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Dennis Kutzen
Known Participant
February 14, 2020
Question

Different alpha information than combined channels.

  • February 14, 2020
  • 1 reply
  • 649 views

I'm trying to do something that would be easy in a node based system.  I would like the alpha channel on a exported file to only have the alpha information from one layer of the composite.  To be overly simple, I would like the RGB channel to be a still, and the alpha channel to be animated.  This is to create a segment that could be keyed later in a broadcast environment.

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1 reply

R
Community Expert
Rick GerardCommunity Expert
Community Expert
February 14, 2020

I am not sure what you are asking for. You can pull an alpha channel from any layer and apply it to any other layer with Set Matte or a half dozen other effects. 

 

Once a video is rendered the alpha channel affects all pixels. I don't know of any way to render a movie with an alpha channel the pokes a hole in an image when it is on but does not when it is off. 

D
Dennis KutzenAuthor
Known Participant
February 14, 2020

What I'm doing is preparing an elements to be used by someone else in an edit.  Here's a simplified idea.  Let's say you have an all white logo or graphic, and you are fading on different parts at different times.  At this point the information in the RGB channels is identical to the alpha channel.  If you are halfway thru a fade, the alpha channel has the white at 50%, which is revealing that part of the RGB which is also at halfway to white.  So the net result of the key is 25% white.  Ideally, you'd want a fully solid still in the RGB channels, and the animation in the alpha.

R
Community Expert
Rick GerardCommunity Expert
Community Expert
February 16, 2020

As far as I know or can tell by experimenting you can't add an alpha channel to any kind of RGB image and have that alpha channel not affect the RGB channels. If you make the alpha transparent the RGB goes away.

 

You can render a separate alpha only file and use that to control the alpha of any other layer. 

 

In some apps, you can use multiple channels from EXR files to do other things, but that's a lot more complicated than most editors are comfortable with.

 

 

 

 

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