The first step in reducing the working set of triangles to be processed is to cull ("select from a group") those that are completely outside of the view volume as we noted previously. This process is called "trivial rejection," because relative to clipping, this is a fairly simple operation.
A test is performed to determine if the x, y, and z coordinates of a triangle's three vertices are completely outside of the view volume (frustum). In addition, the triangle's three vertices have to be completely outside the view volume on the same side of the view frustum, otherwise it could possible for a part of the triangle to pass through the view frustum, even though its three vertices lie completely outside the frustum.