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The Black Art of Chroma Key Compositing
If
you’re combining live action and animated elements within a scene, compositing
will play a large role in your final product. You’ll most likely begin
by shooting video on a blue or green screen. The background color is then
removed using a chroma keying process. 2. Give your blue surface a slight curve. A very slight curve on the vertical axis will help soften the light and lessen visible highlights on the screen. 3. Use flat lighting on the screen. The screen should be lit softly from above and possibly the sides. Never point lights at the screen from behind (or too near) the camera. The goal is to introduce no contrast variations. A video tool called a waveform monitor is used to measure the brightness and color of the screen to make sure it is consistent throughout. Any good production group will use one. If you use any glossy materials on your screen, you’re sunk. 4. When you light the talent, don’t relight the screen. Dramatic and flattering lighting is often antithetical to good bluescreen lighting. Keep the talent far enough from the screen so that you can light them without their lights hitting the screen. Never point talent lights at the screen — you won’t be able to key later. And remember to recheck your waveform monitor after you’ve lit everything. 5. Choose a professional, high-quality tape format. You’ll need a format that can handle highly saturated, complicated images. This requirement precludes the use of consumer formats such as VHS and Hi-8. The new DV format is a better choice, although it deals with color space in a somewhat limited fashion, and can create pixelated artifacts around the edges of foreground objects, making the objects difficult to key. Betacam is a viable low end, with Digital Betacam, Digital-S, and film being the preferred format choices. 6. Narrow your depth of field. You may want the talent in focus, but keep the screen as blurry as possible in order to hide the inevitable imperfections in the screen and make keying much simpler. 7. Kick the talent (no, don’t hurt them). A warm-colored light (try an orange gel) placed behind the talent pointing at his or her back will help create a strong, very unblue edge on the talent, which will keep fingers and hair form disappearing in the key. 8. Avoid the key color in any foreground objects. Yes this sounds simple, but trust me — it isn’t. Blue shirts, socks, even eyes can disappear in the key, as will anything that reflects blue (particularly white and metallics). If you have to use blue elements (such as the Wing Commander uniforms), shoot on a green screen. 9. Fix problems in preproduction. Generally, what you’re paying for with high-end post tools is the ability to screw up in production. Terminator 2 might have cost one third as much if it hadn’t been rushed through production so quickly. If you don’t have access to high-end tools, following these tips (and others like them) are the only chance you’ll have to get a good key. You have time and intelligence, which can offer you almost all of the advantages of money as long as you use them well. Also, I don’t recommend putting too much faith in the equipment to hide your mistakes. 10. It’s important that the live-action video matches the lighting present in the rendered background elements. Otherwise, even the cleanest composite will stand out, and the video will look fake and gimmicky. Giving the animator a diagram of your lighting setup and a tape of the shoot are a requisite. The original design spec for Journeyman Project 3 called for a blue time-travel suit. When developers realized that wouldn’t work in a bluescreen shoot, the suit was quickly repainted green.
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