How do I get a pure alpha layer?

[Mylenium]

I have an luma matte layer, I want to convert it to a pure alpha layer. That is, I want to convert the luminance to alpha and discard the RGB information from the layer. Basically I want the layer to appear totally transparent in RGB mode and only be visible in Alpha mode. The reason, among others, is to produce some glow overlays that take advantage of the "use alpha channel" rather than the colour channel.

I tried the Combine Channels filter, which came very close, but still insisted on outputting RGB data even though I specifically told it "Alpha Only" as its output. That was the closest I could get, but is there any way to get a pure alpha channel on a layer?

Huh? That's mathematically impossible. An Alpha channel only defines the opacity/ blending of the fill pixels. It cannot exist on its own. Even transparent pixels are pixels, they are just not visible. You cannot have pixels that are "more transparent" than the already transparent ones. You need to use matting techniques and duplicates of your other layers to get the desired effect.

Mylenium

[Soviut]

Well, what I want to achieve is the same as if I went into photoshop, created a matte layer, and only painted on the matte layer. In RGB mode this would look totally transparent, but it would still have alpha information.

[Mylenium]

Well, what I want to achieve is the same as if I went into photoshop, created a matte layer, and only painted on the matte layer. In RGB mode this would look totally transparent, but it would still have alpha information.

Na, you are misunderstanding that. PS transparency handling is much different in that respect and technically you are painting on a custom channel, not the Alpha as it's understood in AE (if you were painting on the Alpha, it would affect all layers). Sorry, can't work this way in AE. You have to wrap your head around the conceptual differences in AE and think of another solution.

Mylenium

[Soviut]

Yeah, it makes sense, its just too bad, hehe.

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