Combining alphas from cryptomatte in overlapping elements in After Effects

[SonyBoy]

The title explains the basic problem -- sometimes I want to render objects in a single pass that I then isolate in post using their cryptomattes. In situations where these elements overlap in the image, I am not getting 100 percent alpha, no matter how I interpret the footage (there is alpha, but whether I interpret with it, without it makes no odds as I am using the cryptos to create alpha. Should it be possible to get this to work properly?

I'm not totally sure as it's been quite a while since I worked in Nuke, but I believe in these scenarios there is a way to combine two alphas and get a clean result using the Disjoint Over mode of a merge node. Is there a similar method available in After Effects?


* Update: I've found that using the matte from the cryptos as an alpha versus a luma matte (as I was previously doing, thinking there was no difference) seems to give better results. No idea why this should be the case? Either way, I'd still be interested in knowing if there is a way to get a Disjoint Over in AE.

[BenWall]

Hi Sonyboy,
I didn't use AE for a while, but it seems to me in Nuke RGB pass are generally loaded as srgb while alpha are really computed as linear.
Not sure about that, but it might explain the difference between mattes.