those that would normally be encountered, in order to test 
the algorithms at large angles. 
The film was marked with three reference points which 
remained in the same positions on the film throughout the 
image capture with the CID camera. To test system 
performance, four anatomic features were chosen on the first 
image as it was displayed on the monitor. The upper incisor 
is an example of the type of feature selected. In addition, 
one or more specific points on each feature were selected 
and stored using a cursor also shown on the screen. For 
example, the edge of the upper incisor was one such point. 
A region around the entire tooth was selected using a 
moveable displayed window, and the digital "patch" stored on 
disk. The three subsequent rotated images were displayed on 
the monitor, and the same four points and patches chosen as 
closely as possible, given the changes in images appearance 
due to the rotation. After all feature points and image 
patches were chosen, the boundary detection software 
processed all the sub-images. 
The main goal of this testing procedure was to determine the 
accuracy with which feature boundaries could be extracted 
and fitted together in the presences of rotation and 
translation. However, because each anatomic point was 
selected independently on each of the four orientations of 
the x-ray, pointing error was a component of the final 
misclosure in point coordinates. In order to separate the 
pointing error from the transformation error, the following 
procedure was developed. 
On the first film, the x and y location of each point, for 
example the incisor edge, was taken to be correct. Then, 
the first film was transformed onto the second film, using a 
four parameter transformation on the three fixed reference 
points. The coordinates of the anatomic feature point (i.e. 
incisor edge) were transformed to the coordinate system of 
the second film, and inserted as an artificial point into 
the digital representation of the second film. This process 
was repeated for all points on the second, third, and fourth 
films. 
Then, each feature boundary was fit to its boundary on the 
first film as determined by the boundary detection 
algorithm. For example, the boundary for the upper incisor 
of the second film was transformed to the corresponding 
boundary on the first film, and the coordinates of the 
anatomic point selected on the second film (i.e. incisor 
edge) were then reported in the coordinate system of the 
first film. In addition, the artificial point on the second 
film was transformed, using the same parameters, to the 
coordinate system of the first. 
If the fit on feature boundaries were exact, the coordinates 
of the artificial point would be the same as on the first 
film, and the anatomic point would differ only because of 
pointing error. However, because the fits were not exact 
due to boundary changes under rotation, and algorithmic 
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