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Bold lines: Front polygon 
Figure 4. A schema of procedure of visibility sorting. 
3.2.3 Visibility Sorting of TPM: Visibility sorting of 
o 
Tpr :- ATpr,,Tpr,,...,Tpr,) from a 3-D viewpoint O can be 
n 
executed in the following steps. 
Step 1: Generate base triangles Tri :- (Tri,, Tri, ,..., Tri, ! . Let 
the 2-D viewpoint of O be o. 
Step 2: Execute visibility sorting for 7ri , according to the 
procedure described in 3.2.2. 
Step 3: Sort the prisms in the same order of sorted sequence of 
corresponding base triangles. 
4. TRUE ORTHOIMAGE GENERATION BASED ON 
VISIBILITY SORTING 
4.1 Generation of a True Orthoimage 
Visibility sorting can be utilized in true orthoimage generation 
in the following sequences. 
Step 1: Generate TPM and execute visibility sorting. 
Step 2: Prepare output orthoimage buffer and occlusion buffer 
on the memory space and initialize them with value 0 (see 
Figure 5 (1)). Occlusion buffer should have the same width and 
height with the original image. 
Step 3: Get the first prism from the visibility sorting sequence. 
Generate orthoimage for the top of the prism. In the occlusion 
buffer, record the area where the prism is projected in the 
original image, with non-zero pixel value (see Figure 5 (2)). 
Step 4: Get the next prism and execute the same procedure in 
the Step 3. Before resampling of an orthoimage pixel from the 
original image pixel, check the corresponding occlusion buffer 
pixel if it is 0. If not, the orthoimage pixel should remain as the 
625 
value 0. If the resampled pixel value is 0, change it to some 
proper low pixel value, such as 1, to distinguish it from pixels 
in occluded area. Repeat this step until all prisms are processed 
(see Figure 5 (3)-(6)). 
In the resultant orthoimage, pixels of occluded area remain 
value 0. 
Orthoimage Occlusion buffer TPM 
    
   
  
    
     
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
(1)Start 
Occlusion buffer 
The area where Prism 
1 are projected in the 
original image is 
recorded. 
Orthoimage 
      
  
      
TPM 
  
  
  
  
  
  
  
  
  
  
(2)Ortho-rectification for Prism 1 
Orthoimage Occlusion buffer 
   
(3)Ortho-rectification for Prism 2 
Orthoimage Occlusion buffer 
! This part is not ortho- 
rectified because it is 
occlutded by Prism 1. 
D 
  
(4)Ortho-rectification for Prism 2 
Orthoimage Occlusion buffer 
    
  
  
  
  
  
  
  
  
  
  
  
  
Orthoimage Occlusion buffer 
(6) Final Status 
Figure 5. The schema of procedure of true orthoimage 
generation.