For example, on the first film, the upper incisor was chosen 
as a feature, and reduced to a boundary representation by 
the system (Figure 3). The corner of the tooth was chosen 
as the point at which tooth displacement would be reported. 
On the second digital image, the system extracted a larger 
patch from the region of the image where the feature had 
been located on the first image. This patch was searched 
until the incisor boundary was found. Note on Figure 3 the 
change in feature appearance and orientation that occurred 
during the one year interval between films. In addition, the 
extracted feature boundaries vary due to smearing of the 
edges on the films themselves. The two boundaries were 
matched using the algorithm described in Section 4, and the 
displacement of the point on the feature was computed. 
This process was repeated for the remaining features on the 
second film, and for the third film. Note that the timepoint 
three boundary in Figure 3 is quite different, due to the 
placement of an orthodontic appliance on the tooth. For 
comparison purposes, the same feature points were digitized 
on each of the x-ray films themselves with a 0.1 mm 
resolution Summagraphics digitizer. The registration points 
were also digitized, and the feature point displacements 
reported in the anatomic coordinate system defined by the 
registration points. 
5.2 Analysis 
The anatomic feature comparisons were performed with four, 
five, and six parameter transformations, and the six 
parameter solution was again the best. As a result of the 
boundary length tests performed in Section 6.3, a larger 
number of feature boundary pixels (25) were used in these 
tests. The average error in computed point displacement was 
0.3 pixels (0.21mm at the image scale), slightly smaller 
than the overall average from Section 4. The reduction in 
error is likely due to the larger number of boundary pixels 
included in the feature transformations. The results 
indicate that the matching algorithm performs well, even in 
the presence of changes in feature shape and orientation due 
to anatomic growth. There was no significant difference in 
the results of the transformation of the timepoint 2 film 
onto the timepoint 1 film, compared to transformation of the 
timepoint 3 film.However, in this case the improvement over 
the 5 parameter transformation was significant at the .01 
level, indicating the non-orthogonality of feature growth 
over the two year period. 
These results do not have the effects of pointing error 
removed, as did the tests in Section 4. When a feature was 
initially selected on the first x-ray of the series, a 
feature point was designated as the point at which 
displacement would be computed. For example, the 
displacement of the upper incisor would be reported at the 
tooth edge. Assume that this point selection was exact, and 
only limited by the resolution of the digital image. When 
the x-rays were manually digitized to compute the comparison 
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