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set layer alpha to black

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Xav VonL
set layer alpha to black
on Aug 26, 2014 at 3:24:47 pm

Hi folks, I have a pretty simple problem and surprisingly couldn't find the answer... I must have missed something...

As the title reads, I need to be able to set the alpha of a specific layer or composition to constant black or in other words to not have it appear in the alpha so it's not showing in the alpha of the main comp whatsoever(I mean, without losing the layer in the RGB, of course...)
How can I achieve this in AE ?

thanks a lot


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Dave LaRonde
Re: set layer alpha to black
on Aug 26, 2014 at 4:24:56 pm

If you want a layer to be black, you can use the Fill effect to make it black. If you don't want a layer to appear if you render the comp, you either turn off its visibility eyeball icon or you set opacity to 0%.

Otherwise, I'm not really sure what the question means. If I got it wrong, please try restating the problem in more straightforward language.

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


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Xav VonL
Re: set layer alpha to black
on Aug 26, 2014 at 5:16:03 pm

Alright let's rewind an start over again.

I have a composition in which I have a layer that I want to see in the RGB but I don't want this layer to show inthe alpha channel whatsoever.

now the more detailes in case it helps if you wonder why the heck I'd like a layer to not show in the alpha :

1.let's say I have 3 layers, foreground, middle and background.
all those 3 layers are quite heavy compositions so I wand to pre-render them.

2. the final look of the foreground depends on the background because in my case I use transparency and blending modes

3. so I am pre-rendering the background, pre-rendering the middle comp, but when I want to pre-render the foreground I need to have the background behind it without having it show in the alpha of my output fo pre-rendered foreground..... in NUKE it would be easy or in a 3D software it would be like applying a constant black alpha shader only affectibg ALPHA pass but not the rgb...... but I need to handle this in AE and I can't find how..... basically I want to mute the alpha of a layer without hiding it from rbg.

Anyone ?


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Kevin Camp
Re: set layer alpha to black
on Aug 26, 2014 at 5:27:11 pm

can you set the blending mode of the foreground layer to 'stencil alpha'?

that should use it's alpha over all layer's bellow it.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW


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Xav VonL
Re: set layer alpha to black
on Aug 26, 2014 at 5:41:32 pm

The stencil alpha trick could have been a workaround in some cases but in my case I think it would not be a suitable solution.... too many layers and comps to deal with...

my foreground is... let's say a collection of overlapping 3d solids with different blending modes this means I would need to add a "pre-comp level" by pre-rendering my foreground against background and use the stencil alpha of the foreground over the pre-rendered foreground again.... and this in 200+ comps..... headaches :) ... while it would be so easy to just be able to tell a layer not to affect alpha..... there must be some way of doing this in AE.


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Kevin Camp
Re: set layer alpha to black
on Aug 26, 2014 at 6:33:43 pm

the easiest way is probably to do a separate render for the matte... so render the rgb with all the layers how you have them now, then hide the background layer(s) or any layers that you don't want in the alpha and then render the alpha channel.

you could potentially duplicate all the layers that you need to use in the alpha and move them to the bottom of the comp. then for all the layers above those layers click the 'preserve underlying transparency' option (it's the 'T' between blending modes and track mattes in the modes panel on the timeline).

there is also a very slim possibility that using a bunch of set matte effects on a solid at the top of your comp could allow you to combine the alphas of your alpha layers to create a single matte layer that you could then use stencil alpha on to knock them all out. but that a long shot, since the set matte effect (like all compound effects in ae) will only look at the layer's original (or pre-effect) alpha and the effect expects the layers to be the same size or you will get unexpected results.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW


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Kevin Camp
Re: set layer alpha to black
on Aug 26, 2014 at 6:42:52 pm

if a separate alpha won't work, then the easiest way would be to duplicate the comp, hide the layers to not be included in the alpha, then bring that duplicate into the original comp and use it either with the stencil alpha blending mode or with the preserve underlying transparency option.

my other two options are more tedious.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW


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Xav VonL
Re: set layer alpha to black
on Aug 27, 2014 at 11:07:45 am

Thanks Kevin.... the Preserve Transparency is a feature I had been overlooking so far and I'm sure I'll use it again from now on...

It took some changes in the approach of the comps but I could use this as a less painful workaround that what I had thought of so far, even if I'm still missing the option to simply mute one layer's alpha..... this would have been so easy :/

Thanks a lot anyway, now this has revealed another weird issue but I'm opening a new thread for this......


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Conrad Olson
Re: set layer alpha to black
on Aug 28, 2014 at 2:42:08 am

I don't have AE in front of me right now but I think you should be able to use the Set Matte effect. There are a few effects here that might do the trick:

http://helpx.adobe.com/after-effects/using/channel-effects.html

---

conradolson.com


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Xav VonL
Re: set layer alpha to black
on Aug 28, 2014 at 9:24:21 am

This is where I thought I would find what I am looking for but I'm no expert with these effects and didn't find the solution in there so far :( but thanks for the hint


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