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When NOT to use Green Screen

Background screens come in many colors — are you sure green is the right one for you?

One sight that’s ubiquitous in behind-the-scenes footage from movie sets is the green screen. Green screens are a way to insert backgrounds into a scene while avoiding the time-consuming and expensive process of rotoscoping (painting digitally frame by frame) around people.

This method takes advantage of the fact that green is the least common color in human skin and clothing. VFX software can then isolate the green color to separate the background from the foreground people. This is called chroma keying or simply “keying.” But… green screens are not right for every situation — which is why starting a dialogue with your VFX shop before you shoot is so important.

For instance, a performer like Kermit the Frog should obviously be shot in front of a blue screen. Blue screens were once the most common way to insert a background; meteorologists were famous for doing their forecasts in front of a blue screen. Historically, the blue channel in film had the least grain resulting in cleaner edges, but now video compresses the blue channel, and the green channel is a higher quality with the absence of film grain. The transition to video and digital capture has shifted the majority of the industry to use green over blue except when the foreground subject dictates otherwise.

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If we are shooting elements separately in order to keep the actors safe — like fire, explosions or bullet ricochets — we isolate these elements by using a black screen. Black screens maintain the subtle luminance in the edges of bright elements. When you see a bullet ricochet off of something or a gun shooting sparks and smoke, that element was probably isolated using a black screen before it was added to the shot in post.

So within a particular scene, the actors might be shot against a green screen while the explosion is shot against a black screen. We can pull those elements together and composite them at the highest quality.

I've never heard of a situation where a red screen was used, although it would work for shooting the Blue Man Group if Kermit was a special guest on their show.

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Excessive spill or reflection of a screen’s color on your subject can be avoided by keeping six feet between the actors and the screen when possible. Sometimes the background to be replaced is a phone screen with fingers typing over an image to be inserted later. In this case, I wouldn't suggest a green screen because the fingers are too close to the screen and green light will be reflected onto the skin tones. For these cases we suggest a 50% grey screen image which illuminates the fingers and also allows us to capture the screen's natural reflections for realism. Grey has no chroma (color) information, so we use the luminance (brightness) to separate the elements.

I often meet with the DP (director of photography) and gaffer (the crew responsible for lighting a scene) during pre-production to discuss the use of any screens and ensure that post-production process goes smoothly, and avoids expensive surprises.

Having conversations with a VFX team before a shoot is good due diligence. Any serious VFX shop will be happy to have a friendly conversation before a shoot. Or if you find yourself on a shoot and unsure about something, snap a photo and send it over to me for some real-time advice.

Five minutes on the phone can save $5,000 in unnecessary post-production labor.

Visual Effects for Film & TV - Movie magic for Filmmakers:

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Originally published: 08/24/2021

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