Exporting transparent PNG

Is there anyway to make sure the image background is transparent in an exported PNG?

I would also like the drop shadows (from the color libary) to be alpha blended against the transparent background (so that it can appear on a variety of solid backgrounds in a web page without nasty anti-alias defects).

I start the code with size() to mandate the image size... should I have specified background transparency then?

Did you try:

background(None)