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Seperate RGBA Channels - Possible?

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Sean Dunn
Seperate RGBA Channels - Possible?
on Mar 16, 2017 at 7:35:41 pm

I'm doing some compositing in AE, but need to send out an alpha to a colorist. Is there a way to export an RGBA image sequence from after effects similar to Nuke?

When I do it just exports as white. Even when I export an alpha image sequence it's not a true alpha.


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Dave LaRonde
Re: Seperate RGBA Channels - Possible?
on Mar 16, 2017 at 9:16:50 pm

First, the basics -- does your composition contain transparency?
Click the checkerboard icon at the bottom of the comp window. If you see the familiar Photoshop background indicating transparency, you've got it. No checkerboard visible? You don't.

If you DO have transparency the question becomes, "What do you need to deliver?"
Separate matte & fill files? A file with transparency? Straight or Premultiplied?

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Sean Dunn
Re: Seperate RGBA Channels - Possible?
on Mar 16, 2017 at 9:32:33 pm

The composition technically doesn't contain an alpha, but ideally what I'd like to do is cancel out the alpha from every layer but my matte layer. Retaining the RGB channels and only letting the alpha layer render out, similar to shuffling out an alpha in nuke.


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Dave LaRonde
Re: Seperate RGBA Channels - Possible?
on Mar 16, 2017 at 9:37:37 pm
Last Edited By Dave LaRonde on Mar 16, 2017 at 9:54:56 pm

AE ain't Nuke. If you've got a matte layer, duplicate it, put it in its own comp and render it out.

Since we're not sure what KIND of matte it is -- alpha, inverted alpha, luma or inverted luma -- it's tough to give more detailed advice.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Kevin Camp
Re: Seperate RGBA Channels - Possible?
on Mar 17, 2017 at 4:43:21 pm

as dave points out, when rendering in AE the alpha channel is derived from transparency within the comp. without that transparency, it will render a solid alpha (white).

if you have a matte layer, you can either send that matte to the colorist as Dave mentions, or apply that matte to your comp and render a sequence with the alpha.

there are several ways to apply the matte layer in a comp, in this case i think i'd put the matte layer as the top comp layer, then from the layer blending modes set it to 'stencil luma' or 'silhouette luma' depending if the matte is white on black, or black on white. then you can render that transparency as an alpha.

if you want to render the alpha as a separate file(s) with a RGB file(s), you can set that up as a second output module when you render... essentially render once, but to 2 separate outputs.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW


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