It depends on where you want to use your transparent file. Many formats dont allow transparency. If you want it for web export to a photoshop sequence and open in photoshop to make a gif with transparency.
You will not get transparency in a video player like QuickTime or Windows Media Player. They don't support transparency. Bring your file back into AE or into Premiere Pro and you will see the transparency.
Unless you are completely understand video formats you should be using the Lossless With Alpha Output Module preset to render your project with transparency. The only change that you should make is that you should change the color mode to Straight Alpha (nobody uses pre-multiplied and I don't have any idea why it is the default so I changed it by editing the preset)
Here's the important part, and I'll say it again. Unless you completely understand video formats do not mess with the presets.
It would also help us a bunch of you explained how you plan to use the rendered file. Media players, the web, HTML 5 players do not support alpha channels and will give you a black background.
It worked! Thank you guys!
Not much help for someone after you. No mention of what you were doing or what worked.
Make sure that it is transparent by pressing this.
Pick a format like QuickTime and make sure that it supports Channels RGB + Alpha
From there, whatever you render out should be transparent.
So in theory you could have a complete green background, then layer this video on top of it and the rest should be transparent!
I hope this answers your question.
^ Self advertisement, why not.
Hey! I may have your answer. It certainly was for me. IF you have the AUTO INJEST box checked in the Media Browser in Premiere, it won't matter how to have rendered your alpha channel, because when you bring it into Premiere it will be transcoded to whatever the default codec is. I was bringing in perfect alpha channels, just like I had done for years and I noticed they would work for part of a second and then the background would turn black. I turned off auto injest and everything works as advertised. Good luck.