I've gotten myself a bit confused by googling around on the topic of saving transparency in an exported avi formatted video.
So, have come to the horses mouth to hopefully dispell the confusion.
First, is it possible to export an avi format from AE CS5 that has transparent background... That is, so that I might put it over another video in Premier and be able to see thru it in places, to a track below?
I've tried it with what appears (to me) to be the necessary settings in the render que but when rendered the result is; wherever it was transparent in AE it is now opaque, and simply an opaque black color.
So, is it only possilbe to do this in *.mov or *.flv or some other format, but not *.avi?
I wouldn't recommend using any AVIs, if you need transparency. It is pretty much limited to "Uncompressed" or commercial CoDecs such as the ones from Blackmagic or Cineform. If you have none of the latetr, file handling and performance wil lbe a nightmare. Instead, use image sequences. Traditionalyl, TGA sequences work well in any editing program, but TIFFS and PNGs should just the same allow you to carry over the transparency while allowing you to play in realtime without extensive timeline renders in Premiere...