Re: Alpha channel AE cs3?

Also keep in mind that if you are looking at the file in Quicktime, you will see a black background, since QT is not using the alpha channel. If you import the rendered file back in to AE, the background should disappear since AE will use the alpha channel to remove the background. Also, remember that AE can create two different types of alpha channels. Straight and Pre-multiplied. The default is Pre-multiplied, which includes your comp background color. Straight is preferable and cleaner in most cases, but looks pretty horrible if you just look at it in QT, since it does not include the background color.

Also if you are exporting to flash FLV, to then import into a Flash project, you will want to make sure that you are using at least a Flash 8 encoder and turn on the option for transparency. Otherwise, the flash encoder will discard the alpha channel (versions earlier than 8 didn't have support for alphas).
You only need to do render flash with alpha if you intend on keeping the alpha channel so that your imported video sits seamlessly over a background that is in your Flash project.

Erik