Ah yes, rendering over a background PICTURE - but this is the first time someone mentioned THAT bit. All that was said so far was that you had to render over black, which is not the case.

I tried, and you're right. If there's a picture in the BGPicture node, the alpha channel doesn't work.

Sooooo I have made some experiments and noticed a weird thing:

If I plug the picture into the normally grey background colour node instead of using the Background Picture AND I use the Wacro Delete detached nodes, it works. AS LONG AS I DON'T LEAVE THE MATERIAL ROOM. As soon as I leave the material room, the nodes are rewired to use the Background Picture, and in that case: No Alpha.

So if I render the pic while still in the Material Room I CAN get a png with transparent background even when using a background picture AND if I save it as a TIFF I get a picture that contains an alpha channel.

So there is a workaround but it's clunky and I agree it's a bug.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.