The Digital Cinema Society presented a session on compositing with Bob Kertesz and Steve Wright. At Big Vision Studios, an HD Production resource in Burbank, the two experts spoke to a crowd of cinematographers and engineers with a model greenscreen set-up behind them, utilizing a new Panasonic P-2 VariCam 3700 camera, Panasonic AJ-HPM110 P2 recorder to generate a background and Ultimatte’s new 4:2:2 hardware.

Kertesz was the first speaker, and he focused on compositing from the cinematographer’s perspective. The first question he posed was: which is it, blue or green? “In my experience, you can get a cleaner, nicer matte off of blue, especially if the subject is blond and especially if the blond is out of a bottle,” he said. “Bottled blonds go strawberry with green screen.” If blue is so much better, why does everyone shoot green? It’s about the money, said Kertesz. “Green is a much larger component of video so it requires half the light, smaller crew, less air-conditioning,” he said. “That’s how green became the standard – not because it’s better but because it’s cheaper to shoot.”

As an aside, Kertesz noted that “Paul Newman’s remarkably blue eyes did fine on blue-screen,” but contact lenses make the eyes look like two holes. Kertesz pointed out that he composites live and therefore has no previs on set and has to rely on the waveform monitor and his eye. To avoid headaches, he made the following suggestions: Make sure the screen is lit very flatly and evenly, and do so before the subject is there. Using colored bulbs is dangerous. “There’s no light meter on earth that will read it properly,” he said. “And using colored bulbs, the screen will be overlit and you can’t read the floor. Color Kinos are good for quick and nasty and if the shot is from the waist up. But it’s a huge trade-off.” They do come in handy when lighting a big screen: on Vanilla Sky, Kertesz was faced with a 360 degree green-screen, 450 feet horizontal and 60 feet high. “The only way to light a green-screen that big was with color Kinos,” he said. “Otherwise, stick to tungsten.”

For shooting outside, he advised minimizing direct sun exposure to the screen because “the sun sucks out the color.” “The problem is if it’s outside and it’s cloudy and clouds are moving fast, the temperature constantly changes,” he said. “I’ve had great experiences just using sunlight to light – on the downside, you’re at the mercy of the weather.

Talking about the difference between a classic and digital blue/green screen, he noted that most post compositing software looks for the blue/green difference. “The wider the difference, the easier the matte will cut,” he said. “It’s very difficult to make a paint or cloth that has the proper separation. As you add pigments, the luminence of the paint gets darker and requires more light. There has to be a balance between paint and screen manufacturers as to what it is.” He recommends Composite Components for the best screens; the company also sells matching gloves, gaffer’s tape, hoods, and body suits.

LED lights were briefly discussed. Kertesz described a curtain with embedded reflective media from Reflect Media, developed by the BBC. “A ring light goes around the lens with a dimmer on it,” he explained. “What happens is it’ll reflect very nice blue or green back to the camera with no spill. The downside is that it’s almost impossible to get that ring nodal. Because it’s slightly offset there will always be a slight gray shadow down one edge of the subject. You can get rid of it in post, but if you push the matte harder, subtle detail disappears. It’s a terrific system for shooting puppies, product photography like jewelry, stuff close to green. But you have to be prepared to go in and correct the non-nodal shadow. It’s great if you have the same set everyday- like a shopping channel. You tweak it and go. It’s not so easy on a run-and-gun situation.”

If you’re using a switcher chroma-keyer, said Kertesz, by all means light with the opposite colored (rose or yellow) lights. “But that’s no longer necessary unless you’re finishing in a TV station,” he stressed. “Use the back and side light in the same color. If the Ultimatte sees a yellow gel, it’ll think it’s part of the scene. All decent post production software takes care of the bounce and the flare. The back and side light – you just need a kiss of it, just to kill the blue or green wraparound.”

When asked about wardrobe, Kertesz explained “the way to think about the process is that you’re removing a certain color.” “Greens will be removed from yellows, which are half green,” he said. “If someone comes out in a yellow slicker, unless you’re careful, that’ll go orange. You have to think ahead about the wardrobe. Nothing with too much primary color: cyan and aqua are colors that will change. A lot of times people don’t care, but if you’re maintaining wardrobe continuity, it can matter. If you use a little edge light, a little backlight to separate from the green. I like to shoot my subject 8 to 10 feet away from the green screen-– and you only need it to be around them. Once the person crosses something not green, the only way to deal with it is frame-by-frame and a world of hurt.”

A head-to-toe shot is an increased degree of difficulty. “If you light the floor and wall the same, the floor will be hotter because of the angle of reflection,” he noted. “Whatever you’re using to light the scene, the floor will inevitably be lighter. “Use a pola-screen,” he suggested. “A handy trick is to shove it under the lens until the floor gets as dark as possible.”

If you’re shooting a car on green-screen, says Kertesz, you’re also in a world of pain. “It’s been done but you have 8 million curved shiny reflective surfaces picking up the green from five stages over,” he said. “Matting a car is a very difficult endeavor. You have to set things up with lots of soft light and white cards. It’s a whole complex process.”

More advice: Let the background image motivate how you light the subject. “If it’s animation, have them tell you the direction the light will be coming from,” he said. “That’s how you sell it, baby. Matching the lighting on your subject to the lighting of the background is what it’s all about.”

Final piece of advice: “I’ve stopped telling people it won’t work,” he said. “Everything can be fixed. What I do tell them is, that it’ll be very expensive to do. That gets the producer’s attention.”

Next came Steve Wright , author of two books on compositing including “Compositing Visual Effects,” an introductory book to compositing. He spoke about compositing from a post production point of view. “I’m the guy you pay to fix your shots,” he said. Having pulled over 1,000 blue/green screen shots, he showed a gallery of “appalling green screens from some big motion pictures.”

Steve Wright

“With modern keyers, the skin tones and hair issues are much less of an issue,” said Wright. “Green is easier to light because tungsten throws out more green than blue photons.” He noted that outdoors, the sky is a big blue tarp, and that blue spill is also less offensive to the eye than green spill. The downside of blue? “The blue channel in film and video have the most noise,” he said. “With a blue-screen, you’ll get more sizzle and the compositor has to pound it down, which removes fine-edged details like motion blur and hair. Blue is the worst choice for noisy mattes.”

Purity of color was another important topic. “What you want is lots of green and very little blue and red, or lots of blue and very little green and red,” he said. “Dangers in over-lighting the backing creates major spill-suppression issues. Tungsten is good for green-screen.” He also warned against colored lights or gels, noting that they can land on the talent. “Compositing software has to rip it off and it does weird things to the skin,” he said. “The keyer will see that as partly green-screened and makes it transparent.”

Why not use dimmers? “Color temperature!,” answered Wright. “Remember Mr. Purity. Don’t use dimmers on the tungsten because you’ll shift light towards the red. The human eye turns all colored lights to look white and normal, which is why you can’t judge the luminosity and have to use meters.”

Like Kertesz, Wright said to light talent for their intended composited environment, follow the 8 to 10 foot rule and minimize how much lighting on the talent spills onto the background. “Watch reflective objects and don’t counter with the yellow and magenta,” he said. “Modern keyers are tuned to remove the spill, so don’t try to help it. You’ll actually hurt it.” Add a couple of soft lights coming onto the talent from the side, to lighten up the edges and match overall luminosity, advised Wright.

With regard to the green screen itself, Wright noted you can use paint or fabrics… but don’t go to the hardware store and buy green paint. “Use special paint,” he said. “And put on two coats of paint or it won’t work. Painted surfaces tend to have glare that fabrics don’t. If you use fabric, use the designated special fabric. If you have multiple panels, make sure they’re the same color. Get gaffer’s tape that matches the color or you’ll get holes in the image. Otherwise, I’ll have to fix it.”

The head-to-toe issue introduces two problems: spill and bounce from the back-light and scuffed floors that lose their purity, so you get a bad key on the floor. “Wipe your feet,” said Wright. “You can use mylar to get a mirrored reflection of the green screen, but you’ll also get a mirrored reflection of the talent, which is a fairly easy roto to do.”

If you’re shooting film, use the finest film grain stock you can, he said. “Otherwise, it’s cottage cheese,” he added. “Never use filters because it blocks light I need to use the key. I’ll filter it after the composite. The polarizing filter to knock off glare: yes, please use this. You’re blocking out bad light that can ruin the green screen.”

Wright nixed the idea of 16mm film because of lack of pin-registration and the grain size, four times larger than 35mm. For videographerse, turn off the edge sharpening! “The edge sharpening algorithms introduce edge ringing artifacts that wreak havoc with the mattes you’re going to pull,” said Wright. “Don’t use filters again and don’t use DVCAM. DVCAM cuts down on the amount of color information and it doesn’t look great to the computer.”

“The big issues are brightness, purity, uniformity,” he said. “That’s what you’re aiming for. And, by the way, don’t put a green person on a green screen. You laugh, but I got a commercial of a girl in a blue polka dotted dress bouncing on a trampoline in front of a blue screen. Lots of bouncing dots.”

Yes, shoot a clean plate of the green screen. No, don’t smoke the set. “In compositing, it’s all about the edges,” he said. “I can add smoke in the composite. I can add motion blur in the composite. I can put a tobacco filter…after I’ve made the composite please. Or you will pay.”

Echoing Kertesz’ last piece of advice, Wright said, “We can fix anything.” “One of the problems in the industry is everyone knows that,” he said. “It’s true, but it’ll cost you. It’ll cost more to light well on the set, but you’ll have a better quality matte.”