Green Screen Lighting

Lighting:

Lighting is VERY IMPORTANT. If it’s not right, it won’t work. While lightingour set, we had the advantage of using a GVG SEG with componentChromaKey option, so we could get immediate feedback on our lighting.Failing that, if you’re running MovieShop 3.x or lower, you could try thecrudechromakey effect while setting lights, but it’s really pretty crummy. (Ithinkthat’s why they dropped it, Steve!)

You don’t really have to be able to monitor the effects live. I’ve done thisacouple of times without that possibility. Just follow the rules on thelightingreally carefully.

LIGHTING THE SCREEN:
Actually, the screen doesn’t have to be as perfectly illuminated as most folks might think. You can use three scoops, which of course gives three intersecting softcircles. No problem. However, your talent needs to be a distance from the screen — 6-8 feet is best. This allows you to light the talent without casting a shadow on the screen, which MAY goof you up.

Light the screen as evenly as possible, but DO NOT OVER ILLUMINATE!
Your enemy is green or blue spill on your talent’s back. The more light you pour on thescreen, the more light reflects off. 3d folks, this is radiosity! It’s better to have even, medium illumination. While the illumination can vary, don’t let there be any really dark areas.

LIGHTING THE FOREGROUND:
Talent should be 6-8 feet in front of screen, if possible. WATCH OUT for any reflective surfaces, they will vanish when they pick up the BG. Watchout also for thin stringy stuff, which won’t tend to compose well. Use the standard three-point lighting approach of key, fill, back — except use astronger back light than you might usually use. Looking at the scene on a monitor, watch for any green or blue spill or reflections. One last noe, the talent SHOULD NOT wear any green or blue! Seems obvious… othewise items will be keyed out and you can have someone with a transparent hole in their body.

Proper lighting is much more important than the software for chroma key. There are ways to prevent spill of color on the person by using the right lighting. Depending on the skin, hair, clothes and eye color of the person a choice of blue or green screen should be made.

Keep in mind: Background should be evenly lit & Make notes of your lights positions. It will help to match CG background lighting. Since the color we see is the one not absorbed by the surface. That color will spill on to the person. Colored walls or camera angles may get areas of the person that are have color spill. If scene doesn’t show feet, avoid blue/green floor Set lights for person separate from background. Quite useful to use some light from behind the person. This will help with hair (hardest to clean). Make two takes of each scene. One with just the blue/green background. And one with the person over it. Most advance software requires this. Therefor you must always keep a locked camera to make sure that both passes match.

When lighting your key surface make sure your bounce light is not directed at the camera. Bounce light causes glare, and glare means your surface is not evenly lit from the point of view of the camera. Angle of incidence equals the angle of reflection… For example, if your camera faces the key surface directly, set your lights at 45 degree angles to the key surface. This will ensure that the bounce light bounces toward the other light and not at the camera. Gelling the lights with the same color as the key will also increase the saturation of the light helping the key as well.

Key color spill is usually to be avoided, especially on the subject to be keyed, but sometimes a little ambient spill in areas which normally reflect light is not necessarily a bad thing: A normal (non-key) background usually reflects some light in a scene, especially on shiny surfaces. These surfaces will reflect the key color in blue screen shot as well. Using the background to be keyed, mirroring it in AFX, blurring it, and using positioning this blurred and mirrored image in the places where the key color spills can sell these areas as being normal reflected light. It’s more work, certainly, but the subtle effects can really be convincing to the eye.

Software Tips/Notes
Use After Effects and the Color Diff key. Ultimatte plugin for After Effects (AFX). Once a good matte is pulled you can process it to black and white hi-con with the After Effects production bundle tools.

Software usually doesn’t require you to shoot a screen correction plate, it always helps, but a decent matte can almost always be pulled without one. (It just might take a LOT longer.) If you make a garbage matte around the subject you can usually correct for uneven blue/green screen lighting. (Again, more time.) The spill suppresser in after effects works well. Ultimatte does a great job of suppressing the spill color, and is available in composer and after effects.

Ultimatte is the best keyer ever used…

One thing to avoid is compression, it goes after the blue channel of the image. This makes it more difficult to pull a nice blue screen. Even DigiBeta can be harmful. Another problem from tape is 4:2:2 chroma aliasing. Color is sampled at half luminance resolution, which can sometimes lead to “steppy” edges. ISFX Matte Wizard (AE) is REALLY good for cleaning up ragged bluscreen and doing lightwraps.

Use a combination of both the Difference Matte and the Color Difference Key filters in AFX ProPack for very convincing results. Difference Key usually works well for most of the image but falls apart on places with fine detail (hair). (The Color Difference key is as good as anything, but it isn’t as intuitive as it should be.) The Difference Matte works much better in these areas. You can make separate Comps in AFX using each of these keys to the best of their ability, and then use the masking tool to target areas which are better matted by one filter as opposed to the other. This technique also works for areas where the key light is uneven: Key out most of the scene using a filter set to one color, and use a different key color for spot areas.

Green Screen Tips/Techniques

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