Keying Inside and Outside

You do not need a Motion Template for this effect!

Scene: You have a green screen clip and you want to replace not only the green screen but also the subject with replacement media.

I came across this problem on Apple’s discussion board (FCPX forum) and thought it was an interesting problem. It’s actually quite simple, but there is a catch if you don’t know what to look for.

For this demonstration, I will be using a free sample (practice) green screen clip available from hollywoodcamerawork.com (Godiva Medium). Here’s a frame:

This is a relatively awful green screen. The green isn’t “even”. It is very difficult to pull a decent key especially in the area of the sheer fabric. It is, however, a very good clip to learn how to key and if you successfully pull a good key, then you will have learned something useful!

The effect I want is to have our model replaced with fireworks as a silhouette effect and the green background replaced with an icy waterfall (fire and ice ๐Ÿ˜‰ ). Two mattes from one clip.

If this question hadn’t been asked, this is not something I would have thought of by myself… It’s actually rather cool!

How this is accomplished is rather simple:

1) apply they keyer to the green screen footage โ€” invert it! โ€” and overlay it over the fireworks clip.

(When you invert a keyed green screen, you get the original green screen back as everything that is not green is masked!)

2) select the green screen clip and fireworks clip and make a compound clip.

3) apply the Keyer to the compound clip and overlay it over the icy waterfall clip.

Pretty simple provided 1) you get a good key and 2) you know how to get around the non-transparent “white out” where the green screen used to be. This is what it looks like:

This is a WTF moment. It’s supposed to be transparent. So what happened?

It turns out that the provided Keyer effect in Final Cut Pro automatically sets a 46% Spill Level (I don’t even know what this is because it is separate from the Spill Suppression โ€” it seems to be the “primary” spill removal tool; it’s documented that it will fill the green with a light gray color instead of transparency.) The solution at this stage is to simply set the Spill Level parameter to 0% and that will clear out the white-out to transparency again.

When completed, you will end up with a compound clip containing your green screen footage (with inverted key) over the media used to fill your subject, and placed over another clip used to contain the media used to replace the green screen. It will look great! And, you didn’t need to use a special Motion template to accomplish it.


That said ๐Ÿ˜‰ โ€”

If you use SC KeyFX scKey Replace, you only need one clip. “scKey Replace” allows you to select any color (use shift-click and/or drag to include more) very much as you would in a “paint” application like Photoshop. You can stack the effects on a single clip and use the drop zones to fill the parts. There is a feature that will allow you to blend the replacement media with the original media using Blend Modes if you wish. With it, you can pull off an even more interesting effect of lighting up your subject with the filling media.

In the first frame: throughout most of the play through, the subject model is silhouetted. However, when the “flash” is close to her face, you can see the fireworks “light up” her face briefly. It’s really impressive to see!

Model’s face is silhouetted

Model’s face shows some color detail. (It is more obvious in playback.)

You cannot get this kind of effect from the basic Keyer effect since the blend modes provided by scKey Replace are not available in Keyer.


See SC KeyFX Tips for more information.