Clipping must account for all of the viewport's boundaries. The Cohen-Sutherland Method is used to clip lines to a viewport. An outcode is determined by the position of an endpoint and is a 4-bit value indicating whether it is outside of the viewport to the top (MSB), bottom, right, or left (LSB) side(s).
The algorithm can be summarized as follows:

1. Compute outcode for point P1
2. Compute outcode for point P0
3. Trivial accept? (both endpoints inside of viewport)

   Yes -- draw the line

4. Trivial reject? (both endpoints outside and on the same side of one of the viewport edges)

   Yes -- do not draw and exit

5. Is P0 inside the viewport?

   Yes -- swap P0 <--> P1

6. Is P0 > Ymax (top) ?

   Yes -- clip at intersection and move P0 to new point

7. Is P0 < Ymin (bottom) ?

   Yes -- clip at intersection and move P0 to new point

8. Is P0 > Xmax (right) ?

   Yes -- clip at intersection and move P0 to new point

9. Is P0 < Xmin (left) ?

   Yes -- clip at intersection and move P0 to new point

10. Branch back to Step 2

The worst case will iterate four times through the algorithm.

Clipping objects other than straight lines is rather difficult and/or computationally expensive. Clipping polygons involves ownership and adjacency problems as well as dealing with gaps formed when mesh surfaces are created and rotated.

Refer to Section 3.9.3 in the text (p 103).

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Last Updated 8/8/1999