Master Thesis - Fachbereich Informatik

4.5. MEASURING POINT DETECTION 85
(a)
(b) (c)
Figure 4.22: (a) Edge response of an angular oriented (transparent) tube edge. The characteristic
peaks at the ends of transparent tube edges do not have to lie on one line perpendicular
to the x-axis. The red line visualizes the slight angular orientation of the tube edge. (b) Example
detection result with k = 1 orientations. (c) Corresponding result with k = 3 orientations.
template. Figure 4.22(a) visualizes the edge response of a slightly angular tube edge of a
transparent tube (left side). In such a situation no template will fit the edge perfectly. This
can be critical if the edge contrast is poor. In this case, as mentioned before, the stronger
weighting of the template ends helps to support a match at the real tube boundary instead
of at a background edge. With an angular tube edge, a symmetric template can not be
shifted over the image in a way it matches both edge ends. Thus, the cross-correlation
score is significantly smaller and the probability increases that a background edge yields
a larger score.
A little rotation of the template can overcome this problem. Therefore, the bank of
templates is extended by k − 1 rotated versions of each template. It turned out that it is
sufficient to rotate each template by ±2 degrees to cover the range of expected deviations
from the ideal symmetric model. Thus, k = 3 has been used throughout the experiments.
It is assumed that larger angular deviations can not occur due to the guide bars.
Model Knowledge Optimization The number of templates to be checked each time on
the left and right side increases with the number of rotations. Instead of 2 × 10 templates