This works on the geometry, right. But the problem is, that the the lower part of the warlock in my testcase (see images above) are NOT clipped. I don't get the reason why it is'nt clipped, although the geometry writes into the zBuffer. I also tried to fiddle with zFunc and so on, but it doesn't help.

Misteriously, the self shadowing parts of the warlock behind the box aren't rendered..