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if you want to remove the black background that is blended with the image you need to use some form of matte extraction procedure (like keying) to export it without it. if this is the case I would recommend this free plugin to knock out black background: http://www.videocopilot.net/blog/2014/05/new-plug-in-color-vibrance/ . it does other things too, but it has a checkbox to knock out the black
first we should establish that your sequence does not contain alpha information to begin with. import your sequence to After effects. do you see in the project panel thumbnail a mention of alpha? did the provider of this sequence give you information about its transparency or lack of?
you can create a composition from this sequence and click on the toggle transparency switch to verify it does have transparency
if it does have transparency and you want to export it with transparency, then make sure you are exporting the alpha channel. some format support it, some don't
in the Channels options: make sure it's set to RGB+Alpha
in the Color options: you may want to use straight alpha or premultiplied (matted) alpha - it depends on what your workflow needs are.
after you rendered the footage, note that video players won't show you if there is or isn't transparent background (it will show you the background color which usually is black). the only way to make sure is reimport it to Ae and see for yourself.
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You need to study up on rendering. I wold suggest that you type rendering transparent background in the search help field at the top right conner of AE and check out the resources. Here are the basics: Basics of rendering and exporting in After Effects CC
If you are doing this often I would suggest that you go to the bottom of the Render Cue and set up a new custom preset by selecting Make Template:
This will give you this dialogue:
Name the new setting PNG with Alpha and click the edit button then choose PNG Sequence from he format options, RGB + Alpha from the Channels options, and then you have the choice of Millions of Colors+ or Trillions of Colors+. Personally I always want as high a bit depth as I can get so I choose Trillions of Colors+. If you are always only working with 8 Bit color in AE then Millions of Colors+ would be a good option.
You should notice that in CC2017 the PNG sequence defaults to Straight Alpha for the color mode. If you choose another format I would suggest that you always choose Straight Alpha. Premultiplied with Alpha will give you a better looking preview in some media players but AE and Premiere Pro and all other compositing apps work natively assuming Straight Alpha channels. You will get better edge behavior with Straight alphas.
Once you have the new template created you can simply select it from the dropdown. I've actually named my preset PNG 16bit Transparency:
Now, providing you have set up your comp properly you can simply render. To check your comp for a transparent background click on the Toggle Transparency Grid icon just to the left of the Active Camera button at the bottom of the Comp Panel.
Here's a note. If you open the rendered image sequence in a media browser you will see a black background, but if you bring it into AE or your NLE you'll see a transparent background.
Thanks friends, Monday, or i have if time even tomorrow I do tests and let you know.