Track matte in Nuke

[MasterGeneralist]

Hello Nuke Users,
Looking for information on how to create in Nuke the track matte effect in After Effects, where the luma values of a node/layer create an alpha for another.
I’ve got a half-ass solution going with a merge>colorCorret>Keyer, then plug that into mask input of yet another node to get my results.
Suggestions? Thanks
Comp’ing by myself in Seattle

[Ramazan]

you can do it with a single merge node if you want to use the aplha channel.

so you have A over B in the merge, pipe in the node who’s alpha you want to use as the ‘track matte’ into mask in the merge then turn on the mask checkbox and select rgba.alpha in the mask channel list. this requires the ‘trackmatte’ to be a premult though.

if you just had a non premult image (ie in afx setting the track matte to be the front of a layer), then just put it through keyer node and pipe this into mask of the merge, the default of the keyer will set the matte output of the keyer to rgb.alpha and so again, turn on the mask checkbox in the merge.

[virtualmagician]

Hi pgill,

it’s months later 🙂 but I just started using Nuke and i think this is a very common problem for people switching from After Effects.

I am very used to use non-premult images as trackmattes in afx and now i am trying to convert my workflow to Nuke. I think this issue hasn’t been covered a lot in tutorials, maybe because it is just not the way Nuke users are doing it? 🙂

I made a screenshot of my Nuke script (which doesn’t work the way i want). Maybe you can help me out?

Thanks in advance and kind regards,
Giso Spijkerman

[marios]

You don’t need to luma key your matte. Just use a copy node and nominate the red, green, or blue channel as your alpha. Assuming your matte image doesn’t have an alpha embedded. That will also put an alpha channel into your stream. Hope this helps, the hardest part of switching from AE to Nuke is learning how the alphas work in your script. AE just takes care of it for you.

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