Page 153 - DCAP504_Computer Graphics

Computer Graphics

Polygons clip against convex clipping windows in the Sutherland-Hodgeman polygon clipping
algorithm. It does so by clipping the subject polygon against each clip edge, thereby creating
intermediate subject polygons. The Sutherland-Hodgeman algorithm can easily extend to three
dimensions.
Disadvantages of Sutherland-Hodgeman Algorithm

Some of the disadvantages of Sutherland-Hodgeman algorithm are as follows:
1. Sutherland-Hodgeman algorithm constantly generates a single output polygon even though the
clipped polygon is concave and is expected to produce multiple polygons.
2. The polygons are linked with overlapping segments along the edge of the clipping rectangle. The
result may not be desired because it depends on the application.
Weiler-Atherton Algorithm
The Weiler-Atherton algorithm can clip a concave polygon. The clipping region is called as clip polygon
and the polygon to be clipped is referred to as subject polygon. In this algorithm, you have an arbitrary
vertex of the subject polygon. You need to trace around its border in the clockwise direction until you
find an intersection with the clip polygon. While tracing, you need to:
1. Record the intersection point when the edge enters the clip polygon. Continue to trace the subject
polygon.
2. Record the intersection point when the edge leaves the clip polygon. Make a right turn to follow the
clip polygon. You need to consider the clip polygon as subject polygon and vice versa and then proceed
as before.
When the path of traversal forms a sub-polygon, you output the sub-polygon as a part of the overall
result. You need to continue tracing the rest of the subject polygon from the intersection point that
marks the beginning of an untraced edge. When the entire border of the subject polygon can be traced
once then the algorithm terminates.
The Weiler-Atherton algorithm is mainly used in modeling to find the union, intersection and
difference of the polygons. Consider two polygons M and N with the vertices as shown in the figure
10.5.

Figure 10.5: Weiler-Atherton Algorithm

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