The easiest way after activating the View Transparency button in the Comp Panel to verify you do have transparency, is to add your Comp to the Render Cue and pick Lossless with Alpha from the output module options.
To understand rendering type rendering in the Search Help field at the top right corner of AE and Study Up. Rendering is NOT automatically right for every situation and you need to have a good understanding of how it all works.
To render an alpha channel you must select a codec that supports it. Under colors in render settings, you will see Millions of colors +. It's the plus sign that indicates the codec supports alpha. Prores 4444 and Animation support alpha. I don't know if any of the codecs that use Flash wrappers support alpha, never used Flash.
Thanks guys. I have tried Millions of colors+ with several formats but I wonder why no one seems to support flv anymore including camtasia? I tried converting to flv but the background shows black. Older youtube tutorials show a flv option in AE.
Rick, not sure where to find "View Transparency button in the Comp Panel"...Could not locate that button.
It's actually called Toggle Transparency grid. It looks like a little checkerboard and it is just left of the Active Camera selector.
NO MEDIA PLAYER will give you a transparent background, it will always look black. A video with an alpha channel will only show up transparent if you bring it into an app that supports alpha channels like AE or Premiere Pro. You cannot have a transparent background in the QuickTime player or in the Windows Media Player. They do not support it.
Did you try the Lossless with Alpha preset? Did you read up on rendering? You cannot just poke around until you get it, you have to read up and understand what you are reading.
If you are trying to render an FLV file for flash then you must use CS6 or earlier. A better approach if you are headed for flash is to render a png sequence with transparency and load that into flash. No matter what you do, embedding a video in a flash file is going to take some bandwidth to play back and it's not going to be compatible with mobile devices. It would really help if we knew what you were trying to accomplish. I still have no idea what kind of product you are trying to get AE to deliver.