es). 
s- 
s of 
take 
de 
ements. 
nee 
Fig. 3: Incomplete fire screen DTM 
3. CONSTRUCTION OF THE OBJECT 
If we put up only such screen elements which can't be hidden (partially or 
totally) by others, we don't have to care about comparisons between pairs of 
elements when considered as randomly distributed objects! The regular struc- 
ture of our DTM grid does allow this. 
Fig. 3 shows: Screen element X12 could hide screen element Y12 or £X22 3; 
therefore X12 must be set up earlier than Y12 and X22. . More general: 
X1j elements can hide X2j elements; X2j elements may hide X3j elements 
and.so on. A similar rule is valid for the Yij elements. So we can deduce 
priority rules based on grid-indices rather than on euklidean distances. A 
three-dimensional problem is reduced to two dimensions therewith. 
But a threatening question occurs: What is the role played by the viewing 
point? The rules of hiding indicated above aren't general. From a viewing 
point far in the background of fig. 3, we see: X1j screen elements may be 
hidden by X2j elements, and so on. The Xij - rule is just reversed. The 
general rule for setting up screen elements may be seen in fig. 4, which is 
a groundplan of fig. 3: the viewing point P is chosen in mesh (2,2) 
  
  
  
  
X S xr 
e n3 w Re c 
iz4 en) BTE 
i-3 : … X3j 
i2 o e X2j 
i- x1j 
  
  
  
  
  
  
Fig. 4: Groundplan of the incomplete DIM shown in fig. 3 
X3j - elements precede X4j - elements but 
X2j - elements precede X1j - elements 
Yi3 - elements precede Yi4 - elements & Yis - elements 
Yi2 - elements precede Yil - elements