Suppose your looking down the center of the view frustrum, lets pretend there is a 4 sided poly to the left that runs lengthwise down the Z axis from lets say -10Z to 50Z so that part of the poly is behind the near frustrum plane. Obviously (if your picturing the same thing I am) the clip of importance is going to be the poly clipped to the left plane of the frustrum. That will be the most restrictive clip.

Is there any methodology that I can use to make such a determination or do I have to clip that poly against every plane in the frustrum provided it is not entirely behind any one plane and if it is the case that I have to clip against every poly do you just take the output of one clip, replace any necesary vertices and then clip against the next plane????

Thanks for your advice and patience.