DV Chromakeys

Michael Kuhlman
April 2004

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When the consumer DV format came out in 1995, it truly was a milestone. 500-line resolution and far clearer color than analog S-VHS and Hi-8, all on a tiny tape that could be digitally copied or captured to a computer through a single FireWire cable, edited and dubbed back to the camcorder without losing any quality. Indeed, with the advent of faster processors, higher-speed hard-drives and ever increasing memory, not to mention hardware-accelerator boards, computers have risen to the challenge of video editing with a ferocity that leaves even a so-called "state-of-the-art" 500MHz Pentium II from 1998 in the dust. Despite all this technological prowess, when it comes to chromakeying, we're still confined to the basics: replacing the background key color with a new background. The trick is to leave the foreground subject intact as a clean, pure composite, with no telltale hint that the foreground subject is even part of a composite, making the illusion complete. Pre-CGI (computer generated graphics), filmmakers grappled with these same issues. Even post-CGI filmmakers have to be careful when photographing their actors against blue or green screen to ensure that their actors will composite cleanly into the yet-to-be-produced FX backgrounds in post. The process of creating clean composites is as much an art as a science, and demands tight quality control over every element seen in the frame.

Chromakeying is the process of selecting a color (or small range of colors) in a video signal and making that color transparent. You place your foreground subject against some type of plain-color background, blue or green being the color backgrounds commonly used. After your software makes the color transparent, you can put anything you want into the background.

Where consumer DV tends to fall short is in the way it displays and records color. Both consumer DV and higher-priced broadcast digital formats record video in component form (YUV), meaning that the color picture we see has a luminance signal ("Y") and two color-difference signals ("Y-minus red" and "Y-minus blue"). The Luminance signal contains the brightness and contrast information of the picture and is black and white when viewed by itself. Expensive broadcast digital formats like Digital Betacam sample color as 4:2:2, meaning the luminance portion of the signal is sampled at twice the resolution ("4") of the two color-difference signals ("2:2"). Consumer DV, in order to cram as much signal onto the tiny DV videotape and the 25 Mbps DV data stream, samples color at 4:1:1 or 4(Y):1(U):1(V) or 4(Y):1(Y-red):1(Y-blue). So where is the green?

The green, it turns out, is interpreted from the luminance channel. This is why most digital keying is done with green screens nowadays. Imperfect keying results in green-fringing where the key background is "leaking" onto our subject (especially visible around a person's hair) and the edges may be generally jagged and indistinct. This ruins the illusion just as surely as a microphone dipping into the shot would. Most of us are limited by economics to the 4:1:1 color space of DV, but the situation is not nearly as bleak as some would like you to believe.

Lighting our green background as evenly as possible is the secret to a good key. In a controlled studio environment, this is usually a matter of placing the subject far enough away from the green screen so that the subject's own shadow does not fall anywhere on the screen behind them and then taking great pains to flood the screen with enough light from multiple sources to perfectly and evenly illuminate it. The larger the screen, and the more light you place on the screen, the better. On location outside, matters are a bit trickier, as the sun is the dominant light source. But, with trial-and-error, by placing the background as evenly to the direction of the sun as possible, quality keys can still be achieved. Using the blue sky as a quality chromakey background almost always invites disaster, because there is a subtle gradation in the sky from blue to bluish-white. If you're on a shoestring budget, and depending on your desired effect, you may be able to get away with using the sky as a blue background. Just be prepared to do a lot of tweaking in your software to get this right.

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