V iQ oo 
For the feature tracking tests, the initial film ofa 
patient was converted to a digital image with the CID camera 
and displayed on a monitor. The operator moved a cursor 
(also displayed on the screen) through the image, choosing 
and storing anatomic features of interest. Also, a 
rectangular "patch" boundary was displayed on the monitor 
(Figure 1). 
  
Figure 1 - Feature Selection 
The patch could be enlarged or reduced as necessary in order 
to cover as much of the feature as the operator deemed 
necessary to uniquely identify it. Each patch was stored as 
a subset of the original image, along with the coordinates 
of the patch boundaries and the coordinates of any 
identified points of interest within the patch. These might 
include the apex of a tooth root, the tip of an incisor, or 
the center of the pituitary fossa (sella). 
After the operator completed identifying points of interest 
on the baseline film, any remaining films for the patient 
were processed automatically. Each x-ray film was digitized 
with the CID camera, and the resulting digital images were 
Stored in the system and scanned by the software, tracking 
the points already identified on the baseline image. 
3. FEATURE BOUNDARY DETECTION AND TRANSFORMATION 
Straightforward correlation methods match digital image sub- 
pictures on the basis of pixel to pixel grey level 
differences. However, if the features being matched differ 
in shape, size, and orientation, the correlation method does 
not work well. Several refinements to the basic correlation 
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