Green-screen key, also called a chroma-key, is the process of removing a background color from the image so that you can place an actor in front of another background.
NOTE: The reason this process is necessary is that no camera shoots an image with depth. Because we can’t tell our software to: “remove everything that is more than ten feet away from the talent,” we use chroma-key instead.
Premiere Pro provides several different ways to create this key. In this tutorial, I’ll show you the preferred method: the Ultra key.
THE ULTRA KEY
The big benefit to Ultra Key is that it is fast and looks great.
This is Lisa. We want to remove the green background and replace it with something more surreal.
NOTE: The flatter the green-screen background, the better. Paint is better than fabric. Smooth, even lighting, no wrinkles, and no shadows will make your keying work in post-production simple.
In the Timeline, put the background clip on V1 and the foreground (green-screen) clip on V2. In this example, I’m not using audio, but the process is the same whether you have audio or not. Select the V2 (green-screen) clip.
In the Effects tab, double-click Keying > Ultra Key to apply it to the selected clip. (You can also drag the effect on top of the V2 clip.) This applies the keying effect to the selected clip.
In the Effect Controls tab, click the twirl-down arrow next to Ultra key.
Click the eyedropper next to Key Color. This allows you to sample the background color of the image so the Ultra key knows what to remove. (The default color is a dark gray.)
Then, click near the face of the subject; but not so close that you run the risk of getting either skin or hair as part of the sample.
KEY TRICK: Press the Command key when you click the eyedropper, which allows the eyedropper to select a 5×5 pixel square, rather than the default 1 pixel under the eyedropper. This makes for more accurate samples. Notice, when you press the Command key, the eyedropper gets “fatter.”
NOTE: There are two schools of thought on where to select the background color. One side says to select something in the mid-range of the background, not too dark or light. My point of view is to select the color near the face. If the face keys well, people won’t even notice other problems. If the face doesn’t key well, nothing else you do will matter. You are welcome to form your own opinion.
Instantly, we have a pretty darn good looking key. But, before moving on, check the matte to be sure it really IS OK.
Change the Output menu from Composite (the final result) to Alpha Channel. This displays the key in shades of gray. Your goal is to get the foreground solid white (opaque), while the background is solid black (transparent). Any shades of gray will be translucent and, in general, you want to avoid translucency.
This foreground image was shot and lit pretty well, notice there is a clear delineation between foreground (white) and background (black.)
Switch the Output to Composite and move on. We’re done here.
TWEAKING THE KEY
However, not all keys are that easy to pull. Let’s try something more difficult.
Here’s a key with a non-green background, wrinkles in the fabric, and uneven lighting. In other words, a normal key.
NOTE: Just because the green background needs to be lit evenly and flat, does NOT mean your talent needs flat lighting. One of the secrets I’ve learned about lighting for green-screen is that you can make your talent look truly dramatic, just keep the background lighting flat and put at least ten feet between the talent and the background.
I’m a big fan of having a process to follow when creating effects. This is especially true of green-screen work. So, here’s mine. To begin, follow the steps above, select the color and switch to Alpha Channel mode.
Hmmm… notice all the white “dust” in the background? Not good. Here’s how to fix it. We are going to take this in three steps:
Twirl down Matte Generation, then twirl down Transparency. This slider controls the foreground separation. Slide this until you find the point where the foreground becomes transparent, then move the slider a little bit until the foreground is truly solid white. Check carefully for subtle shadows, which will allow the background to bleed through.
Twirl down Pedestal and do the same thing, except now you are adjusting the background to be solid black. Switch the Output back to Composite and take a look at the finished results.
These two controls should allow you to nail the key.
TWEAKING THE EDGES
If you need to adjust the edges to remove excessive spill, or halos, twirl down Matte Cleanup.
Here, start by GENTLY adjusting Choke. This crops in the edges of the image and a little goes a long way.
Then, add a bit of blur to the edges with Soften. Again, small amounts are always the best option.
To remove green around the edges, twirl down Spill Suppression and tweak Desaturate. Remember, the easiest way to avoid excessive spill is to separate your actors from the background.
NOTE: There are color correction settings with this filter, but we have far more control over color using the Three-Way Color Corrector; which is what I use.
When you need a great key in a hurry, your new best friend is the Ultra key.
Final Cut Pro X 10.4
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