1: end point. Only one arc is tied to it. This might occur 
if no further measurement is possible, or re- 
quired, e. g. at the border of the area of inte- 
rest. 
2: break point. Important points, such as points on edges 
(which may not be included in the graph) may di- 
vide an arc into two arcs. 
3: boundary point. If the boundary of an area of interest is 
included in the wire model, the intersection with 
a given plane results in a node of degree 3. 
4: intersection point of two planes in the interior of the 
area of interest. 
5: intersection point of three planes at the boundary. This 
is very unlikely. 
6: intersection point of three planes on the surface. This 
also is a very unlikely situation on arbitrary 
surfaces. 
The last two cases are explicitely included in the data 
structure in order to be able to handle simulated data. 
Nodes of higher order degree are resolved by logically 
splitting them into nodes of lower degree while keeping the 
coordinates unchanged. 
F 
Figure 2: 
Example of a wire model 
      
    
The list of planes contains planes of two types. The 
boundary of the area of interest is defined by a polygon in 
one of the two images which together with the corresponding 
projection centre builds up a set of boundary planes. On the 
other hand the user can read in a separate list of planes 
which together with the surface implicitely define the wire 
model. The orientation of the planes in object space is de- 
fined by the equation a°x+b*y+c*‘z+d=0, the parameters a, b, 
c and d being contained in the plane-description. For each 
plane a pointer to the first arc in the list of aros is 
stored. With the pointers p(next node) in the list of aros 
one is able to connect all arcs belonging to one plane in 
order to build up profiles. Thus the list of planes has the 
following form: 
L.plane = (description, p(first arc), typel 
The notion “plane” results from the present application 
of this data structure. Of course also more complicated sur- 
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