Er.... As far as I know - but I'm no animation expert - you render it as single frames and save as tiff or png, both of which has an alpha channel. I THINK there is some animation-format that supports transparency too, but I don't know for sure.

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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.