WHAT’S GREEN SCREEN & HOW DOES IT WORK?
Green screen is a visual effects (VFX) technique where two images or video streams are layered—i.e. composited—together. Think about behind-the-scenes clips or bloopers reels from Hollywood movies. It’s hard to miss the sheer amount of green you see on set.
Green screen basically lets you drop in whatever background images you want behind the actors and/or foreground. It’s used in film production (and also in news and weather reports) to relatively simply place the desired background behind the subject/actor/presenter. When a background isn’t available—like a fictional, alien, historic, futuristic or even just hard-to-access location—green screen comes to the rescue!
After the footage is shot, the compositors take over:
- The new background is composited (i.e. two images or video streams are layered together) into the shot.
- The chroma key singles out the selected colour (usually the green) and digitally removes it by rendering it transparent. This lets the other image to show through.
- When used with more sophisticated 3D techniques, this process can add any new element (smoke, fire, rain, etc.) to complex moving shots.
AN IMPORTANT NOTE: WHY GREEN?
Technically, you can use any colour background. A vibrant, almost neon green is the standard choice because it’s strong and usually a distinctly different colour from anything on the subject (e.g. the actor’s clothes, eyes, hair, accessories).
But green doesn’t work for everything. You wouldn’t be able to film Kermit the Frog against a green screen—he’d disappear! In this case, you’d typically use a blue screen, the “second-in-line” colour.
The key rule is no matching colours! The background has to be a completely different colour from the subject. Otherwise, if let’s say your actor is wearing a bright green tie in front of a green screen, he’ll end up with a transparent strip down his chest where the tie is supposed to be!
HOW TO SET UP A GREEN SCREEN SHOOT
- No green or reflective surfaces.
- Avoid green like the plague. Don’t have the subject wear or hold anything green or else the areas will be transparent once chroma-keyed.
- Reflective materials are also a no-no. Shiny objects (e.g. glasses, large jewelry, props, etc.) will pick up the green from the screen and will also be rendered transparent. Very small jewelry is usually okay though.
- Have makeup available.
- Actors can look pale and sickly against a green background. You might need some makeup adjustments.
- Allow depth.
- For a full shot of your actor, you’ll need to back up the camera to allow the full figure shot plus a good separation between the subject and the green screen (usually at least 6 feet to avoid “spill”).
- Depending on the lens, you’ll typically want 25-30 feet depth for a full-length shot that doesn’t shoot off the edges of the background.
- Use a “coved” green screen for best results.
- To show your actor/subject from head to toe, the green screen has to continue down the wall and onto the floor under their feet.
- A cove (a curved corner where the wall meets the floor) will smooth out the transition from wall to floor. You want to avoid shadows and hard lines that cause gradient changes in the green background.
- Light evenly and softly.
- Light the green screen as smoothly as possible to give an even texture and gradient. The more even the lighting, the better and easier it’ll be to manipulate the material.
- Keep the green screen itself as clean and smooth as possible to maintain a consistent colour range. If the screen is fabric, make sure there aren’t any wrinkles. If painted, keep extra paint on hand to touch up any scuffs and scratches.
- Light separately.
- The green background should be lit separately from the subject to avoid: a) the subject casting shadows on the background (causing uneven gradient) and b) a green hue bouncing off the subject, which will cause problems when removing the background.
- This is also why you’ll want at least six feet separation between background and subject.
- Keep the camera still.
- For simple green screen shots, keep the camera completely static. Lock down the camera so the subject doesn’t look like it’s vibrating or bouncing against the background when the camera moves even slightly.
- Zooms also look strange. Your subject will look like it’s shrinking or growing in relation to the background.
- You can use motion control and motion trackers for more sophisticated 3D shots, but it’s a lot more work for the VFX team.
- Use Chroma Key software that works for you.
- Research and consider software based on the shots needed, the editing system, budget, and user experience. Your options for Chroma Key software will essentially depend on the type of shot.
- There’s free, user-friendly software for simpler shots, but they probably won’t be able to pull off the more sophisticated effects you see in feature films.
And that’s pretty much all you need to know to get started! It might seem complicated, but these tips will help your shoot go smoothly. So go ahead and set your story on an alien planet. With time and practice, filming with green screen can open up an entirely new, almost unlimited, creative playground.
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