So we may formulate the hiding rule for screen elements: 
eo 
-— 
Xij - elements precede Xkj - elements if i is nearer to k 
Yij - elements precede Yil - elements if j is nearer to 1 
In a program, index loops should start near the projection center and run 
away using positive increments for positive coordinate direction, negative in- 
crements for negative direction. So each quadrant (referred to the projection 
centre as origin) is characterized by its own increment pair. 
But: How do screen elements of different direction intervene? 
Unfortunately Xij - elements and Yij - elements aren't independent. 
If we want to display only profilelines our rules are sufficient, we may dis- 
regard the complementary screen set. For displaying both grid directions, we 
have to choose one direction as dominant, the other as subordinate, corres- 
ponding to the nesting of index loops. So we scan one quadrant of the grid. 
We put onto each grid point a pair of screen elements as fig. 5 shows: 
  
  
  
  
  
  
  
  
Y 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
first pair second pair next pair last pair 
£ 
Fig. 8: Gert 
n the first quadrant 
ng up pairs of screen elements 
e 
We see: no member of any pair can obscure his partner, no pair can be hidden 
later one, 
4 
nd 
We continue with the 2 Quadrant: 
  
  
  
  
  
  
  
  
o 
o 
o 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
| 
i 
i 
  
  
  
  
  
  
  
  
first pair second pair next pair last pair 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   
     
Fig. 6: Setting up pairs of screen elements 
in the second quadrant 
And so on 
E A = 
^ o Oo eit veri. à 0 
fied E 1 7 
i d. 
* * * * 2 
first pair second pair next pair last pair 
T. 7 
Fan 
- wid a 
> 
C 
3 
» 
ct