Greenscreen and Other A-Over-B Composites
Most editors regularly encounter a greenscreen composite, or another situation in which a foreground must be integrated with a separate background. Although greenscreen keying is the stereotypical role of the compositor, to me pulling the key is only the first step toward the real work of the VFX compositor—matching a foreground and background convincingly (see Figure 2). That task can involve all four of what I consider the fundamental building blocks of compositing:
- Keying: Creating a selection from a solid-color background (see Figure 3a, 3b, and 3c)
- Rotoscoping: Cleaning up where the greenscreen isn't complete or perfect
- Motion tracking: Working with a camera and objects in motion
- Color and image matching: Fully integrating foreground with background
Any editing software lets you pull a key, but getting the lighting to match and the edges to behave, as well as clearing rigs and other unwanted debris from the moving shot, requires a tool like After Effects. Luckily, all these tasks are quite doable with After Effects, with fundamental techniques that you can learn.
Figure 2 Chapter 1 of After Effects Studio Techniques introduces a "simple" A-over-B shot, inserting a can into the background scene. Without matching camera motion, scene lighting, and color, as well as the grain of the source footage and the smokiness of the original scene, even such a simple comp fails. These steps are accomplished much more easily and effectively in After Effects than in an NLE.
Figure 3c Chapter 6 of the book, which deals specifically with keying, demonstrates why many bluescreen and greenscreen shots require more than a one-pass, one-button approach. In After Effects, you can go as far as needed in order to break a challenging source matte into manageable sections, even dividing each of those sections into multiple passes. Figure 3a breaks down a single foreground figure into multiple pieces for optimal keying. Figure 3b uses multiple passes; it's also possible to use both, and then evaluate the result against the target background (Figure 3c).
Once you can handle this type of composite, other bread-and-butter effects that I cover in depth in the book, including shots such as sky replacements and removing items from a moving camera shot, or even inserting a 3D computer graphics object, are well within reach.
But another type of After Effects shot, one that's more directly pertinent to the editing process, is even simpler. Let's take a look.