What Is a Garbage Matte?

Unfortunately, locations and chroma backgrounds aren't always the best suited for a shot, but you have to work with what you have. For instance, if you have a distant shot of subjects in front of a chroma tarp, you may be limited to the available size for the scene. This would leave edges of the frame, hardware, lights, or cables exposed in the shot around the subject. All of these items and edges have to be removed before you can use the blue-screen footage in a composite.

A garbage matte is a solid-layer mask that covers up the majority of "garbage" around the blue-screened subject—staying clear of any motion paths that may get covered up. This helps to minimize the amount of rotoscoping necessary to remove the remaining unwanted material.

A garbage matte is usually a single, static layer that overlays the blue-screen footage in an After Effects composite. Placed in combination with solid-colored layers that track the movement in the frame, a garbage matte can help you cover up a great deal.

The project used in this chapter is a blue-screen shot from a crane above the actors walking across a chroma tarp. The lighting was natural daylight with a few large reflectors to soften any hard shadows on the actors. The problem was that the chroma tarp was too small for the entire required scene, so a garbage matte and rotoscoping were required to fill in the gaps and outlying areas beyond the tarp in the frame.

A Blue-Screen/Green-Screen Primer

Proper lighting for blue/green screens varies for the location and shoot. If it is an outdoor shot that S doesn't require capturing the actors'shadows, then the screen is placed well behind the actors 2 with reflectors flooding it with light. Most blue/green screen shots are done in a controlled studio £ environment that virtually emulates optimum natural lighting conditions. Keeping the chroma ^ background evenly and brightly lit is important so that you can eliminate as many shadows, wrings kles, or imperfections as possible and give an even space for proper removal.The actors are usually < placed at least 10 to 15 feet in front of a well-lit chroma background to keep the background u evenly lit and have the minimum amount of spill onto the back and edges of the actors.

Why use blue rather than green—or vice versa? Green has an obvious advantage over blue for complete and separate keying from a background, because this shade of green doesn't exist in nature. However,the blue is much more tolerant to spill onto the actor, because the light does exist in nature and is less discernable in a composite.This is why blue has been preferred for close-range keying,where the shadows and close proximity of the actors mean that they come in physical contact with the chroma background.

Today's software matte and keying plug-ins (commonly referred to as "keyers") are more capable of accurately blending edge tolerances and background-to-foreground color correction than their predecessors of only a few years ago.You can learn more about these software plug-ins for After Effects and Photoshop in Chapter 9,"Matte and Keying Plug-ins."

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