Module 6: Backgrounds and Green Screens

Green Screen and Chroma Keys

There are digital graphic images, and scenes in movies and TV shows, that involve placing a person or object over a graphic background or a scene shot elsewhere. A common example is a weather forecaster who appears on TV in front of a weather map. This is done by shooting the forecaster in front of a special green background, called a green screen. A chroma key is used to target areas in the frame that have that particular green and “key them out”—make them transparent— which leaves the person on a transparent background. The forecaster, minus the background, is then layered (composited) over a digital weather map. The green color is a special hue not otherwise found in nature, so you don’t accidentally key out, say, a green tie. In some cases a blue background is used instead, which may work better with subjects that are green; blue is also used for traditional film opticals.

This technique is called a green-screen or blue-screen shot or, more generically, a process or matte shot. For situations other than live television, the chroma key is usually done in postproduction, and the keyer is part of the editing system. Ultimatte is a common professional chroma key system and is available as a plug-in for various applications.

Green-screen shots are not hard to do, but they need to be lit and framed carefully. Green

background material is available in several forms, including cloth that can be hung on a frame, pop- up panels for close shots, and paint for covering a large background wall. When using a small screen, keep the camera back and shoot with a long lens to maximize coverage. It’s important that the green background be evenly lit with no dark shadows or creases in the material, though some keying programs are more forgiving than others of background irregularities. For typical green-screen work shot on video, lighting the background with tungsten-balanced light should work fine, but for film and blue-screen work, filters or bulbs better matched to the screen color are often used. Don’t underexpose the background, since you want the color to be vivid.

Avoid getting any of the green background color on the subject, since that may become transparent after the key. Keep as much separation as possible between the subject and the background and avoid green light from the background reflecting on the subject (called spill—most keying software includes some spill suppression). If you see green tones in someone’s skin, reposition him or set a flag. If objects are reflecting the background, try dulling spray or soap. Fine details, like frizzy hair, feathers, and anything with small holes can sometimes be hard to key. Make sure your subject isn’t wearing green (or blue for a blue screen) or jewelry or shiny fabrics that may pick up the background. Don’t use smoke or diffusion that would soften edges. Using a backlight to put a bright rim on the subject can help define his edges.

The Chromatte system uses a special screen that reflects back light from an LED light ring around the lens. This is fast to set up (since you don’t need other light for the background) and works in situations where the subject is very close to the screen (but keep the camera back). Get your white balance before turning up the light ring.

Locked-down shots with no camera movement are the easiest to do. You can put actors on a treadmill, stationary bike, or turntable to simulate camera or subject movement. Footage shot to be superimposed behind the subject is called a background plate. If the camera moves you’ll want the background to move also, which may require motion tracking. Orange tape marks are put on the green screen for reference points for tracking. Do digital mockups or storyboards to plan your shots.

As a rule, keys work best with HD and with 4:2:2 color sampling (or even better, 4:4:4). That said, many successful keys have been done in SD, and even with formats like DV that are 4:1:1 or 4:2:0 (for more, see the previous section). Make sure the detail/enhancement on the camera isn’t set too high. Bring a laptop with keying software to the set to see how the chroma key looks.

Fig. 5-16. Chroma key. The subject is shot in front of a green or blue background, which is then keyed out. The subject can then be composited on any background. NLEs usually include a chroma key effect or you may get better results with specialized software or plug-ins such as this Serious Magic product. (Serious Magic)

See Progressive and Interlace Scanning on p. 11 before reading this section.

It’s very easy to convert from progressive to interlace. One frame is simply divided into two fields (this is PsF; see p. 602). This is done when you’ve shot using a progressive format but are distributing the movie in an interlaced one, such as 50i or 60i.

Creating progressive footage from interlace is more complex. This may be done when distributing interlaced material on the Web, for example, or when extracting still frames. Static images aren’t a problem—you can just combine the two fields. But when there’s any movement between fields they will show edge tear when you combine them (see Figs. 1-11 and 5-17). Some deinterlacing methods just throw away one of the fields and double the other (line doubling), which lowers resolution and can result in unsmooth motion. Some use field blending to interpolate between the two fields (averaging them), which may also lower resolution. “Smart” deinterlacers can do field blending when there’s motion between two fields but leave static images unchanged. For more, see p. 600.

In terms of video, moiré (pronounced “mwa-ray”) is a type of visual artifact that can appear as weird rippling lines or unnatural color fringes. Sometimes you’ll see moiré in video when people wear clothing that has a finely spaced checked, striped, or herringbone pattern. Moiré patterns often show up on brick walls, along with odd-colored areas that may seem to move. When you convert from a higher definition format to a lower definition one, various types of aliasing can result, including moiré and “jaggies” (stair steps on diagonal lines).

With the explosion of DSLRs, moiré is often showing up in scenes that would look fine if shot with traditional video cameras. Even hair, or clothing that has no visible pattern, may end up with moiré when shot with a DSLR. This happens in part because the DSLR’s sensor is designed to shoot stills at a much higher resolution than HD or SD video. To create the video image, instead of doing a high-quality downconversion, as you might do in postproduction with specialized software or hardware, many DSLRs simply skip some of the sensor’s horizontal lines of pixels. The resulting image forces together pixels that should have been separated by some distance, causing artifacts.

Fig. 5-17. Interlace and deinterlace. (A, B) Two consecutive fields from 60i footage of a basketball

being thrown. (C) Here we see both fields together as one frame. You can clearly see that each field has half the resolution (half the horizontal lines), and that the ball has moved between the two fields. Makes for an ugly frame. (D) This deinterlaced frame was made by deleting the first field and filling in the gap it left. We can create new pixels for the gap by “interpolating” (essentially averaging) the lines on either side of each missing line. Though this creates a single frame that could be used for a progressive display, it’s not as clean or sharp as true progressive footage shot in a progressive format. Deinterlacing works best when there’s relatively little camera or subject movement. See also Fig. 1-11.

Though it may seem counterintuitive, the way to minimize this problem is essentially to lower the resolution of the picture at the sensor. With a DSLR, be sure that any sharpness settings are turned down. The camera may have a picture profile for a softer, mellower look. Sometimes you have to shoot a shot slightly out of focus to get rid of a particularly bothersome pattern. True video cameras avoid moiré by using an optical low pass filter (OLPF), which softens the image, removing very fine details that can cause artifacts (if you’re shooting with a Canon EOS 5D Mark II DSLR, you could use Mosaic Engineering’s VAF-5D2 Optical Anti-Aliasing filter). Higher-end cameras that shoot both stills and video, like the RED Epic, avoid artifacts by not doing line skipping when they downconvert.

If you’re on a shoot and you’re seeing moiré in the viewfinder, it may be in the image or it may just be in the viewfinder. Try a different monitor to check. If it’s in the image, to minimize the artifacts try shooting from a different angle and not moving the camera. You may also need to change wardrobe or other items that are causing issues.

Footage that has moiré in it can be massaged in post to try to soften the most objectionable parts, but it’s a cumbersome process that usually delivers mixed results. Along with poor audio, moiré is one of the chief drawbacks of shooting with DSLRs.

Fig. 5-18. Moiré. The metal gate on the storefront has only horizontal slats, but this video image shot with a DSLR (Canon EOS 5D Mark II with full-frame sensor) shows a fan pattern of moving dark lines. This is one of several types of moiré that can occur in video, particularly with footage from DSLRs. (David Leitner)

As described on p. 11, interlace involves scanning the picture from top to bottom, capturing each frame in two parts: one field (half of the frame) first, and the second field fractions of a second later. Progressive formats, on the other hand, capture the entire frame at the same instant in time.

Actually, the part about progressive is not entirely true. CCD sensors can capture an entire

progressive frame at once, as can CMOS cameras equipped with global shutters. However, many CMOS cameras have rolling shutters, which scan from top to bottom. There’s still only one field— it’s truly progressive—but the scan starts at the top of the frame and takes a certain amount of time to get to the bottom. The result is that fast pans and movements can seem to wobble or distort. Straight vertical lines may seem to bend during a fast pan (sometimes called skew; see Fig. 5-19). Some people call the overall effect “jello-cam.”

Some sensors scan faster than others, so you may not have a problem. To avoid rolling shutter issues, do relatively slow camera movements. Avoid whip pans. Keep in mind that fast events, like a strobe light, lightning, or photographer’s flash may happen faster than the sensor’s scan, leaving the top or the bottom of the frame dark.

Various software solutions minimize rolling shutter artifacts in postproduction, including Adobe After Effects, the Foundry’s RollingShutter plug-in, and CoreMelt’s Lock & Load.

Fig. 5-19. Rolling shutter and skew. (A) On cameras that have a rolling shutter, the sensor scans from top to bottom. If the camera or an object moves quickly, the top of an object may be in one part of the frame at the beginning of the scan, with the bottom of the object in a different place by the time the scan finishes. (B) The recorded frame shows the object tilted or skewed, even though it was in fact vertical. The slower a particular camera scans, the worse the problem.

As discussed elsewhere, different types of video noise can become apparent in the image when it is underexposed, enlarged, processed at a low bit depth, etc. Sometimes you can reduce the appearance of noise simply by darkening blacks in postproduction. Various types of noise-reducing software are available either as part of your NLE or as a plug-in, such as Neat Video.