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Video Made on a Mac: Greenscreen Solutions

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The popularity of color (or chroma) key technology has grown immensely, bringing the technology and tools within reach of most content creators. By color keying (or removing) a colored background, you can create an alpha channel for the foreground footage layer. This allows for two or more images to be composited together to create a new image. You’ll find this technique used for a wide range of tasks, from special effects and feature films to corporate video and the local weather report.
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Planning Greenscreen Projects

Great-looking compositing is more than just filters and presets (FIGURE 5.1). It all begins with high-quality footage that is properly acquired. The most common techniques involve shooting talent against a blue or green screen. While software tools are increasingly forgiving of poorly shot footage, the most professional results start with well-executed acquisition. But before you can execute, you must plan.

Figure 5.1 The figure on the left is properly shot greenscreen footage. The screen is evenly lit with no shadows. After the color key is performed, the foreground subject can be composited over a background image.

Budgeting Greenscreen

If you hear the words, “We’ll fix it in post...” your greenscreen project is doomed. Poorly shot footage adds significant time to the postproduction stages of your budget. The end results will cost more and may never look right. But let’s assume your footage is wonderfully shot with great exposure, an even background, and no shadows. Your budget can still take a hit. Why you ask? By our estimation there are four aspects to the greenscreen keying process that make your project more complex (Figure 5.2).

Figure 5.2 The greenscreen footage with the program’s host is color keyed over multiple graphic layers. Applications like Apple Motion can be used to create background plates for use in greenscreen projects. In this example, the third-party plug-in dvMatte Blast is used to create the key.

  • All foreground footage needs to be keyed. This means keying the shots using a plug-in or specialized software application.
  • A background image needs to be located or created. When footage is shot practical (over a real background), it’s ready to go. Once you choose to key, you need to come up with at least twice as much footage to use.
  • Layers must be color corrected to match. You’ll often need to balance the color and exposure to match one another. This means more time spent getting the layers to look like they were originally shot at the same time, in the same location, with the same camera (which is even harder than it sounds).
  • Everything must be rendered. Color keys can be a time-consuming process. For most folks, time is money. Well-composited keying shots take a while to render—all of which adds up to machine usage and staff charges.

Successful Lighting

Effective keying relies on good lighting. It is essential to minimize variation in colors for the backdrop. This means that you must evenly light the background to avoid hot spots. Additionally, you’ll want well-lit subjects that don’t cast shadows on the backdrop. Sounds a bit challenging? Here are a few tips on doing it right:

  • Spill is bad. It is essential that your backdrop be evenly lit (Figure 5.4). You’ll also want to place the foreground person or object far enough away from the backdrop (with their own lighting) to avoid problems. If you put the foreground and background too close, you’ll get shadows on the backdrop and color spill on the foreground.
  • Figure 5.4 The use of several, evenly distributed lights helps minimize hot spots. If you have variation in your background, it will be harder to key.
    ©istockphoto

  • Get rid of hot spots. You should avoid variation in brightness on the backdrop so you can get a better chroma key. To do this, look for hot spots. Simply turn down the exposure of your camera and look through the viewfinder. Your hot spots will be quite visible. Adjust your lighting by softening it, and then set your exposure back to a normal level. While shooting, keep an eye on zebra patterns to avoid variations in exposure in the background.
  • Light with softboxes or professional fluorescent lights. The use of these specialty light types will better enable you to get an evenly lit backdrop (Figure 5.5). The goal is to spread an even amount of light across the surface of the backdrop.
  • Figure 5.5 The use of -professional-grade fluorescent lights is common practice for chroma key lighting because they can produce even light without hot spots.

  • Match the foreground to the background. If possible, you should know what your keyed footage will be composited with so you can adjust lighting (Figure 5.6). The most believable keys try to match the lighting of the replacement background in the final composite. Make sure that the light is shining from the same direction (and with the same color and intensity) as the light shining on the background plate you plan to use.
  • Figure 5.6 The chroma key footage was lit to look like an outdoor shot (notice the directional light hitting the top of the subject’s head). After keying, the color temperature of the footage was further modified to help match the shot to the background.

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