Changing mask into alpha channel

[vvvideo]

I have a simple problem but didn't find a solution with google or tutorials. I have a composition with two layers. One is a 3D-animation, the second one is a black mask on top of it to hide the backgroaund. It shall be replaced later in Premiere Pro. When I export it as RGB + alpha, ist is only black but not a transparent alpha channel. Here is my question: Is it possible to simply turn the black mask into a "real" alpha channel in After Effects?

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[Mike_Abbott]

If you've rendered this correctly - and from your description it reads like you have - your file does have an alpha channel. If you're viewing it in a typical 'consumer level' video player - VLC, Windows media player etc - they won't show the transparency / alpha, so you'll see black.

Import the rendered file back into After Effects. Here are a few things that are useful to know:

1. When the file is selected in the project panel, look at the thumbnail and information to the right of it. The last line will say: Millions of Colours...  but if it also has a "+" that's telling you it has additional channels of data = an alpha channel.

2. In the project panel menu (the three short lines in the tab) you'll find the option for 'thumbnail transparency grid'. This will show the transparency in the thumbnail.

3. If you what to check the transparency at full size: double click the movie in the project panel and it will open in the 'footage window'.  Toward the bottom lower left you'll find a 'transparency grid' icon - black and grey checkers. That can be toggled on / off to show transparency.  The same icon is also available in the Composition and Layer windows.

So, please check it and report back. Does your file actually have an alpha?

[vvvideo]

Hallo Mike!

Thank you for your answer! I imported the file back into AE. It lookes like it has no alpha. Here are two screenshots:

[Mike_Abbott]

Are you rendering via Media Encoder or Render Queue? What format / codec?

Its an 8 bit per channel project?

[Mike_Abbott]

...All of a sudden I realised we might be overthinking this...  : )

Have you set up an luma track matte between your two layers?  Your first screenshot isn't big enough for me to visually check.

NB: Edited: was 'alpha track matte' - I meant to write 'luma track matte'

[vvvideo]

I tried different settings, but I didn`t succeed. Here are my last ones, I hope this is what you meant:

[Mike_Abbott · Accepted solution]

OK - I can see what's going on. I thought your transparency was correctly set up. My wrong assumption.

What to do:

Layer 1: 'Mode' menu should be 'normal'.

Layer 2: 'TrkMat' menu should be 'Luma Matte "name of layer 1" '

(Note: The visibility 'eye ball' for Layer 1 will automatically be switched off when you select 'Luma matte' for layer 2. That is correct. Leave it switched off.

You should now have created your transparency = an alpha channel. Check it with the transparency grid button I mentioned above. Render this out as before and you should be good to go.

[vvvideo]

Great! Thank you very much! You fixed it! 🙂