OK, ghonma sounds like he knows what he is talking about, and I have tremendous respect for bagginsbill as a master of Poser nodes.

Never the less, on the face of it, my tests seem to directly contradict both of them.

I am willing to accept that I have done something wrong here, but I would appreciate it if someone could tell me what.

The bottom image was rendered over a background in Poser, and exported as "B1.PNG".

In the top image the figure was rendered over a black background and exported as "B2.PNG". In PaintShop Pro 5 I loaded the background photo (clouds-002.jpg), then pasted B2.PNG as a new layer, then applied the alpha mask. You can see the results above, a very pronounced black fringe in the composite image. This image was PNG and rendered over black, so by either ghonma's theory or bagginsbill's, should not have any fringe, if I have understood them correctly.

Just to be sure I tried a different method. This time I first opened B2.PNG in PSP5, then converted clouds-002.jpg to a PNG,  reloaded clouds-002.PNG, pasted it as a new layer, loaded the alpha mask, inverted the mask. The results were identical to the first test, a pronounced fringe.

So am I using the wrong method to composite? Is this something that needs to be done in Photoshop, rather than PaintShop Pro? Some other explanation? At the moment I am still getting far superior results rendering over a background image, rendering over black only seems to make things worse.

What am I doing wrong?