FCP X: Create a Chroma-key
I’ve covered chroma-keying (also called green-screen keys) in my FCP X video training, but haven’t written about it. So, time to fix that lack.
This article looks at how to chroma-key using Final Cut Pro X. (This article shows how to chroma-key using Final Cut Pro 7.)
First, the best thing you can do to improve the quality of your keys is to improve how you shoot them. Here are seven basic production rules:
- Actors should be at least 10 feet in front of the green screen. This avoids light from the background “spilling” around their body or shoulders.
- In general, don’t cast shadows on the green screen. Be very careful shooting feet.
- The green background should be as smooth as possible. Paint is always better than fabric; avoid wrinkles and folds.
- The green background should be lit smoothly, both from side to side and top to bottom. I try to have the green background display between 40-50% level on the waveform monitor.
- There is NO relationship between how the background is lit and how your actors are lit. This example will illustrate that.
- Light your background for smoothness. Light your actors for drama.
- Don’t worry about having the green background fill the frame. It only needs to completely surround the edges of your actors. Garbage mattes are used to get rid of the junk.
SETTING UP THE KEY
The green screen image is always placed above the background. You can place either the green screen or background image into the Primary Storyline. I find it easier to put the background in the Primary Storyline, because it makes editing the green screen image easier. But this is purely personal choice.
Select the green screen clip.
From the Effects Browser > Keying category, double-click the Keyer effect, which applies it to the selected clip. (You can also drag the effect on top of the clip, if you forgot to select the green-screen clip first.)
Don’t panic if your image looks weird – we will fix it.
Click the Sample Color icon. This allows fine-tuning the selection of the background color.
In the green-screen image, drag to select a representative section of the background. I try to get close to the face, but not so close that I accidentally select loose hair or skin.
Your key should look better immediately. Most of the time, you can probably stop here. But there are three other adjustments that can make your key look even better:
* Cleaning up the matte
* Edge adjustments
* Light wrap
Click the Matte button to display your key as a white foreground on a black background.
Your goal is the make the foreground solid white, which means opaque, and the background solid black, which means transparent. Adjust the Fill Holes and Edge Distance sliders until your key looks solid. (For REALLY bad keys, you’ll need to also adjust Color Selection, mentioned below.)
If an edge is too pronounced, or needs help, click the Edges icon.
Then, click and drag a line from the foreground to the background in the Canvas. Drag the midpoint slider (where my cursor is) until the edge looks the best it can. Different video formats make this easy (ProRes), while others (HDV, AVCHD) make this much harder. Perfection is impossible – do the best you can.
New with the 10.0.3 release are four additional tweaks at the bottom of the keyer filter:
* Color Selection
* Matte Tools
* Spill Suppression
* Light Wrap
The first three are designed to clean up poorly shot keys – read the FCP X Help files to learn how these work. (I used the Color Selection tools to clean up the key I show later in this article.)
Light wrap, though, is aesthetic. What it does is blend colors from the background into the edges of the foreground, to make the entire key look more “organic,” as if the foreground and background were actually in the same space.
This is a subtle effect, but very cool.
Twirl down Light Wrap and adjust the Amount slider and watch what happens. Drag the other sliders around and see what happens. The nice thing about this setting is that when it looks good to you, it is good. The amount of the effect is totally up to you.
Remember, Light Wrap only affects the edges of the foreground and should be used subtly.
When you are done, you have a great looking key!
Sometimes, however, you don’t have, um, perhaps, the best green-screen image to work with. Here, for example, there are lighting instruments in the foreground, with a completely inadequately lit green screen in the background. (Sigh… just awful.)
Once you pull the key – which is film-speak for creating a green-screen shot, as I described above – and get it looking as good as possible, there’s one more step: adding a garbage matte to get rid of all the garbage.
Once you get your key looking as good as you can – which in this case isn’t all that good – drag the Mask effect (Effects > Keying > Mask) on top of the green-screen clip.
NOTE: The Mask effect should always be added after the Keying effect, so that the Mask is below the Keyer in the Inspector.
Then, drag each of the four circles to create a shape such that your foreground image is contained inside it, and everything you want to exclude is outside. Here, for instance, we removed the light stand and the tearing around the top of the image. I’ve found this Mask effect works best when applied to a connected clip, though Apple cleaned up a bug that prevented it from working when applied to a clip in the Primary Storyline.
However, the big limitation of the Mask effect is that you only have four points to work with. That’s where a free effect comes in, which allows you to create far more flexible shapes with it.
It’s written by Alex Gollner and is available on his website – alex4d.wordpress.com/fcpx/ – I recommend his effects highly.
Apple totally rewrote the chroma-key filter in FCP X — it is far better than the filter supplied with FCP 7 — which allows us to create some amazing effects. Have fun playing with it.
LEARN MORE: Want to learn more about chroma-keying, and luma keying, in FCP X? Check out this chapter of my FCP X video training.