DV Chromakeys

From the magical CGI composites of Hollywood to the meteorologist pointing to an animated map, the one essential element that links them is the key. Perhaps you’d like to make your two-year old child appear like Godzilla on top of a neighborhood of unsuspecting barbecue dads. In the olden days, that kind of composite was an expensive and time-consuming process of optical printing, re-photographing film images and hand-drawing mattes to replace the previously photographed foreground object, one frame at a time. We know an easier way.

Same as it Ever Was

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.

4:1:1 vs. 4:2:2 – Color Matters

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 is Everything

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.


Avoid Same-Color Clothing

This may be obvious to some, but there are still times, even in the broadcast world, when the subject wears clothing that is the same color as the background. In post, the subject’s dress or suit magically disappears. Fortunately, this doesn’t result in naked embarrassment, but it does allow the background to show right through them, making them look like a (mostly) invisible person. There are two solutions at this point: reshoot or pay someone to go through the video frame-by-frame and fix the shot. Always direct your foreground subject to wear a color that is completely different from your key background.

And that ’80s-style mullet hairdo? Pat it down and get rid of the frizz. Hairs and stray fibers around the outside edge of the subject’s sweater do not key well. A good rule to adhere to here is: Keep the lines on your foreground subject simple and smooth. A triangular-shaped Star Destroyer will key over a planet a lot better than a fuzzy space octopus with tiny flailing arms and appendages.

Try to limit your subject’s movements as well. A quickly moving subject carries with it a motion blur, where the subject and key background blend together a bit. The worst way to reveal your composite to your audience is a to show a huge colored fringe every time your subject quickly moves. You’ll notice this phenomenon even during the weather, when the weatherman quickly points at something. This anomaly is worse in 4:1:1 DV.

Software

Your keying software is important as well. Often, the Chromakey Effect that comes standard with your editor is the most fundamental of keyers, basically only allowing you to select a range of colors to key. More advanced compositing software, such as discreet combustion or the specialized Ultimatte AdvantEdge will give you much better keys. They aren’t magic, however amazing they seem, and the fundamentals we’ve discussed here still apply.

With proper care and a vivid imagination, there are no limits to what kind of spectacular visual effects you can achieve through the magic of chromakeying. Just be sure to wave “Hi” to George Lucas on your way to special effects superstardom.

Mike Kuhlman owns and operates a video production company.

Sidebar: DV Video Fundamentals

Component Video: In consumer DV, like most professional digital and even some professional analog video formats like Betacam SP, video is recorded in component form, with the brightness or black-and-white portion of the video signal (known as “Y”) recorded separately from two color-difference (“Y-minus red” and “Y-minus blue”) signals. The resulting picture is clean and free of cross-color artifacts, most noticably the pinkish moire sheen that occurs when, say, black lines going in the same direction appear too closely next to each other. A component video picture is like a window on the world, clear, vivid and with natural-looking color.

Sidebar: See YUV

If you own a DVD player and a television with component video inputs and outputs, try hooking up each of the three video cables that make up the component color picture one at a time. They are most often color coded green (Y), blue (Cb) and red (Cr). As you connect and disconnect each one, you’ll see exactly how each component of the signal combine to make the color picture you see.

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