etc.