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Figure 5. Match points of hip roof (red dots) 
regular shape except one of the gable roofs above the garage 
which has seven vertices and the distance between some of the 
vertices is very small as shown in Figure 6. There are no trees 
near the building except some shadow and a deck in the 
backyard. As can be seen, the building roof extraction was quite 
successful and all roof points were extracted and matched 
correctly (red dots) and all roof facets were created. The 3D 
view of the reconstructed building is shown in Figure 6. 
    
    
(a) Matched roof points : 
(b) 3D view of reconstructed building 
Figure 6. Matched points of complex building roof 
   
  
   
    
    
   
    
   
    
  
   
    
  
  
  
  
  
  
    
  
   
   
   
   
   
   
   
   
  
  
   
   
   
   
  
  
   
  
   
  
  
  
  
  
  
    
       
Figure 7 shows the extraction result of a building with more 
complex structure. The building has 15 facets and most of them 
have reasonable size. Due to the low sun angle the image was 
captured, there are some shadows cast on the roof which caused 
some problem for one of the hip roofs above the garage. As 
noticed, two points (green dots) were not matched since they 
are not visible in three oblique images. As a result, two facets 
on both sides of the valley line between these two points (green 
line) were not created. 
  
Figure 7. Building reconstruction of complex building 
To investigate the applicability of the developed approach, a 
large residential building was tested. The building has four 
units under one roof and very complex roof structure. There are 
6 different shapes of facets and 30 roof facets totally. The most 
complex facet has 10 vertices and adjoins 8 different facets. As 
shown in Figure 8, most roof points were extracted except two 
and 26 out of 30 roof facets were reconstructed successfully. 
The reason that four facets were not extracted is that two ridge 
lines were not extracted due to the low contrast along the ridge 
lines (blue lines). This caused the gable roof facets on both 
sides of the ridge lines not generated correctly. Two points 
(green dots) were not extracted because the valley line between 
them was not extracted by the Canny operator due to the low 
contrast. 
  
Figure 8. Building reconstruction of large complex building