Sorry, but your question is a bit confusing.
Are you asking if you can overwrite the original file with two files? I hope the answer is "no".
Are you asking if AE can create a second file of just the matte? Yes: You can set AE to render Alpha Only and it will create a standard black-and-white matte.
1. I have a greenscreen scene
2. a scene backgrond.
3. I make the key, and
I would like to export a single DPX sequence with the scene composed (backgrond and foregrond) and inside the matte of foregrond.
In Nuke this work is done by the node "shuffle copy".
Is it possible in After CC?
Yes: this is done all the time in AE. Simply add your background layer to the AE composition with the chroma key footage. Then render a new DPX sequence. Very simple.
my colorist needs a unique composed sequence DPX with the transparency of foregrond,
so it can be adjusted separately the 2 scenes composed.
I know that you can export the alpa in a second sequence, but the question
is like to have in a single DPX sequence with 2 scenes composed and matte.
I want the information of the matte in a single DPX sequence composed,
where we will have (in a single DPX sequence): Final compositing+alpha channel of foregrond.
In Nuke this work is done by the node "shuffle copy" or "copy".
You will need three DPX sequences:
- The subject on green screen
- A matte for the subject
- The background
You have the subject and the background. You need the matte. You can make one in AE by keying the subject, then rendering the alpha channel only. You will then create your matte. And if you want ONE DPX SEQUENCE ONLY, you will have to take all three and put them end to end, creating one long DPX sequence. There isn't a way to create a single file that will do all things, as it seems that you hope AE will do.
After color correction, you may have to bring the color-corrected sequences back into AE for final compositing. In AE, you can add Light Wrap, a process that helps you incorporate the edges of the subject into the background, and you avoid the look of a television weatherman in front of a weather map.
Or, since you've used Nuke before, you can use it again in a workflow that seems to be more comfortable for you.
Thank you very much Dave,
then in AE you can not do
what you do in Nuke with the node "shuffle copy" or "copy".
As I feared!
AE can only generate an alpha channel from transparent areas inside the comp. There is no channel rendering based on layers. This does sound like a great idea for a plug-in though. Developers, are you listening???
A workaround would be to pre-compose your keyed footage and render both the composite and the pre-comp. That's what I have done in the past for color correction. The colorist just loads up both clips so the matte can be used to separate the layers for color grading.
I think you can do this in AE.
1. Make a new comp that has your final scene as the layer.
2. Drop in your alpha as a new layer and set it to be the first layer's alpha/luma matte.
3. Render to DPX with alpha, alpha interpreted as straight.
This should give you a DPX with the full image in RGB and your alpha in place. To check the output, import the rendered files back in the AE and interpret with alpha ignored and you should see the comped scene.
Thank you very much