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Green Screen Keying in Final Cut Pro

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gs1.pngIf you've watched the weatherman on your local evening news, you know what a green-screen or blue-screen does.  The chroma keying process is the most popular way to extract talent or props from a moving image and composite them into another image -- like an animated weather map, or a virtual set.  Chroma keying allows your software to cleanly and automatically separate subjects from the background, while retaining their full range of detail.

Final Cut ships with a simple but powerful set of tools for pulling keys from green screen footage.  The keying tools in Final Cut Pro work based on the same engine as the tools in Motion, so you're free to work in whichever environment you're more comfortable with.

In today's article, a quick two-part guide to shooting reasonably good green screen footage and pulling the key in post.

Here's a short video tutorial on a creative way of using green screen.

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Shooting The Footage
Key Points:
  • Green screen vs. blue screen: The better technical choice depends partly on your subject matter and partly on the codec you're using, but green screens preserve more detailed color information in formats that involve chroma subsampling -- which is to say most everything except film.
  • Don't shoot DV or HDV if you can avoid it.  These formats use really intense subsampling, which leads to blocky edges around your subject when you pull your key.
  • Modern chroma keying algorithms are fairly forgiving.  While a well-lit, perfectly flat backdrop will still yield the most reliable results, you can often pull an acceptable key from cheapo green fabric from your local fabric store plus shop lights. 
  • The more uniform the lighting on the backdrop, the better; also, try to pull your subject away from the screen as best you can.  Shadows on the backdrop can throw off your key, and the closer the subject to the backdrop, the more green/blue light will "spill," or get reflected back onto your subject.  Ideally, you'll use an entirely separate set of lights for your subject and for your backdrop, but ...
  • Always test your setup before shooting all of your footage.  Capture a clip of your actual talent on the actual lit set, pull a quick key using FCP's quick keying tools, and you'll be able to see any major production-related problems in time to fix them.
Pulling the Key in Final Cut
FCP and Motion use the same keying engine, but Motion's robust masking tools will save you tons of time if you need to do any serious garbage masking (i.e. removing unwanted elements from the edges of your subject).  Here, then, are instructions for both:

Final Cut Pro
  1. Arrange your clips in the Timeline, so you can see what you're doing.  The greenscreen footage should be above the background footage.
  2. If you shot your footage in DV or HDV, clean up the chroma channels for keying.  Apply a 4:1:1 Chroma Smoothing filter - Effects -> Video Filters -> Key -> 4:1:1 Chroma Smoothing.
  3. Apply the main key.  Effects -> Video Filters -> Key -> Primatte RT.  Don't use the Blue/Green Screen filter; it normally gives unacceptable results.
  4. Use the eyedropper under Filter Controls to tell the Primatte RT engine exactly which range of color corresponds to your green screen.  Simply click the eyedropper, then click on the greenscreen in the Canvas.
  5. Adjust the sliders under the Primatte RT Filter Controls.  Functionally speaking, Noise Removal and Matte Density both affect the balance between preserving subject detail and cleaning out subtle variations from the background.  Spill Suppression attempts to compensate for green/blue light bounced back into your foreground elements.  As you'll see when you adjust the Spill Suppression, it'll shift your foreground from a green tint through a neutral tone to a magenta tint (or blue/orange if you're using a blue screen).
  6. If your key still looks unacceptable, the Matte Magic filter will ordinarily clean it up.  But be warned: Matte Magic takes forever to render.  Effects -> Video Filters -> Matte -> Matte Magic.
  7. If you need to mask out part of your image -- for example, if a boom mic is visible, or the edge of the green screen is visible -- you'll want to use a Garbage Matte to fix it.  Effects-> Video Filters -> Matte -> Four-Point (or Eight-Point) Garbage Matte.  These are a pain: you need to click the crosshair beside each point in the Filter Controls, then click the point on the image where you want that edge of the matte to end.  Think about Motion if you need to do much garbage masking/matteing.
Motion
  1. Set your project duration to match the duration of the clip you're keying.
  2. Go through Steps 1-6 in the Final Cut instructions above.  The process is exactly the same, except that in Motion, the filters are located under the Filters drop-down, there is no Chroma Smoothing filter built into Motion, and the filter controls are located in the Filters Inspector. gs4.pnggs5.png

  3. To draw a garbage mask in Motion, use the Mask tools.  The Bezier Mask tool is probably your best choice.  Simply outline the part of the image that you want to retain.gs2.png
  4. If you need to animate your mask, click the Record button (or press "A" on the keyboard).  Now, when you move your playhead and adjust the mask's outline, the mask will automatically place a keyframe and animate its shape.gs3.png

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Creating a Multiclip in Final Cut Pro 7 was the previous entry in this blog.

Multiclip Editing in Final Cut Pro Part 2 is the next entry in this blog.

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