Clipping planes and MSAA

I’ve run across a strange problem on a GF7950GX2 (169.39):
Every polygon edge that is created by clipping with a user-defined clipping plane, gets no antialiasing. Is this defined behaviour or a bug? I assume, that the “new” border-fragments get no correct sub-pixel mask.

I believe the GF6 series cards and probably also the GF7 cards do the clipping by user defined clipping planes in the fragment shader using the discard instruction. This would explain why there is no MSAA on those fragments since the correct sub pixel mask will be missing this way.

The proper way would be for the card to do the clipping before the fragment shaders (which will be done on GF8 and later since AFAIK the Geometry Shader requires this)

[ | vector_math (3d math library) | software renderer ]

IIRC user-defined clip planes are implemented as a per-fragment test on your graphics card, so that is to be expected.