Hi I am currently going through the past exam problems and I am stuck on this clipping problem. Could you give me some hint on how to solve it?
If we clip a polygon to a window, it is inadequate to merely perform repeated half-space clipping on each polygon line segment independently. Give an example illustrating why independent clipping of polygon line segments may fail to correctly clip a convex polygon. A picture alone is inadequate; you should also give a short answer describing the picture and why it illustrates the problem, noting both the incorrectly clipped polygon and what the correctly clipped polygon should be.
Hint: If you were clipping a polygon to a single half-space, such as the set of points $(x,y)$ with $y>0$, then repeated half-space clipping would work. In order to come up with an example where the procedure fails, the polygon will have to interact with at least two different sides of the window...