this original question is apparently from 2008 lol, but i'll answer anyways hehe
( explained extra simple so people who don't know much about transparency can understand too )
one way to approach it, which applies for cta, ic4 and ct6 or any other video for that matter, is to make whatever you do on a green screen background, then import it to the reallusion pop video converter, from there you key out the green and export it as an iwidget. take the iwidget and embed it into the widget cast software and make a widget .....that widget will hold the transparency, i did something similar on my myspace page, www.myspace.com/rickcrespo..... if you look at the player up top that spans the whole page, it's a widget cast widget .....and the dancer on the left dancing on the player has a transparent background remove the player and he'd be dancing in air ......... another way to do it ....is to make an animated GIF in the same fashion by keying out the green ( or blue ) background ..you will not have an audio option though and you would need another software to compile the gif's into one animating gif. , Iclone has the ability to export transparent png images but you would still have to cnvert them to gif and compile them to make an animated gif You have to consider that wherever you use these transparent images will have their own limitations , so you do have to follow some sort of pipeline to a format thats friendly with wherever you post it, as not all sites accept gif and not all sites accept html embeds. The transparency is a web code thing, not a video thing, at least on the web, as in video, chroma keying is for letting the video behind show ...and on the web its to let any page its posted on show from behind ....same concepts but different approaches. there are other ways of approaching this as well, ie - exporting alpha channels...but i find the pop converter does a very clean chroma key and keeps life simple ~ Good luck ;)
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"To define Tao is to defile it" - Lao Tzu