Interpret with alpha, then remove alpha?

[Steve5D7C]

Is there any freaking way in AE to bring in footage (multichannel EXRs) with alpha, then IGNORE the alpha for compositing purposes? I've been looking for this solution for years.

I want to be able to bring in my footage ONCE, then composite WITHOUT ALPHA, but still have the alpha available for final stenciling. This just does not seem possible in AE.

When extracting channels from the EXRs with the Extractor effect there is an "unmult" checkbox, but it does not remove the alpha. Presumably it unmults the edge pixels with the import color that's set in the interpret footage menu. But, even with this unchecked, comps STILL result in unwanted alpha stacking on edge pixels.

So, the ONLY solution I've come up with is to import all my sequences twice, once with alpha ignored (for comping) and once with it premultiplied. The second one is only used as an alpha stencil where appropriate in my comp flow.

This is incredibly hacky and inconvenient, especially when dealing with endless versions of a shot. There's got to be a better way that I don't know about, right?

RIGHT?

[Kevin-Monahan]

Hey @Steve5D7C,

Thanks for the question. I'm Kevin from Adobe Support. I'm a moderator here. I'll ask the developers if there is a better workflow given your needs. I hope we can assist you in getting an answer back for you and for the benefit of the community as soon as possible. Sorry for the hassle. Thx, ^KM

[JohnColombo]

Hi @Steve5D7C,

I'd expect you would be able to include the alpha as a channel that EXtratoR could see so that you wouldn't need to have each EXR imported twice, but @fnordware would be the best person to answer questions related to EXR channel handling. Perhaps the Unmult option of EXtrator is doing something incorrect or different that what you would expect.

- John, After Effects Engineering Team 

[Steve5D7C]

Thank you! TBH it's never occurred to me before that the Extractor plugin could extract the alpha in the same way that it does any other channel (partially because I always rely on the Create ProEXR Layer Comps script to auto shuffle my channels out, and this ignore the alpha channel). Bringing in footage with alpha ignored and then shuffling the alpha with Extractor is exactly how Nuke works, and a quick test shows it seems to do the job just fine. Thanks for the answer!

[fnordware]

If the EXR is imported with alpha set to premultiplied, you just comp that footage over a black solid and you'll have the same result as ignore alpha. If you're using EXtractoR, you can comp your extracted footage over black or use Shift Channels to set the alpha to Full On before EXtractoR is applied.

[fnordware]

It's true that the ProEXR Comp Creator doesn't really set up the AE comp they way you need to to comp multiple passes, mostly because it doesn't know how you intend to use each pass. In general you want to:

1.  Use Shift Channels to set the alpha to Full On for each layer, revealing the jaggy unpremultiplied pixels.
2.  Comp all the passes in jaggy-land.
3. Add a layer to the top that has the alpha intact and set it to Stencil Alpha mode.

Of course, that comp should be a pre-comp. You don't want that Stencil Alpha affecting your background or whatever.

[Steve5D7C]

Both of these would also work, but are extra effort. I had already realized that comping over black would give the same result as ignoring alpha, but that's an extra step I was trying to avoid (bit of a pain if you've got 10 or 20 layers in an EXR). Sounds like adding Shift Channels would also require extra work (I expect this could be scripted). Extracting the alpha with EXtractoR seems to work, and simply requires manually creating an extra comp to do that, then that comp is your alpha source. Any reason not to go that route?

[fnordware]

Yes, if you interpret with ignore alpha, you can get it back with EXtractoR. That would be the fewest steps.

BUT…in general you do want that UnMult (same as AE interpreting as premultiplied) to happen at some point because AE works with un-premultiplied pixels internally. If you comp your passes without the jaggies and then apply a Stencil Alpha, technically you'll have a little black border around the edges. It may be too subtle to notice in many cases, and I'm sure the vast majority of AE users are not doing what I describe, but I'd have to say they are doing it wrong. I wish I had a better way to disseminate this info.

[fnordware]

Another way to do it would be to interpret with ignore alpha, comping all your passes with non-jaggy edges and no alpha. Then downstream in the next comp you apply EXtractoR with (copy) for RGB, pull in the alpha and check the UnMult box.