a face-aspect consists of four edges, the first- 
figure is 4. Others are on the analogy of this. 
2) If the edge in face-aspect is straight line, the 5) If a face-aspect is composed of more than 
code is defined as 1, or as 0. 
3) If each pair of opposite edge is parallel, the of many primitives by Boolean operator. The 
code is defined as |, or as 0. 
its code is defined as 10c, or it is an ellipse and 
its code is defined as 10e. 
four edges, it possibly is an unit or intersection 
coding principle is in the same way as above. 
We build up a base of face-aspect code for 
  
    
    
  
1 
  
  
  
eleven face-aspect, and their codes correspond 
tol. 4111111, 2. 4111110 or 4111101, 3. 
4101011 or 4010111, 4. 4101001 or 4010110, 
5. 4000011, 6. 3100 or 3001 or 3010, 7. 3111, 
8. 3011, 3110 or 3101, 9. 10e, 10. 10c, 11. 20. 
2.2. Coding Mergence Principle 
  
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In some case, face-aspect can correctly be 
interpreted into volume-primitive using face- 
aspect code stored in base. However, a 
primitive-aspects in other case maybe be two or 
several face-aspect mergence. So it is necessary 
to study the mergence principle of face-aspect. 
When view point is located in general position, 
  
  
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we result in twelve face-aspect mergence 
relations, which describe the projection of two 
neighbor face-aspects, and then classify these 
mergence codes into a base of mergence 
code[Fig. 4]. In base of mergence code, the 
  
  
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mergence code means the mergence of two 
ordinal numbers describing face-aspect in base 
of face-aspect code. For example, mergence 
code 11 means two face-aspect codes are 
4111111 because ordinal number 1 stands for 
face-aspect 4111111. Others are on the analogy 
  
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of this. 
A O—0 e O—9 
  
  
    
   
   
       
  
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Fig. 3 Face-aspect code 
| Fig. 2 Projection of primitives 
| 4) If the code of face-aspect is 10, it shows the 
| face possibly is a circle or an ellipse. In this case, 
| . curvature should be computed. If curvature is 
equal in any point, it shows the face is a circle, Fig. 5 Multiple faces mergence 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
emxe-o | 
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Fig. 4 Face-aspect mergence code 
  
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