Polygon Clipping

[Hearn & Baker: p. 237-242. Foley & van Dam: p. 124-127]
The Sutherland-Hodgman algorithm can be used to clip any polygon (convex or concave) against any convex clipping polygon. The algorithm clips against one edge at a time, producing a new vertex list each time. The following figure assumes the most common case, a rectangular clipping window.

The algorithm can be summarized as follows: 

  for each side of clipping window
     for each edge of polygon
        output points based upon the following table
Example
 
 

 

 

 

 

Using Outcodes for Trivial Accept and Reject

In many cases, two trivial tests can be used to quickly determine whether a polygon is completely inside or outside the viewing window. This then allows us to skip the above clipping procedure.  The trivial tests require first computing outcodes.

A vertex outcode consists of four bits: TBRL, where:

Trivial accept:    all vertices are inside (all outcodes are 0000)
Trivial reject:     all vertices are outside with respect to any given side(bitwise AND is not 0000)