Page 102 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Lab on Computer Graphics

Notes
Figure 6.2: A Non-simple Polygon

The third step accepts a sorted list of points and connects them according to the odd-party
rule. For example, given the list [p 1 ; p 2 ; p 3 ; p 4 … p 2n-1 ; p 2n ], it draws the lines p 1 → p 2 ; p 3 → p 4 ;
…. ; p 2n-1 → p 2n . A decision must be taken as to whether the edges should be displayed or not:
given that p 1 = (x 1 , y) and p 2 = (x 2 , y), should we display the line (x 1 , y, x 2 , y) or just the interior
points (x 1 + 1, y, x 2 – 1, y).

Consider the polygon in Figure 6.2. The edge Table 6.1 for such a polygon would be:

Table 6.1: Edge Table

1
Edge Y min Y max X of Y min X of Y max
m
E1 11 11 6 11 0
E2 7 11 15 11 –1
E3 3 7 15 15 0
E4 3 3 10 15 –
E5 3 5 10 10 0
E6 5 5 10 6 –
E7 3 5 6 6 0
E8 3 3 3 6 –
E9 3 7 3 3 0
E10 7 11 3 7 1

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