Clipping is performed near the end of the graphics pipeline , just before rasterization.
There are several advantages to performing clipping at this stage in the pipeline. If we were to attempt to clip the object earlier, we would have to clip against a more complex object-space volume. If we tried to clip the polygon after the perspective divide, we would have to deal with the complicated problem of clipping a line that passes through the point at infinity . The actual clipping is quite simple. We loop through each of the vertices and test whether they are inside or outside of the plane.