@LUCKINESS Well, I don't know that, but a workaround would be, to just render with a completely green background, and then make green in something like Gimp or Photoshop transparent. Allthough, this limits the color of models you can render, but you could actually also make any other color transparent.
Then I also have a question. I'm rendering 360 Pictures for a gif. I have a not so good Pc, and because of this, every picture needs 20 or more minutes to render. Since I don't want to render through the night, I wanted to know, if i can just put my Pc to sleep mode. Will this stop the rendering process, or will it just pause the process?
Edit: Somewhen I needed to turn off my pc, so.. I put it into sleep mode. Today I turned it on, and yes, it kept rendering! But, due to the 100% CPU usage,which I wanted to use for something else, I needed to stop the process...