That 1 pixel business on the edge is definitely a sometime thing. I just posted a Poser 8 IDL render in my Gallery here and no such effect. In fact, I haven't seen it for a few days. My worst case, which persists, is where the backdrop is a single, very large square, with no polygonal subdivision, i.e., just four vertices. Then I even get the second thread's first render block to come back totally transparent!

Whichway