You’ve seen it in countless Hollywood movies. And it’s easier than ever to do yourself. I’m talking about compositing: the process of combining a foreground subject and a background “plate” to create a single “composite” image. By shooting a subject against a colored background and then using software to remove the color, you can place your subject in any environment you like. This process is called “keying” or “pulling a key.”
These days, anyone can purchase an inexpensive collapsible green or blue screen (or paint a wall or even hang a sheet), set up a few lights, and make someone appear to be relaxing in a Parisian café, hanging from the ledge of a high-rise building, or jumping from an exploding plane.
The quality of the key you can pull depends both on the shot and on how you process it “in post." An evenly lit colored background (usually blue or green) that is well separated from your subject makes for an ideal key. Of course, if you are using a green bedsheet and a couple of hardware-store utility lights in your garage, you may not be able to create a perfectly smooth, evenly lit background. But you may still be able to pull a good key.
If you are editing your video in iMovie, you’ll find that the built-in keyer (available if you have checked Show Advanced Tools in the Preferences) works well if you start with a good shot—but if the initial result isn’t good, you're stuck, because you don’t have any controls for adjusting the key.
That’s where Final Cut Pro X comes in. With its automatic keyer and advanced manual settings, you can turn a mediocre key into a great key in just a few steps. And, since Final Cut Pro X can open your iMovie project, if you started your project in iMovie, you can take it further in Final Cut Pro X. I'll show you how.
Keying a clip
To key a video clip of a subject that was shot against a colored background in Final Cut Pro X, first edit the replacement background into your project (drag it from your event to your project, and click the Append button or press the E key), and then connect the clip to be keyed above it (by selecting it and clicking the Connect button or pressing the Q key). Now, open the Effects browser (by clicking the icon or pressing Command-5), select the Keying category, and then drag the Keyer effect onto your clip in the Timeline.
The Keyer effect will analyze your video clip and immediately attempt to key out the background. On evenly lit shots with good separation between the foreground subject and the colored background, you may not need to do anything else. But if the key isn’t perfect, it’s easy to adjust.
Evaluating the key
Open the Inspector (click the Inspector button or press Command-4), and if necessary, click the disclosure triangle next to the word Keyer to reveal the parameters for the Keyer effect.
To evaluate the automatic key, check the composite image in the Viewer. Use the Fit pop-up menu at the top right to zoom in closer and inspect the edge detail. Even better, click the middle View button in the Inspector—the one with the white head on the black background—to see the matte that the key created.
The Matte view uses shades of gray to indicate what parts of the shot are being removed: The darker the shade of gray, the more that area is cut out of the shot. So, the matte for a good key should have a completely black background and a completely white foreground, and it should be gray in semitransparent areas. If the interior of your subject isn’t solid white, if the background isn’t jet black, or if the edges of your subject don’t look smooth, you can try a manual key.
The Strength parameter in the Keyer effect determines how much automatic keying Final Cut Pro uses. To key manually, the first step is to set the Strength slider to 0.
The next step is to tell Final Cut Pro what color values to remove. You accomplish this with the two tools in the Refine Key section of the Inspector: Sample Color and Edges. Start by clicking the Sample Color icon; then, in the Viewer, drag out a rectangle over the green background. If the background is unevenly lit, choose an area close to the subject, ideally close to difficult areas to key, such as hair. As soon as you draw the box, all the colors inside that box are removed. If necessary, you can add more boxes to remove more of the background. Tip: If you are having trouble at the edges of the frame, use the Mask effect to mask out them out.
With the background color removed, the second step is to improve the key along the edges of your subject. Back in the Inspector, select the Edges tool; then, in the Viewer, drag a line that crosses from the background to your subject. Adjust the end points and the slider in between to keep as much detail as possible while removing as much background color as you can. I prefer to use this tool while in Matte view.
The final step is to clean up any green areas that may still appear around the edges of your subject. The light on the green screen can reflect back onto the subject, spilling green light around the edges. The Spill Level parameter attempts to fix this automatically by adding the complementary color (in this case, magenta) to neutralize the spill. If you set the Spill Level to zero, you can clearly see the green spill. If you increase the Spill Level too much, you introduce a magenta cast to the entire foreground subject.
Drag the slider to find the optimal level, where you eliminate the green spill without introducing a color cast.
The Keyer effect in Final Cut Pro X often produces great results automatically, and its powerful manual controls let you refine your key so that even a poorly lit shot will key nicely. If you've never keyed a shot before, or if you are having trouble doing so in iMovie, give it a shot in Final Cut Pro.