I am a novice :)...
I brought in an avi file to After Effects with a subject shot against a green screen.
I succesfully got all the green out and my background is all now black (I don't have anoter backgroung layer, my goal here is key out the background in AE so I can use the resulting file in Premiere.
When I export the clip back to an avi file and then import it into Premiere and layer it on top of another image I still have all the black in my 1st avi file.
I have two questions?
1. Am I doing something wrong or is this just not possible.
2. Semi unrelated - is it possible to export the file as h.264 from AE and then use that inside Premiere CS4 instead of my avi file.
1) You need to export to a format that supports alpha channels. Try QT:Animation 100% quality, RGB+Alpha, Millions+ (the + designates the alpha).
2) You shouldn't use H264, since it is a heavily compressed lossy codec. (each time you render to it, you're losing quality). When you pass footage back and forth between programs, you should use uncompressed or lossless compression. When you're ready to deliver to web or dvd or whatever, then you use the lossy compression.
Also note, that in Premiere you may need to manually enable the Alpha channel in the video track's transparency options. Sometimes it doesn't show up correctly, even though the channel is properly imported.