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Exercises 287

from alignment of segment references. Use as segment reference the upper edge of the
tank cannon.

6.14 Use an attributed string matching approach for the detection of P waves in ECG
signals, selecting one of the waves as prototype. Perform experiments using the column
G signal of ECG Signals.xls, characterised by the presence of multiple P waves.

6.15 An impulsive signal sometimes appears buried in noise. The signal impulses can be
described by:

a) Draw the state diagram corresponding Lo these rules.
b) What is the probability of the sequence uudud appearing, and what are the sequences
most favoured by the transition probabilities?
c) The presence of noise enhances the occurrence rate of saw-tooth sequences of the
udud type. Modify the transition probabilities so that such sequences have always a
probability of occurring below 0.05.

6.16 Consider the chain code description of object contours, presented in section 6.1.2.

a) Write the string descriptions of the shapes above, assuming that the description starts
with the upper left point.
b) Assuming that all sides are equal, as for the square, what is the correspondence rule
between descriptions of the same object rotated by multiples of 4S0?
c) Taking into consideration the descriptions of the triangles, how should one modify
the previous rule in order to obtain the same chain code descriptions of any object
rotated by multiples of 45"?
d) How must one modify the algorithm that computes the Levenshtein distance of such
chain code strings, so that the matching score obtained is invariant to object rotations
by multiples of 4S0?

6.17 Consider the tree description of cupboards, as exemplified in Figure 6.6. Discard the
body primitive and the o, i relations. Assume that the cupboards are built in a modular
way, with modules constituted by at most two pieces (e.g., a module can have two
shelves, another can have a drawer either on the top or on the bottom, etc.).
Furthermore, there are equivalent modules; for instance, a module with a top shelf is
considered equivalent to another one with a bottom shelf. Develop a discrete relaxation