Effect of reverting the keyer, background colour, transparency

Wait... I tried a little example, and it did not quite work, but I have an alternative, that differs only in step 4:

4' - Place the desired foreground above the desired background in a new timeline. Add the clip from the previous steps above the other two, and change its Blend Mode to "Stencil Alpha". Then select the two top clips (i.e. the "mask" and the new foreground) and make them a compound clip. (*)

(*) the compound clip is so that the blend mode from the "mask" does not affect the background.

@Luis, you can actually do this much easier!

Add Highlighter to the green screen clip. Move the OSCs to the corners. Use the Color Mask on the green screen and set the Blend Mode to Silhouette Alpha (selecting the non-green screen material). Set the Color Mask Softness to 150.

Add another Highlighter. Move the OSCs to the corners. Use the Color Mask on the *alpha* region and set Invert Masks. Set the Fill Color to blue (any color will do) with the Blend Mode Normal. Set the Color Mask softness to 0.

Place this clip over one of the background clips.

Create a compound clip of the two and go back to the storyline.

To the compound, add another Highlighter, OSCs to corners. Select the Color you used. Set the blend mode to Silhouette Alpha (I know this is counter-intuitive, but it works). Place clip over secondary background.

No need to change the clip to PR4x4 or do any exporting/importing. (It actually annoys me that making a compound clip is necessary!) Everything is done in the storyline.

Now, if you want, you can take the original green screen clip, key the green out (again), overlay it on top and set the Opacity of the clip to about 6% to "ghost in" the original subject:

There is an even easier method if you don't mind using drop zones. The steps would be:

Add Highlighter and key the green screen — Blend Mode Silhouette Alpha — Softness 150 (OSCs to corners).

Add Highlighter - Apply Media. Select the Alpha (keyed out with Highlighter) — Softness 0. Select Invert Masks. Choose a clip to add to the drop zone. Select Apply Media (option). If you want to "ghost" the original subject, dial down the Opacity slightly.

Add another Highlighter - Apply Media. Select the Alpha (again) — Softness 0. Add a Clip to the drop zone and select the Apply Media option. No compound. (Order of operations not all that important — order of Effects is!)

Hmm,

Maybe I don't understand correctly the 'what & why'

Here what I would do. Do the key on the clip (green is transparent - proves 4444 timeline), make a compound out of this.

Use this compound as 'Image Mask'

https://youtu.be/bWbNnSISejI

I still don't get it.

If I look at the OP's screenshot and his question:

The only thing he has to do is to put the green screen clip into a 4444 compound apply the colour key there.

Add his colour solid to the main project and apply the image mask.

That's 5 clicks!

You have this almost right — the order of clips is correct. "Regular" Key all three green screens. Set the top Clip to Compositing > Blend Mode > Stencil Alpha. The top duplicate lion clip is your "cutout".

Select the 3 green screen clips and make a compound clip.

That should get you what you want.

Sorry for all the other confusing stuff...!

Please post screenshots of the viewer and the timeline.

@spherico — The effects I'm referring to are not included with Final Cut Pro. They're effects I made. Using them as chroma keys is not something I've gone into a great deal, so I took the opportunity to quickly illustrate a couple of different methods.

Let me see if I understand: you want to replace both the green background and the foreground?

+1

Actually, you can! Image Mask can be used with a video clip, not just a still.

It sounds to me that you are already half way there or more.

1 - First, change the current project in order to support transparency:

a) press Command-J to open the Project Inspector;

b) click Modify

c) Choose ProRes 4444 as the render format (this is necessary, otherwise an empty background will export as solid background, not transparent)

2 - Remove the background layer, and uncheck the invert mask, so you get only the foreground. Export this as ProRes 4444.

You get a movie that has only the foreground element, and is otherwise transparent.

3 - Import the movie you created in step 2 - this will be your mask!!!!

4 - Place the desired foreground above the desired background in a new timeline. Apply Image Mask to the foreground, select the clip from the previous steps, and Alpha as the source channel.

DONE! 😎

Now even I understand 😃