 <p>Just don't bother with user clip planes. They are emulated in weird ways across drivers. Why do you really need them? If it is to optimize something you are going to get a small gain on some HW and pay a horrible price on other. If you need it for some algorithm do it yourself using a texkill and it will be reliable everywhere. Probably not the answer you wanted, but in general, just don't use it and rethink your approach. </p>