1) All remaining patches are culled against the viewing frustum.

2) Given the current frustum, compute the visible geometry.

3) The forming principle of frustum cone - like cup ( container ) is studied.

4) Since we defined the frustum, we know the angle at which the sides of the frustum meet - this is the field of view we used to create the projection matrix in the first place.

5) All vertices are clipped by the front plane of the viewing frustum.

6) To make use of these coordinates,[http://sentencedict.com/frustum.html] we must first re-define them in terms of the visible area of the viewing frustum.

7) To understand this, we must first take a brief look at the characteristics of the frustum.

8) To do this we clip the patches' bounding boxes against all six sides of the viewing frustum .

9) First a rectangle which covers the projection of the viewing frustum onto the ground plane is calculated. All patches outside that rectangle will surely not be visible.

10) This will help the 3 D engine's performance by doing per - object frustum culling.