Z3 re-visited

I believe 3dlabs 16x Superscene antialiasing has an overflow buffer like you mention. They apparently don't keep track of full coverages masks to allow for sort-independent transparency though. If they did that feature would be advertised. I think one of the B3D articles made mention of this overflow buffer.

 

The problem with such overflow buffers is, of course, that you have to dimension them for worst-case behaviour (=when maximum # of samples needed is reached for too many pixels) or else add the infrastructure needed to do the 'stall rendering->allocate/swap out memory->resume rendering' roundtrip whenever the overflow buffer overflows. If you do neither, you get unpredictable gliches during rendering.