Cantores de la capilla

In the recent past, working with blue screen or green screen required considerable resources, for example, finding an available screen. If you knew the people at your local public access station, you might have been able to use theirs. If not, or if the screen was already booked by someone else, you would have to spend the money to rent time at a production facility. Now, prosumer technology makes it affordable to set up a good quality screen in your home studio or even on location. Many film and video equipment stores sell chroma key screens at accessible prices. A company called Lastolite sells collapsible chroma key backgrounds that

it advertises as "completely crease free." Creased backgrounds can be a problem for chroma key effects because the creases produce shadows, making some areas darker and others lighter—this results in a multicolored background instead of a solid color background (solid backgrounds are easier to key out, because they contain only one color to identify and remove). Working with a creased background isn't the end of the world, but avoiding creases might save you a few gray hairs.

To create the chroma key effects for this book, I used a 5x7 foot background, from a Colombian company called Botero, that folds into a 26-inch disc for travel and storage. Some collapsible backgrounds are also reversible, blue on one side and green on the other, which lets you switch colors depending on your actor's wardrobe. (As of this writing, a single-color Botero chroma key background costs $63.50; a reversible model, such as that in Figure 5-2, costs $79.95.) If you shoot a man wearing a blue suit in front of a blue screen, parts of his suit may become transparent when you key out the screen (you may have noticed that this happens on television sometimes when a new person does the weather forecast and doesn't quite know what to wear).

Figure 5-2. The Botero chroma key screen, shown here folded, is blue on one side and green on the other. The same company sells slightly less expensive versions in either green or blue.

Various companies also sell reversible chroma key curtains as well as rolls of chroma key "seamless" paper backgrounds. ("Seamless" means that the material curves onto the floor, without making a distinct angle that would produce shadows.) Other companies, and some hobby stores, also sell large pieces of blue or green cloth you can use as a chroma key background at a much lower price; just be sure to stretch the

material tight enough to avoid wrinkles and creases. If you're looking to create a large and permanent chroma key stage, you can use chroma key paint. Rosco International sells specially formulated chroma key paint for $44.95 a gallon. According to the company, a gallon of paint covers 300 square feet. A dedicated chroma key stage can be helpful if you want to record large areas of action, such as a person running across the frame, or if you want to record a number of people in the frame together.

Note: If you're planning a chroma key effect to composite a person into an outdoor scene, shooting the outdoor footage on a cloudy day can make lighting much easier. As described in the last chapter, cloudy weather produces an even light without shadows. It's much easier to match the lighting in a shadow-free clip, because you don't have to worry as much about the direction of the light source. We purposely waited for a cloudy day to shoot the street scene exteriors for this book (we shot in New York in December, when cloudy days are plentiful—if you're working in a sunnier climate you may not have this option). In addition to the issue of shadows, the color temperature of an outdoor scene varies by time of day and is especially noticeable in shots that contain bright sunlight. If the color cast of the actor doesn't match the color cast of the background clip, the audience will notice. Cloudy days create an even, flat light that's much easier to reproduce in front of a chroma key screen.

Just as a crease-free background makes chroma key effects easier to execute, so does evenly lighting your background. Even if you work with the perfect solid-color background, shadows or inadequate lighting can make key effects more difficult, if not impossible.

5.1.1.1. The background

An editing system will likely interpret lighter and darker areas as multiple colors, creating more than one color for you to key out. If an editing system interprets some of the darker areas of your background as similar in color to darker areas of your actor's hair or wardrobe, the editing system will key out pieces of your actor along with the chroma key background you're trying to remove. This can be really frustrating unless you're doing it on purpose, but fortunately, there are ways around the problem.

If you have Final Cut Pro installed on a laptop, the software's documentation suggests bringing your laptop to the shoot, connecting your camera via the FireWire port, and then using Final Cut's waveform monitor and vectorscopes to ensure your background is evenly lit (you can perform similar tests using Premiere Pro, Avid XPress DV, and Sony Vegas). Waveform monitors and vectorscopes (addressed in detail in Chapter 16) measure the color and brightness information in a video signal, and will reliably point out any

potentially problematic areas in your shot. You can use the software to measure color information with, and without, your actor standing in front of a chroma key background. As they say, an ounce of prevention is worth a pound of cure. Even if it takes considerable time and effort to finetune your lighting before you start to shoot, it can save you immeasurable frustration in the long run if it makes your effect easier to execute (not to mention the money you save by not having to reshoot the scene to correct a problem). Like everything else, the process will go more quickly as you get more practice.

To light the chroma key background in Figure 5-3, we placed very powerful 600-watt tungsten lights at either end and positioned them to light the background as evenly as possible. As you can see, the light is not perfectly even. Some areas are slightly brighter than others, but there are no problematic areas of extreme darkness or shadow.

Figure 5-3. The trick to chroma key is good lighting. If a green or blue screen background isn't lit brightly enough, it becomes much harder to key out.

5.1.1.2. The actor

Once you have your background evenly lit, it's time to light your actor, preferably without casting shadows on your carefully lit background. One preventative measure you can take is positioning your actor at a distance from the background, as in Figure 5-4. This not only helps prevent shadows, but it also helps to prevent the color of your screen from reflecting, or spilling, onto your actor. If blue or green light reflects off your background onto a person in the scene, you may inadvertently key out parts of his body along with the background during the Figure 5-3. The trick to chroma key is good lighting. If a green or blue screen background isn't lit brightly enough, it becomes much harder to key key process.

Figure 5-4. To light our actor, we placed a Chimera (it's the large light in the left of this photo) as far from the background as possible. Our goal was to evenly light Luca as he ran past the screen, without

casting shadows on the background.

It's important to try and match the lighting of the clip you plan to composite your actor into. If you're compositing a person into a shot containing a weather map, or another type of animated background that isn't designed to look realistic, matching the lighting scheme won't be too much of an issue. If, however, you're attempting to create a realistic looking composite, the lighting on your actor needs to match the lighting in your background video clip. If you plan on compositing a person into footage that contains any type of shadows, it's important to keep the direction and the darkness of those shadows in mind. If you have noticeably flat lighting on a person's face, and dark shadows in the clip you're compositing that person into (or vice versa), the shadows won't match. As a result, the composite won't look believable to the audience, even if your chroma key is otherwise technically flawless. You can work with color correction tools to compensate for differences in color temperature or overall brightness (again, these are addressed in Chapter 16), but shadows and the direction of a light source are not elements you can readily change using

prosumer video-editing systems.

Cleaner edges yield cleaner key effects. As a result, a person with neatly arranged hair is much easier to key out of a background than a person with wild wisps of hair going off in every direction—the fine strands can result in a fuzzy edge that's harder for a computer to separate from a chroma key background. (If the computer cannot effectively separate a foreground object from the background chroma key screen, it leaves a thin halo of color around the edges.) Shaggy wardrobe materials, like a fuzzy sweater, a fleecy jacket, or clothing with fringes can be especially hard to work with as well. Some types of dangling jewelry, such as fine strands of gold in a necklace or earrings, can also be problematic if they hang away from your actor's body.

Clean edges are particularly important when you're working with DV. Prosumer mini DV is a lightly compressed format—the compression takes place in the camera, and most of the time audiences and filmmakers never notice. The compression, however, makes it even harder to separate fine or fuzzy edges from a background using a chroma key effect. If your story absolutely requires hard-to-key images, such the hair or wardrobe elements just described, or if it contains translucent visuals like smoke, a format other than DV might make a better choice.

Uncompressed standard-definition formats like Betacam SP or more lightly compressed high-definition formats allow more latitude in key effects. Don't worry though: if you're careful about hair, clothing, and lighting you can still get good results with DV. If you're not sure, do some tests. Shoot some footage of your actor in a dress rehearsal, and try out the composite techniques you'd like to use. With a little experimentation, forethought, and some effort, you'll get the results you're looking for.

Note: If you find that your original framing causes you difficulty when it comes time to edit your

composite sequence together, you can use the scale, mask, and reposition techniques described in Chapter 12. Chapter 12 also shows you step by step how to create very slick composite effects, including chroma key.