answer to the second question is that AVI files will not work for a moving background for the web. You have to convert your AE media be it AVI, MOV, MPEG etc to a SWF file in order to use it with flash. AE can do this but take in mind this is a new feature in AE and it does not support 3D, cameras, lights, and a lot of the FX you’d use. My suggestion is to get your hands on a copy of “sqeeze” That way you can output from AE using all the wonderful toys into a Mpeg, AVI, mov etc.. Squeeze then converts your media into a SWF file there. And at this point cant tell the difference between cameras, lights etc.. hope that helps
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