Digital Video - April 2008 - (Page 32) KEY MASTER PULLING THE “IMPOSSIBLE” KEY IN YOUR NLE. BY REV. JOHN JACKMAN mazing technology, bluescreen. Without it, most of the cool visual effects in movies and television just wouldn’t be possible. But as anyone who has done much work with bluescreen techniques can testify, it isn’t always as easy as it looks. Narrative applications that involve large, difficult-to-light screen areas can suddenly create challenges that your NLE’s built-in keyer doesn’t handle well. Bluescreen work is a tightrope walk, a matter of balancing many conflicting factors. It’s rarely perfect. By the way, I’ll use the term “bluescreen” to refer to all color-based compositing, whether the screen color is blue, green, or even red. The tough problems in keying more commonly originate from haste or carelessness in production — uneven lighting on the background, conflicting costume choices and the basic poorly exposed shot are far more common causes of expletives in post. Once it’s “in the can,” it’s usually extremely difficult or expensive to go back and reshoot. So how do you deal with problem footage? Professional compositors rarely create a composite shot using a single filter layer. As a base, they will usually use a garbage matte to remove extraneous bluescreen (so the keying process doesn’t have to deal with it), an edge matte to create the cleanest possible edge for the subject, and a core matte to take care of the rest of bluescreen further away from the edges. Usually, the mattes will be “pulled” as discrete grayscale files that can be manipulated separately by other filters, then combined and applied to the foreground footage manually in a compositing program like After Effects, Apple Shake or Autodesk Combustion. It’s not uncommon to have a final matte that is composed of 6 or 8 submattes. While this may seem like a lot of trouble to the average editor, it’s how top quality keyed composites are done. Often the compositor will find that one keyer works best on edges, while another cleans up the rest of the background better. For instance, with an unevenly lit background where the foreground has motion blur, the compositor may use Keylight to create the edge matte with transparent motion blur, but find that the varying tones of the rest of the unevenly lit background can best be keyed using Primatte or a color range keyer. 32 dv april 2008 A But you can actually do a “poor man’s” version of this multilayer process in most current NLEs without sinking a fortune into specialized plug-ins and separate compositing programs. The first part of the secret is to use multiple applications of keying plug-ins to render narrow sections of the troublesome background transparent, rather than depending on a single application with very aggressive settings. Color-based keyers are really designed to deal effectively with a very narrow color range; when pushed too far, their performance suffers. The second is to also use your NLE’s Garbage Matte function to get rid of portions of the background that aren’t really part of the action — not just to get rid of extraneous set pieces and offthe-screen areas. In many cases, a bit of garbage matting can get rid of the most troublesome, poorly lit edges of the bluescreen through brute force and substantially reduce the range of color that the keyers must deal with. Let’s try this with a less-than-perfect foreground plate — though this sample is nowhere near as bad as it gets! The background is unevenly lit, the subject is too close to the screen so there’s a shadow, and she’s wearing a dark green sweater in front of a green background. The combination of factors mean that this shot won’t key properly with the basic keyers built into most NLE’s. The first step is to use a garbage matte — necessary anyway here due to the extraneous “offscreen” material. However, we can also use the garbage matte to get rid of the darkest areas of the edge of the screen, as well. Then test the different keyers that are available in your NLE package to see which one performs best on a specific scene; with the garbage matte cleanup, one of them may be able to do the job. If the results are less than optimal, move on to step two. Step two is to use multiple applications of the filter, each one keying out a narrow band of hue and value. Let’s try the color keyer in Premiere Pro, which is a really basic filter. After applying the filter in Premiere (or Final Cut or Vegas or Avid, the filters aren’t that different), use the eyedropper to select the brightest area of backwww.dv.com http://www.dv.com
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