Figure 4. Intersecting the line segment planes with the 
XY-plane 
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A 4 1 
UA 
4 
Weighted sum of line segments 
  
0 A 
  
  
0 25 50 75 100 125 150 175 
Intersection angle with XY-plane 
Figure 5. Finding main directions (all images) 
Figure 3. The segmentation result using Object Related Ima 
building. 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
       
      
       
         
          
  
     
ge Segmentation (ORIS) on six image patches of one 
3.5 Searching for the main structures 
Having found the main directions in object space, i.e., the 
vanishing directions in the images, a 3D clustering of the 
line segments that contribute to these directions is carried 
out. The clustering of intersecting line segments is done 
for all images simultaneously. The aim of the evaluation 
of the clusters is to determine the correspondence 
between image lines (Fig. 6). Geometrical rules like 
parallelity,  perpendicularity and connectivity are 
incorporated in the clustering algorithm. 
In Fig. 7, all line segments belonging to a main 
direction are projected on a cluster plane and in Fig. 8 
only clusters that have at least five intersecting lines. The 
cluster planes are evaluated using MDL to remove 
contradictions and to find the most probable solution. The 
found 3D lines are back-projected on an image in Fig. 9.